![As shown earlier, the Banyan switch is internally block-
ing. However, the Banyan switch possesses an interesting
feature. Namely, internal blocking can be avoided if the
cells arriving at the Banyan switch’s input ports are sorted
in ascending order by their destination addresses. The
Batcher–Banyan switch takes advantage of this fact and
uses the Batcher soring network to sort the cells, thereby
making the Batcher–Banyan switch internally nonblock-
ing. The Starlite switch, designed by Bellcore, is based on
the Batcher–Banyan architecture (16).
Crossbar Switches. The crossbar switch interconnects N
inputs and N outputs into a fully meshed topology; that is,
there are N2
cross points within the switch (14). (See
Fig. 12.) Because it is always possible to establish a con-
nection between any arbitrary input and output pair, inter-
nal blocking is impossible in a crossbar switch.
The architecture of the crossbar switch has some advan-
tages. First, it uses a simple two-state cross-point switch
(open and connected state), which is easy to implement.
Second, the modularity of the switch design allows simple
expansion. One can build a larger switch by simply adding
more cross-point switches. Lastly, compared to Banyan-
based switches, the crossbar switch design results in low
transfer latency, because it has the smallest number of
connecting points between input and output ports. One
disadvantage to this design, however, is the fact that it
uses the maximum number of cross points (cross-point
switches) needed to implement an N N switch.
The knockout switch by ATT Bell Labs is a nonblock-
ing switch based on the crossbar design (17,18). It has N
inputs and N outputs and consists of a crossbar-based
switch with a bus interface module at each output (Fig. 12).
Nonblocking Buffered Switches
Although some switches such as Batcher–Banyan and
crossbar switches are internally nonblocking, two or
more cells may still contend for the same output port in a
nonblocking switch, resulting in the dropping of all but one
cell. In order to prevent such loss, the buffering of cells by
the switch is necessary. Figure 13 illustrates that buffers
may be placed (1) in the inputs to the switch, (2) in the
outputs to the switch, or (3) within the switching fabric
itself, as a shared buffer (14). Some switches put buffers in
both the input and output ports of a switch.
The first approach to eliminating output contention is to
place buffers in the output ports of the switch (14). In the
worst case, cells arriving simultaneously at all input ports
can be destined for a single output port. To ensure that no
cells are lost in this case, the cell transfer must be per-
formed at N times the speed of the input links, and the
switch must be able to write N cells into the output buffer
during one cell transmission time. Examples of output
buffered switches include the knockout switch by ATT
Bell Labs, the Siemens Newbridge MainStreetXpress
switches, the ATML’s VIRATA switch, and Bay Networks’
Lattis switch.
The second approach to buffering in ATM switches is to
place the buffers in the input ports of the switch (14). Each
input has a dedicated buffer, and cells that would otherwise
be blocked at the output ports of the switch are stored in
input buffers. Commercial examples of switches with input
buffers as well as output buffers are IBM’s 8285 Nways
switches, and Cisco’s Lightstream 2020 switches.
A third approach is to use a shared buffer within the
switch fabric. In a shared buffer switch, there is no buffer at
the input or output ports (14). Arriving cells are immedi-
ately injected into the switch. When output contention
happens, the winning cell goes through the switch, while
the losing cells are stored for later transmission in a shared
buffer common to all of the input ports. Cells just arriving at
the switch join buffered cells in competition for available
outputs. Because more cells are available to select from, it is
possible that fewer output ports will be idle when using the
shared buffer scheme. Thus, the shared buffer switch can
achieve high throughput. However, one drawback is that
cells may be delivered out of sequence because cells that
arrived more recently may win over buffered cells during
contention (19). Another drawback is the increase in the
number of input and output ports internal to the switch.
The Starlite switch with trap by Bellcore is an example of
the shared buffer switch architecture (16). Other examples
of shared buffer switches include Cisco’s Lightstream 1010
switches, IBM’s Prizma switches, Hitachi’s 5001 switches,
and Lucent’s ATM cell switches.
CONTINUING RESEARCH IN ATM NETWORKS
ATM is continuously evolving, and its attractive ability to
support broadband integrated services with strict quality of
service guarantees has motivated the integration of ATM
and existing widely deployed networks. Recent additions to
ATM research and technology include, but are not limited
to, seamless integration with existing LANs [e.g., LAN
emulation (20)], efficient support for traditional Internet
IP networking [e.g., IP over ATM (21), IP switching (22)],
and further development of flow and congestion control
Figure 11. Batcher–Banyan switch.
Figure 12. A knockout (crossbar) switch.
ASYNCHRONOUS TRANSFER MODE NETWORKS 9](https://image.slidesharecdn.com/88948-250108001425-9f4ecb2b/85/Get-Wiley-Encyclopedia-of-Computer-Science-and-Engineering-5-Volume-Set-1st-Edition-Benjamin-W-Wah-free-all-chapters-17-320.jpg)
![algorithms to support existing data services [e.g., ABR flow
control (12)]. Research on topics related to ATM networks is
currently proceeding and will undoubtedly continue to
proceed as the technology matures.
BIBLIOGRAPHY
1. CCITT Recommendation I-Series. Geneva: International Tele-
phone and Telegraph Consultative Committee.
2. J. B. Kim, T. Suda and M. Yoshimura, International standar-
dization of B-ISDN, Comput. Networks ISDN Syst., 27: 1994.
3. CCITT Recommendation G-Series. Geneva: International Tel-
ephone and Telegraph Consultative Committee.
4. ATM Forum Technical Specifications [Online]. Available www:
www.atmforum.com
5. Report of ANSI T1S1.5/91-292, Simple and Efficient Adapta-
tion Layer (SEAL), August 1991.
6. Report of ANSI T1S1.5/91-449, AAL5—A New High Speed
Data Transfer, November 1991.
7. CCITT Recommendation Q-Series. Geneva: International Tel-
ephone and Telegraph Consultative Committee.
8. J. Bae and T. Suda, Survey of traffic control schemes and
protocols in ATM networks, Proc. IEEE, 79: 1991.
9. B. J. Vickers et al., Congestion control and resource manage-
ment in diverse ATM environments, IECEJ J., J76-B-I (11):
1993.
10. J. S. Turner, New directions in communications (or which way
to the information age?), IEEE Commun. Mag., 25 (10): 1986.
11. G. Gallassi, G. Rigolio, and L. Fratta, ATM: Bandwidth
assignment and bandwidth enforcement policies. Proc.
GLOBECOM’89.
12. ATM Forum, ATM Forum Traffic management specification
version 4.0, af-tm-0056.000, April 1996, Mountain View, CA:
ATM Forum.
13. Quantum Flow Control version 2.0, Flow Control Consortium,
FCC-SPEC-95-1, [Online], July 1995. http://www.qfc.org
14. Y. Oie et al., Survey of switching techniques in high-speed
networks and their performance, Int. J. Satellite Commun., 9:
285–303, 1991.
15. M. De Prycker and M. De Somer, Performance of a service
independent switching network with distributed control, IEEE
J. Select. Areas Commun., 5: 1293–1301, 1987.
16. A. Huang and S. Knauer, Starlite: A wideband digital switch.
Proc. IEEE GLOBECOM’84, 1984.
17. K. Y. Eng, A photonic knockout switch for high-speed packet
networks, IEEE J. Select. Areas Commun., 6: 1107–1116, 1988.
18. Y. S. Yeh, M. G. Hluchyj, and A. S. Acampora, The knockout
switch: A simple, modular architecture for high-performance
packet switching, IEEE J. Select. Areas Commun., 5: 1274–
1283, 1987.
19. J. Y. Hui and E. Arthurs, A broadband packet switch for
integrated transport, IEEE J. Select. Areas Commun., 5:
1264–1273, 1987.
20. ATM Forum, LAN emulation over ATM version1.0. AF-LANE-
0021, 1995, Mountain View, CA: ATM Forum.
21. IETF, IP over ATM: A framework document, RFC-1932, 1996.
22. Ipsilon Corporation, IP switching: The intelligence of
routing, The Performance of Switching [Online]. Available
www.ipsiolon.com
TATSUYA SUDA
University of California, Irvine
Irvine, California
Figure 13. Nonblocking buffered
switches.
10 ASYNCHRONOUS TRANSFER MODE NETWORKS](https://image.slidesharecdn.com/88948-250108001425-9f4ecb2b/85/Get-Wiley-Encyclopedia-of-Computer-Science-and-Engineering-5-Volume-Set-1st-Edition-Benjamin-W-Wah-free-all-chapters-18-320.jpg)
![improvements in memory size, clock speeds, functionality,
and overall computer performance.
Personal Computers
Landau (1) defines personal computers as electronic
machines that can be owned and operated by individuals
for home and business applications such as word proces-
sing, games, finance, and electronic communications.
Hamacher et al. (3) explain that rapidly advancing very
large-scale integrated circuit (VLSI) technology has
resulted in dramatic reductions in the cost of computer
hardware. The greatest impact has been in the area of small
computing machines, where it has led to an expanding
market for personal computers.
The idea of a personally owned computer is fairly new.
The computational power available in handheld toys today
was only available through large, costly computers in
the late 1950s and early 1960s. Vendors such as Atari,
Commodore, and Compaq made simple computer games
household items. Performance improvements in memory,
throughput, and processing power by companies such as
IBM, Intel, and Apple made facilities such as spreadsheets
for home budgets, automated tax programs, word proces-
sing, and three-dimensional virtual games common house-
hold items. The introduction of Microsoft’s Disk Operating
System (DOS) and Windows has also added to the accep-
tance of the personal computers through access to software
applications. Improvements in computer technology offer
continual improvements, often multiple times a year. The
durability and portability of these computers is beginning
to allow them to replace specialized aircraft computers
that had strict weight, size, power, and functionality
requirements.
AVIONICS
In the early years of aircraft flight, technological innovation
was directed at improving flight performance through
rapid design improvements in aircraft propulsion and
airframes. Secondary development energies went to areas
such as navigation, communication, munitions delivery,
and target detection. The secondary functionality of
aircraft evolved into the field of avionics. Avionics now
provides greater overall performance and accounts for a
greater share of aircraft lifecycle costs than either propul-
sion or airframe components.
Landau (1) definesavionics [avi(ation) þ (electr)onics] as
the branch of electronics dealing with the development and
use of electronic equipment in aviation and astronautics.
The field of avionics has evolved rapidly as electronics has
improved all aspects of aircraft flight. New advances in
these disciplines require avionics to control flight stability,
which was traditionally the pilot’s role.
Aircraft Antennas
An important aspect of avionics is receiving and transmit-
ting electromagnetic signals. Antennas are devices for
transmitting and receiving radio-frequency (RF) energy
from other aircraft, space applications, or ground applica-
tions. Perry and Geppert (4) illustrate the aircraft electro-
magnetic spectrum, influenced by the placement and usage
of numerous antennas on a commercial aircraft. Golden (5)
illustrates simple antenna characteristics of dipole, horn,
cavity-backed spiral, parabola, parabolic cylinder, and
Cassegrain antennas.
Radiation pattern characteristics include elevation and
azimuth. The typical antenna specifications are polariza-
tion, beam width, gain, bandwidth, and frequency limit.
Computers are becoming increasingly important for the
new generation of antennas, which include phased-array
antennas and smart-skin antennas. For phased-array
antennas, computers are needed to configure the array
elements to provide direction and range requirements
between the radar pulses. Smart-skin antennas comprise
the entire aircraft’s exterior fuselage surface and wings.
Computers are used to configure the portion of the aircraft
surface needed for some sensor function. The computer also
handles sensor function prioritization and deinterleaving
of conflicting transmissions.
Aircraft Sensors
Sensors, the eyes and ears of an aircraft, are electronic
devices for measuring external and internal environmental
conditions. Sensors on aircraft include devices for sending
and receiving RF energy. These types of sensors include
radar, radio, and warning receivers. Another group of
sensors are the infrared (IR) sensors, which include lasers
and heat-sensitive sensors. Sensors are also used to mea-
sure direct analog inputs; altimeters and airspeed indica-
tors are examples. Many of the sensors used on aircraft
have their own built-in computers for serving their own
functional requirements such as data preprocessing, filter-
ing, and analysis. Sensors can also be part of a computer
Figure 1. Typical aircraft central computer.
2 AIRCRAFT COMPUTERS](https://image.slidesharecdn.com/88948-250108001425-9f4ecb2b/85/Get-Wiley-Encyclopedia-of-Computer-Science-and-Engineering-5-Volume-Set-1st-Edition-Benjamin-W-Wah-free-all-chapters-20-320.jpg)
![foot pedal movements. Today, hydraulic control, the use of
fluids for activation, is typical. Aircraft control surfaces are
connected to stick and foot pedals through hydraulic lines.
Pistons in the control surfaces are pushed or pulled by
associated similar pistons in the stick or foot pedal. The
control surfaces move accordingly.
Electronic control is the use of electronic devices, such as
motors or relays, to regulate a system. A motor is turned on
by a switch, and it quickly changes control surfaces by
pulling or pushing a lever on the surface. Automatic control
is a system-initiated control, which is a system-initiated
response to a known set of environmental conditions. Auto-
matic control was used for early versions of automatic pilot
systems, which tied flight control feedback systems to
altitude and direction indicators. The pilot sets his desired
course and altitude, which is maintained through the flight
control’s automatic feedback system.
To understand the need for computers in these control
techniques, it is important to note the progression of the
complexity of the techniques. The earliest techniques con-
nected the pilot directly to his control surfaces. As the
aircraft functionality increased, the pilot’s workload also
increased, requiring his (or his aircrew’s) being free to
perform other duties. Additionally, flight characteristics
became more complex, requiring more frequent and instan-
taneous control adjustments. The use of computers helped
offset and balance the increased workload in aircraft. The
application of computers to flight control provides a means
for processing and responding to multiple complex flight
control requirements.
Aircraft Computer Hardware
For aircraft computers, hardware includes the processors,
buses, and peripheral devices inputting to and outputting
from the computers. Landau (1) defines hardware as appa-
ratus used for controlling a spacecraft; the mechanical,
magnetic, and electronic design, structure, and devices of
a computer; and the electronic or mechanical equipment
that uses cassettes, disks, and so on. The computers used
on an aircraft are called processors. The processor takes
inputs from peripheral devices and provides specific com-
putational services for the aircraft.
There are many types and functions of processors on an
aircraft. The most obvious processor is the central compu-
ter, also called the mission computer. The central computer
provides direct control and display to the aircrew. The
federated architecture (discussed in more detail later) is
based on the central computer directing the scheduling and
tasking of all the aircraft subsystems. Other noteworthy
computers are the data processing and signal processing
computers of the radar subsystem and the computer of the
inertial navigation system. Processors are in almost every
component of the aircraft. Through the use of an embedded
processor, isolated components can perform independent
functions as well as self-diagnostics.
Distributed processors offer improved aircraft perfor-
mance and, in some cases, redundant processing capability.
Parallel processors are two or more processors configured
to increase processing power by sharing tasks. The
workload of the shared processing activity is distributed
among the pooled processors to decrease the time it takes to
form solutions. Usually, one of the processors acts as the
lead processor, or master, while the other processor(s) act
as slave(s). The master processor schedules the tasking and
integrates the final results, which is particularly useful on
aircraft in that processors are distributed throughout the
aircraft. Some of these computers can be configured to be
parallel processors, offering improved performance and
redundancy. Aircraft system redundancy is important
because it allows distributed parallel processors to be
reconfigured when there is a system failure. Reconfigur-
able computers are processors that can be reprogrammed
to perform different functions and activities. Before com-
puters, it was very difficult to modify systems to adapt to
their changing requirements. A reconfigurable computer
can be dynamically reprogrammed to handle a critical
situation, and then it can be returned to its original
configuration.
Aircraft Buses
Buses are links between computers (processors), sensors,
and related subsystems for transferring data inputs and
outputs. Fink and Christiansen (8) describe two primary
buses as data buses and address buses. To complete the
function of an MPU, a microprocessor must access memory
and peripheral devices, which is accomplished by placing
data on a bus, either an address bus or a data bus, depend-
ing on the function of the operation. The standard 16-bit
microprocessor requires a 16-line parallel bus for each
function. An alternative is to multiplex the address or
data bus to reduce the number of pin connections. Common
buses in aircraft are the Military Standard 1553 Bus (Mil-
Std-1553) and the General-Purpose Interface Bus (GPIB),
which is the IEEE Standard 488 Bus.
Aircraft Software
Landau (1) defines software as the programs, routines, and
so on for a computer. The advent of software has provided
great flexibility and adaptability to almost every aspect of
life, which is especially true in all areas of aerospace
sciences, where flight control, flight safety, in-flight enter-
tainment, navigation, and communications are continu-
ously being improved by software upgrades.
Operation Flight Programs. An operational flight pro-
gram (OFP) is the software of an aircraft embedded com-
puter system. An OFP is associated with an aircraft’s
primary flight processors, including the central computer,
vertical and multiple display processors, data processors,
signal processors, and warning receivers. Many OFPs in
use today require dedicated software integrated support
environments toupgrade and maintain them asthe mission
requirements of their parent aircraft are modified. The
software integrated support environment [also called avio-
nics integrated support environment (AISE), centralized
software support activity (CSSA), and software integration
laboratory (SIL)] not only allows an OFP to be updated and
maintained, but also provides capabilities to perform unit
AIRCRAFT COMPUTERS 5](https://image.slidesharecdn.com/88948-250108001425-9f4ecb2b/85/Get-Wiley-Encyclopedia-of-Computer-Science-and-Engineering-5-Volume-Set-1st-Edition-Benjamin-W-Wah-free-all-chapters-23-320.jpg)
![dictionary scene (22–26). Lew (3) noted these recent trends
stating:
It has been claimed that with the move from paper to online
dictionaries, restrictions of space would disappear. That, how-
ever, is a simplification at best. While storage space may indeed
become irrelevant, there are still severe restrictions as to how
much information can be displayed at a time. In fact, even the
bestcurrently availabledisplaydevicesare stilleasilybeatenby
the old-fashioned printed paper in terms of visual resolution. So
space-saving issues will still be with for at least as long as the
visual modality is primarily used for information transfer from
dictionary to user. . .on-screen presentation of entries has much
to offer. . .to the researcher by way of convenience, including a
potential to log responses automatically, thus obviating the
need for the laborious paperwork and keyboarding at the
data entry stage, as well as allowing ‘‘unobtrusive observation’’.
(p. 157)
The equivalence of on-screen and paper formats should
not be taken for granted, as Laufer (27) found significant
and substantial differences in word recall scores between
marginal paper glosses and on-screen pop-up window
glosses.
DOING LEXICOGRAPHY IN AN ELECTRONIC AGE
Tono (28) predicted the advantages of online media using
machine translation, saying ‘‘Electronic dictionaries have
great potential for adjusting the user interface to users’
skill level[s] so that learners with different needs and skills
can access information in. . . different way[s].’’ (p. 216)
First of all, one must note that electronic dictionaries
have developed based on a healthy integration of develop-
ments in computerized corpus linguistics and modern
technology, used to enhance learning in many fields,
particularly computer-assisted language learning
(or CALL) or computer-mediated communications
(CMC).
Laufer and Kimmel (2) provide a clear summary of this
field, noting that
If the consumer is to benefit from the lexicographer’s product,
the dictionary should be both useful and usable. We suggest a
definition of dictionary usefulness as the extent to which a
dictionary is helpful in providing the necessary information
to its user. Dictionary usability, on the other hand, can be
defined as the willingness on the part of the consumer to use
the dictionary in question and his/her satisfaction from it.
Studies of dictionary use by L2 learners . . . reveal that dic-
tionary usefulness and dictionary usability do not necessarily
go hand in hand. (pp. 361–362)
Laufer and Levitzky-Aviad’s (4) study recommends
working toward designing a bilingualized electronic dic-
tionary (BED) more clear and useful for second language
production. Whereas conventional bilingual L1-L2 diction-
aries list translation options for L1 words without explain-
ing differences between them or giving much information
about how to use various functions, Laufer and Levitzky-
Aviad (4) examined the usefulness of an electronic Hebrew-
English-English (L1-L2-L2) minidictionary designed for
production. Their results demonstrated the superiority of
fully bilingualized L1-L2-L2 dictionaries and some unique
advantages of the electronic format. Their literature review
provides a good overview of this field:
Surveys of dictionary use indicate that the majority of foreign
language learners prefer bilingual L2-L1 dictionaries and use
them mainly to find the meaning of unknown foreign (L2) words
(Atkins 1985; Piotrowsky 1989). However, if learners writing in
L2 need an L2 word designating a familiar L1 concept, they do
not readily turn to an L1-L2 dictionary for help. The reason for
this may lie in a serious limitation of most L1-L2 bilingual
dictionaries. They rarely differentiate between the possible L2
translations of the L1 word, nor do they provide information
regarding the use of each translation option. . . An electronic
dictionary can fulfill the above requirements since it can com-
bine the features of an L2-L1bilingual dictionary, an L1-L2
bilingual dictionary and an L2 monolingual dictionary. The
advent of electronic dictionaries has already inspired research
into their use and their usefulness as on-line helping tools and
as contributors to incidental vocabulary learning. The built in
log files can keep track of words looked up, type of dictionary
information selected (definition, translation, example, etc.), the
number of times each word was looked up, and the time spent on
task completion. (pp. 1–2)
Although most electronic dictionaries do auto-
archiving of any new words by means of their history
search function, most online dictionaries do not have
a means of tracking student use, except for programs
like Wordchamp.com or Rikai.com, which give students a
way to archive words they have double-clicked. These
words may later be seen, printed, and reviewed. In
fact, Wordchamp.com, by far the most sophisticated online
electronic dictionary and vocabulary development pro-
gram, allows users to make online flashcards with sentence
examples and links to online texts where target words are
found in context. It can also automatically generate about
10 types of online vocabulary quizzes and provides a free
course management system (CMS) for monitoring stu-
dents’ work online. Wordchamp’s Webreader provides the
most versatile online glossing engine known, already for
over 100 languages, with more being added regularly.
Teachers need to show learners how to best inte-
grate the use of such portable and online dictionaries to
make them maximally effective for their language develop-
ment, in both receptive and productive aspects. Chun (12)
noted that learners who could read online text with ‘‘access
to both internally (instructor-created) glossed words as well
as externally glossed words. . . recalled a significantly
greater number of important ideas than when they read
an online text and had access only to an external (portable
electronic) dictionary’’ (p. 75).
Loucky (29) also examined how to best maximize L2
vocabulary development by using a depth of lexical proces-
sing (DLP) scale and vocabulary learning strategies (VLSs)
taxonomy together with online CALL resources and sys-
tematic instruction in the use of such strategies. It used
40 of the 58 VLSs identified in Schmitt’s earlier taxonomy.
An electronic dictionary use survey (see Appendix) was
designed to solicit information about how students used
various computerized functions of electronic or online
2 COMPUTERIZED DICTIONARIES](https://image.slidesharecdn.com/88948-250108001425-9f4ecb2b/85/Get-Wiley-Encyclopedia-of-Computer-Science-and-Engineering-5-Volume-Set-1st-Edition-Benjamin-W-Wah-free-all-chapters-31-320.jpg)
![dictionaries at each major phase of lexical processing to
help learners maximize processing in the following eight
stages of vocabulary learning: (1) assessing degree of word
knowledge, (2) accessing new word meanings, (3) archiving
new information for study, (4) analyzing word parts and
origins, (5) anchoring new words in short-term memory,
(6) associating words in related groups for long-term reten-
tion, (7) activating words through productive written or
oral use, and (8) reviewing/recycling and then retesting
them. Portable devices or online programs that could moni-
tor and guide learners in using these essential strategies
should be further developed.
In Loucky’s (7) findings, despite being one grade level
higher in their proficiency, English majors were out-
performed on all types of electronic dictionaries by Com-
puter majors. The author concluded that familiarity with
computerized equipment or computer literacy must have
accounted for this, and therefore should be carefully con-
sidered when developing or using electronic dictionary
programs of any sort for language or content learning.
His study compared vocabulary learning rates of Japanese
college freshmen and functions of 25 kinds of electronic
dictionaries, charting advantages, disadvantages, and
comments about the use of each (for details, see Loucky
(7) Table 1 and Appendix 3; Loucky (8) Tables 1 and 2.
For a comparative chart of six most popular EDs for
English-Japanese use, see www.wordtankcentral.com/
compare.html).
Generally speaking, language learners prefer access to
both first and second language information, and beginning
to intermediate level learners are in need of both kinds of
data, making monolingual dictionaries alone insufficient
for their needs. As Laufer and Hadar (30) and others have
shown the benefits of learners using fully bilingualized
dictionaries, the important research question is to try to
determine which kinds of electronic portable, software, or
online dictionaries offer the best support for their needs.
Grace (31) found that sentence-level translations should be
included in dictionaries, as learners having these showed
better short- and long-term retention of correct word mean-
ings. This finding suggests a close relationship exists
between processing new terms more deeply, verifying their
meanings, and retaining them.
Loucky (32) has researched many electronic dictionaries
and software programs, and more recently organized links
to over 2500 web dictionaries, which are now all accessible
from the site http://www.call4all.us///home/_all.php?fi=d.
His aim was to find which kind of EDs could offer the most
language learning benefits, considering such educational
factors as: (1) better learning rates, (2) faster speed of
access, (3) greater help in pronunciation and increased
comprehensibility, (4) providing learner satisfaction with
ease of use, or user-friendliness, and (5) complete enough
meanings to be adequate for understanding various read-
ing contexts.
As expected, among learners of a common major, more
proficient students from four levels tested tended to use
EDs of all types more often and at faster rates than less
language-proficient students did. In brief, the author’s
studies and observations and those of others he has cited
[e.g., Lew (3)] have repeatedly shown the clear benefits of
using EDs for more rapid accessing of new target vocabu-
lary. They also point out the need for further study of
archiving, and other lexical processing steps to investigate
the combined effect of how much computers can enhance
overall lexical and language development when used more
intelligently and systematically at each crucial stage of first
or second language learning. Regular use of portable or
online electronic dictionaries in a systematic way that uses
these most essential phases of vocabulary acquisition cer-
tainly does seem to help stimulate vocabulary learning and
retention, when combined with proper activation and recy-
cling habits that maximize interactive use of the target
language. A systematic taxonomy of vocabulary learning
strategies (VLSs) incorporating a 10-phase set of specific
recyclable strategies is given by Loucky (7,29) to help
advance research and better maximize foreign language
vocabulary development (available at http://www.call4all.
us///home/_all.php?fi=../misc/forms).
A summary of Laufer and Levitzky-Aviad’s (4) findings
is useful for designers, sellers, and users of electronic
dictionaries to keep in mind, as their study showed that:
‘‘the best dictionaries for L2 written production were the
L1-L2-L2 dictionaries. . . Even though the scores received
with the paper version of the L1-L2-L2 dictionary were just
as good, the electronic dictionary was viewed more favor-
ably than the paper alternative by more learners. Hence, in
terms of usefulness together with user preference, the
electronic version fared best’’ (p. 5). Such results should
motivate CALL engineers and lexicographers to produce
fully bilingualized electronic dictionaries (as well as print
versions), specifically designed not merely to access recep-
tive information to understand word meanings better, but
also for L2 production, to practically enable students to
actively use new terms appropriately as quickly as possible.
SURVEYING USE OF ELECTRONIC DICTIONARIES
To more thoroughly analyze and compare the types of
dictionaries being used by Japanese college students in
three college engineering classes, two kinds of surveys
were designed by Loucky (29). The first was a general
survey about purchase, use, and preferences regarding
electronic dictionaries. The second survey (shown in the
Appendix) asked questions about how various computer-
ized functions were used at each major phase of lexical
processing. The aim was to help learners maximize these
eight essential phases of vocabulary learning: (1) assessing
degree of word knowledge; (2) accessing new word mean-
ings; (3) archiving new information for study; (4) analyzing
word parts and origins; (5) anchoring new words in
short-term memory; (6) associating words in related groups
for long-term retention; (7) activating words through pro-
ductive written or oral use; and (8) reviewing/recycling and
re-testing them. After re-evaluating how well new words
are learned by post-tests, any words not fully understood
should be remet through planned re-encounters, retellings,
and activities that encourage learners to repeat the voca-
bulary learning cycle again so that relearning and reacti-
vation can take place.
COMPUTERIZED DICTIONARIES 3](https://image.slidesharecdn.com/88948-250108001425-9f4ecb2b/85/Get-Wiley-Encyclopedia-of-Computer-Science-and-Engineering-5-Volume-Set-1st-Edition-Benjamin-W-Wah-free-all-chapters-32-320.jpg)
![The first survey described Japanese college students’
preferences and reasons for purchasing EDs. The second
showed self-reported use of PEDS and how their respective
functions were seen to aid in different phases of L2 voca-
bulary learning. Students compared their use to that of
print dictionaries. A majority of East Asian students sur-
veyed expressed a preference for using mobile or online
dictionaries rather than carry bulkier book dictionaries,
although a few English students carry both. These ED
preferences and patterns of use need more investigation,
but probably hold true wherever the level of economic
development is sufficient to support their purchase, as
well as the use and availability of Internet access to online
dictionary and Webreader glossing functions.
Kobayashi (33) compared the use of pocket electronic
versus printed dictionaries to examine the effects of their
use on LPSs used. The three major strategies she distin-
guished were consulting, inferring versus ignoring new
terms. She found that ‘‘Pocket electronic dictionaries
(PEDs) are rapidly becoming popular among L2 learners.
Although many L2 learners depend on dictionaries, the
prevalent view among L2 researchers and educators is that
learners should use dictionaries sparsely. They encourage
students to use another lexical processing strategy (LPS),
contextual guessing, for better vocabulary learning and
reading comprehension. [But] are dictionaries indeed so
harmful?’’ (p. 2).
As some educators and researchers have been con-
cerned about the pedagogical value of EDs because of their
perceived limitations, such as insufficient information
provided, the possibility of discouraging contextual gues-
sing, and a supposed negative impact on word retention
(34-38), these educators’ and researchers’ concerns require
more investigation. So far, however, language learners’
preference for them, and EDs’ rapidly improving functions
appear to be scuttling most of these previous claims.
Although native readers have far larger working vocabul-
aries to guess from context, most second language readers
prefer and benefit greatly from having both monolingual
and bilingual/mother tongue glosses available to them.
Kobayashi (39) found that
1. More than two-thirds of the students owned a PED, and
most of those who owned a PED exclusively used it regard-
less of purposes.
2. The PEDs owned by most students cost $100–$400, were
of high quality, and did not have the disadvantages
identified in other studies, such as brief definitions, the
absence of examples, and inaccurate information.
3. Most students were satisfied with their PEDs, especially
with their portability, and ease to look up a word, and ease
to change from one dictionary to another.
4. The perceived disadvantages included the relative una-
vailability (or inaccessibility) of detailed usage informa-
tion, examples, and grammatical information.
5. PEDs enabled students to use different types of diction-
aries in different places.
6. Although both PED users and PD users depended on
dictionaries, PED users used dictionaries more often.
This was especially the case with smaller vocabulary
size students.
7. PD users and PED users did not significantly differ
in terms of their LPS use, except for the sheer frequency
of dictionary consultation.
8. There was a possibility that PED users consulted diction-
aries at the expense of contextual guessing.
9. Although students depended on dictionaries, whether
PEDs or PDs, they also used guessing strategies fre-
quently. They often used a dictionary to confirm guessed
meaning. This was particularly the case with successful
students.
10. Larger and smaller vocabulary size students differed in
their use of LPSs such as basic dictionary use, extended
dictionary use for meaning, extended dictionary use for
usage, extended dictionary use for grammatical informa-
tion, lookup strategies, note-taking strategies, guessing
strategies using immediate context, guessing strategies
using wider context, combined use of LPSs, and selective
use of LPSs.
11. Higher and lower reading ability students differed in their
use of LPSs such as basic dictionary use, extended dic-
tionaryuseformeaning,extendeddictionaryuseforusage,
extended dictionary use for grammatical information,
lookup strategies, self-initiation, note-taking strategies,
SYSTRANSoft 15 Language Pairs
http://www.systransoft.com/
Can translate a web page, a file (TXT, RTF, or HTML) or up to 150 words of text.
Quality varies by language pair and subject matter. Check results with a human
translator.
Tarjim (Registration Required) 1 Language Pair
http://tarjim.ajeeb.com/ English Arabic
Wordchamp.com Over 100 Language Pairs
http://wordchamp.com
Free to all.
Instant Glossing; Auto-Archiving; Online
Flashcard and Test Creation; Files can be
shared internationally between distance
learners, as well as internally within intact
classes, using its currently free Course
Management System (CMS).
*
Free unless stated otherwise. Summarized from site by author.
COMPUTERIZED DICTIONARIES 5](https://image.slidesharecdn.com/88948-250108001425-9f4ecb2b/85/Get-Wiley-Encyclopedia-of-Computer-Science-and-Engineering-5-Volume-Set-1st-Edition-Benjamin-W-Wah-free-all-chapters-34-320.jpg)
![GENERAL TRENDS AND FUTURE FRUITFUL
RESEARCH AREAS
Major benefits of using portable devices include their mobi-
lity and instant archiving or storage in history memos for
future useful quick review. Web dictionaries like those
organized at the author’s site, however, provide much
more potential, as one can copy and paste between any of
over 2000 online lexicons organized there for over 500
language pairs. www.CALL4ALL.us provides a ‘‘Virtual
Rosetta Stone,’’ not only of the full range of monolingual
and multilingual web dictionaries, but also a vast language
education links library for studying most of these lan-
guages as well.
Another main advantage of modern translation technol-
ogy is that it is much more efficient. One saves a lot of time,
as there is no more turning of book pages and searching for
words endlessly. Words you are looking for are at your
fingertips, just one click away. Each online dictionary has
400,000 entries, for example, in the case of Ectaco pro-
grams, and far more are freely available from web diction-
aries organized at www.CALL4ALL.us’s dictionaries page
at http://www.call4all.us///home/_all.php?fi=d. Recommen-
dations for integrating the use of web dictionaries with
language learning programs online are given in Loucky
(32). The 10 types of sites are organized to help teachers and
students more efficiently combine the benefits of electronic
and online dictionaries with CALL websites to produce
more effective and enjoyable content and language learning
lessons.
The general trends over the past 10 years have been for
PEDs to become more prevalent because of their speedy
access to language meanings, grammar data, collocations/
corpus examples, compact size, improved features, and
convenience of use as well as economical pricing. Some
feature as many as 32 lexicons or more, pronunciation
support, Internet connectivity, review games, automatic
history of searches for review, and so on. Translation soft-
ware and CD-ROM dictionaries, being more expensive and
limited to the location of one’s PC, have not become as
popular. Web and phone dictionaries appear to be the ‘‘tool
of choice’’ of most students, as these functions are often
provided at their schools or included in their cell phone
services at little or no extra charge. Assistive reading pens
made by Quickionary also offer promise to those who can
afford them. They also seem to enhance learners’ interest
and motivation levels, and thus help to contribute to higher
levels of vocabulary retention, although how to best do
so online is a major question in need of further study.
Some of the most promising online glossing programs being
tested now can be recommended for further research in this
area: 1) Wordchamp.com, 2) Rikai.com, 3) Wordsurfing.-
com, and 4) Babelfish.com.
CONCLUSIONS AND RECOMMENDATIONS
To conclude, CALL and website e-learning developers need
to remember that teachers need to be able to scale their
language and vocabulary learning activities from those
that require simpler and easier processing for lower level
students, to activities that require deeper and more com-
plex lexical processing for more advanced language lear-
ners using various kinds of EDs, both online and offline,
whether stationary or mobile. It is also important for
teachers to give more clear guidance about particular kinds
of EDs, especially including good online programs for
learning, to help maximize the use of their functions for
education. We can only help maximize each program’s
effectiveness if students learn how to use their various
functions as efficiently as possible to help them at each
stage of processing new words outlined above. Further
helpful guidelines and goals to examine when seeking to
integrate new insights and innovations from CALL into the
field of foreign language reading and vocabulary develop-
ment are given by Sokmen (46). In her words, among the
many areas in need of further systematic research in this
field, ‘‘we need to take advantage of the possibilities inher-
ent in computer-assisted learning, especially hypertext
linking, and create software which is based on sound prin-
ciples of vocabulary acquisition theory . . . programs which
specialize on a useful corpus. . . provide. . .[for] expanded
rehearsal, and engage the learner on deeper levels and in a
variety of ways as they practice vocabulary. There is also
the fairly unchartered world of the Internet as a source for
meaningful activities for the classroom and for the inde-
pendent learner’’ (p. 257).
In this way, using proven portable devices, multimedia
translation software, and well-designed, interactive web-
sites as much as possible, language learning can be made
much more interesting and effective as these CALL
resources are all used as tools for developing more balanced
communication skills, which emphasize blending active
production and interactive, content-based learning with
authentic tasks and materials made much more accessible,
comprehensible, and memorable with the help of modern
technology. All in all, we can be quite optimistic about the
future of EDs, as de Schryver (25) is. Listing 118 ‘‘lexico-
graphers’ dreams’’ in summarized tables, he masterfully
‘‘incorporates almost every speculation ever made about
electronic dictionaries (EDs)’’ (p. 61) in Roby’s terms (47).
Roby (47) further notes that not only technical hard-
ware, but also human ‘‘fleshware’’ is the most crucial ele-
ment when designing EDs, otherwise users may drown in
a sea of data. One cannot drink efficiently from a fire hose.
As he states, ‘‘Sophisticated software and huge hardware
cannot guarantee the quality of an electronic dictionary. . .
Good online dictionaries will be equipped with ‘spigots’
that allow users to draw manageable amounts of
information. . . Information must be internalized for it to
be considered knowledge.’’ In the vast reaches of virtual
e-learning cyberspace, one does indeed require a common
gold standard compass, or better yet, a virtual Rosetta
Stone for language learning, such as those helpful sites
provided here.
As second language learners venture into ‘‘terra incog-
nita’’ they do need clear maps and strategies to improve
their navigation on various WebQuests for knowledge.
Roby (47, p. 63) correctly asserts that ‘‘Dictionaries can
be guides because they ‘potentially intersect with every text
of the language: in a sense all texts lead to the dictionary’
(quoting Nathan). . . Learners can make forays into cyber-
8 COMPUTERIZED DICTIONARIES](https://image.slidesharecdn.com/88948-250108001425-9f4ecb2b/85/Get-Wiley-Encyclopedia-of-Computer-Science-and-Engineering-5-Volume-Set-1st-Edition-Benjamin-W-Wah-free-all-chapters-37-320.jpg)
![space with an electronic dictionary as a navigational [tool].
And in a real sense, one can expect to see portable, wireless
dictionaries that will both allow physical mobility and
afford Internet access.’’ (In fact, most mobile phones and
WiFi laptops already do).
Tailoring computerized dictionaries to effectively sup-
port learners’ needs will require specific attention to their
types, functions, and uses to best guide learners and tea-
chers to most effective integration of these portable and
online tools into language and science education. Research
is showing us that all future EDs would do well to include
preorganized categories of terms, searchable by topic and
semantic field. Five examples of these already found online
include: 1) UCREL’s Semantic Analysis System located at
http://www.comp.lancs.ac.uk/ucrel/usas/ with 21 major
A–Z discourse fields; 2) Variation in English Words and
Phrases (VIEW) at http://view.byu.edu/; 3) this writer’s
bilingualized Semantic Field Keyword Approach covering
about 2000 intermediate to advanced terms in nine aca-
demic disciplines found at: http://www.call4all.us///misc/
sfka.php; 4) ThinkMap’s Visual Thesaurus at http://
www.visualthesaurus.com/index.jsp?vt ; and 5) Wordnet
found at http://wordnet.princeton.edu/. This writer’s
www.CALL4ALL.us site helps to integrate essential, com-
mon core vocabulary in many of these academic disciplines
with most web dictionaries for 500 major world language
pairs. For an overview, see its site map at ( http://www.
call4all.us///home/_all.php?fi=0) or see Loucky (32,48,49).
In the final analysis, probably what learners are guided
to do with new terms will prove to be a more important
learning factor than multimedia glossing and text concor-
dancer options alone can provide. New technologies do
indeed offer more powerful resources than ever before for
independent or classroom study of languages. Word learn-
ing options will probably be best maximized when comput-
ing power is used to enhance learners’ access to various
types of EDs of high quality simultaneously in all fields,
while likewise providing them with the means to auto-
archive and organize new target vocabulary as they are
shown how to actively use these new terms productively as
soon as possible.
APPENDIX
Survey of Computerized Bilingual Dictionaries (27)
Name your Book Dictionary or Electronic/Compu-
terized Bilingual Dictionary:
Model #: Cost:
NAME:
Accessing Archiving
Time: ____________minutes
(for 15 Laufer Hadar
terms)
ID/YEAR: Reading Level:
a. Grade:
b. Headwords:
c. %VLS Used:
d. DLP Level:
e. AVQ/IP:
1. Assessing Vocabulary Size:
Check your manual to see how many words it has for
a. English:
b. Japanese—(or other L1):
c. Kanji Study—
d. How many words do you think you know in English?
2. Accessing—Frequency of Use—How many times
do you use it each day?
a. For English to Japanese what % of the time?
b. For Japanese to English, what % of the time?
c. To check unknown Kanji, what % of the time?
3. Archiving—How do you record new words found?
a. In my textbook in the margins
b. On paper or in a Vocabulary Notebook
c. I don’t record new words
d. My CBD can record and save new words I’ve looked
up. If so, tell how:
e. Can it do Automatic Recording and Review (of last 1–
20 words) (called a History Search)
f. Can you save and store new words manually?
g. Can you Save and Print Text Files or Notes on new
words?
4. Analyzing Special Functions or Features—Does
your CBD have any Special Functions or Features which
help you to break up new words into parts to better
understand their grammar, origins or meaning?
If so, please try to explain how to use them and tell how
often you do so. (Use Manual)
Does it give special information about word parts, gram-
mar, or the origin of words?
Does it give any common phrases? _____Yes ______No
____Not Sure
Does it give any sentence examples? ____Yes ____No
____Not Sure
5. Anchoring New Words in Memory—Does your Elec-
tronic Dictionary have any special Visual Images or
Auditory Sounds or other special functions to help illus-
trate new word meanings, forms or use to help you better
remember them? ___Yes _____No
If so, tell what these special functions are and try to
explain how they work to help you fix new words in
your memory.
6. Associating Functions—Does your Electronic
Dictionary help you to organize your vocabulary
learning in any way?
For example, can you put words into Study Groups?
Do you organize your vocabulary learning or notebook
in any special order or way to help you remember
new words? Do you group any words together to
better remember or learn them? If so, please tell
how you do so.
COMPUTERIZED DICTIONARIES 9](https://image.slidesharecdn.com/88948-250108001425-9f4ecb2b/85/Get-Wiley-Encyclopedia-of-Computer-Science-and-Engineering-5-Volume-Set-1st-Edition-Benjamin-W-Wah-free-all-chapters-38-320.jpg)
![nontrivial. Algorithms for modulation recognition have
been described in various papers, of which Refs. 19–22
are representative examples.
A related idea is based on the observation that the signal
waveforms generated by practical radio transmitters will
differ in subtle ways depending on implementation details
and component tolerances, and that these differences can
be sufficient to distinguish between transmitters that are
very similar or even nominally identical. Various techni-
ques have also been proposed to extract and measure
appropriately selected features from a signal and use sta-
tistical tests to determine whether the feature measure-
ments match those of previously observed signals (23, 24).
Radar ES Signal Processing. Various analog and digital
approaches have been used in radar ES receivers to detect
signals and measure their parameters. Descriptions and
performance analyses of the more common ones have been
published 25–27. The radar ES receivers used for current
radar ES systems deployed for the self-protection of plat-
forms such as aircraft and surface ships generate pulse
descriptor words (PDWs) for each radar pulse that is
received. Each PDW consists of digital data that represents
the principal signal parameters, typically frequency,
power, time of arrival, pulse duration, and if available,
angle of arrival and modulation type (phase or frequency).
Early implementations made extensive use of analog tech-
niques to generate PDWs, but more recent implementa-
tions are making increasingly extensive use of digital
techniques.
Pulse train deinterleaving is required because the
pulses that are received from the various radars in the
signal environment will be interleaved in time (i.e., in a
sequence of received radar pulses there is no certainty that
for a given pulse in the sequence, the previous or next
pulses in the sequence will be from the same radar). Dein-
terleaving is typically performed in a two-stage process.
First, clustering is performed as pulses are received to form
clusters or groups of pulses having similar characteristics.
A subset of the signal parameters contained in the PDWs,
typically frequency, angle of arrival, and pulse duration,
are used in this stage. The second stage involves analyzing
the time relationships [Pulse Repetition Interval (PRI)
deinterleaving] between the pulses collected in each cluster
to identify patterns that are consistent with the hypothesis
that they were transmitted by a single radar. In addition to
the radar PRI behavior, the radar scan pattern can be
inferred by examining the time history of the measured
power of received pulses in a deinterleaved pulse train. For
example, a radar that is performing a circular scan will
illuminate the platform carrying the ES system with its
main beam response at uniform intervals in time.
Emitter identification involves comparing the various
parameters that have been measured for each of the
resultant deinterleaved pulse trains with those in an EW
library and identifying the best match.
In practice, many potential difficulties may occur. The
PDWs generated by the receiver will contain errors that
result from various sources. At least some clusters formed
in the first stage will have broad ranges. For example, a
large frequency range may be needed to accommodate a
frequency agile radar. Consequently, some clusters may
overlap. Accurate PRI deinterleaving can be very difficult
to perform with limited signal data sets; many modern
radars have complex PRI staggers (i.e., the time intervals
between successive pulses transmitted by a radar vary
randomly or follow patterns that repeat only over a long
period). Deinterleaving errors can result in the pulse train
transmitted by such a radar being fragmented into two or
more partial pulse trains. Finally, EW databases can have
errors, be incomplete, or as a result of ambiguities, may be
unable to provide a unique identification.
More sophisticated approaches are being investigated
for the extraction of features that can be used to provide
additional information for the classification and identifica-
tion of radar signals. For radars that use frequency or phase
modulation to improve range resolution, knowledge of the
type of modulation waveform and its parameters is useful
for classification purposes. Also, the waveforms trans-
mitted by radar systems often have distinctive features,
which are sometimes referred to as unintentional modula-
tion on pulse (UMOP). Various techniques have been pro-
posed for the extraction and processing of waveform
features for signal identification.
ELECTRONIC ATTACK
EA, which is also known as Electronic Countermeasures,
involves actions intended to degrade the ability of an
adversary to make use of the electromagnetic spectrum.
It may be active or passive in nature.
EA Against Communication Signals
EA against communication signals can be carried out as
deception operations or jamming.
Deception operations involve the transmission of sig-
nals to mislead the enemy intentionally. For example,
after a ground formation has been redeployed for opera-
tions elsewhere, simulated radio traffic may be main-
tained to give the impression that the formation is still
in its original location. Another technique involves the
transmission of messages that contain misleading infor-
mation in the expectation that the message content will be
recovered and used by the adversary. Deception opera-
tions must be carefully designed and organized to be
convincing; the information provided should be consistent
with other information that the intended recipient
believes to be true. Large-scale deception operations
that involve carefully coordinated activities can influence
an adversary’s strategic planning with decisive effect.
Several accounts of highly successful Allied deception
operations in WW2 have been published (5, 28).
Jamming is intended to prevent an adversary from
reliably receiving his communication signals by the trans-
mission of signals that interfere with their reception. In the
simplest form, a jammer consists of an antenna, power
amplifier, and signal generator programmed to produce a
signal with an appropriately chosen waveform. It is also
possible to use a conventional transmitter or radio as an
improvised jammer. Jamming systems are often deployed
with an adjunct ES capability to ascertain the frequencies
6 ELECTRONIC WARFARE](https://image.slidesharecdn.com/88948-250108001425-9f4ecb2b/85/Get-Wiley-Encyclopedia-of-Computer-Science-and-Engineering-5-Volume-Set-1st-Edition-Benjamin-W-Wah-free-all-chapters-47-320.jpg)
![ambitions. He is so lately from the hands of his Creator that he has
not even yet conceived the idea of accumulation. The other day I
told him that he might take a quantity of seed and surplus plants,
and sell them, and he would not. “I, how shall I sell?” he said, “I am
a gardener. This thing is done by Johnson-sahib,” and he looked at
me with amusement. I called him by a foolish name and told him
that he should surely sell, and get money; but he shook his head still
smiling. “By your honour’s favour,” he said, “month by month I find
ten rupees. From this there is food twice a day and clothes, and two
or three rupees to go by the hand of an old man who comes from
my people. It is enough. What more?” I mentioned the future.
“Old?” he cried, “God knows if I will be old. At this time I am a work-
doing wallah. When I am old and your honours are gone to Belaat,
[2] I also will go, and live with my people.”
“And they will, without doubt, give you food and clothes?” I asked.
“According as there is,” he said, “without doubt they will give it,” and
went on with his work.
Here, if you like, was a person of short views and unvexed
philosophy. A lecture upon the importance of copper coins trembled
on my lips, but I held it back. A base aim is a poor exchange for a
lesson in content, and I held it back, wondering whether my servant
might not be better off than I, in all that he could do without.
Alas for the poor people who have to pay at the rate of a dollar a
day and mind their own business into the bargain! Never can they
know one of the greatest pleasures of life, to be served by a serving
people. There is a spark of patriarchal joy, long extinct west of Suez,
in the simple old interpretation which still holds here, of the relation
of master and servant, scolding and praise, favour and wrath; a
lifelong wage and occasionally a little medicine are still the portion of
the servant-folk, accepted as a matter of course, and “Thou wilt not
hear orders?” ever a serious reproach. To all of us Outlanders of the
East, it is one of the consolations of exile, and to some of us a keen
and constant pleasure to be the centre and source of prosperity for
these others, a simple, graphic, pressing opportunity to do justice](https://image.slidesharecdn.com/88948-250108001425-9f4ecb2b/85/Get-Wiley-Encyclopedia-of-Computer-Science-and-Engineering-5-Volume-Set-1st-Edition-Benjamin-W-Wah-free-all-chapters-64-320.jpg)
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- 5. Wiley Encyclopedia of Computer Science and Engineering 5 Volume Set 1st Edition Benjamin W. Wah Digital Instant Download Author(s): Benjamin W. Wah ISBN(s): 9780471383932, 0471383937 Edition: 1 File Details: PDF, 47.57 MB Year: 2009 Language: english
- 7. Wiley Encyclopedia of Computer Science and Engineering FullTitle of Book: Wiley Encyclopedia Of Computer Science And Engineering Editor(s): Wah Publisher: Wiley-interscience YearPublished: Nov., 2008 ISBN-10: 0471383937 ISBN-13: 978-0471383932 Size& Format: 2362 pages
- 8. • Applications • Computer Vision • Computing Milieux • Data • Foundation and Theory • Hardware and Architecture • Image Processing and Visualization • Intelligent Systems • IS • Parallel and Distributed Systems • Software
- 9. A ASYNCHRONOUS TRANSFER MODE NETWORKS Asynchronous transfer mode, or ATM, is anetwork transfer technique capable of supporting a wide variety of multi- media applications with diverse service and performance requirements. It supports traffic bandwidths ranging from a few kilobits per second (e.g., a text terminal) to several hundred megabits per second (e.g., high-definition video) and traffic types ranging from continuous, fixed-rate traffic (e.g., traditional telephony and file transfer) to highly bursty traffic (e.g., interactive data and video). Because of its support for such a wide range of traffic, ATM was designated by the telecommunication standardization sec- tor of the International Telecommunications Union (ITU-T, formerly CCITT) as the multiplexing and switching tech- nique for Broadband, or high-speed, ISDN (B-ISDN) (1). ATM is a form of packet-switching technology. That is, ATM networks transmit their information in small, fixed- length packets called cells, each of which contains 48 octets (or bytes) of data and 5 octets of header information. The small, fixed cell size was chosen to facilitate the rapid processing of packets in hardware and to minimize the amount of time required to fill a single packet. This is particularly important for real-time applications such as voice and video that require short packetization delays. ATM is also connection-oriented. In other words, a virtual circuit must be established before a call can take place, where a call is defined as the transfer of information between two or more endpoints. The establishment of a virtual circuit entails the initiation of a signaling process, during which a route is selected according to the call’s quality of service requirements, connection identifiers at each switch on the route are established, and network resources such as bandwidth and buffer space may be reserved for the connection. Another important characteristic of ATM is that its network functions are typically implemented in hardware. With the introduction of high-speed fiber optic transmis- sion lines, the communication bottleneck has shifted from the communication links to the processing at switching nodes and at terminal equipment. Hardware implementa- tion is necessary to overcome this bottleneck because it minimizes the cell-processing overhead, thereby allowing the network to match link rates on the order of gigabits per second. Finally, as its name indicates, ATM is asynchronous. Time is slotted into cell-sized intervals, and slots are assigned to calls in an asynchronous, demand-based man- ner. Because slots are allocated to calls on demand, ATM can easily accommodate traffic whose bit rate fluctuates over time. Moreover, in ATM, no bandwidth is consumed unless information is actually transmitted. ATM also gains bandwidth efficiency by being able to multiplex bursty traffic sources statistically. Because bursty traffic does not require continuous allocation of the bandwidth at its peak rate, statistical multiplexing allows a large number of bursty sources to share the network’s bandwidth. Since its birth in the mid-1980s, ATM has been fortified by a number of robust standards and realized by a signifi- cant number of network equipment manufacturers. Inter- national standards-making bodies such as the ITU and independent consortia like the ATM Forum have developed a significant body of standards and implementation agree- ments for ATM (1,4). As networks and network services continue to evolve toward greater speeds and diversities, ATM will undoubtedly continue to proliferate. ATM STANDARDS The telecommunication standardization sector of the ITU, the international standards agency commissioned by the United Nations for the global standardization of telecom- munications, has developed a number of standards for ATM networks. Other standards bodies and consortia (e.g., the ATM Forum, ANSI) have also contributed to the develop- ment of ATM standards. This section presents an overview of the standards, with particular emphasis on the protocol reference model used by ATM (2). Protocol Reference Model The B-ISDN protocol reference model, defined in ITU-T recommendation I.321, is shown in Fig. 1(1). The purpose of the protocol reference model is to clarify the functions that ATM networks perform by grouping them into a set of interrelated, function-specific layers and planes. The refer- ence model consists of a user plane, a control plane, and a management plane. Within the user and control planes is a hierarchical set of layers. The user plane defines a set of functions for the transfer of user information between communication endpoints; the control plane defines control functions such as call establishment, call maintenance, and call release; and the management plane defines the opera- tions necessary to control information flow between planes and layers and to maintain accurate and fault-tolerant network operation. Within the user and control planes, there are three layers: the physical layer, the ATM layer, and the ATM adaptation layer (AAL). Figure 2 summarizes the functions of each layer (1). The physical layer performs primarily bit- level functions, the ATM layer is primarily responsible for the switching of ATM cells, and the ATM adaptation layer is responsible for the conversion of higher-layer protocol frames into ATM cells. The functions that the physical, ATM, and adaptation layers perform are described in more detail next. Physical Layer The physical layer is divided into two sublayers: the phy- sical medium sublayer and the transmission convergence sublayer (1). 1 Wiley Encyclopedia of Computer Science and Engineering, edited by Benjamin Wah. Copyright # 2008 John Wiley & Sons, Inc.
- 10. Physical Medium Sublayer. The physical medium (PM) sublayer performs medium-dependent functions. For example, it provides bit transmission capabilities including bit alignment, line coding and electrical/optical conversion. The PM sublayer is also responsible for bit timing (i.e., the insertion and extraction of bit timing information). The PM sublayer currently supports two types of interface: optical and electrical. Transmission Convergence Sublayer. Above the physical medium sublayer is the transmission convergence (TC) sublayer, which is primarily responsible for the framing of data transported over the physical medium. The ITU-T recommendation specifies two options for TC sublayer transmission frame structure: cell-based and synchronous digital hierarchy (SDH). In the cell-based case, cells are transported continuously without any regular frame struc- ture. Under SDH, cells are carried in a special frame structure based on the North American SONET (synchro- nous optical network) protocol (3). Regardless of which transmission frame structure is used, the TC sublayer is responsible for the following four functions: cell rate decou- pling, header error control, cell delineation, and transmis- sion frame adaptation. Cell rate decoupling is the insertion of idle cells at the sending side to adapt the ATM cell stream’s rate to the rate of the transmission path. Header error control is the insertion of an 8-bit CRC in the ATM cell header to protect the contents of the ATM cell header. Cell delineation is the detection of cell boundaries. Transmis- sion frame adaptation is the encapsulation of departing cells into an appropriate framing structure (either cell- based or SDH-based). ATM Layer The ATM layer lies atop the physical layer and specifies the functions required for the switching and flow control of ATM cells (1). There are two interfaces in an ATM network: the user- network interface (UNI) between the ATM endpoint and the ATM switch, and the network-network interface (NNI) between two ATM switches. Although a 48-octet cell pay- load is used at both interfaces, the 5-octet cell header differs slightly at these interfaces. Figure 3 shows the cell header structures used at the UNI and NNI (1). At the UNI, the header contains a 4-bit generic flow control (GFC) field, a 24-bit label field containing virtual path identifier (VPI) and virtual channel identifier (VCI) subfields (8 bits for the VPI and 16 bits forthe VCI), a2-bit payloadtype (PT) field,a 1-bit cell loss priority (CLP) field, and an 8-bit header error check (HEC) field. The cell header for an NNI cell is identical to that for the UNI cell, except that it lacks the GFC field; these four bits are used for an additional 4 VPI bits in the NNI cell header. The VCI and VPI fields are identifier values for virtual channel (VC) and virtual path (VP), respectively. A virtual channel connects two ATM communication endpoints. A virtual path connects two ATM devices, which can be switches or endpoints, and several virtual channels may be multiplexed onto the same virtual path. The 2-bit PT field identifies whether the cell payload contains data or control information. The CLP bit is used by the user for explicit indication of cell loss priority. If the value of the CLP is 1, then the cell is subject to discarding in case of congestion. The HEC field is an 8-bit CRC that protects the contents of the cell header. The GFC field, which appears only at the UNI, is used to assist the customer premises network in controlling the traffic flow. At the time of writ- ing, the exact procedures for use of this field have not been agreed upon. Figure 1. Protocol reference model for ATM. Figure 2. Functions of each layer in the protocol reference model. 2 ASYNCHRONOUS TRANSFER MODE NETWORKS
- 11. ATM Layer Functions The primary function of the ATM layer is VPI/VCI transla- tion. As ATM cells arrive at ATM switches, the VPI and VCI values contained in their headers are examined by the switch to determine which outport port should be used to forward the cell. In the process, the switch translates the cell’s original VPI and VCI values into new outgoing VPI and VCI values, which are used in turn by the next ATM switch to send the cell toward its intended destination. The table used to perform this translation is initialized during the establishment of the call. An ATM switch may either be a VP switch, in which case it translates only the VPI values contained in cell headers, or it may be a VP/VC switch, in which case it translates the incoming VPI/VCI value into an outgoing VPI/VCI pair. Because VPI and VCI values do not represent a unique end- to-end virtual connection, they can be reused at different switches through the network. This is important because the VPI and VCI fields are limited in length and would be quickly exhausted if they were used simply as destination addresses. The ATM layer supports two types of virtual connec- tions: switched virtual connections (SVC) and permanent, or semipermanent, virtual connections (PVC). Switched virtual connections are established and torn down dyna- mically by an ATM signaling procedure. That is, they exist only for the duration of a single call. Permanent virtual connections, on the other hand, are established by network administrators and continue to exist as long as the admin- istrator leaves them up, even if they are not used to trans- mit data. Other important functions of the ATM layer include cell multiplexing and demultiplexing, cell header creation and extraction, and generic flow control. Cell multiplexing is the merging of cells from several calls onto a single trans- mission path, cell header creation is the attachment of a 5- octet cell header to each 48-octet block of user payload, and generic flow control is used at the UNI to prevent short- term overload conditions from occurring within the net- work. ATM Layer Service Categories The ATM Forum and ITU-T have defined several distinct service categories at the ATM layer (1,4). The categories defined by the ATM Forum include constant bit rate (CBR), real-time variable bit rate (VBR-rt), non-real-time variable bit rate (VBR-nrt), available bit rate (ABR), and unspecified bit rate (UBR). ITU-T defines four service categories, namely, deterministic bit rate (DBR), statistical bit rate (SBR), available bit rate (ABR), and ATM block transfer (ABT). The first of the three ITU-T service categories correspond roughly to the ATM Forum’s CBR, VBR, and ABR classifications, respectively. The fourth service cate- gory, ABT, is solely defined by ITU-T and is intended for bursty data applications. The UBR category defined by the ATM Forum is for calls that request no quality of service guarantees at all. Figure 4 lists the ATM service categories, their quality of service (QoS) parameters, and the traffic descriptors required by the service category during call establishment (1,4). The constant bit rate (or deterministic bit rate) service category provides a very strict QoS guarantee. It is targeted at real-time applications, such as voice and raw video, which mandate severe restrictions on delay, delay variance (jitter), and cell loss rate. The only traffic descriptors required by the CBR service are the peak cell rate and the cell delay variation tolerance. A fixed amount of band- width, determined primarily by the call’s peak cell rate, is reserved for each CBR connection. The real-time variable bit rate (or statistical bit rate) service category is intended for real-time bursty applica- tions (e.g., compressed video), which also require strict QoS guarantees. The primary difference between CBR and VBR-rt is in the traffic descriptors they use. The VBR-rt service requires the specification of the sustained (or aver- age) cell rate and burst tolerance (i.e., burst length) in addition to the peak cell rate and the cell delay variation Figure 3. ATM cell header structure. Figure 4. ATM layer service categories. ASYNCHRONOUS TRANSFER MODE NETWORKS 3
- 12. tolerance. The ATM Forum also defines a VBR-nrt service category, in which cell delay variance is not guaranteed. The available bit rate service category is defined to exploit the network’s unused bandwidth. It is intended for non-real-time data applications in which the source is amenable to enforced adjustment of its transmission rate. A minimum cell rate is reserved for the ABR connection and therefore guaranteed by the network. When the network has unused bandwidth, ABR sources are allowed to increase their cell rates up to an allowed cell rate (ACR), a value that is periodically updated by the ABR flow control mechanism (to be described in the section entitled ‘‘ATM Traffic Control’’). The value of ACR always falls between the minimum and the peak cell rate for the connection and is determined by the network. The ATM Forum defines another service category for non-real-time applications called the unspecified bit rate (UBR) service category. The UBR service is entirely best effort; the call is provided with no QoS guarantees. The ITU-T also defines an additional service category for non- real-time data applications. The ATM block transfer ser- vice category is intended for the transmission of short bursts, or blocks, of data. Before transmitting a block, the source requests a reservation of bandwidth from the network. If the ABT service is being used with the immedi- ate transmission option (ABT/IT), the block of data is sent at the same time as the reservation request. If bandwidth is not available for transporting the block, then it is simply discarded, and the source must retransmit it. In the ABT service with delayed transmission (ABT/DT), the source waits for a confirmation from the network that enough bandwidth is available before transmitting the block of data. In both cases, the network temporarily reserves bandwidth according to the peak cell rate for each block. Immediately after transporting the block, the network releases the reserved bandwidth. ATM Adaptation Layer The ATM adaptation layer, which resides atop the ATM layer, is responsible for mapping the requirements of higher layer protocols onto the ATM network (1). It oper- ates in ATM devices at the edge of the ATM network and is totally absent in ATM switches. The adaptation layer is divided into two sublayers: the convergence sublayer (CS), which performs error detection and handling, timing, and clock recovery; and the segmentation and reassembly (SAR) sublayer, which performs segmentation of conver- gence sublayer protocol data units (PDUs) into ATM cell- sized SAR sublayer service data units (SDUs) and vice versa. In order to support different service requirements, the ITU-T has proposed four AAL-specific service classes. Figure 5 depicts the four service classes defined in recom- mendation I.362 (1). Note that even though these AAL service classes are similar in many ways to the ATM layer service categories defined in the previous section, they are not the same; each exists at a different layer of the protocol reference model, and each requires a different set of func- tions. AAL service class A corresponds to constant bit rate services with a timing relation required between source and destination. The connection mode is connection- oriented. The CBR audio and video belong to this class. Class B corresponds to variable bit rate (VBR) services. This class also requires timing between source and desti- nation, and its mode is connection-oriented. The VBR audio and video are examples of class B services. Class C also corresponds to VBR connection-oriented services, but the timing between source and destination needs not be related. Class C includes connection-oriented data transfer such as X.25, signaling, and future high-speed data ser- vices. Class D corresponds to connectionless services. Con- nectionless data services such as those supported by LANs and MANs are examples of class D services. Four AAL types (Types 1, 2, 3/4, and 5), each with a unique SAR sublayer and CS sublayer, are defined to support the four service classes. AAL Type 1 supports constant bit rate services (class A), and AAL Type 2 sup- ports variable bit rate services with a timing relation between source and destination (class B). AAL Type 3/4 was originally specified as two different AAL types (Type 3 and Type 4), but because of their inherent similarities, they were eventually merged to support both class C and class D services. AAL Type 5 also supports class C and class D services. AAL Type 5. Currently, the most widely used adaptation layer is AAL Type 5. AAL Type 5 supports connection- oriented and connectionless services in which there is no timing relation between source and destination (classes C and D). Its functionality was intentionally made simple in order to support high-speed data transfer. AAL Type 5 assumes that the layers above the ATM adaptation layer can perform error recovery, retransmission, and sequence numbering when required, and thus, it does not provide these functions. Therefore, only nonassured operation is provided; lost or corrupted AAL Type 5 packets will not be corrected by retransmission. Figure 6 depicts the SAR-SDU format for AAL Type 5 (5,6). The SAR sublayer of AAL Type 5 performs segmenta- tion of a CS-PDU into a size suitable for the SAR-SDU payload. Unlike other AAL types, Type 5 devotes the entire 48-octet payload of the ATM cell to the SAR-SDU; there is no overhead. An AAL specific flag (end-of-frame) in the Figure 5. Service classification for AAL. Figure 6. SAR-SDU format for AAL Type 5. 4 ASYNCHRONOUS TRANSFER MODE NETWORKS
- 13. ATM PT field of the cell header is set when the last cell of a CS-PDU is sent. The reassembly of CS-PDU frames at the destination is controlled by using this flag. Figure 7 depicts the CS-PDU format for AAL Type 5 (5,6). It contains the user data payload, along with any necessary padding bits (PAD) and a CS-PDU trailer, which are added by the CS sublayer when it receives the user information from the higher layer. The CS-PDU is padded using 0 to 47 bytes of PAD field to make the length of the CS- PDU an integral multiple of 48 bytes (the size of the SAR- SDU payload). At the receiving end, a reassembled PDU is passed to the CS sublayer from the SAR sublayer, and CRC values are then calculated and compared. If there is no error, the PAD field is removed by using the value of length field (LF) in the CS-PDU trailer, and user data is passed to the higher layer. If an error is detected, the erroneous information is either delivered to the user or discarded according to the user’s choice. The use of the CF field is for further study. AAL Type 1. AAL Type 1 supports constant bit rate services with a fixed timing relation between source and destination users (class A). At the SAR sublayer, it defines a 48-octet service data unit (SDU), which contains 47 octets of user payload, 4 bits for a sequence number, and a 4-bit CRC value to detect errors in the sequence number field. AAL Type 1 performs the following services at the CS sublayer: forward error correction to ensure high quality of audio and video applications, clock recovery by monitoring the buffer filling, explicit time indication by inserting a time stamp in the CS-PDU, and handling of lost and misinserted cells that are recognized by the SAR. At the time of writing, the CS- PDU format has not been decided. AAL Type 2. AAL Type 2 supports variable bit rate services with a timing relation between source and desti- nation (class B). AAL Type 2 is nearly identical to AAL Type 1, except that it transfers service data units at a variable bit rate, not at a constant bit rate. Furthermore, AAL Type 2 accepts variable length CS-PDUs, and thus, there may exist some SAR-SDUs that are not completely filled with user data. The CS sublayer for AAL Type 2 performs the following functions: forward error correction for audio and video services, clock recovery by inserting a time stamp in the CS-PDU, and handling of lost and misinserted cells. At the time of writing, both the SAR-SDU and CS-PDU for- mats for AAL Type 2 are still under discussion. AAL Type 3/4. AAL Type 3/4 mainly supports services that require no timing relation between the source and destination (classes C and D). At the SAR sublayer, it defines a 48-octet service data unit, with 44 octets of user payload; a 2-bit payload type field to indicate whether the SDU is at the beginning, middle, or end of a CS-PDU; a 4-bit cell sequence number; a 10-bit multiplexing identifier that allows several CS-PDUs to be multiplexed over asingle VC; a 6-bit cell payload length indicator; and a 10-bit CRC code that covers the payload. The CS-PDU format allows for up to 65535 octets of user payload and contains a header and trailer to delineate the PDU. The functions that AAL Type 3/4 performs include seg- mentation and reassembly of variable-length user data and error handling. It supports message mode (for framed data transfer) as well as streaming mode (for streamed data transfer). Because Type 3/4 is mainly intended for data services, it provides a retransmission mechanism if neces- sary. ATM Signaling ATM follows the principle of out-of-band signaling that was established for N-ISDN. In other words, signaling and data channels are separate. The main purposes of signaling are (1) to establish, maintain, and release ATM virtual con- nections and (2) to negotiate (or renegotiate) the traffic parameters of new (or existing) connections (7). The ATM signaling standards support the creation of point-to-point as well as multicast connections. Typically, certain VCI and VPI values are reserved by ATM networks for signaling messages. If additional signaling VCs are required, they may be established through the process of metasignaling. ATM TRAFFIC CONTROL The control of ATM traffic is complicated as a result of ATM’s high-link speed and small cell size, the diverse service requirements of ATM applications, and the diverse characteristics of ATM traffic. Furthermore, the configura- tion and size of the ATM environment, either local or wide area, has a significant impact on the choice of traffic control mechanisms. The factor that most complicates traffic control in ATM is its high-link speed. Typical ATM link speeds are 155.52 Mbit/s and 622.08 Mbit/s. At these high-link speeds, 53- byte ATM cells must be switched at rates greater than one cell per 2.726 ms or 0.682 ms, respectively. It is apparent that the cell processing required by traffic control must perform at speeds comparable to these cell-switching rates. Thus, traffic control should be simple and efficient, without excessive software processing. Such high speeds render many traditional traffic control mechanisms inadequate for use in ATM because of their reactive nature. Traditional reactive traffic control mechanisms attempt to control network congestion by responding to it after it occurs and usually involves sending Figure 7. CS-PDU format, segmentation and reassembly of AAL Type 5. ASYNCHRONOUS TRANSFER MODE NETWORKS 5
- 14. feedback to the source in the form of a choke packet. However, a large bandwidth-delay product (i.e., the amount of traffic that can be sent in a single propagation delay time) renders many reactive control schemes ineffec- tive in high-speed networks. When a node receives feed- back, it may have already transmitted a large amount of data. Consider a cross-continental 622 Mbit/s connection with a propagation delay of 20 ms (propagation-bandwidth product of 12.4 Mbit). If a node at one end of the connection experiences congestion and attempts to throttle the source at the other end by sending it a feedback packet, the source will already have transmitted over 12 Mb of information before feedback arrives. This example illustrates the inef- fectiveness of traditional reactive traffic control mechan- isms in high-speed networks and argues for novel mechanisms that take into account high propagation-band- width products. Not only is traffic control complicated by high speeds, but it also is made more difficult by the diverse QoS require- ments of ATM applications. For example, many applica- tions have strict delay requirements and must be delivered within a specified amount of time. Other applications have strict loss requirements and must be delivered reliably without an inordinate amount of loss. Traffic controls must address the diverse requirements of such applica- tions. Another factor complicating traffic control in ATM net- works is the diversity of ATM traffic characteristics. In ATM networks, continuous bit rate traffic is accompanied by bursty traffic. Bursty traffic generates cells at a peak rate for a very short period of time and then immediately becomes less active, generating fewer cells. To improve the efficiency of ATM network utilization, bursty calls should be allocated an amount of bandwidth that is less than their peak rate. This allows the network to multiplex more calls by taking advantage of the small probability that a large number of bursty calls will be simultaneously active. This type of multiplexing is referred to as statistical multiplex- ing. The problem then becomes one of determining how best to multiplex bursty calls statistically such that the number of cells dropped as a result of excessive burstiness is balanced with the number of bursty traffic streams allowed. Addressing the unique demands of bursty traffic is an important function of ATM traffic control. For these reasons, many traffic control mechanisms developed for existing networks may not be applicable to ATM networks, and therefore novel forms of traffic control are required (8,9). One such class of novel mechanisms that work well in high-speed networks falls under the heading of preventive control mechanisms. Preventive control attempts to manage congestion by preventing it before it occurs. Preventive traffic control is targeted primarily at real-time traffic. Another class of traffic control mechan- isms has been targeted toward non-real-time data traffic and relies on novel reactive feedback mechanisms. Preventive Traffic Control Preventive control for ATM has two major components: call admission control and usage parameter control (8). Admis- sion control determines whether to accept or reject a new call at the time of call set-up. This decision is based on the traffic characteristics of the new call and the current net- work load. Usage parameter control enforces the traffic parameters of the call after it has been accepted into the network. This enforcement is necessary to ensure that the call’s actual traffic flow conforms with that reported during call admission. Before describing call admission and usage parameter control in more detail, it is important to first discuss the nature of multimedia traffic. Most ATM traffic belongs to one of two general classes of traffic: continuous traffic and bursty traffic. Sources of continuous traffic (e.g., constant bit rate video, voice without silence detection) are easily handled because their resource utilization is predictable and they can be deterministically multiplexed. However, bursty traffic (e.g., voice with silence detection, variable bit rate video) is characterized by its unpredictability, and this kind of traffic complicates preventive traffic control. Burstiness is a parameter describing how densely or sparsely cell arrivals occur. There are a number of ways to express traffic burstiness, the most typical of which are the ratio of peak bit rate to average bit rate and the average burst length. Several other measures of burstiness have also been proposed (8). It is well known that burstiness plays a critical role in determining network performance, and thus, it is critical for traffic control mechanisms to reduce the negative impact of bursty traffic. Call Admission Control. Call admission control is the process by which the network decides whether to accept or reject a new call. When a new call requests access to the network, it provides a set of traffic descriptors (e.g., peak rate, average rate, average burst length) and a set of quality of service requirements (e.g., acceptable cell loss rate, acceptable cell delay variance, acceptable delay). The net- work then determines, through signaling, if it has enough resources (e.g., bandwidth, buffer space) to support the new call’s requirements. If it does, the call is immediately accepted and allowed to transmit data into the network. Otherwise it is rejected. Call admission control prevents network congestion by limiting the number of active con- nections in the network to a level where the network resources are adequate to maintain quality of service guar- antees. One of the most common ways for an ATM network to make a call admission decision is to use the call’s traffic descriptors and quality of service requirements to predict the ‘‘equivalent bandwidth’’ required by the call. The equivalent bandwidth determines how many resources need to be reserved by the network to support the new call at its requested quality of service. For continuous, constant bit rate calls, determining the equivalent band- width is simple. It is merely equal to the peak bit rate of the call. For bursty connections, however, the process of deter- mining the equivalent bandwidth should take into account such factors as a call’s burstiness ratio (the ratio of peak bit rate to average bit rate), burst length, and burst interarri- val time. The equivalent bandwidth for bursty connections must be chosen carefully to ameliorate congestion and cell loss while maximizing the number of connections that can be statistically multiplexed. 6 ASYNCHRONOUS TRANSFER MODE NETWORKS
- 15. Usage Parameter Control. Call admission control is responsible for admitting or rejecting new calls. However, call admission by itself is ineffective if the call does not transmit data according to the traffic parameters it pro- vided. Users may intentionally or accidentally exceed the traffic parameters declared during call admission, thereby overloading the network. In order to prevent the network users from violating their traffic contracts and causing the network to enter a congested state, each call’s traffic flow is monitored and, if necessary, restricted. This is the purpose of usage parameter control. (Usage parameter control is also commonly referred to as policing, bandwidth enforce- ment, or flow enforcement.) To monitor a call’s traffic efficiently, the usage para- meter control function must be located as close as possible to the actual source of the traffic. An ideal usage parameter control mechanism should have the ability to detect para- meter-violating cells, appear transparent to connections respecting their admission parameters, and rapidly respond to parameter violations. It should also be simple, fast, and cost effective to implement in hardware. To meet these requirements, several mechanisms have been pro- posed and implemented (8). The leaky bucket mechanism (originally proposed in Ref. 10) is a typical usage parameter control mechanism used for ATM networks. It can simultaneously enforce the average bandwidth and the burst factor of a traffic source. One possible implementation of the leaky bucket mechan- ism is to control the traffic flow by means of tokens. A conceptual model for the leaky bucket mechanism is illustrated in Fig. 5. In Fig. 8, an arriving cell first enters a queue. If the queue is full, cells are simply discarded. To enter the net- work, a cell must first obtain a token from the token pool; if there is no token, a cell must wait in the queue until a new token is generated. Tokens are generated at a fixed rate corresponding to the average bit rate declared during call admission. If the number of tokens in the token pool exceeds some predefined threshold value, token generation stops. This threshold value corresponds to the burstiness of the transmission declared at call admission time; for larger threshold values, a greater degree of burstiness is allowed. This method enforces the average input rate while allowing for a certain degree of burstiness. One disadvantage of the leaky bucket mechanism is that the bandwidth enforcement introduced by the token pool is in effect even when the network load is light and there is no need for enforcement. Another disadvantage of the leaky bucket mechanism is that it may mistake nonviolating cells for violating cells. When traffic is bursty, a large number of cells may be generated in a short period of time, while conforming to the traffic parameters claimed at the time of call admission. In such situations, none of these cells should be considered violating cells. Yet in actual practice, leaky bucket may erroneously identify such cells as violations of admission parameters. A virtual leaky bucket mechanism (also referred to as a marking method) alleviates these disadvantages (11). In this mechanism, violating cells, rather than being discarded or buffered, are permitted to enter the network at a lower priority (CLP ¼ 1). These violating cells are discarded only when they arrive at a congested node. If there are no congested nodes along the routes to their destinations, the violating cells are trans- mitted without being discarded. The virtual leaky bucket mechanism can easily be implemented using the leaky bucket method described earlier. When the queue length exceeds a threshold, cells are marked as ‘‘droppable’’ instead ofbeingdiscarded. The virtual leaky bucket method not only allows the user to take advantageof alight network load but also allows a larger margin of error in determining the token pool parameters. Reactive Traffic Control Preventive control is appropriate for most types of ATM traffic. However, there are cases where reactive control is beneficial. For instance, reactive control isuseful for service classes like ABR, which allow sources to use bandwidth not being used by calls in other service classes. Such a service would be impossible with preventive control because the amount of unused bandwidth in the network changes dynamically, and the sources can only be made aware of the amount through reactive feedback. There are two major classes of reactive traffic control mechanisms: rate-based and credit-based (12,13). Most rate-based traffic control mechanisms establish a closed feedback loop in which the source periodically transmits special control cells, called resource management cells, to the destination (or destinations). The destination closes the feedback loop by returning the resource management cells to the source. As the feedback cells traverse the network, the intermediate switches examine their current conges- tion state and mark the feedback cells accordingly. When the source receives a returning feedback cell, it adjusts its rate, either by decreasing it in the case of network conges- tion or increasing it in the case of network underuse. An example of a rate-based ABR algorithm is the Enhanced Proportional Rate Control Algorithm (EPRCA), which was proposed, developed, and tested through the course of ATM Forum activities (12). Credit-based mechanisms use link-by-link traffic con- trol to eliminate loss and optimize use. Intermediate switches exchange resource management cells that contain ‘‘credits,’’ which reflect the amount of buffer space available at the next downstream switch. A source cannot transmit a new data cell unless it has received at least one credit from its downstream neighbor. An example of a credit-based mechanism is the Quantum Flow Control (QFC) algorithm, developed by a consortium of reseachers and ATM equip- ment manufacturers (13). Figure 8. Leaky bucket mechanism. ASYNCHRONOUS TRANSFER MODE NETWORKS 7
- 16. HARDWARE SWITCH ARCHITECTURES FOR ATM NETWORKS In ATM networks, information is segmented into fixed- length cells, and cells are asynchronously transmitted through the network. To match the transmission speed of the network links and to minimize the protocol proces- sing overhead, ATM performs the switching of cells in hardware-switching fabrics, unlike traditional packet switching networks, where switching is largely performed in software. A large number of designs has been proposed and imple- mented for ATM switches (14). Although many differences exist, ATM switch architectures can be broadly classified into two categories: asynchronous time division (ATD) and space-division architectures. Asynchronous Time Division Switches The ATD, or single path, architectures provide a single, multiplexed path through the ATM switch for all cells. Typically a bus or ring is used. Figure 9 shows the basic structure of the ATM switch proposed in (15). In Fig. 6, four input ports are connected to four output ports by a time- division multiplexing (TDM) bus. Each input port is allo- cated a fixed time slot on the TDM bus, and the bus is designated to operate at a speed equal to the sum of the incoming bit rates at all input ports. The TDM slot sizes are fixed and equal in length to the time it takes to transmit one ATM cell. Thus, during one TDM cycle, the four input ports can transfer four ATM cells to four output ports. In ATD switches, the maximum throughput is deter- mined by a single, multiplexed path. Switches with N input ports and N output ports must run at a rate N times faster than the transmission links. Therefore, the total through- put of ATD ATM switches is bounded by the current cap- abilities of device logic technology. Commercial examples of ATD switches are the Fore Systems ASX switch and Digi- tal’s VNswitch. Space-Division Switches To eliminate the single-path limitation and increase total throughput, space-division ATM switches implement mul- tiple paths through switching fabrics. Most space-division switches are based on multistage interconnection net- works, where small switching elements (usually 2 2 cross-point switches) are organized into stages and provide multiple paths through a switching fabric. Rather than being multiplexed onto a single path, ATM cells are space- switched through the fabric. Three typical types of space- division switches are described next. Banyan Switches. Banyan switches are examples of space-division switches. An N N Banyan switch is con- structed by arranging a number of binary switching ele- ments into several stages (log2N stages). Figure 10 depicts an 8 8 self-routing Banyan switch (14). The switch fabric is composed of twelve 2 2 switching elements assembled into three stages. From any of the eight input ports, it is possible to reach all the eight output ports. One desirable characteristic of the Banyan switch is that it is self-routing. Because each cross-point switch has only two output lines, only one bit is required to specify the correct output path. Very simply, if the desired output addresses of a ATM cell is stored in the cell header in binary code, routing decisions for the cell can be made at each cross-point switch by examining the appropriate bit of the destination address. Although the Banyan switch is simple and possesses attractive features such as modularity, which makes it suitable for VLSI implementation, it also has some disad- vantages. One of its disadvantages is that it is internally blocking. In other words, cells destined for different output ports may contend for a common link within the switch. This results in blocking all cells that wish to use that link, except for one. Hence, the Banyan switch is referred to as a blocking switch. In Fig. 10, three cells are shown arriving on input ports 1, 3, and 4 with destination port addresses of 0, 1, and 5, respectively. The cell destined for output port 0 and the cell destined for output port 1 end up contending for the link between the second and third stages. As a result, only one of them (the cell from input port 1 in this example) actually reaches its destination (output port 0), while the other is blocked. Batcher–Banyan Switches. Another example of space- division switches is the Batcher–Banyan switch (14). (See Fig. 11.) It consists of two multistage interconnection networks: a Banyan self-routing network and a Batcher sorting network. In the Batcher–Banyan switch, the incom- ing cells first enter the sorting network, which takes the cells and sorts them into ascending order according to their output addresses. Cells then enter the Banyan network, which routes the cells to their correct output ports. Figure 9. A 4 4 asynchronous time division switch. Figure 10. A 8 8 Banyan switch with binary switching ele- ments. 8 ASYNCHRONOUS TRANSFER MODE NETWORKS
- 17. As shown earlier, the Banyan switch is internally block- ing. However, the Banyan switch possesses an interesting feature. Namely, internal blocking can be avoided if the cells arriving at the Banyan switch’s input ports are sorted in ascending order by their destination addresses. The Batcher–Banyan switch takes advantage of this fact and uses the Batcher soring network to sort the cells, thereby making the Batcher–Banyan switch internally nonblock- ing. The Starlite switch, designed by Bellcore, is based on the Batcher–Banyan architecture (16). Crossbar Switches. The crossbar switch interconnects N inputs and N outputs into a fully meshed topology; that is, there are N2 cross points within the switch (14). (See Fig. 12.) Because it is always possible to establish a con- nection between any arbitrary input and output pair, inter- nal blocking is impossible in a crossbar switch. The architecture of the crossbar switch has some advan- tages. First, it uses a simple two-state cross-point switch (open and connected state), which is easy to implement. Second, the modularity of the switch design allows simple expansion. One can build a larger switch by simply adding more cross-point switches. Lastly, compared to Banyan- based switches, the crossbar switch design results in low transfer latency, because it has the smallest number of connecting points between input and output ports. One disadvantage to this design, however, is the fact that it uses the maximum number of cross points (cross-point switches) needed to implement an N N switch. The knockout switch by ATT Bell Labs is a nonblock- ing switch based on the crossbar design (17,18). It has N inputs and N outputs and consists of a crossbar-based switch with a bus interface module at each output (Fig. 12). Nonblocking Buffered Switches Although some switches such as Batcher–Banyan and crossbar switches are internally nonblocking, two or more cells may still contend for the same output port in a nonblocking switch, resulting in the dropping of all but one cell. In order to prevent such loss, the buffering of cells by the switch is necessary. Figure 13 illustrates that buffers may be placed (1) in the inputs to the switch, (2) in the outputs to the switch, or (3) within the switching fabric itself, as a shared buffer (14). Some switches put buffers in both the input and output ports of a switch. The first approach to eliminating output contention is to place buffers in the output ports of the switch (14). In the worst case, cells arriving simultaneously at all input ports can be destined for a single output port. To ensure that no cells are lost in this case, the cell transfer must be per- formed at N times the speed of the input links, and the switch must be able to write N cells into the output buffer during one cell transmission time. Examples of output buffered switches include the knockout switch by ATT Bell Labs, the Siemens Newbridge MainStreetXpress switches, the ATML’s VIRATA switch, and Bay Networks’ Lattis switch. The second approach to buffering in ATM switches is to place the buffers in the input ports of the switch (14). Each input has a dedicated buffer, and cells that would otherwise be blocked at the output ports of the switch are stored in input buffers. Commercial examples of switches with input buffers as well as output buffers are IBM’s 8285 Nways switches, and Cisco’s Lightstream 2020 switches. A third approach is to use a shared buffer within the switch fabric. In a shared buffer switch, there is no buffer at the input or output ports (14). Arriving cells are immedi- ately injected into the switch. When output contention happens, the winning cell goes through the switch, while the losing cells are stored for later transmission in a shared buffer common to all of the input ports. Cells just arriving at the switch join buffered cells in competition for available outputs. Because more cells are available to select from, it is possible that fewer output ports will be idle when using the shared buffer scheme. Thus, the shared buffer switch can achieve high throughput. However, one drawback is that cells may be delivered out of sequence because cells that arrived more recently may win over buffered cells during contention (19). Another drawback is the increase in the number of input and output ports internal to the switch. The Starlite switch with trap by Bellcore is an example of the shared buffer switch architecture (16). Other examples of shared buffer switches include Cisco’s Lightstream 1010 switches, IBM’s Prizma switches, Hitachi’s 5001 switches, and Lucent’s ATM cell switches. CONTINUING RESEARCH IN ATM NETWORKS ATM is continuously evolving, and its attractive ability to support broadband integrated services with strict quality of service guarantees has motivated the integration of ATM and existing widely deployed networks. Recent additions to ATM research and technology include, but are not limited to, seamless integration with existing LANs [e.g., LAN emulation (20)], efficient support for traditional Internet IP networking [e.g., IP over ATM (21), IP switching (22)], and further development of flow and congestion control Figure 11. Batcher–Banyan switch. Figure 12. A knockout (crossbar) switch. ASYNCHRONOUS TRANSFER MODE NETWORKS 9
- 18. algorithms to support existing data services [e.g., ABR flow control (12)]. Research on topics related to ATM networks is currently proceeding and will undoubtedly continue to proceed as the technology matures. BIBLIOGRAPHY 1. CCITT Recommendation I-Series. Geneva: International Tele- phone and Telegraph Consultative Committee. 2. J. B. Kim, T. Suda and M. Yoshimura, International standar- dization of B-ISDN, Comput. Networks ISDN Syst., 27: 1994. 3. CCITT Recommendation G-Series. Geneva: International Tel- ephone and Telegraph Consultative Committee. 4. ATM Forum Technical Specifications [Online]. Available www: www.atmforum.com 5. Report of ANSI T1S1.5/91-292, Simple and Efficient Adapta- tion Layer (SEAL), August 1991. 6. Report of ANSI T1S1.5/91-449, AAL5—A New High Speed Data Transfer, November 1991. 7. CCITT Recommendation Q-Series. Geneva: International Tel- ephone and Telegraph Consultative Committee. 8. J. Bae and T. Suda, Survey of traffic control schemes and protocols in ATM networks, Proc. IEEE, 79: 1991. 9. B. J. Vickers et al., Congestion control and resource manage- ment in diverse ATM environments, IECEJ J., J76-B-I (11): 1993. 10. J. S. Turner, New directions in communications (or which way to the information age?), IEEE Commun. Mag., 25 (10): 1986. 11. G. Gallassi, G. Rigolio, and L. Fratta, ATM: Bandwidth assignment and bandwidth enforcement policies. Proc. GLOBECOM’89. 12. ATM Forum, ATM Forum Traffic management specification version 4.0, af-tm-0056.000, April 1996, Mountain View, CA: ATM Forum. 13. Quantum Flow Control version 2.0, Flow Control Consortium, FCC-SPEC-95-1, [Online], July 1995. http://www.qfc.org 14. Y. Oie et al., Survey of switching techniques in high-speed networks and their performance, Int. J. Satellite Commun., 9: 285–303, 1991. 15. M. De Prycker and M. De Somer, Performance of a service independent switching network with distributed control, IEEE J. Select. Areas Commun., 5: 1293–1301, 1987. 16. A. Huang and S. Knauer, Starlite: A wideband digital switch. Proc. IEEE GLOBECOM’84, 1984. 17. K. Y. Eng, A photonic knockout switch for high-speed packet networks, IEEE J. Select. Areas Commun., 6: 1107–1116, 1988. 18. Y. S. Yeh, M. G. Hluchyj, and A. S. Acampora, The knockout switch: A simple, modular architecture for high-performance packet switching, IEEE J. Select. Areas Commun., 5: 1274– 1283, 1987. 19. J. Y. Hui and E. Arthurs, A broadband packet switch for integrated transport, IEEE J. Select. Areas Commun., 5: 1264–1273, 1987. 20. ATM Forum, LAN emulation over ATM version1.0. AF-LANE- 0021, 1995, Mountain View, CA: ATM Forum. 21. IETF, IP over ATM: A framework document, RFC-1932, 1996. 22. Ipsilon Corporation, IP switching: The intelligence of routing, The Performance of Switching [Online]. Available www.ipsiolon.com TATSUYA SUDA University of California, Irvine Irvine, California Figure 13. Nonblocking buffered switches. 10 ASYNCHRONOUS TRANSFER MODE NETWORKS
- 19. A AIRCRAFT COMPUTERS AIRCRAFT ANALOG COMPUTERS Early aircraft computers were used to take continuous streams of inputs to provide flight assistance. Examples of aircraft analog inputs are fuel gauge readings, throttle settings, and altitude indicators. Landau (1) defines an analog computer as a computer for processing data repre- sented by a continuous physical variable, such as electric current. Analog computers monitor these inputs and imple- ment a predetermined service when some set of inputs calls for a flight control adjustment. For example, when fuel levels are below a certain point, the analog computer would read a low fuel level in the aircraft’s main fuel tanks and would initiate the pumping of fuel from reserve tanks or the balancing of fuel between wing fuel tanks. Some of the first applications of analog computers to aircraft applications were for automatic pilot applications, where these analog machines took flight control inputs to hold altitude and course. The analog computers use operational amplifiers to build the functionality of summers, adders, subtracters, and integrators on the electric signals. Aircraft Digital Computers As the technologies used to build digital computers evolved, digital computers became smaller, lighter, and less power- hungry, and produced less heat. This improvement made them increasingly acceptable for aircraft applications. Digital computers are synonymous with stored-program computers. A stored-program computer has the flexibility of being able to accomplish multiple different tasks simply by changing the stored program. Analog computers are hard-wired to perform one and only one function. Analog computers’ data, as defined earlier, are continuous physical variables. Analog computers may be able to recognize and process numerous physical variables, but each variable has its unique characteristics that must be handled during processing by the analog computer. The range of output values for the analog computer is bounded as a given voltage range; if they exceed this range, they saturate. Digital computers are not constrained by physical vari- ables. All the inputs and outputs of the digital computer are in a digital representation. The processing logic and algorithms performed by the computer work in a single representation of the cumulative data. It is not uncommon to see aircraft applications that have analog-to-digital and digital-to-analog signal converters. This method is more efficient than having the conversions done within the computers. Analog signals to the digital computer are converted to digital format, where they are quickly processed digitally and returned to the analog device through a digital-to-analog converter as an analog output for that device to act upon. These digital computers are smaller, more powerful, and easier to integrate into multi- ple areas of aircraft applications. Landau (1) defines a digital computer as a computer for processing data represented by discrete, localized physical signals, such as the presence or absence of an electric current. These signals are represented as a series of bits with word lengths of 16, 32, and 64 bits. See micro- computers for further discussion. Wakerly (2) shows number systems and codes used to process binary digits in digital computers. Some impor- tant number systems used in digital computers are binary, octal, and hexadecimal numbers. He also shows conver- sion between these and base-10 numbers, as well as simple mathematical operations such as addition, subtraction, division, and multiplication. The American Standard Code for Information Interchange (ASCII) of the American National Standard Institute (ANSI) is also presented, which is Standard No. X3.4-1968 for numerals, symbols, characters, and control codes used in automatic data processing machines, including computers. Figure 1 shows a typical aircraft central computer. Microcomputers The improvements in size, speed, and cost through compu- ter technologies continually implement new computer con- sumer products. Many of these products were unavailable to the average consumer until recently. These same break- throughs provide enormous functional improvements in aircraft computing. Landau (1) defines microcomputers as very small, relatively inexpensive computers whose central processing unit (CPU) is a microprocessor. A microprocessor (also called MPU or central processing unit) communicates with other devices in the system through wires (or fiber optics) called lines. Each device has a unique address, represented in binary format, which the MPU recognizes. The number of lines is also the address size in bits. Early MPU machines had 8-bit addresses. Machines of 1970 to 1980 typically had 16-bit addresses; modern MPU machines have 256 bits. Common terminology for an MPU is random access memory (RAM), read only memory (ROM), input-output, clock, and interrupts. RAM is volatile storage. It holds both data and instructions for the MPU. ROM may hold both instructions and data. The key point of ROM is that it is nonvolatile. Typically, in an MPU, there is no operational difference between RAM and ROM other than its volatility. Input-output is how data are transferred to and from the microcomputer. Output may be from the MPU, ROM, or RAM. Input may be from the MPU or the RAM. The clock of an MPU synchronizes the execution of the MPU instruc- tions. Interrupts are inputs to the MPU that cause it to (temporarily) suspend one activity in order to perform a more important activity. An important family of MPUs that greatly improved the performance of aircraft computers is the Motorola M6800 family of microcomputers. This family offered a series of 1 Wiley Encyclopedia of Computer Science and Engineering, edited by Benjamin Wah. Copyright # 2008 John Wiley Sons, Inc.
- 20. improvements in memory size, clock speeds, functionality, and overall computer performance. Personal Computers Landau (1) defines personal computers as electronic machines that can be owned and operated by individuals for home and business applications such as word proces- sing, games, finance, and electronic communications. Hamacher et al. (3) explain that rapidly advancing very large-scale integrated circuit (VLSI) technology has resulted in dramatic reductions in the cost of computer hardware. The greatest impact has been in the area of small computing machines, where it has led to an expanding market for personal computers. The idea of a personally owned computer is fairly new. The computational power available in handheld toys today was only available through large, costly computers in the late 1950s and early 1960s. Vendors such as Atari, Commodore, and Compaq made simple computer games household items. Performance improvements in memory, throughput, and processing power by companies such as IBM, Intel, and Apple made facilities such as spreadsheets for home budgets, automated tax programs, word proces- sing, and three-dimensional virtual games common house- hold items. The introduction of Microsoft’s Disk Operating System (DOS) and Windows has also added to the accep- tance of the personal computers through access to software applications. Improvements in computer technology offer continual improvements, often multiple times a year. The durability and portability of these computers is beginning to allow them to replace specialized aircraft computers that had strict weight, size, power, and functionality requirements. AVIONICS In the early years of aircraft flight, technological innovation was directed at improving flight performance through rapid design improvements in aircraft propulsion and airframes. Secondary development energies went to areas such as navigation, communication, munitions delivery, and target detection. The secondary functionality of aircraft evolved into the field of avionics. Avionics now provides greater overall performance and accounts for a greater share of aircraft lifecycle costs than either propul- sion or airframe components. Landau (1) definesavionics [avi(ation) þ (electr)onics] as the branch of electronics dealing with the development and use of electronic equipment in aviation and astronautics. The field of avionics has evolved rapidly as electronics has improved all aspects of aircraft flight. New advances in these disciplines require avionics to control flight stability, which was traditionally the pilot’s role. Aircraft Antennas An important aspect of avionics is receiving and transmit- ting electromagnetic signals. Antennas are devices for transmitting and receiving radio-frequency (RF) energy from other aircraft, space applications, or ground applica- tions. Perry and Geppert (4) illustrate the aircraft electro- magnetic spectrum, influenced by the placement and usage of numerous antennas on a commercial aircraft. Golden (5) illustrates simple antenna characteristics of dipole, horn, cavity-backed spiral, parabola, parabolic cylinder, and Cassegrain antennas. Radiation pattern characteristics include elevation and azimuth. The typical antenna specifications are polariza- tion, beam width, gain, bandwidth, and frequency limit. Computers are becoming increasingly important for the new generation of antennas, which include phased-array antennas and smart-skin antennas. For phased-array antennas, computers are needed to configure the array elements to provide direction and range requirements between the radar pulses. Smart-skin antennas comprise the entire aircraft’s exterior fuselage surface and wings. Computers are used to configure the portion of the aircraft surface needed for some sensor function. The computer also handles sensor function prioritization and deinterleaving of conflicting transmissions. Aircraft Sensors Sensors, the eyes and ears of an aircraft, are electronic devices for measuring external and internal environmental conditions. Sensors on aircraft include devices for sending and receiving RF energy. These types of sensors include radar, radio, and warning receivers. Another group of sensors are the infrared (IR) sensors, which include lasers and heat-sensitive sensors. Sensors are also used to mea- sure direct analog inputs; altimeters and airspeed indica- tors are examples. Many of the sensors used on aircraft have their own built-in computers for serving their own functional requirements such as data preprocessing, filter- ing, and analysis. Sensors can also be part of a computer Figure 1. Typical aircraft central computer. 2 AIRCRAFT COMPUTERS
- 21. interface suite that provides key aircraft computers with the direct environmental inputs they need to function. Aircraft Radar Radar (radio detection and ranging) is a sensor that trans- mits RF energy to detect air and ground objects and deter- mines parameters such as the range, velocity, and direction of these objects. The aircraft radar serves as its primary sensor. Several services are provided by modern aircraft radar, including tracking, mapping, scanning, and identi- fication. Golden (5) states that radar is tasked either to detect the presence of a target or to determine its location. Depending on the function emphasized, a radar system might be classified as a search or tracking radar. Stimson (6) describes the decibel (named after Alexander Graham Bell) as one of the most widely used terms in the designand description of radar systems. The decibel (dB)is a logarithmic unit originally devised to express power ratios, but also used to express a variety of other ratios. The power ratioindBisexpressedas10 log10 P2/P1,whereP2 andP1 are the power levels being compared. Expressed in terms of voltage, the gain is (V2/V1)2 dB provided the input voltage V1 and output voltage V2 are across equal resistances. Stimson (6) also explains the concept of the pulse repeti- tion frequency (PRF), which is the rate at which a radar system’s pulses are transmitted: the number of pulses per second. The interpulse period T of a radar is given by T ¼ 1=PRF. For a PRF of 100 Hz, the interpulse period would be 0.01 s. The Doppler Effect, as described by Stimson (6), is a shift in the frequency of a radiated wave, reflected or received by an object in motion. By sensing Doppler frequencies, radar not only can measure range rates, but can also separate target echoes from clutter, or can produce high-resolution ground maps. Computers are required by an aircraft radar to make numerous and timely calculations with the received radar data, and to configure the radar to meet the aircrew’s needs. Aircraft Data Fusion Data fusion is a method for integrating data from multiple sources in order to give a comprehensive solution to a problem (multiple inputs, single output). For aircraft com- puters, data fusion specifically deals with integrating data from multiple sensors such as radar and infrared sensors. For example, in ground mapping, radar gives good surface parameters, whereas the infrared sensor provides the height and size of items in the surface area being investi- gated. The aircraft computer takes the best inputs from each sensor, provides a common reference frame to inte- grate these inputs, and returns a more comprehensive solution than either single sensor could have given. Data fusion is becoming increasingly important as air- crafts’ evolving functionality depends on off-board data (information) sources. New information such as weather, flight path re-routing, potential threats, target assignment, and enroute fuel availability are communicated to the air- craft from its command and control environment. The air- craft computer can now expand its own solution with these off-board sources. Aircraft Navigation Navigation is the science of determining present location, desired location, obstacles between these locations, and best courses to take to reach these locations. An interesting pioneer of aircraft navigation was James Harold Doolittle (1886–1993). Best known for his aircraft-carrier-based bomber raid on Tokyo in World War II, General Doolittle received his Master’s and Doctor of Science degrees in aeronautics from Massachusetts Institute of Technology, where he developed instrumental blind flying in 1929. He made navigation history by taking off, flying a set course, and landing without seeing the ground. For a modern aircraft, with continuous changes in altitude, air- speed, and course, navigation is a challenge. Aircraft com- puters help meet this challenge by processing the multiple inputs and suggesting aircrew actions to maintain course, avoid collision and weather, conserve fuel, and suggest alternative flight solutions. An important development in aircraft navigation is the Kalman filter. Welch and Bishop (7) state that in 1960, R.E. Kalman published his famous paper describing a recursive solution to the discrete-data linear filtering problem. Since that time, due in large part to advances in digital comput- ing, the Kalman filter has been the subject of extensive research and application, particularly in the area of auton- omous or assisted navigation. The Kalman filter is a set of mathematical equations that provides an efficient compu- tational (recursive) implementation of the least-squares method. The filter is very powerful in several aspects: It supports estimation of past, present, and even future states, and it can do so even when the precise nature of the modeled system is unknown. The global positioning system (GPS) is a satellite refer- ence system that uses multiple satellite inputs to determine location. Many modern systems, including aircraft, are equipped with GPS receivers, which allow the system access to the network of GPS satellites and the GPS ser- vices. Depending on the quality and privileges of the GPS receiver, the system can have an instantaneous input of its current location, course, and speed within centimeters of accuracy. GPS receivers, another type of aircraft computer, can also be programmed to inform aircrews of services related to their flight plan. Before the GPS receiver, the inertial navigation systems (INS) were the primary navigation system on aircraft. Fink and Christiansen (8) describe inertial navigation as the most widely used ‘‘self-contained’’ technology. In the case of an aircraft, the INS is contained within the aircraft, and is not dependent on outside inputs. Accelerometers con- stantly sense the vehicle’s movements and convert them, by double integration, into distance traveled. To reduce errors caused by vehicle attitude, the accelerometers are mounted on a gyroscopically controlled stable platform. Aircraft Communications Communication technologies on aircraft are predominately radio communication. This technology allows aircrews to communicate with ground controllers and other aircraft. Aircraft computers help establish, secure, and amplify these important communication channels. AIRCRAFT COMPUTERS 3
- 22. These communication technologies are becoming increasingly important as aircraft become interoperable. As the dependency of aircraft on interoperability increases, the requirements to provide better, more reliable, secure point-to-point aircraft communication also increases. The aircraft computer plays a significant role in meeting this challenge by formatting and regulating this increased flow of information. Aircraft Displays Displays are visual monitors in aircraft that present desired data to aircrews and passengers. Adam and Gibson (9) illustrate F-15E displays used in the Gulf War. These illustrations show heads-up displays (HUDs), vertical situation displays, radar warning receivers, and low- altitude navigation and targeting system (Lantirn) displays typical of modern fighter aircraft. Sweet (10) illustrates the displays of a Boeing 777, showing the digital bus interface to the flight-deck panels and an optical-fiber data distribution interface that meets industry standards. Aircraft Instrumentation Instrumentation of an aircraft means installing data col- lection and analysis equipment to collect information about the aircraft’s performance. Instrumentation equipment includes various recorders for collecting real-time flight parameters such as position and airspeed. Instruments also capture flight control inputs, environmental para- meters, and any anomalies encountered in flight test or in routine flight. One method of overcoming this limitation is to link flight instruments to ground recording systems, which are not limited in their data recording capacities. A key issue here is the bandwidth between the aircraft being tested and its ground (recording) station. This bandwidth is limited and places important limitations on what can be recorded. This type of data link is also limited to the range of the link, limiting the aircraft’s range and altitude during this type of flight test. Aircraft computers are used both in processing the dataastheyarebeingcollected onthe aircraft and in analyzing the data after they have been collected. Aircraft Embedded Information Systems Embedded information system is the latest terminology for an embedded computer system. The software of the embedded computer system is now referred to as embedded information. The purpose of the aircraft embedded infor- mation system is to process flight inputs (such as sensor and flight control) into usable flight information for further flight system or aircrew use. The embedded information system is a good example of the merging of two camps of computer science applications. The first, and larger, camp is the management of information systems (MIS). The MIS dealt primarily with large volumes of information, with primary applications in business and banking. The timing requirements of processing these large information records are measured in minutes or hours. The second camp is the real-time embedded computer camp, which was concerned with processing a much smaller set of data, but in a very timely fashion. The real-time camp’s timing requirement is in microseconds. These camps are now merging, because their requirements are converging. MIS increasingly needs real-time performance,while real-timesystemsarerequired to handle increased data processing workloads. The embedded information system addresses both needs. Aircraft and the Year 2000 The year 2000 (Y2K) was a major concern for the aircraft computer industry. Many of the embedded computers on aircraft and aircraft support functions were vulnerable to Y2K faults because of their age. The basic problem with those computers was that a year was represented by its low- order two digits. Instead of the year having four digits, these computers saved processing power by using the last two digits of the calendar year. For example, 1999 is repre- sented as 99, which is not a problem until you reach the year 2000, represented as 00. Even with this representation, problems are limited to those algorithms sensitive to calen- dar dates. An obvious problem is when an algorithm divides by the calendar date, which is division by 0. Division by 0 is an illegal computer operation, causing problems such as infinite loops, execution termination, and system failure. The most commonly mentioned issue is the subtraction of dates todetermine time durations andtocomparedates.The problem is not that the computer programs fail in a very obvious way (e.g., divide-by-zero check) but rather that the program computes an incorrect result without any warning or indication of error. Lefkon and Payne (11) discuss Y2K and how to make embedded computers Y2K-compliant. Aircraft Application Program Interfaces An application programming interface (API) is conven- tionally defined as an interface used by one program to make use of the services of another program. The human interface to a system is usually referred to as the user interface, or, less commonly, the human–computer inter- face. Application programs are software written to solve specific problems. For example, the embedded computer software that paints the artificial horizon on a heads-up display is an application program. A switch that turns the artificial horizon on or off is an API. Gal-Oz and Isaacs (12) discuss APIs and how to relieve bottlenecks of software debugging. Aircraft Control Landau (1) defines a control as an instrument or apparatus used to regulate a mechanism or a device used to adjust or control a system. There are two concepts with control. One is the act of control. The other is the type of device used to enact control. An example of an act of control is when a pilot initiates changes to throttle and stick settings to alter flight path. The devices of control, in this case, are the throttle and stick. Control can be active or passive. Active control is force- sensitive. Passive control is displacement-sensitive. Mechanical control is the use of mechanical devices, such as levers or cams, to regulate a system. The earliest form of mechanical flight control was wires or cables, used to activate ailerons and stabilizers through pilot stick and 4 AIRCRAFT COMPUTERS
- 23. foot pedal movements. Today, hydraulic control, the use of fluids for activation, is typical. Aircraft control surfaces are connected to stick and foot pedals through hydraulic lines. Pistons in the control surfaces are pushed or pulled by associated similar pistons in the stick or foot pedal. The control surfaces move accordingly. Electronic control is the use of electronic devices, such as motors or relays, to regulate a system. A motor is turned on by a switch, and it quickly changes control surfaces by pulling or pushing a lever on the surface. Automatic control is a system-initiated control, which is a system-initiated response to a known set of environmental conditions. Auto- matic control was used for early versions of automatic pilot systems, which tied flight control feedback systems to altitude and direction indicators. The pilot sets his desired course and altitude, which is maintained through the flight control’s automatic feedback system. To understand the need for computers in these control techniques, it is important to note the progression of the complexity of the techniques. The earliest techniques con- nected the pilot directly to his control surfaces. As the aircraft functionality increased, the pilot’s workload also increased, requiring his (or his aircrew’s) being free to perform other duties. Additionally, flight characteristics became more complex, requiring more frequent and instan- taneous control adjustments. The use of computers helped offset and balance the increased workload in aircraft. The application of computers to flight control provides a means for processing and responding to multiple complex flight control requirements. Aircraft Computer Hardware For aircraft computers, hardware includes the processors, buses, and peripheral devices inputting to and outputting from the computers. Landau (1) defines hardware as appa- ratus used for controlling a spacecraft; the mechanical, magnetic, and electronic design, structure, and devices of a computer; and the electronic or mechanical equipment that uses cassettes, disks, and so on. The computers used on an aircraft are called processors. The processor takes inputs from peripheral devices and provides specific com- putational services for the aircraft. There are many types and functions of processors on an aircraft. The most obvious processor is the central compu- ter, also called the mission computer. The central computer provides direct control and display to the aircrew. The federated architecture (discussed in more detail later) is based on the central computer directing the scheduling and tasking of all the aircraft subsystems. Other noteworthy computers are the data processing and signal processing computers of the radar subsystem and the computer of the inertial navigation system. Processors are in almost every component of the aircraft. Through the use of an embedded processor, isolated components can perform independent functions as well as self-diagnostics. Distributed processors offer improved aircraft perfor- mance and, in some cases, redundant processing capability. Parallel processors are two or more processors configured to increase processing power by sharing tasks. The workload of the shared processing activity is distributed among the pooled processors to decrease the time it takes to form solutions. Usually, one of the processors acts as the lead processor, or master, while the other processor(s) act as slave(s). The master processor schedules the tasking and integrates the final results, which is particularly useful on aircraft in that processors are distributed throughout the aircraft. Some of these computers can be configured to be parallel processors, offering improved performance and redundancy. Aircraft system redundancy is important because it allows distributed parallel processors to be reconfigured when there is a system failure. Reconfigur- able computers are processors that can be reprogrammed to perform different functions and activities. Before com- puters, it was very difficult to modify systems to adapt to their changing requirements. A reconfigurable computer can be dynamically reprogrammed to handle a critical situation, and then it can be returned to its original configuration. Aircraft Buses Buses are links between computers (processors), sensors, and related subsystems for transferring data inputs and outputs. Fink and Christiansen (8) describe two primary buses as data buses and address buses. To complete the function of an MPU, a microprocessor must access memory and peripheral devices, which is accomplished by placing data on a bus, either an address bus or a data bus, depend- ing on the function of the operation. The standard 16-bit microprocessor requires a 16-line parallel bus for each function. An alternative is to multiplex the address or data bus to reduce the number of pin connections. Common buses in aircraft are the Military Standard 1553 Bus (Mil- Std-1553) and the General-Purpose Interface Bus (GPIB), which is the IEEE Standard 488 Bus. Aircraft Software Landau (1) defines software as the programs, routines, and so on for a computer. The advent of software has provided great flexibility and adaptability to almost every aspect of life, which is especially true in all areas of aerospace sciences, where flight control, flight safety, in-flight enter- tainment, navigation, and communications are continu- ously being improved by software upgrades. Operation Flight Programs. An operational flight pro- gram (OFP) is the software of an aircraft embedded com- puter system. An OFP is associated with an aircraft’s primary flight processors, including the central computer, vertical and multiple display processors, data processors, signal processors, and warning receivers. Many OFPs in use today require dedicated software integrated support environments toupgrade and maintain them asthe mission requirements of their parent aircraft are modified. The software integrated support environment [also called avio- nics integrated support environment (AISE), centralized software support activity (CSSA), and software integration laboratory (SIL)] not only allows an OFP to be updated and maintained, but also provides capabilities to perform unit AIRCRAFT COMPUTERS 5
- 24. testing, subsystem testing, and some of the integrated system testing. Assembly Language. Assembly language is a machine (processor) language that represents inputs and outputs as digital data and that enables the machine to perform operations with those data. For a good understanding of the Motorola 6800 Assembler Language, refer to Bishop (13). According to Seidman and Flores (14), the lowest-level (closest to machine) language available to most computers is assembly language. When one writes a program in assembly code, alphanumeric characters are used instead of binary code. A special program called an assembler (provided with the machine) is designed to take the assem- bly statements and convert them to machine code. Assem- bly language is unique among programming languages in its one-to-one correspondence between the machine code statements produced by the assembler and the original assembly statements. In general, each line of assembly code assembles into one machine statement. Higher-Order Languages. Higher-order languages (HOLs) are computer languages that facilitate human language structures to perform machine-level functions. Seidman and Flores (14) discuss the level of discourse of a pro- gramming language as its distance from the underlying properties of the machine on which it is implemented. A low-level language is close to the machine, and hence provides access to its facilities almost directly; a high-level language is far from the machine, and hence insulated from the machine’s peculiarities. A language may provide both high-level and low-level constructs. Weakly typed languages are usually high-level, but often provide some way of calling low-level subroutines. Strongly typed lan- guages are always high-level, and they provide means for defining entities that more closely match the real-world objects being modeled. Fortran is a low-level language that can be made to function as a high-level language by use of subroutines designed for the application. APL, Sobol, and SETL (a set-theoretic language) are high-level languages with fundamental data types that pervade their language. Pascal, Cobol, C, and PL/I are all relatively low-level lan- guages, in which the correspondence between a program and the computations it causes to be executed is fairly obvious. Ada is an interesting example of a language with both low-level properties and high-level properties. Ada provides quite explicit mechanisms for specifying the layout of data structures in storage, for accessing particular machine locations, and even for communicating with machine interrupt routines, thus facilitating low-level requirements. Ada’s strong typing qualities, however, also qualify it as a high-level language. High-level languages have far more expressive power than low-level languages, and the modes of expression are well integrated into the language. One can write quite short programs that accomplish very complex operations. Gonzalez (15) developed an Ada Programmer’s Handbook that presents the terminology of the HOL Ada and exam- ples of its use. He also highlights some of the common programmer errors and examples of those errors. Sodhi (16) discusses the advantages of using Ada. Important discussions of software lifecycle engineering and main- tenance are presented, and the concept of configuration management is presented. The package concept is one of the most important devel- opments to be found in modern programming languages, such as Ada, Modula-2, Turbo Pascal, Cþþ, and Eiffel. The designersofthe different languageshavenot agreed onwhat terms to use for this concept: Package, module, unit, and class are commonly used. It is generally agreed, however, that the package (as in Ada) is the essential programming tool to be used for going beyond the programming of very simple class exercises to what is generally called software engineering or building production systems. Packages and package-like mechanisms are important tools used in soft- ware engineering to produce production systems. Feldman (17) illustrates the use of Ada packages to solve problems. Databases. Database are essential adjuncts to computer programming. Databases allow aircraft computer appli- cations the ability to carry pertinent information (such as flight plans or navigation waypoints) into their missions, rather than generating them enroute. Databases also allow the aircrew to collect performance information about the aircraft’s various subsystems, providing a capability to adjust the aircraft in flight and avoid system failures. Elmasri and Navathe (18) define a database as a collec- tion of related data. Data are described as known facts that can be recorded and have implicit meaning. A simple example consists of the names, telephone numbers, and addresses of an indexed address book. A database manage- ment system (DBMS) is a collection of programs that enable users to create and maintain a database. The DBMS is hence a general-purpose software system that facilitates the processes of defining, constructing, and manipulating databases for various applications. Verification and Validation. A significant portion of the aircraft computer’s lifecycle cost is system and software testing, performed in various combinations of unit-level, subsystem-level, integrated-system-level, developmental, and operational testing. These types of tests occur fre- quently throughout the life of an aircraft system because there are frequent upgrades and modifications to the air- craft and its various subsystems. It is possible to isolate acceptance testing to particular subsystems when minor changes are made, but this is the exception. Usually, any change made to a subsystem affects other multiple parts of the system. As aircraft become increasingly dependent on computers (which add complexity by the nature of their interdependences), and as their subsystems become increasingly integrated, the impact of change also increases drastically. Cook (19) shows that a promising technology to help understand the impact of aircraft com- puter change is the Advanced Avionics Verification and Validation (AAVV) program developed by the Air Force Research Laboratory. Sommerville (20) develops the concepts of program ver- ification and validation. Verification involves checking that the program conforms to its specification. Validation involves checking that the program as implemented meets the expectations of the user. 6 AIRCRAFT COMPUTERS
- 25. Figure 2 shows an aircraft avionics support bench, which includes real components from the aircraft such as the FCC line replaceable unit (LRU) sitting on top of the pictured equipment. Additional equipment includes the buses, cooling, and power connection interfaces, along with monitoring and displays. On these types of benches, it is common to emulate system and subsystem responses with testing computers such as the single-board computers illustrated. Figure 3 shows another verification and validation asset called the workstation-based support environment. This environment allows an integrated view of the aircraft’s performance by providing simulations of the aircraft’s controls and displays on computer workstations. The simulation is interfaced with stick and throttle controls, vertical situation displays, and touch-screen avionics switch panels. Object-Oriented Technology. Object-oriented (OO) tech- nology is one of the most popular computer topics of the 1990s. OO languages such as Cþþ and Ada 95 offer tre- mendous opportunities to capture complex representations of data and then save these representations in reusable objects. Instead of using several variables and interactions to describe some item or event, this same item or event is described as an object. The object contains its variables, control-flow representations, and data-flow representa- tions. The object is a separable program unit, which can be reused, reengineered, and archived as a program unit. The power of this type of programming is that when large libraries of OO programming units are created, they can be called on to greatly reduce the workload of computer soft- wareprogramming. Gabel (21) says that OO technology lets an object (a software entity consisting of the data for an action and the associated action) be reused in different parts of the application, much as an engineered hardware product can use a standard type of resistor or micropro- cessor. Elmasri and Navathe (18) describe an OO database as an approach with the flexibility to handle complex requirements without being limited by the data types and query languages available in traditional database systems. Open System Architecture. Open system architecture is a design methodology that keeps options for updating sys- tems open by providing liberal interfacing standards. Ralston and Reilly (22) state that open architectures per- tain primarily to personal computers. An open architecture is one that allows the installation of additional logic cards in the computer chassis beyond those used with the most primitive configuration of the system. The cards are inserted into slots in the computer’s motherboard—the main logic board that holds its CPU and memory chips. A computer vendor that adopts such a design knows that, because the characteristics of the motherboard will be public knowledge, other vendors that wish to do so can design and market customized logic cards. Open system architectures are increasingly important in modern air- craft applications because of the constant need to upgrade these systems and use the latest technical innovations. It is extremely difficult to predict interconnection and growth requirements for next-generation aircraft, which is exactly what an open architecture attempts to avoid the need for. Client-Server Systems. A client-server system is one in which one computer provides services to another computer on a network. Ralston and Reilly (22) describe the file- server approach as an example of client-server interaction. Clients executing on the local machine forward all file requests (e.g., open, close, read, write, and seek) to the remote file server. The server accepts a client’s requests, performs its associated operation, and returns a response to the client. Indeed, if the client software is structured transparently, the client need not even be aware that files being accessed physically reside on machines located else- where on the network. Client-server systems are being applied on modern aircraft, where highly distributed resources and their aircrew and passenger services are networked to application computers. Subsystems. The major subsystems of an aircraft are its airframe, power plant, avionics, landing gear, and controls. Landau (1) defines a subsystem as any system that is part of Figure 2. An aircraft avionics support bench. Figure 3. A workstation-based aircraft avionics support envi- ronment. AIRCRAFT COMPUTERS 7
- 26. alarger system. Many of the subsystems on an aircraft have one or more processors associated with them. It is a complex task to isolate and test the assorted subsystems. Another layer of testing below subsystem testing is unit testing. A unit of a subsystem performs a function for it. For example, in the radar subsystem, the units include its signal processor and its data processor. In order to test a system adequately, each of its lowest-level items (units) must be tested. As the units affect and depend on each other, another layer of testing addresses that layer of dependences. In the same fashion, subsystem testing is performed and integrated with associated subsystems. It is important to test not only at the unit and the subsystem level, but at the system and operational level. The system level is where the subsystems are brought together to offer the system functionality. System integration is the process of connecting subsystem components into greater levels of system functionality until the complete system is realized. The operational level of testing is where the subsystem is exercised in its actual use. Line Replaceable Units. LRUs are subsystems or subsys- tem components that are self-contained in durable boxes containing interface connections for data, control, and power. Many LRUs also contain built-in test (BIT) capabil- ities that notify air and maintenance crews when a failure occurs. A powerful feature of LRUs is that functionality can be compartmentalized. When a failure is detected, the LRU can easily be pulled and replaced, restoring the aircraft to service within moments of detection. Graceful Degradation. All systems must have plans to address partial or catastrophic failure. System failure in flight controls is often catastrophic, whereas system failure in avionics can be recovered from. For this reason, most flight-critical systems have built-in redundant capabilities (sometimes multiple layers of redundancy), which are auto- matically activated when the main system or subsystem fails. Degraded system behavior occurs when the main system fails and backup systems are activated. The critical nature of system failure requires immediate activation of backup systems and recognition by all related subsystems of the new state of operation. Graceful degradation is the capability of aircraft computers to continue operating after incurring system failure. Graceful degradation is less than optimal performance, and may activate several layers of decreasing performance before the system fails. The value of graceful degradation is that the aircrew has time to respond to the system failure before a catastrophic failure occurs. AEROSPACE Computer technologies have helped provide a continuum of improvements in aircraft performance that has allowed the airspace where aircraft operate to increase in range and altitude. Landau (1) defines aerospace as the Earth’s atmo- sphere and the space outside it, considered as one contin- uous field. Because of its rapidly increasing domain of air and space travel, the U. S. Air Force is beginning to refer to itself as the U. S. Aerospace Force. Modern air-space vehi- cles are becoming increasingly dependent on information gleaned from ground stations, satellites, other air-space vehicles, and onboard sensors to perform their mission. These vehicles use signals across the electromagnetic spec- trum. Antennas can be found in multiple locations on wings, the fuselage, tails, and draglines. If antennas are located too close together, their signals can interfere with each other, called crossed frequency transmission. This interference reduces the efficiency of each affected antenna. Placement of multiple antennas requires mini- mizing the effects of crossed frequency transmissions. Techniques for minimization include antenna placement, filtering, and timing, which presents another challenge for aircraft computers to sort and process these multiple sig- nals. Perry and Geppert (4) show how the aircraft electro- magnetic spectrum is becoming busy, and thus, dangerous for aerospace communications. Legacy Systems Legacy systems are fielded aircraft, or aircraft that are in active use. Probably the only nonlegacy aircraft are experi- mental or prototype versions. Legacy aircraft are often associated with aging issues, more commonly known as parts obsolescence. A growing problem in these systems is the obsolescence of entire components, including the many computers used on them. Aircraft, like many other systems, are designed with expected lifetimes of 10 to 15 years. Because of the high replacement costs, lifetimes are often doubled and tripled by rebuilding and updating the air- craft. To reduce costs, as many of the original aircraft components as possible are kept. Problems develop when these components are no longer produced or stockpiled. Sometimes, subsystems and their interfaces have to be completely redesigned and produced at great cost in order to keep an aircraft in service. System architectures and standard interfaces are constantly being modified to address these issues. Aircraft evolve during their lifetimes to a more open architecture. This open architecture, in turn, allows the aircraft components to be more easily replaced, thus making further evolution less expensive. Unmanned Air Vehicles Unmanned air vehicles (UAVs) are aircraft that are flown without aircrews. Their use is becoming increasingly pop- ular for military applications. Many of the new capabilities of UAVs come from the improved computers. These com- puters allow the vehicles to have increased levels of auton- omy and to perform missions that once required piloted aircraft. Some of these missions include reconnaissance and surveillance. These same types of missions are finding increasing commercial importance. UAVs offer tremen- dous advantages in lifecycle cost reductions because of their small size, ease of operation, and ability to be adapted to missions. MAN–MACHINE SYSTEMS An aircraft is an example of a man–machine system. Other examples are automobiles and boats. These machines 8 AIRCRAFT COMPUTERS
- 27. have the common attribute of being driven by a human. Landau (1) defines man–machine systems as sets of manu- ally performed and machine-performed functions, oper- ated in conjunction to perform an operation. The aircraft computer is constantly changing the role of the human in the aircraft machine. The earliest aircraft required the constant attention of the pilot. Improved flight control devices allowed the pilot freedom for leisure or for other tasks. Modern aircraft computers have continued the trend of making the aircraft more the machine and less the man system. Human Factors of Aircraft Computers Human factors is the science of optimal conditions for human comfort and health in the human environment. The human factors of aircraft computers include the posi- tioning of the controls and displays associated with the aircrew’s workloads. They also provide monitoring and adjustment of the aircraft human environment, including temperature, oxygen level, and cabin pressure. Man–Machine Interface The man–machine interface is the place where man’s inter- actions with the aircraft coordinate with the machine functionality of the aircraft. An example of a man–machine interface is the API, which is where a person provides inputs to and receives outputs from computers. These types of interfaces include keyboards (with standard ASCII char- acter representation), mouse pads, dials, switches, and many varieties of monitors. A significant interface in air- craft comprises their associated controls and displays, which provide access to the flight controls, the sensor suite, the environmental conditions, and the aircraft diagnostics through the aircraft’s central computer. Control sticks, buttons, switches, and displays are designed based on human standards and requirements such as seat height, lighting, accessibility, and ease of use. Voice-Activated Systems. Voice-activated systems are interfaces to aircraft controls that recognize and respond to aircrew’s verbal instructions. A voice-activated input provides multiple input possibilities beyond the limited capabilities of hands and feet. Voice-activated systems have specified sets of word commands and are trained to recognize a specific operator’s voice. Aircraft Computer Visual Verification Visual verification is the process of physically verifying (through sight) the correct aircraft response to environ- mental stimuli. This visual verification is often a testing requirement. It is usually done through the acceptance test procedure (ATP) and visual inspections of displays through a checklist of system and subsystem inputs. Until recently, visual verification has been a requirement for pilots, who have desired the capability to see every possibility that their aircraft might encounter. This requirement is becom- ing increasingly difficult to implement because of the grow- ing complexity and workload of the aircraft’s computers and their associated controls and displays. In the late 1980s to early 1990s, it required about 2 weeks to visually verify the suite of an advanced fighter system’s avionics. This verification can no longer be accomplished at all with current verification and validation techniques. Several months would be required to achieve some level of confi- dence that today’s modern fighters are flight-safe. Air Traffic Control Air traffic control is the profession of monitoring and controlling aircraft traffic through an interconnected ground-based communication and radar system. Perry (23) describes the present capabilities and problems in air traffic control. He also discusses the future require- ments for this very necessary public service. Air traffic controllers view sophisticated displays, which track multi- ple aircraft variables such as position, altitude, velocity, and heading. Air traffic control computers review these variables and give the controllers continuous knowledge of the status of each aircraft. These computers continuously update and display the aircraft in the ground-based radar range. When potential emergency situations, such as collision, develop, the computer highlights the involved aircraft on the displays, with plenty of lead time for the controller to correct each aircraft’s position. AIRCRAFT CONTROL AND COMPUTERS D’ Azzo and Houpis (24) give a good explanation of the complexity of what is needed for an aircraft control sys- tem. The feedback control system used to keep an airplane on a predetermined course or heading is necessary for the navigation of commercial airliners. Despite poor weather conditions and lack of visibility, the airplane must maintain a specified heading and altitude in order to reach its destination safely. In addition, in spite of rough air, the trip must be made as smooth and comfortable as possible for the passengers and crew. The problem is considerably complicated by the fact that the airplane has six degrees of freedom, which makes control more difficult than control of a ship, whose motion is limited to the surface of the water. A flight controller is used to control aircraft motion. Two typical signals to the system are the correct flight path, which is set by the pilot, and the level position of the airplane. The ultimately controlled variable is the actual course and position of the airplane. The output of the control system, the controlled variable, is the aircraft heading. In conventional aircraft, three primary control surfaces are used to control the physical three-dimensional atti- tude of the airplane: the elevators, the rudder, and the ailerons. A directional gyroscope (gyro) is used as the error-measuring device. Two gyros must be used to pro- vide control of both heading and attitude of the airplane. The error that appears in the gyro as an angular displace- ment between the rotor and case is translated into a voltage by various methods, including the use of transdu- cers such as potentiometers, synchros, transformers, or microsyns. Selection of the method used depends on the AIRCRAFT COMPUTERS 9
- 28. preference of the gyro manufacturer and the sensitivity required. Additional stabilization for the aircraft can be provided in the control system by rate feedback. In other words, in addition to the primary feedback, which is the position of the airplane, another signal proportional to the angular rate of rotation of the airplane around the vertical axis is fed back in order to achieve a stable response. A rate gyro is used to supply this signal. This additional stabili- zation may be absolutely necessary for some of the newer high-speed aircraft. In reading through this example, it should be obvious that as the complexity of the control feedback system of the aircraft increases, a need for computer processing to eval- uate the feedback and to adjust or recommend flight control adjustments exists. Additional feedback may come from global positioning, from ground-based navigation systems through radio inputs, and from other aircraft. The compu- ter is able to integrate these inputs into the onboard flight control inputs and provide improved recommendations for stable flight. REAL-TIME SYSTEMS The computers on aircraft are required to perform their functions within short times. Flight control systems must make fine adjustments quickly in order to main- tain stable flight. Sensor suites must detect and analyze potential threats before it is too late. Cabin pressure and oxygen must be regulated as altitude changes. All these activities, plus many others on aircraft, must happen in real time. Nielsen (25) defines a real-time system as a controlled (by software or firmware) system that performs all of its process functions within specified time constraints. A real-time system usually includes a set of independent hardware devices that operate at widely differing speeds. These devices must be controlled so that the system as a whole is not dependent on the speed of the slowest device. Hatley and Pirbhai (26) describe timing as one of the most critical aspects of modern real-time systems. Often, the system’s response must occur within milliseconds of a given input event, and every second it must respond to many such events in many different ways. Flight-Critical Systems Flight-critical systems are those activities of an aircraft that must be completed without error in order to maintain life and flight. The aircraft flight controls, engines, landing gear, and cabin environment are examples of flight-critical systems. Failures in any of these systems can have cata- strophic results. Flight-critical systems are held to tight levels of performance expectations, and often have redun- dant backups in case of failure. Federated Systems Federated systems are loosely coupled distributed systems frequently used in aircraft system architectures to tie multiple processors in multiple subsystems together. The loose coupling allows the multiple subsystems to operate somewhat autonomously, but have the advantage of the shared resources of the other subsystems. A typical aircraft federated system might include its central computer, its INS, its radar system, and its air-vehicle management system. The INS provides the radar with the aircraft’s present position, which is reported to the pilot through displays put forth by the central computer. The pilot adjusts his course through the air-vehicle management system, which is updated by the INS, and the cycle is repeated. These subsystems perform their individual func- tionality while providing services to each other. Cyclic Executive A cyclic executive on an aircraft computer provides a means to schedule and prioritize all the functions of the computer. The executive routine assigns the functions and operations to be performed by the computer. These assign- ments are given a specific amount of clock time to be performed. If the assignment does not complete its task in its allocated time, it is held in a wait state until its next clock period. From the beginning of the clock period to its end is one clock cycle. High-priority functions are assigned faster clock cycles, whereas low-priority functions are assigned slower cycles. For example, the high-priority executive function might be assigned a speed of 100 cycles per second, whereas some lower-priority function might have 5 cycles per second to complete its tasks. Sometimes, the latter might take several clock cycles to perform a task. An additional feature of cyclic executives is that they are equipped with interrupts, which allow higher-priority systems to break into the executive assignments for system-level assigned tasking. There are several types of scheduling methodologies that provide performance improvements in cyclic exe- cutives. One of the more prominent is rate monotonic analysis (RMA), which determines the time requirement for each function and the spare time slots, and then makes time assignments. THE NETWORK-CENTRIC AIRCRAFT In the age of the World Wide Web (www), it is hard to imagine the concept of platform-centric systems, such as many of the aircraft that are in service today. These air- craft were built with the requirement to be self-sufficient, safe, and survivable. Dependency on off-board inputs was minimized as advanced avionics technologies allowed air- craft to assess and respond to their environment flight dynamics independently. These aircraft have been con- ceived, created, and maintained right up to this new information age. It takes significant effort to open the architectures of these aircraft, in order for their existing capabilities to be enhanced by outside information. For- tunately, the adaptability and flexibility of aircraft com- puters makes this process possible for many of these aircraft. The modern aircraft (conceived, created, and main- tained since the mid-1990s) is a network-centric aircraft. These aircraft take full advantage of the platform-centric 10 AIRCRAFT COMPUTERS
- 29. systems with independent suites of avionics and aircraft computers. However, they have the additional ability to adapt to their environmental flight dynamics, which is possible because these systems have access to the most recent information about their environment. They can interactively communicate with other aircraft entering and leaving their environment, as well as take advantage of the information services available in that environment. The aircraft computers work very much the same as in the platform-centric aircraft, but with improved and broader information than was available before (27,28). The network-centric aircraft can take full advantage of route changes caused by heavy air traffic, threats, or weather. It can send its systems self-diagnostics ahead to maintenance crews, who can have parts and resources available reducing the service re-cycling time of the air- craft. It can inform passengers and crew about their indi- vidual travel plans and the options available to them as they arrive at their destinations. It can help air traffic controllers and flight planners manage the dynamic work- load of the many aircraft in service. BIBLIOGRAPHY 1. S. Landou, Webster Illustrated Contemporary Dictionary, Encyclopedic Edition. Chicago: J. G. Ferguson, 1992. 2. J. F. Wakerly, Digital Design Principles and Practices. Engle- wood Cliffs, NJ: Prentice-Hall, 1985, pp. 1–48, 53–138. 3. V. C. Hamacher, Z. G. Vranesic, and S. G. Zaky, Computer Organization, 2nd ed. New York: McGraw-Hill, 1984. 4. T. Perry and L. Geppert, Do portable electronics endanger flight, IEEE Spectrum, 33(9): 26–33, 1996. 5. A. Golden, Radar Electronic Warfare. Washington: AIAA Edu- cation Series, 1987. 6. G. W. Stimson, Introduction to Airborne Radar. El Segundo, CA: Hughes Aircraft, 1983, pp. 107, 151–231. 7. G. Welch and G. Bishop, An introduction to the Kalman filter, Department of Computer Science, University of North Caro- lina at Chapel Hill, Chapel Hill, NC, http://www.cs.unc.edu/ ~welch/media/pdf/kalman.pdf, 1997. 8. D.Finkand D.Christiansen, Electronics Engineers’ Handbook, 3rd ed., New York: McGraw-Hill, 1989. 9. J. Adam and T. Gibson, Warfare in the information age, IEEE Spectrum, 28(9): 26–42, 1991. 10. W. Sweet, The glass cockpit, IEEE Spectrum, 32(9): 30–38, 1995. 11. D. Lefkon and B. Payne, Making embedded systems year 2000 compliant, IEEE Spectrum, 35(6): 74–79, 1998. 12. S. Gal-Oz and M. Isaacs, Automate the bottleneck in embedded system design, IEEE Spectrum, 35(8): 62–67, 1998. 13. R. Bishop, Basic Microprocessors and the 6800. Hasbrouck Heights, NJ: Hayden, 1979. 14. A. Seidman and I. Flores, The Handbook of Computers and Computing. New York: Van Norstrand Reinhold, 1984, pp. 327–502. 15. D. W. Gonzalez, Ada Programmer’s Handbook. Redwood City, CA: Benjamin/Cummings, 1991. 16. J. Sodhi, Managing Ada Projects. Blue Ridge Summit, PA: TAB Books, 1990. 17. M. B. Feldman and E. B. Koffman, Ada Problem Solving and Program Design. Reading, MA: Addison-Wesley, 1992. 18. R. Elmasri and S. B. Navathe, Fundamentals of Database Design, 2nd ed. Redwood City, CA: Benjamin/Cummings,1994. 19. R. Cook, The advanced avionics verification and validation II final report, Air Force Research Laboratory Technical Report ASC-99-2078, Wright-Patterson AFB. 20. I. Sommerville, Software Engineering, 3rd ed. Reading, MA: Addison-Wesley, 1989. 21. D. Gabel, Software engineering, IEEE Spectrum, 31(1): 38–41, 1994. 22. A. Ralston and E. Reilly, Encyclopedia of Computer Science. New York: Van Nostrand Reinhold, 1993. 23. T. Perry, In search of the future of air traffic control, IEEE Spectrum, 34(8): 18–35, 1997. 24. J. J. D’ Azzo and C. H. Houpis, Linear Control System Analysis and Design, 2nd ed. New York: McGraw-Hill, 1981, pp. 143– 146. 25. K. Nielsen, Ada in Distributed Real-Time Systems. New York: Intertext, 1990. 26. D. J. Hatley and I. A. Pirbhai, Strategies for Real-Time System Specification. New York: Dorset House, 1988. 27. D. S. Alberts, J. J. Garstka, and F. P. Stein, Network Centric Warfare. Washington D.C.: CCRP Publication Series, 2000. 28. D. S. Alberts and R. E. Hayes, Power to the Edge. Washington D.C.: CCRP Publication Series, 2003. FURTHER READING G. Buttazo, Hard Real-Time Computing Systems. Norwell, MA: Kluwer, 1997. R. Comerford, PCs and workstations, IEEE Spectrum, 30(1): 26– 29, 1993. D. Dooling, Aerospace and military, IEEE Spectrum, 35(1): 90–94, 1998. J. Juliussen and D. Dooling, Small computers, aerospace mili- tary, IEEE Spectrum, 32(1): 44–47, 76–79, 1995. K. Kavi, Real-Time Systems, Abstractions, Languages, and Design Methodologies. Los Alamitos, CA: IEEE Computer Society Press, 1992. P. Laplante, Real-Time Systems Design and Analysis, an Engineer’s Handbook. Piscataway, NJ: IEEE Press, 1997. M. S. Roden, Analog and Digital Communication Systems, 2nd ed. Englewood Cliffs, NJ: Prentice-Hall, 1985. H. Taub, Digital Circuits and Microprocessors. New York: McGraw-Hill, 1982. C. Weitzman, Distributed Micro/Minicomputer. Englewood Cliffs, NJ: Prentice-Hall, 1980. CHARLES P. SATTERTHWAITE United States Air Force Wright-Patterson AFB, Ohio. AIRCRAFT COMPUTERS 11
- 30. C COMPUTERIZED DICTIONARIES: INTEGRATING PORTABLE DEVICES, TRANSLATION SOFTWARE, AND WEB DICTIONARIES TO MAXIMIZE LEARNING BACKGROUND STUDIES ON BILINGUAL AND ELECTRONIC DICTIONARIES Many articles comparing various types of dictionaries may be found in the first fully annotated bibliographic review of studies in this broad field of lexicography (the making of dictionaries, whether print or electronic), entitled Pedagogical Lexicography Today by Dolezal and McCreary (1), under either the learner dictionary category or under traditional dictionaries designed for native read- ers. Articles on learner dictionaries are grouped by their central focus, namely by whether they are mainly dealing with bilingual (giving first language or L1 translations), bilingualized (including both L1 and L2 information), or only monolingual (providing only English-to-English or other L2 to/from L2 definitions) explanations of target language (TL) vocabulary. Laufer and Kimmel (2) des- cribed patterns of use, comparing a particular dictionary’s degree of accessibility versus difficulty for learners, finding that ‘‘Each learner was classified by his favorite look-up pattern. . .on the basis of these, we argue that the bilin- gualised dictionary is very effective as it is compatible with all types of individual preferences.’’ (p. 361) (for more information on computerized dictionary writing systems, see http://nlp.fi.muni.cz/dws06/). Lexical computing is a field of most concern to language teachers, computational linguists, and lexicographers involved in making dictionary writing systems (DWS), software for writing and producing a dictionary. It might include an editor, a database, a web interface, and various management tools (for allocating work, etc.), operating with a dictionary grammar, which specifies the internal structure of the dictionary. Robert Lew (3), whose disserta- tion provides a massive database for further research in this field, considered the receptive use of bilingual, mono- lingual, and semi-bilingual dictionaries by Polish learners of English, asking the most basic question for language teachers and dictionary designers (lexicographers) to consider, namely the question of which dictionary is best for whom? Other studies have compared the use of various types of glosses, such as ‘‘(paper, electronic textual, electronic pictorial, electronic, and video) on reading com- prehension, translation, the number of words looked up, time-on-task and satisfaction of dictionary users. Others investigated incidental vocabulary learning via computer glosses, as reported by Laufer and Levitzky-Aviad (4). Loucky (5–8) compared Japanese college students’ accessing speeds for portable devices with using software or mobile phone dictionaries. Akbulut (9–11) compared the supposed advantage that adding various types of multimedia glossing might bring to language learners. Two crucial findings are well summarized in Chun (12): ‘‘. . .previous studies have found that L2 vocabulary is remembered better when learners look up picture or video glosses in addition to translations of unfamiliar words, but that when given the choice, learners tend to prefer and use the simple translation of words. . . In summary, research during the last ten years (1995–2005) has found that bilingual dictionaries and multimedia glosses have a more direct impact on vocabulary acquisition than on overall reading comprehension. . . .’’ (pp. 78–81). A history of lexicography and dictionary development in Japan may be found in Nakao’s (13)The State of Bilingual Lexicography in Japan: Learners’ English-Japanese/ Japanese-English Dictionaries. Other researchers who have examined the individual preferences, needs, and skills of dictionary users (both monolingual and bilingual) include Baxter (14), Tomaszczyk (15), Hartmann (16), Piotrowski (17), Atkins and Knowles (18), and Nuccorini (19). Hulstijn and Atkins (20) suggested that use of electro- nic dictionaries be studied more systematically. Laufer and Hill (21) examined how users’ CALL dictionary look-up behaviors affected their retention. Those who design dic- tionaries for language learners, whether traditional text or electronic types of dictionaries, can gain much insight from more individualized, long-term studies done in countries where they have a consumer base. Tomaszczyk (15), who first questioned foreign language learners regarding their preferences and dictionary usage, stated that the vast majority of his close to 450 Polish respondents ‘‘would like their dictionaries to give much more extensive treatment to every type of information. . . would like to have an omnibus dictionary which would cover everything anyone has ever thought of including in dictionaries and encyclopedias’’ (p. 115). Today, Internet search engines seem to do just that, but are often far too broad, especially for limited English proficiency (LEPs) learners to use efficiently. One solution to this problem is to use the writer’s Virtual Language Learning Encyclope- dia site at www.CALL4ALL.us. Providing instant links to most web dictionaries found on its Dictionaries (D) page at http://www.call4all.us///home/_all.php?fi=d, this site enables anyone to find vocabulary information for 500 language pairs systematically, by giving simultaneous instant free access to over 2500 online dictionaries. More- over, this online multilingual dictionary portal now inte- grates the many functions of Wordchamp.com’s versatile Webreader on each of its pages, thereby providing auto- matic glossing from English into over 100 languages for any website, including 40 online newspapers in 10 major languages. Paper Versus Electronic Dictionaries Electronic dictionaries are undoubtedly greatly gaining in popularity, so much so that they will soon dominate the 1 Wiley Encyclopedia of Computer Science and Engineering, edited by Benjamin Wah. Copyright # 2008 John Wiley Sons, Inc.
- 31. dictionary scene (22–26). Lew (3) noted these recent trends stating: It has been claimed that with the move from paper to online dictionaries, restrictions of space would disappear. That, how- ever, is a simplification at best. While storage space may indeed become irrelevant, there are still severe restrictions as to how much information can be displayed at a time. In fact, even the bestcurrently availabledisplaydevicesare stilleasilybeatenby the old-fashioned printed paper in terms of visual resolution. So space-saving issues will still be with for at least as long as the visual modality is primarily used for information transfer from dictionary to user. . .on-screen presentation of entries has much to offer. . .to the researcher by way of convenience, including a potential to log responses automatically, thus obviating the need for the laborious paperwork and keyboarding at the data entry stage, as well as allowing ‘‘unobtrusive observation’’. (p. 157) The equivalence of on-screen and paper formats should not be taken for granted, as Laufer (27) found significant and substantial differences in word recall scores between marginal paper glosses and on-screen pop-up window glosses. DOING LEXICOGRAPHY IN AN ELECTRONIC AGE Tono (28) predicted the advantages of online media using machine translation, saying ‘‘Electronic dictionaries have great potential for adjusting the user interface to users’ skill level[s] so that learners with different needs and skills can access information in. . . different way[s].’’ (p. 216) First of all, one must note that electronic dictionaries have developed based on a healthy integration of develop- ments in computerized corpus linguistics and modern technology, used to enhance learning in many fields, particularly computer-assisted language learning (or CALL) or computer-mediated communications (CMC). Laufer and Kimmel (2) provide a clear summary of this field, noting that If the consumer is to benefit from the lexicographer’s product, the dictionary should be both useful and usable. We suggest a definition of dictionary usefulness as the extent to which a dictionary is helpful in providing the necessary information to its user. Dictionary usability, on the other hand, can be defined as the willingness on the part of the consumer to use the dictionary in question and his/her satisfaction from it. Studies of dictionary use by L2 learners . . . reveal that dic- tionary usefulness and dictionary usability do not necessarily go hand in hand. (pp. 361–362) Laufer and Levitzky-Aviad’s (4) study recommends working toward designing a bilingualized electronic dic- tionary (BED) more clear and useful for second language production. Whereas conventional bilingual L1-L2 diction- aries list translation options for L1 words without explain- ing differences between them or giving much information about how to use various functions, Laufer and Levitzky- Aviad (4) examined the usefulness of an electronic Hebrew- English-English (L1-L2-L2) minidictionary designed for production. Their results demonstrated the superiority of fully bilingualized L1-L2-L2 dictionaries and some unique advantages of the electronic format. Their literature review provides a good overview of this field: Surveys of dictionary use indicate that the majority of foreign language learners prefer bilingual L2-L1 dictionaries and use them mainly to find the meaning of unknown foreign (L2) words (Atkins 1985; Piotrowsky 1989). However, if learners writing in L2 need an L2 word designating a familiar L1 concept, they do not readily turn to an L1-L2 dictionary for help. The reason for this may lie in a serious limitation of most L1-L2 bilingual dictionaries. They rarely differentiate between the possible L2 translations of the L1 word, nor do they provide information regarding the use of each translation option. . . An electronic dictionary can fulfill the above requirements since it can com- bine the features of an L2-L1bilingual dictionary, an L1-L2 bilingual dictionary and an L2 monolingual dictionary. The advent of electronic dictionaries has already inspired research into their use and their usefulness as on-line helping tools and as contributors to incidental vocabulary learning. The built in log files can keep track of words looked up, type of dictionary information selected (definition, translation, example, etc.), the number of times each word was looked up, and the time spent on task completion. (pp. 1–2) Although most electronic dictionaries do auto- archiving of any new words by means of their history search function, most online dictionaries do not have a means of tracking student use, except for programs like Wordchamp.com or Rikai.com, which give students a way to archive words they have double-clicked. These words may later be seen, printed, and reviewed. In fact, Wordchamp.com, by far the most sophisticated online electronic dictionary and vocabulary development pro- gram, allows users to make online flashcards with sentence examples and links to online texts where target words are found in context. It can also automatically generate about 10 types of online vocabulary quizzes and provides a free course management system (CMS) for monitoring stu- dents’ work online. Wordchamp’s Webreader provides the most versatile online glossing engine known, already for over 100 languages, with more being added regularly. Teachers need to show learners how to best inte- grate the use of such portable and online dictionaries to make them maximally effective for their language develop- ment, in both receptive and productive aspects. Chun (12) noted that learners who could read online text with ‘‘access to both internally (instructor-created) glossed words as well as externally glossed words. . . recalled a significantly greater number of important ideas than when they read an online text and had access only to an external (portable electronic) dictionary’’ (p. 75). Loucky (29) also examined how to best maximize L2 vocabulary development by using a depth of lexical proces- sing (DLP) scale and vocabulary learning strategies (VLSs) taxonomy together with online CALL resources and sys- tematic instruction in the use of such strategies. It used 40 of the 58 VLSs identified in Schmitt’s earlier taxonomy. An electronic dictionary use survey (see Appendix) was designed to solicit information about how students used various computerized functions of electronic or online 2 COMPUTERIZED DICTIONARIES
- 32. dictionaries at each major phase of lexical processing to help learners maximize processing in the following eight stages of vocabulary learning: (1) assessing degree of word knowledge, (2) accessing new word meanings, (3) archiving new information for study, (4) analyzing word parts and origins, (5) anchoring new words in short-term memory, (6) associating words in related groups for long-term reten- tion, (7) activating words through productive written or oral use, and (8) reviewing/recycling and then retesting them. Portable devices or online programs that could moni- tor and guide learners in using these essential strategies should be further developed. In Loucky’s (7) findings, despite being one grade level higher in their proficiency, English majors were out- performed on all types of electronic dictionaries by Com- puter majors. The author concluded that familiarity with computerized equipment or computer literacy must have accounted for this, and therefore should be carefully con- sidered when developing or using electronic dictionary programs of any sort for language or content learning. His study compared vocabulary learning rates of Japanese college freshmen and functions of 25 kinds of electronic dictionaries, charting advantages, disadvantages, and comments about the use of each (for details, see Loucky (7) Table 1 and Appendix 3; Loucky (8) Tables 1 and 2. For a comparative chart of six most popular EDs for English-Japanese use, see www.wordtankcentral.com/ compare.html). Generally speaking, language learners prefer access to both first and second language information, and beginning to intermediate level learners are in need of both kinds of data, making monolingual dictionaries alone insufficient for their needs. As Laufer and Hadar (30) and others have shown the benefits of learners using fully bilingualized dictionaries, the important research question is to try to determine which kinds of electronic portable, software, or online dictionaries offer the best support for their needs. Grace (31) found that sentence-level translations should be included in dictionaries, as learners having these showed better short- and long-term retention of correct word mean- ings. This finding suggests a close relationship exists between processing new terms more deeply, verifying their meanings, and retaining them. Loucky (32) has researched many electronic dictionaries and software programs, and more recently organized links to over 2500 web dictionaries, which are now all accessible from the site http://www.call4all.us///home/_all.php?fi=d. His aim was to find which kind of EDs could offer the most language learning benefits, considering such educational factors as: (1) better learning rates, (2) faster speed of access, (3) greater help in pronunciation and increased comprehensibility, (4) providing learner satisfaction with ease of use, or user-friendliness, and (5) complete enough meanings to be adequate for understanding various read- ing contexts. As expected, among learners of a common major, more proficient students from four levels tested tended to use EDs of all types more often and at faster rates than less language-proficient students did. In brief, the author’s studies and observations and those of others he has cited [e.g., Lew (3)] have repeatedly shown the clear benefits of using EDs for more rapid accessing of new target vocabu- lary. They also point out the need for further study of archiving, and other lexical processing steps to investigate the combined effect of how much computers can enhance overall lexical and language development when used more intelligently and systematically at each crucial stage of first or second language learning. Regular use of portable or online electronic dictionaries in a systematic way that uses these most essential phases of vocabulary acquisition cer- tainly does seem to help stimulate vocabulary learning and retention, when combined with proper activation and recy- cling habits that maximize interactive use of the target language. A systematic taxonomy of vocabulary learning strategies (VLSs) incorporating a 10-phase set of specific recyclable strategies is given by Loucky (7,29) to help advance research and better maximize foreign language vocabulary development (available at http://www.call4all. us///home/_all.php?fi=../misc/forms). A summary of Laufer and Levitzky-Aviad’s (4) findings is useful for designers, sellers, and users of electronic dictionaries to keep in mind, as their study showed that: ‘‘the best dictionaries for L2 written production were the L1-L2-L2 dictionaries. . . Even though the scores received with the paper version of the L1-L2-L2 dictionary were just as good, the electronic dictionary was viewed more favor- ably than the paper alternative by more learners. Hence, in terms of usefulness together with user preference, the electronic version fared best’’ (p. 5). Such results should motivate CALL engineers and lexicographers to produce fully bilingualized electronic dictionaries (as well as print versions), specifically designed not merely to access recep- tive information to understand word meanings better, but also for L2 production, to practically enable students to actively use new terms appropriately as quickly as possible. SURVEYING USE OF ELECTRONIC DICTIONARIES To more thoroughly analyze and compare the types of dictionaries being used by Japanese college students in three college engineering classes, two kinds of surveys were designed by Loucky (29). The first was a general survey about purchase, use, and preferences regarding electronic dictionaries. The second survey (shown in the Appendix) asked questions about how various computer- ized functions were used at each major phase of lexical processing. The aim was to help learners maximize these eight essential phases of vocabulary learning: (1) assessing degree of word knowledge; (2) accessing new word mean- ings; (3) archiving new information for study; (4) analyzing word parts and origins; (5) anchoring new words in short-term memory; (6) associating words in related groups for long-term retention; (7) activating words through pro- ductive written or oral use; and (8) reviewing/recycling and re-testing them. After re-evaluating how well new words are learned by post-tests, any words not fully understood should be remet through planned re-encounters, retellings, and activities that encourage learners to repeat the voca- bulary learning cycle again so that relearning and reacti- vation can take place. COMPUTERIZED DICTIONARIES 3
- 33. Table 1. Comparative Chart of Some Translation Software* Al Misbar Translation 1 Language Pair http://www.almisbar.com/salam_trans.html Paid Subscription English - Arabic Amikai 13 Language Pairs http://www.amikai.com/products/enterprise/ (under Translation Demo) Free demo version (up to 100 characters) Full version can be customized with dictionaries. Babel Fish 18 Language Pairs http://babelfish.altavista.com/ Can translate a web page or up to 150 words of text. Ectaco LingvoBit 1 Language Pair http://www.poltran.com/ English - Polish Kielikone WebTranSmart 1 Language Pair https://websmart.kielikone.fi/eng/kirjaudu.asp Registration Required Per-word fee must be paid in advance for translations. English - Finnish ParsTranslator 1 Language Pair http://www.parstranslator.com/ English - Farsi PROMT-Online 7 Language Pairs http://translation2.paralink.com/ Reverso 5 Language Pairs http://www.reverso.net/text_translation.asp Can translate text or web pages. Special characters can be inserted onscreen. SDL Enterprise Translation Server 5 Language Pairs http://www.sdl.com/enterprise-translation-server Free demonstration (up to 200 words) Can translate text or web pages. Used by FreeTranslation.com SYSTRANBox 16 Language Pairs http://www.systranbox.com/ Can translate a web page or up to 150 words of text. Used by AOL, Lycos, Terra, Google, Voila, Wanadoo, Free.fr, and others. Check results with a human translator. SYSTRANet 18 Language Pairs http://www.systranet.com/systran/net More tools than SYSTRANsoft More language pairs Quality varies by language pair and subject matter. Check results with a human translator. Must sign up for a password, but delivery of password is in seconds. 4 COMPUTERIZED DICTIONARIES
- 34. The first survey described Japanese college students’ preferences and reasons for purchasing EDs. The second showed self-reported use of PEDS and how their respective functions were seen to aid in different phases of L2 voca- bulary learning. Students compared their use to that of print dictionaries. A majority of East Asian students sur- veyed expressed a preference for using mobile or online dictionaries rather than carry bulkier book dictionaries, although a few English students carry both. These ED preferences and patterns of use need more investigation, but probably hold true wherever the level of economic development is sufficient to support their purchase, as well as the use and availability of Internet access to online dictionary and Webreader glossing functions. Kobayashi (33) compared the use of pocket electronic versus printed dictionaries to examine the effects of their use on LPSs used. The three major strategies she distin- guished were consulting, inferring versus ignoring new terms. She found that ‘‘Pocket electronic dictionaries (PEDs) are rapidly becoming popular among L2 learners. Although many L2 learners depend on dictionaries, the prevalent view among L2 researchers and educators is that learners should use dictionaries sparsely. They encourage students to use another lexical processing strategy (LPS), contextual guessing, for better vocabulary learning and reading comprehension. [But] are dictionaries indeed so harmful?’’ (p. 2). As some educators and researchers have been con- cerned about the pedagogical value of EDs because of their perceived limitations, such as insufficient information provided, the possibility of discouraging contextual gues- sing, and a supposed negative impact on word retention (34-38), these educators’ and researchers’ concerns require more investigation. So far, however, language learners’ preference for them, and EDs’ rapidly improving functions appear to be scuttling most of these previous claims. Although native readers have far larger working vocabul- aries to guess from context, most second language readers prefer and benefit greatly from having both monolingual and bilingual/mother tongue glosses available to them. Kobayashi (39) found that 1. More than two-thirds of the students owned a PED, and most of those who owned a PED exclusively used it regard- less of purposes. 2. The PEDs owned by most students cost $100–$400, were of high quality, and did not have the disadvantages identified in other studies, such as brief definitions, the absence of examples, and inaccurate information. 3. Most students were satisfied with their PEDs, especially with their portability, and ease to look up a word, and ease to change from one dictionary to another. 4. The perceived disadvantages included the relative una- vailability (or inaccessibility) of detailed usage informa- tion, examples, and grammatical information. 5. PEDs enabled students to use different types of diction- aries in different places. 6. Although both PED users and PD users depended on dictionaries, PED users used dictionaries more often. This was especially the case with smaller vocabulary size students. 7. PD users and PED users did not significantly differ in terms of their LPS use, except for the sheer frequency of dictionary consultation. 8. There was a possibility that PED users consulted diction- aries at the expense of contextual guessing. 9. Although students depended on dictionaries, whether PEDs or PDs, they also used guessing strategies fre- quently. They often used a dictionary to confirm guessed meaning. This was particularly the case with successful students. 10. Larger and smaller vocabulary size students differed in their use of LPSs such as basic dictionary use, extended dictionary use for meaning, extended dictionary use for usage, extended dictionary use for grammatical informa- tion, lookup strategies, note-taking strategies, guessing strategies using immediate context, guessing strategies using wider context, combined use of LPSs, and selective use of LPSs. 11. Higher and lower reading ability students differed in their use of LPSs such as basic dictionary use, extended dic- tionaryuseformeaning,extendeddictionaryuseforusage, extended dictionary use for grammatical information, lookup strategies, self-initiation, note-taking strategies, SYSTRANSoft 15 Language Pairs http://www.systransoft.com/ Can translate a web page, a file (TXT, RTF, or HTML) or up to 150 words of text. Quality varies by language pair and subject matter. Check results with a human translator. Tarjim (Registration Required) 1 Language Pair http://tarjim.ajeeb.com/ English Arabic Wordchamp.com Over 100 Language Pairs http://wordchamp.com Free to all. Instant Glossing; Auto-Archiving; Online Flashcard and Test Creation; Files can be shared internationally between distance learners, as well as internally within intact classes, using its currently free Course Management System (CMS). * Free unless stated otherwise. Summarized from site by author. COMPUTERIZED DICTIONARIES 5
- 35. guessing strategies using immediate context, guessing strategies using wider context, and selective use of LPSs (p.2). SURVEYING AND MONITORING USE OF VOCABULARY LEARNING STRATEGIES Vocabulary researchers such as Schmitt (40), Kudo (41), Orita (42), and Loucky (29) have examined more than 50 other effective vocabulary learning strategies, coming up with some useful taxonomies that makers of dic- tionaries should be aware of and seek to maximize in their design of electronic features and functions in particular. Language learners do appear to benefit greatly from spe- cific strategy training in this essential area of language development (43). Loucky (29) has presented useful surveys of CBDs or EDs presented in CALICO Journal. He also included many recommendations for how to properly integrate computer- ized lexicons, both portable and online, into CALL as effectively and enjoyably as possible. He explained a useful taxonomy of VLS for all designers and users of computer- ized dictionaries to help students maximize their learning of target language vocabulary. CALL Journal in December, 2005, highlighted the www.CALL4All.us website, showing how learners and teachers may use its extensive encyclo- pedia of preorganized online dictionaries and language learning links to produce more effective and enjoyable reading and vocabulary learning lessons. These tools include the use of online glossing engines and reading labs, word-surfing games, vocabulary profilers most useful for text analysis and simplification, readability analyzers, and so on. State-of-the-Art Technical Features Probably the company offering the largest variety of func- tions and types of computerized dictionaries for the most languages is Ectaco, whose U.K. site enables one to search for both type of software/platform and particular language pair combination sought. It can be accessed at http:// www.ectaco.co.uk/how-find/. Their programs for handheld, portable devices may be found at http://www.ectaco.co.uk/ Software-for-Pocket-PC/. Electronic Dictionaries Electronic dictionary and electronic translator handhelds are modern, lightweight, and fashionable gadgets with a great variety of features. An electronic translator or dic- tionary is becoming a definite must-have in many areas of business. More expensive devices are based on advanced speech recognition and text-to-speech technologies. Advanced models may include these useful functions: 1) a business organizer, 2) bidirectional, 3) voice recognition or synthesis, 4) extensive vocabularies (up to 1,000,000 words), 5) grammar references, and 6) phrase banks con- taining colloquial expressions and common phrases, irre- gular verbs, and more. Ectaco offers more than 70 titles for over 20 languages at: http://www.ectaco.co.uk/Electronic- Dictionaries/. Translation Software For example, Ectaco has devices featuring a wide range of software products, over 220 titles, translation tools, and learning aids for over 35 languages designed for allstandard computer platforms, such as Windows, Pocket PC, and Palm OS. Manydeviceshavetools forvariouslanguagegoals(e.g., text translators, accent removers, bidirectional talking dic- tionaries,localizationtools,andlanguageofficetools),which include speaking and nonspeaking EOs, voice and travel language translators, handheld PDAs, and software bun- dles for Pocket PCs, Windows, Palm OS, and Cell phones. Although some online dictionaries charge fees, a major- ity are now available for free use. Most of these are now organized at the author’s www.CALL4ALL.us site, under Dictionaries Galore! http://www.call4all.us///home/_all. php?fi=d. Many examples of excellent translation software programs and portable, software, and online dictionaries can be seen and even ordered from these sites directly, or from those shown in Table 1. 1. http://www.ectaco.co.uk/how-find/ (Ectaco). 2. http://www.call4all.us///prod/_order.php?pp=2 (For language learning software, http://www. call4all. us///home/_all. php?fi=d links to most web diction- aries). 3. http://www.wor.com/shopping/ (World of Reading Language Learning Software). 4. http://speedanki.com/ (Speedanki.com offers Kanji Level Tests and flash cards to help one learn and review for national Japanese Proficiency Tests). 5. http://quinlanfaris.com/?cat=3 (Compares technical functions and differences between Seiko and Canon Wordtanks and the Seiko SR-E9000 PEDs). 6. http://flrc.mitre.org/Tools/reports/products_list.pl? LID=199# (Translation software and professional tools used for customized and specialized dictionary creations. Completeness of report is dependent on the completeness of the data entries and is expected to improve rapidly over time. Information is pro- vided by each developer or vendor). 7. http://flrc.mitre.org/Tools/internetMT.pl * (These translation programs are intended for giving a general gist of meaning, not as a substitute for human translation. However, this site is the best quick view of machine translation options online, covering 13 online translation engines). Computerized Dictionaries and Translation Software Programs Available The most detailed and extensive table of translation soft- ware and computerized dictionary products may be found at the Foreign Language Resource Center’s http://flrc. mitre.org/Tools/reports/products_list.pl?LID=202. Informa- tion provided by each developer or vendor at that site includes company, product names and version, and des- criptions of languages and functions included. As about 75 companies are listed, only the list of companies providing these kinds of products will be listed here to make online 6 COMPUTERIZED DICTIONARIES
- 36. searches possible. Computerized translation software companies include the following: ABLE Innovations, Alis Technologies; Acapela Group; Agfa Monotype Corporation; Al-Buraq; Arabeyes; Arabic OCR; arabsun.de; ARA- BVISTA; AramediA; Arava Institute for Environmental Studies; ATA Software Technology Limited; Alchemy Soft- ware Development; Abbyy Software House; Applications Technology; Ascender Corporation; Atril UK, Ltd.; Atten- sity Corporation; Basic Language Systems Corporation; Basis Technology; CACI, Inc.; Ciyasoft Corporation; CIMOS; Automatic Vocalization for Arabic; Automatic- Topic–Detection/ Abstract of Document; Compure, Compu- ter Language Technology; Ectaco; Galtech Soft, Ltd.; GlobalSight Corporation; International Systems Consul- tancy; IBM; Ice-LC Software; Idiom Technologies, Inc.; Jubilant Technologies, Inc.; Language Analysis Systems; Language Engineering Company; Language Weaver, Inc., LLC; Lingua; Linguist’s Software; Lockheed-Martin; Marine Acoustics, Inc.–VoxTec; Paragon Software GmbH piXlogic; Postchi.com; Melingo, Ltd.; MetaTexis Software and Services; Microsoft Corporation; MultiCorpora RD, Inc.; Nattiq Technologies; Nisus Software; NovoDynamics.- com (Detects new application programming interface, API); Paragon Software; Sakhr Software Company; SDL Inter- national; SIL International Publishing Services; Smart Link Corporation; Tavultesoft Pty, Ltd.; Telelingua; THUNDERSTONE SOFTWARE; TM SYSTEMS; TRA- DOS Corporation; Transclick, Inc.; Translation Experts; translation.net; United Nations Educational, Scientific and Cultural Organization (UNESCO); United Nations; Uni- versity of California, Davis; University of Maryland; U.S. Army Intel Center; Verity; WORDFAST; World Health Organization; WorldLanguage Resources; and Xerox–The Document Company. Among the various types of advanced applications provided by innumerable types of software from these companies are multilingual translation; dictionaries; lan- guage learning applications and toolkits; speech recogni- tion; information retrieval; multilingual word processing, spelling, and grammar; optical character recognition with easy insertion into Windows word processing; and web development and browsing. Discussion and Pedagogical Implications Common findings can now be summarized about elec- tronic lexicons from a broad reading of research in the field by Kobayashi (33), Laufer and Hill (44), and Hill and Laufer (45), combined with the author’s findings as follows: 1. PEDs facilitate L2 learning rather than hindering it. Regardless of whether they are using electronic or print dictionaries, successful students use effec- tive lexical processing strategies. Moreover, PEDs facilitate dictionary use. Therefore, the use of PEDs should not be discouraged. 2. Rather than discouraging the use of PEDs, teachers could advise students to use a PED and a PD for different purposes. 3. Dictionary use and contextual guessing are not mutually exclusive. Successful learners use both dictionaries and contextual guessing more often than less successful learners. Dictionary use should not be frowned on for the reason that it hinders contextual guessing. 4. Many LPSs involving dictionary use and guessing are helpful for both vocabulary learning and read- ing. These strategies should be taught to students. a. Teachers should give students instruction in how to use a dictionary effectively, particularly how to look for a variety of information and what diction- aries are available. b. Guessing is also important for vocabulary learning and reading. Teachers should give students instruction in how to guess at word meaning using wider and immediate contexts. c. The ability to use a dictionary selectively is also important.Teachersshouldinstructstudentswhen to use a dictionary and when to turn to other LPSs. 5. Some strategies are more important for vocabulary learning than reading comprehension, and some strategies are more important for reading com- prehension than for vocabulary learning. These strategies should be taught considering the desired skills and purposes of a reader or language learner (29,33). 6. Successful language learners tend to use a much wider variety of effective lexical and text processing strategies than do less proficient, unsuccessful lear- ners, regardless of whether they use electronic or print dictionaries. 7. Teachers often observe that the more frequently EDs are used in a consistent manner with regular archiving and activation of new word information, and the more systematically new vocabulary is used and reviewed, that retention results are better. Quality and amount of review techniques or media functions used by a learner largely determine both their degree of retention and speed and percentage of retrieval of new target terms and language forms. Reaction and retrie- val times can be improved by giving more recent and frequent encounters with target terms, helping to reactivate them by building further memory traces. Along with recy- cling and review techniques to improve recognition and prediction skills, reassessing of learning must be done regularly with frequent individual feedback to maximize motivation and acquisition. CALL should capitalize on these language learning insights to design maximally effi- cient vocabulary learning programs for use both online and with portable devices. When constructing or using online vocabulary learn- ing programs, these same crucial vocabulary learning steps and strategies need to be encouraged by specific questions intext and functions used by the programs. There should also be a tracking or feedback mechanism to help teachers monitor learning, and to guide and prompt lear- ners not to forget to do any of these essential phases of lexical processing. COMPUTERIZED DICTIONARIES 7
- 37. GENERAL TRENDS AND FUTURE FRUITFUL RESEARCH AREAS Major benefits of using portable devices include their mobi- lity and instant archiving or storage in history memos for future useful quick review. Web dictionaries like those organized at the author’s site, however, provide much more potential, as one can copy and paste between any of over 2000 online lexicons organized there for over 500 language pairs. www.CALL4ALL.us provides a ‘‘Virtual Rosetta Stone,’’ not only of the full range of monolingual and multilingual web dictionaries, but also a vast language education links library for studying most of these lan- guages as well. Another main advantage of modern translation technol- ogy is that it is much more efficient. One saves a lot of time, as there is no more turning of book pages and searching for words endlessly. Words you are looking for are at your fingertips, just one click away. Each online dictionary has 400,000 entries, for example, in the case of Ectaco pro- grams, and far more are freely available from web diction- aries organized at www.CALL4ALL.us’s dictionaries page at http://www.call4all.us///home/_all.php?fi=d. Recommen- dations for integrating the use of web dictionaries with language learning programs online are given in Loucky (32). The 10 types of sites are organized to help teachers and students more efficiently combine the benefits of electronic and online dictionaries with CALL websites to produce more effective and enjoyable content and language learning lessons. The general trends over the past 10 years have been for PEDs to become more prevalent because of their speedy access to language meanings, grammar data, collocations/ corpus examples, compact size, improved features, and convenience of use as well as economical pricing. Some feature as many as 32 lexicons or more, pronunciation support, Internet connectivity, review games, automatic history of searches for review, and so on. Translation soft- ware and CD-ROM dictionaries, being more expensive and limited to the location of one’s PC, have not become as popular. Web and phone dictionaries appear to be the ‘‘tool of choice’’ of most students, as these functions are often provided at their schools or included in their cell phone services at little or no extra charge. Assistive reading pens made by Quickionary also offer promise to those who can afford them. They also seem to enhance learners’ interest and motivation levels, and thus help to contribute to higher levels of vocabulary retention, although how to best do so online is a major question in need of further study. Some of the most promising online glossing programs being tested now can be recommended for further research in this area: 1) Wordchamp.com, 2) Rikai.com, 3) Wordsurfing.- com, and 4) Babelfish.com. CONCLUSIONS AND RECOMMENDATIONS To conclude, CALL and website e-learning developers need to remember that teachers need to be able to scale their language and vocabulary learning activities from those that require simpler and easier processing for lower level students, to activities that require deeper and more com- plex lexical processing for more advanced language lear- ners using various kinds of EDs, both online and offline, whether stationary or mobile. It is also important for teachers to give more clear guidance about particular kinds of EDs, especially including good online programs for learning, to help maximize the use of their functions for education. We can only help maximize each program’s effectiveness if students learn how to use their various functions as efficiently as possible to help them at each stage of processing new words outlined above. Further helpful guidelines and goals to examine when seeking to integrate new insights and innovations from CALL into the field of foreign language reading and vocabulary develop- ment are given by Sokmen (46). In her words, among the many areas in need of further systematic research in this field, ‘‘we need to take advantage of the possibilities inher- ent in computer-assisted learning, especially hypertext linking, and create software which is based on sound prin- ciples of vocabulary acquisition theory . . . programs which specialize on a useful corpus. . . provide. . .[for] expanded rehearsal, and engage the learner on deeper levels and in a variety of ways as they practice vocabulary. There is also the fairly unchartered world of the Internet as a source for meaningful activities for the classroom and for the inde- pendent learner’’ (p. 257). In this way, using proven portable devices, multimedia translation software, and well-designed, interactive web- sites as much as possible, language learning can be made much more interesting and effective as these CALL resources are all used as tools for developing more balanced communication skills, which emphasize blending active production and interactive, content-based learning with authentic tasks and materials made much more accessible, comprehensible, and memorable with the help of modern technology. All in all, we can be quite optimistic about the future of EDs, as de Schryver (25) is. Listing 118 ‘‘lexico- graphers’ dreams’’ in summarized tables, he masterfully ‘‘incorporates almost every speculation ever made about electronic dictionaries (EDs)’’ (p. 61) in Roby’s terms (47). Roby (47) further notes that not only technical hard- ware, but also human ‘‘fleshware’’ is the most crucial ele- ment when designing EDs, otherwise users may drown in a sea of data. One cannot drink efficiently from a fire hose. As he states, ‘‘Sophisticated software and huge hardware cannot guarantee the quality of an electronic dictionary. . . Good online dictionaries will be equipped with ‘spigots’ that allow users to draw manageable amounts of information. . . Information must be internalized for it to be considered knowledge.’’ In the vast reaches of virtual e-learning cyberspace, one does indeed require a common gold standard compass, or better yet, a virtual Rosetta Stone for language learning, such as those helpful sites provided here. As second language learners venture into ‘‘terra incog- nita’’ they do need clear maps and strategies to improve their navigation on various WebQuests for knowledge. Roby (47, p. 63) correctly asserts that ‘‘Dictionaries can be guides because they ‘potentially intersect with every text of the language: in a sense all texts lead to the dictionary’ (quoting Nathan). . . Learners can make forays into cyber- 8 COMPUTERIZED DICTIONARIES
- 38. space with an electronic dictionary as a navigational [tool]. And in a real sense, one can expect to see portable, wireless dictionaries that will both allow physical mobility and afford Internet access.’’ (In fact, most mobile phones and WiFi laptops already do). Tailoring computerized dictionaries to effectively sup- port learners’ needs will require specific attention to their types, functions, and uses to best guide learners and tea- chers to most effective integration of these portable and online tools into language and science education. Research is showing us that all future EDs would do well to include preorganized categories of terms, searchable by topic and semantic field. Five examples of these already found online include: 1) UCREL’s Semantic Analysis System located at http://www.comp.lancs.ac.uk/ucrel/usas/ with 21 major A–Z discourse fields; 2) Variation in English Words and Phrases (VIEW) at http://view.byu.edu/; 3) this writer’s bilingualized Semantic Field Keyword Approach covering about 2000 intermediate to advanced terms in nine aca- demic disciplines found at: http://www.call4all.us///misc/ sfka.php; 4) ThinkMap’s Visual Thesaurus at http:// www.visualthesaurus.com/index.jsp?vt ; and 5) Wordnet found at http://wordnet.princeton.edu/. This writer’s www.CALL4ALL.us site helps to integrate essential, com- mon core vocabulary in many of these academic disciplines with most web dictionaries for 500 major world language pairs. For an overview, see its site map at ( http://www. call4all.us///home/_all.php?fi=0) or see Loucky (32,48,49). In the final analysis, probably what learners are guided to do with new terms will prove to be a more important learning factor than multimedia glossing and text concor- dancer options alone can provide. New technologies do indeed offer more powerful resources than ever before for independent or classroom study of languages. Word learn- ing options will probably be best maximized when comput- ing power is used to enhance learners’ access to various types of EDs of high quality simultaneously in all fields, while likewise providing them with the means to auto- archive and organize new target vocabulary as they are shown how to actively use these new terms productively as soon as possible. APPENDIX Survey of Computerized Bilingual Dictionaries (27) Name your Book Dictionary or Electronic/Compu- terized Bilingual Dictionary: Model #: Cost: NAME: Accessing Archiving Time: ____________minutes (for 15 Laufer Hadar terms) ID/YEAR: Reading Level: a. Grade: b. Headwords: c. %VLS Used: d. DLP Level: e. AVQ/IP: 1. Assessing Vocabulary Size: Check your manual to see how many words it has for a. English: b. Japanese—(or other L1): c. Kanji Study— d. How many words do you think you know in English? 2. Accessing—Frequency of Use—How many times do you use it each day? a. For English to Japanese what % of the time? b. For Japanese to English, what % of the time? c. To check unknown Kanji, what % of the time? 3. Archiving—How do you record new words found? a. In my textbook in the margins b. On paper or in a Vocabulary Notebook c. I don’t record new words d. My CBD can record and save new words I’ve looked up. If so, tell how: e. Can it do Automatic Recording and Review (of last 1– 20 words) (called a History Search) f. Can you save and store new words manually? g. Can you Save and Print Text Files or Notes on new words? 4. Analyzing Special Functions or Features—Does your CBD have any Special Functions or Features which help you to break up new words into parts to better understand their grammar, origins or meaning? If so, please try to explain how to use them and tell how often you do so. (Use Manual) Does it give special information about word parts, gram- mar, or the origin of words? Does it give any common phrases? _____Yes ______No ____Not Sure Does it give any sentence examples? ____Yes ____No ____Not Sure 5. Anchoring New Words in Memory—Does your Elec- tronic Dictionary have any special Visual Images or Auditory Sounds or other special functions to help illus- trate new word meanings, forms or use to help you better remember them? ___Yes _____No If so, tell what these special functions are and try to explain how they work to help you fix new words in your memory. 6. Associating Functions—Does your Electronic Dictionary help you to organize your vocabulary learning in any way? For example, can you put words into Study Groups? Do you organize your vocabulary learning or notebook in any special order or way to help you remember new words? Do you group any words together to better remember or learn them? If so, please tell how you do so. COMPUTERIZED DICTIONARIES 9
- 39. If your computerized dictionary, translation website, or software helps you to do this in any way, please tell how: 7. Activating Functions—Does your Electronic Dic- tionary give you any ways to USE new words right away? ____Yes ____No If so, how? Can you think of some ways ON YOUR OWN that you could USE new words you have looked up more actively or creatively? If so, tell how: 8. Review: Do you review any new words after find- ing their meanings? ____No ____Sometimes ____Yes, usually If so, tell how does your Electronic Dictionary help you to review or retest new words? Does your ED/CBD have any Vocabulary Practice Games that you can use for review and practice? If so describe. If it had, what level would you start to study at? Does your CBD have any Special Functions or Features which help you study new words, such as challenge games, memos, word search history, and so on to help you learn, analyze, review or remember new words? ____Yes _____No _____Not Sure If so, please explain how to use them: FURTHER READING G. Cumming S. Cropp, and R. Sussex, On-line lexical resources for language learners: assessment of some approaches to word formation, System, 22 (3): 369–377, 1994. J. H. Hulstijn When do foreign-language readers look up the meaning of unfamiliar words? The influence of task and learner variables, Modern Lang. J. 77 (2): 139–147, 1993. BIBLIOGRAPHY 1. F. T. Dolezal and D. R. McCreary, Pedagogical Lexicography Today: A Critical Bibliography on Learners’ Dictionaries with Special Emphasis on Language Learners and Dictionary Users. Lexicographica, Series Maior 96. Tubingen: Max Niemeyer Verlag, 1999. 2. B. Laufer and M. 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Schmitt, Vocabulary: Description, Acquisition and Pedagogy, Cambridge:Cambridge University Press, 1997, pp. 200–203. 41. Y. Kudo, L2 vocabulary learning strategies. Available: http:// www.nrc.hawaii.edu/networks/NW14/NW14.pd. 42. M. Orita, Vocabulary learning strategies of Japanese EFL learners: their actual use and perception of usefulness, in M. L. Kyuoki (ed.), JACET Annual Review of English Learning and Teaching, 8: 27–41, 2003, Miyazaki, Japan: Miyazaki University. 43. I. Kojic-Sabo and P. Lightbown, Student approaches to voca- bulary learning and their relationship to success, Modern Lang. J., 83 (2): 176–192, 1999. 44. B. Laufer and M. Hill, What lexical information do L2 learners select in a call dictionary and how does it affect word retention?, Lang. Learn. Technol., 3 (2): 58–76, 2000. 45. M. Hill and B. Laufer, Type of task, time-on-task and electronic dictionaries in incidental vocabulary acquisition, Int. Rev. Applied Linguist., 41 (2): 87–106, 2003. 46. A. Sokmen, Current trends in teaching second language vocabulary, in N. Schmitt and M. McCarthy (eds.), Vocabulary: Description, Acquisition and Pedagogy, Cambridge: Cambridge University Press, 1997, pp. 237–257. 47. W. B. Roby, The internet, autonomy, and lexicography: a convergence?, Melanges CRAPEL, No. 28. Centrede Recherche et d’Applications Pédagogiques En Langues, Publications Scientifiques, 2006. 48. J. P. Loucky, Harvesting CALL websites for enjoyable and effective language learning, in The Proceedings of JALT CALL2005,Glocalization:Bringingpeopletogether, Ritsumeikan University, BKC Campus, Shiga, Japan, June 3–5, 2005, pp. 18–22. 49. J. P. Loucky, Developing integrated online English courses for enjoyable reading and effective vocabulary learning, in The Proceedings of JALT CALL 2005, Glocalization: Bringing Peo- ple Together, Ritsumeikan University, BKC Campus, Shiga, Japan, June 3–5, 2005, pp. 165–169. JOHN PAUL LOUCKY Seinan JoGakun University Fukuokaken, Japan COMPUTERIZED DICTIONARIES 11
- 42. E ELECTRONIC WARFARE INTRODUCTION Over the last century, there has been a burgeoning use of the electromagnetic (EM) spectrum for military purposes, including those related to communications, navigation, and targeting. This dependence is embedded in many modern warfare doctrines and technologies, such as: Revolution in military affairs; Network-centric warfare; Information warfare; Rapid decisive operations; Intelligence, surveillance, target acquisition, and reconnaissance; Precision guided weapons. Given the importance of the EM environment to military operations, there is obvious reason for safeguarding its use by friendly forces, denying its use by enemy forces, and defeating enemy efforts to achieve the same objectives. Electronic warfare (EW) encompasses the broad and some- what ill-defined mix of military tactics, techniques, proce- dures, technology, and organizational structures that address these concerns (1, 2). It is also related to some civilian technologies and applications, which include spec- trum monitoring and radio astronomy. Historical experience has repeatedly demonstrated the importance of EW as highlighted by an extensive body of declassified information that pertains to operations by both sides in World War II (WW2)(3–5), and by more recent accounts concerning the Korean, Vietnam, Six-Day and Yom Kippur Wars, and the campaigns in the Falklands, Lebanon, Kosovo, Chechnya, and Iraq (6–11). EW continues to be widely recognized as a powerful force multiplier, and the development and application of EW concepts and technologies consequently remains a high priority (12,13). For the greatest effect, its use is regulated by planning structures that tailor it to situational require- ments and procedures intended to deny the enemy as much knowledge as possible relating to its specific capabilities and deployment structures. For this reason, many aspects of EW are highly classified. Formally, the roles of EW are subdivided into: 1. Electronic support (ES) - taking advantage of signals emitted by an opponent’s systems; 2. Electronic attack (EA) - degrading the ability of an opponent to use his systems; 3. Electronic protection (EP) - safeguarding the effective operation of friendly force electronic systems against enemy EA and ES activities. The following article presents a breakdown of EW in this order, with attention given to both technical system con- cepts and relevant operational doctrine. ELECTRONIC SUPPORT ES, which is also known as electronic support measures, concerns the sensing of communication, radar, and other electromagnetic signals of potential interest. ES sensors perform the following technical functions: 1. Signal detection - determining the presence of a signal; 2. Signal classification - associating the signal with a type of modulation or function; 3. Signal parameter and feature extraction - measuring various signal parameters; such as carrier frequency, power, transmission start and end times, and band- width; 4. Emitter identification - determining the type of sys- tem that the signal is associated with; 5. Signal intercept - recovering the message content from communication signals; 6. EW analysis - inferring the organization and struc- ture of enemy networks, dispositions of forces and operational intent from communications traffic pat- terns and message content; 7. Geo-location - determining the positions of signal emitters. Several points concerning ES deserve emphasis. First, its passive nature has the great advantage that valuable intelligence can be produced without an adversary being aware. Second, the mere suspicion of its use can cause an adversary to restrict its use of communication systems and active sensors, which thereby reduces their operational value. Finally, radar ES systems often can detect a radar transmitter at ranges considerably in excess of the useful range of the radar (14). The organization and processing of information pro- vided by ES sensors is a complex problem. Much value of ES sensor outputs can be lost if information does not reach the appropriate commanders and other potential users in a timely way. Complicating factors include the volume of information, the difficulty of interpreting it, and the need to protect sensitive information concerning ES capabilities. The last point is a very real concern. During WW2, the decryption of German communication signals coded with the Enigma cipher provided immensely valuable intelli- gence to the British. Accordingly, every effort was made to avoid arousing suspicions that the Enigma cipher was anything other than unbreakable. For example, reconnais- sance aircraft would be dispatched to ‘‘find’’ an important convoy whose orders had in fact been revealed by the 1 Wiley Encyclopedia of Computer Science and Engineering, edited by Benjamin Wah. Copyright # 2008 John Wiley Sons, Inc.
- 43. decryption of Enigma messages, which thereby gave the impression that the attack that followed was the direct result of routine aerial reconnaissance (5). The diversity of the roles performed by ES systems has resulted in a significant degree of specialization in the design ofthe systemsthemselvesandtheirorganization andcontrol. Tactical ES Tactical ES is the deployment of an ES capability in direct support of field operations. It typically resides within some form of dedicated EW unit that may be either part of the maneuver force’s echelon or assigned to support it under an operational (OPCON) or tactical (TACON) command and control relationship. Examples of tactical ES are found in land, air, and sea operational environments, where objec- tives include: 1. The intercept, direction finding, and analysis of bat- tlefield communications signals by ground-based assets to determine the composition and geographical distribution of enemy forces and the immediate inten- tions of its elements, from fighter to commander. When ES is performed by an EW unit native to the maneuver force, ‘‘intentions and warnings’’ tip-offs are reported directly to field unit commanders and their staff. The unit may also acquire and disseminate intelligence for consumption strictly within Signals Intelligence (SIGINT) channels (see below) and gen- erate technical information for internal process refinement; 2. The detection and direction finding of battlefield surveillance radars by ground-based radar ES; 3. The detection and analysis by a radar warning recei- ver (RWR) of radar signals associated with enemy target acquisition, tracking, and fire control systems, to provide aircraft pilots with situational awareness and warnings of threats. This information is essential for the timely initiation of suitable countermeasures, which may include a combination of EA and evasive maneuvers; 4. A general surveillance capability by a warship’s radar ES systems to track military, merchant, or clandes- tine ships and fishing vessels using the signals received from their navigation radars. These systems also support self-protection functions against radars associated with threat weapon systems. On larger platforms, there are usually more provisions for ana- lyzing ES information, fusing it with other intelli- gence, and distributing it to other platforms, channels and organizations (including SIGINT). The capability to geo-locate transmitters associated with communication, navigation, and radar systems is particularly important; even approximate indications of the direction of an enemy position or platform provided by direction finding (DF) are valuable from a situational- awareness perspective. Estimates of the positions of individual emitters can be determined by obtaining lines- of-bearing from spatially separated sites and solving for the positions where they intersect. Geo-location is particularly important for communication signals when the message content cannot be extracted because of encryption or other techniques. Appendix 1 provides an overview of various DF techniques that can be used for the geo-location of signal sources by ES systems. An additional EW analysis (EWA) capability is often associated with units that deploy ES assets. EWA is a military intelligence function that specializes in drawing operational inferences from EW data. Its main purpose is to determine the enemy’s ‘‘electronic order of battle,’’ which is a comprehensive representation of its electronics systems, including their identification, geographical disposition, and where possible the association of this equipment with specific units within a command-control structure. An EWA cell may also be responsible for maintaining communication target lists and selecting information for dissemination to Intelligence organizations. TacticalcommunicationsESisaparticularlychallenging problem in urban environments. Multipath propagation effects can be expected to degrade the accuracy of radio- frequency direction-finding systems. Furthermore, opposi- tion forces can be expected to make use of the civilian communications infrastructure, which results in a require- ment to sift rapidly through a large amount of communica- tions traffic to find the signals of interest. Signals Intelligence SIGINT is the strategic application of ES performed under the controlof national intelligence organizations,suchasthe National Security Agency in the U.S., and the Government Communication Headquarters in the U.K. The term relates variously to the type of information produced, the systems used toproduceit,and tothe community thatcontrolsthe ES systems and the analysis and distribution of their products. SIGINT ‘‘products’’ are disseminated via highly classified channels and, except in exceptional circumstances, are released only for use in the wider national or Military Intelligence communities after being ‘‘sanitized’’ of any dis- tinguishing elements that could reveal the source. On the battlefield, there may be some overlap between SIGINT and tactical ES activities and platforms, with EW units some- times tasked to serve both functions simultaneously. SIGINT comprises communications intelligence (COMINT) and electronic intelligence (ELINT). COMINT is concerned with the message content of communication signals, information about communication traffic patterns, and the locations of the associated transmitters, with a strong emphasis on determining higher-level or ‘‘strategic’’ command and control structures. ELINT is the collection of technical or ‘‘parametric’’ information about the radar and other noncommunications equipment (15). ELINT has several important uses. First, theoretical analysis of the signal parameters allows inferences to be drawn about the functions, capabilities, and limitations of the systems associated with the signals, and hence, more broadly, about enemy early warning or targeting capabilities. Second, ELINT data are used to construct emitter libraries or databases that are fundamental to EA and EP operations. For each known type of radar, information is collected on the signal parameters for the 2 ELECTRONIC WARFARE
- 44. various operating modes, the estimated radar perfor- mance, its intended function(s), and the platforms the radar is known to be installed on. An ES system on a ship or tactical aircraft correlates the parameters of observed signals with the database entries to identify the radar systems that transmitted them, and, if an observed signal is associated with a threat, it provides the information needed to select and execute the most appropriate countermeasures. SIGINT operations often involve the use of specialized equipment deployed on either dedicated or multiuse plat- forms, which include satellites, ships, and aircraft. During the Cold War, suitable types of aircraft were extensively modified to perform SIGINT. By operating at altitudes of 10 km or higher, useful ranges could be extended to hundreds of km for the intercept of micro- wave radar signals. Consequently, intelligence could be acquired from aircraft flying at the periphery of the Soviet defense perimeter. For a period, specialized high-altitude aircraft could even conduct operations over Soviet terri- tory by flying above the effective ceiling of interceptor aircraft and ground based antiaircraft weapons. After improved Soviet antiaircraft defenses made overflights impractical, the West hurriedly deployed satellite-based systems (16). In recent years, much interest has been aroused by the idea of integrating ES information derived at different levels (tactical, operational, and strategic) by EW and SIGINT units with similar objectives, but possibly differ- ent reporting mechanisms. For instance, modern strate- gies for Netcentric Warfare involve the accumulation of various kinds of data and intelligence at a central point where it can be fused to produce more complete assess- ments. However, many practical challenges exist in recon- ciling technical possibilities with doctrine. Complicating factors and risks involved with centralized analysis schemes include: 1. The quantity of data generated by advanced ES sys- tems may tax the analysis systems that must sort through it; 2. Delays in the reporting chain, where key information may take longer to reach its ultimate destination after passing through a central accumulation point; 3. The expense and complexity of deploying communi- cation systems with adequate bandwidth; 4. Standardization issues for technical interfaces, and the complexity of both designing and maintaining interfaces for systems that were originally designed for different purposes and may be based on widely differing technologies; 5. Complications that affect the handling and distribu- tion of information resulting from classification issues and, in the case of multinational environ- ments, the willingness of individual nations to declare and release their information to others; 6. The risks of commanders relying too heavily on the formation of a ‘‘complete intelligence picture’’ in lieu of trusting their judgment and intuition, which can lead to decision paralysis. ES System Technologies and Implementation ES systems are typically comprised of antenna, receiver, and processing sub-systems. Early ES systems were often improvisations based on civilian equipment. For example, receivers developed for radio amateurs had relatively good sensitivity and frequency coverage and were widely used by the Allies during WW2. The National HRO, which had excellent frequency resolution, was used to intercept com- munication signals in the medium and high frequency bands. The Hallicrafters S-27, which provided contiguous coverage in the lower portion of the very high frequency (VHF) band, was widely used to receive signals associated with German VHF radar, air-to-air communication, and bombing navigation systems. These receivers, although useful, had significant limitations. Their frequency cover- age was limited, and their effectiveness was heavily depen- dent on the training and skill of the operators. The continued evolution of the technologies used by communication and radar systems has contributed to the development of specialized ES receivers. A fundamental issue concerns the differences in the waveforms used by communication and radar signals. Most communication systems transmit a continuous or near-continuous narrow bandwidth signal during the transmission of a message. A primary goal is to make efficient use of bandwidth to transmit information, which thereby allows the available radio frequency bands to be divided between many users. Communication signals have continued to evolve: 1. The bandwidth and channel spacing associated with conventional narrowband signals has decreased because of developments in more efficient modulation formats and accurate frequency references and synthesizers; 2. Digital modulation techniques are being increasingly used to transmit information in the form of binary data; 3. Time division multiplexing access techniques are being used by some systems, such as those based on the GSM cell phone standard, to provide a way of time sharing bandwidth between multiple users; 4. Classes of spread-spectrum techniques are being used in some military and civilian communication systems. Frequency hopping (FH) systems superpose periodic changesonthecenterfrequencyofatransmittedsignal following a predetermined sequence. These changes typically occur at rates that are tens or hundreds of times per second. The portion of a transmission that corresponds to a dwell at a single frequency is often referred to as a hop. To minimize interference between FH communication systems, careful coordination is needed in the assignment of hop frequencies and/ or the codes that define the hop sequences. Direct sequence spread spectrum (DSSS) uses a different approach. In the basic form, a pseudo-random number (PRN) sequence is used by the transmitter to spread the narrowband information content over a much larger bandwidth. The receiver uses the same PRN sequence to recover the information. Multiple systems ELECTRONIC WARFARE 3
- 45. can share the same bandwidth without seriously interfering with each other if they are assigned differ- ent PRN sequences. Code division multiple access (CDMA) cell phone systems are a major application of DSSS techniques. Because the detection of spread- spectrum signals often requires special techniques (17), these signals are sometimes referred to as low probability of intercept signals. 5. Mobile communication systems and networks have proliferated and are widely used. These systems are based on the idea of dividing a geographical area into cells. Each cell has a base station that performs the functions of relaying messages between the short- range handset radios within the cell and a commu- nication network interface to other carriers, such as the public telephone system network. Cellular tele- phone systems usually operate in the ultra high frequency band. The classic pulsed-radar concept, however, involves the transmission of short duration pulses with relatively large time intervals between successive pulses. This method sidesteps the difficult problem of detecting the relatively weak signals reflected from the target during the simulta- neous transmission of a high power signal. Requirements for range resolution often dictate the use of pulse widths on the order of a microsecond or less, which thereby results in relatively large bandwidths on the order of MHz. The waveforms used by advanced radars have increased in sophistication: 1. Coherent radars transmit signals whose waveforms are precisely defined; 2. Frequency or phase modulation may be used to increase range resolution; 3. The time intervals between successive pulses (pulse repetition interval) may be varied in a periodic or random sequence (pulse repetition interval stagger); 4. Multifunction radars select between different wave- formsdependingonthefunctionalitythatisrequired1 ; Application requirements for high angular resolution and compact antenna dimensions have motivated the extensive use of frequencies above 8 GHz. The differences between radar and communication sig- nals have motivated the development of specialized ES equipment: 1. Communication ES receivers feature extended fre- quency coverage to reduce the need to use different receivers, selective filters for separating signals that are closely spaced in frequency, comprehensive cap- abilities for demodulating the signal message con- tent, and provisions for the measurement of signal parameters; 2. Radar ES receivers emphasize microwave frequency coverage and are optimized for the reception of pulse signals; 3. Specialized radar ES receivers have been developed for strategic and tactical applications. For example, electronic intelligence receivers are designed for the precision measurement of signal parameters, whereas radar warning receivers are designed to provide warnings of threat signals, be simple to use, and satisfy size and cost constraints; 4. Multichannel receivers have been developed to pro- cess multiple signals from antenna arrays with the accurate phase and amplitude matching needed for applications such as direction finding. General trends in all systems include the use of precision frequency references and synthesizers to permit accurate and repeatable tuning, progressive reductions in size, and the use of form factors that permit the convenient installation of multiple receivers in standardized rack con- figurations. Communication ES Signal Processing. The classic com- munication ES receiver implementation is basically a high-quality manually controlled superheterodyne recei- ver. Signal search was performed by the operator manu- ally tuning the receiver through the frequency range known to be used by the adversary’s radios and listening to the outputs of the available demodulator(s) for signals of interest. When such a signal was found, the operator would listen to the demodulated signal and record the observations. If available, a DF system would be tuned to the frequency and measurements obtained for the signal angle of arrival. This process required the attention of a skilled operator and had the additional weakness that short duration transmissions on new frequencies could be missed, particularly if the frequency ranges to be covered could not be divided up among multiple systems and operators. Another weakness concerned the size, weight, and power consumption of the equipment. Modern purpose-designed communication EW receivers provide significant enhancements: 1. Computer controlled operation via standard digital interfaces; 2. Accurate high-speed tuning and reduced phase noise that results from the use of high-quality crystal oscillators as frequency references and sophisticated frequency synthesis techniques; 3. Provisions for phase coherent operation of multiple receivers to allow commonality of hardware between systems used for signal search and DF; 4. Built-in-test functionality; 5. Reduced size, weight, and power consumption. Digital signal processing techniques are being adopted for advanced ES systems. Digital filter bank concepts based on the Fast Fourier Transform algorithm allow a single wideband receiver to process and detect the individual 1 For example, the optimal waveforms for discriminating between a moving target on the ground and the surrounding terrain would be unsuitable for providing extreme range resolution. 4 ELECTRONIC WARFARE
- 46. signals present within a large instantaneous bandwidth. Also, if the system dwells on a fixed center frequency, digital downconverters can be used to extract the narrow- band signals within the receiver bandwidth and software demodulators used to recover the message content from each signal. Advanced wideband communication ES sensors based on digital filter bank techniques have some very desirable advantages: 1. A large frequency range can be scanned quickly; the tuning frequency step size can be orders of magnitude larger than the required frequency resolution. This method substantially reduces or eliminates the like- lihood that a short duration transmission will be missed and can provide some capability for detecting at least some hops transmitted by a frequency hop- ping radio; 2. The use of Constant False Alarm Rate techniques allows the system detection processing parameters to be adjusted automatically to achieve the best possible sensitivity without incurring erroneous signal detec- tions at a rate that exceeds a set value, even if the environmental noise is frequency dependent and time variant (18); 3. Algorithms can beimplemented to determine the type of modulation used by a signal and the modulation parameters; 4. Raw signal data can beacquired and stored for off-line analysis; 5. Demodulators implemented in software can accom- modate a wide range of modulation types; 6. DF functionality can be integrated into the system to provide a measurement of the angle of arrival for each signal that is detected; 7. Reports of signal detections and the measured signal parameters can be automatically stored in a database and transferred to EW analysis and intelligence sys- tems for subsequent processing; 8. Remote controlled or autonomous operation of ES systems is feasible. However, wideband signal processing techniques also incur disadvantages. Early implementations tended to be expensive and have significant performance limitations. A major problem concerns dynamic range, which is a mea- sure of the ability of a system to process strong and weak signals simultaneously. This issue is of considerable importance for wideband communications ES systems because weak signals of interest and strong signals will often coexist in the same frequency range. The dynamic range of a practical system is dependent on the noise and spurious signals, which are generated in the system by various mechanisms. One of the most important of these mechanisms is third order intermodulation distortion. This occurs when two or more signals present within the system bandwidth interact because of nonli- nearities in the system signal processing. The spurious signals that result remain within the system bandwidth and, depending on the size of the input signals and the nature of the system nonlinearities, can be large enough to be detected and interpreted as actual signals in subsequent processing. To avoid this undesirable result, the detection processing must be adjusted to reduce the effective system sensitivity. Thus, the presence of strong input signals tends to degrade the ability of the system to detect and process weak signals usefully. The problem is aggravated as the system bandwidth is increased because the number of strong signals within the system bandwidth can also be expected to increase. Fortunately, progressive advances in radio frequency components, analog-to-digital converters, and digital processor hardware have substantially resolved these issues, particularly when careful system design choices and tradeoffs are made. Nevertheless, a well-designed narrowband receiver may still offer advantages with respect to usable sensitivity and selectivity in a dense signal environment that includes strong signals. In addition to its message content, a communication signal contains information that can be used to classify the type of signal, and, with some limitations, to identify individual emitters. The measurement of the modulation type and para- meters is an important topic for communications ES systems. Conventional communication systems use modu- lation techniques to embed information on a sinusoidal carrier signal. The choice of modulation type and imple- mentation parameters is dependent on application require- ments and various factors, such as the need for interoperability with other radio systems as well as techno- logy and cost constraints. Advances in communication the- ory coupled with the availability of low-cost digital signal processing hardware have motivated the use of sophisti- cated digital modulation techniques to provide favorable trade-offs between bandwidth efficiency, sensitivity to pro- pagationeffects,andhardwareimplementationcosts.Atthe same time, simple, classic modulation techniques, such as analog frequency modulation, remain in widespread use, in part to maintain interoperability with older systems. Knowledge of the modulation type and parameters asso- ciated with a signal is of considerable practical value. Requirements for interoperability have led to the standar- dization of the modulation types used by military radios. For example, the tactical VHF radios used in ground opera- tions typically support analog FM and digital FSK modula- tions in accordance with standards such as MIL-STD-188- 242. If a signal has a modulation type and parameters associated with a communication system known to be used by an adversary, then it can be flagged as a potential signal of interest and prioritized to receive attention. Also, because emitters that are communicating with each other will generally use the same modulation type, this knowl- edge can be used to support or reject hypotheses that concern the membership of a given emitter in a network. Finally, knowledge of the modulation type and parameters facilitates the selection of an appropriate demodulation technique to recover the message content. Becauseof the diversityof modulation standards and the effects of multipath propagation and nonideal radio system implementations, the modulation recognition problem is ELECTRONIC WARFARE 5
- 47. nontrivial. Algorithms for modulation recognition have been described in various papers, of which Refs. 19–22 are representative examples. A related idea is based on the observation that the signal waveforms generated by practical radio transmitters will differ in subtle ways depending on implementation details and component tolerances, and that these differences can be sufficient to distinguish between transmitters that are very similar or even nominally identical. Various techni- ques have also been proposed to extract and measure appropriately selected features from a signal and use sta- tistical tests to determine whether the feature measure- ments match those of previously observed signals (23, 24). Radar ES Signal Processing. Various analog and digital approaches have been used in radar ES receivers to detect signals and measure their parameters. Descriptions and performance analyses of the more common ones have been published 25–27. The radar ES receivers used for current radar ES systems deployed for the self-protection of plat- forms such as aircraft and surface ships generate pulse descriptor words (PDWs) for each radar pulse that is received. Each PDW consists of digital data that represents the principal signal parameters, typically frequency, power, time of arrival, pulse duration, and if available, angle of arrival and modulation type (phase or frequency). Early implementations made extensive use of analog tech- niques to generate PDWs, but more recent implementa- tions are making increasingly extensive use of digital techniques. Pulse train deinterleaving is required because the pulses that are received from the various radars in the signal environment will be interleaved in time (i.e., in a sequence of received radar pulses there is no certainty that for a given pulse in the sequence, the previous or next pulses in the sequence will be from the same radar). Dein- terleaving is typically performed in a two-stage process. First, clustering is performed as pulses are received to form clusters or groups of pulses having similar characteristics. A subset of the signal parameters contained in the PDWs, typically frequency, angle of arrival, and pulse duration, are used in this stage. The second stage involves analyzing the time relationships [Pulse Repetition Interval (PRI) deinterleaving] between the pulses collected in each cluster to identify patterns that are consistent with the hypothesis that they were transmitted by a single radar. In addition to the radar PRI behavior, the radar scan pattern can be inferred by examining the time history of the measured power of received pulses in a deinterleaved pulse train. For example, a radar that is performing a circular scan will illuminate the platform carrying the ES system with its main beam response at uniform intervals in time. Emitter identification involves comparing the various parameters that have been measured for each of the resultant deinterleaved pulse trains with those in an EW library and identifying the best match. In practice, many potential difficulties may occur. The PDWs generated by the receiver will contain errors that result from various sources. At least some clusters formed in the first stage will have broad ranges. For example, a large frequency range may be needed to accommodate a frequency agile radar. Consequently, some clusters may overlap. Accurate PRI deinterleaving can be very difficult to perform with limited signal data sets; many modern radars have complex PRI staggers (i.e., the time intervals between successive pulses transmitted by a radar vary randomly or follow patterns that repeat only over a long period). Deinterleaving errors can result in the pulse train transmitted by such a radar being fragmented into two or more partial pulse trains. Finally, EW databases can have errors, be incomplete, or as a result of ambiguities, may be unable to provide a unique identification. More sophisticated approaches are being investigated for the extraction of features that can be used to provide additional information for the classification and identifica- tion of radar signals. For radars that use frequency or phase modulation to improve range resolution, knowledge of the type of modulation waveform and its parameters is useful for classification purposes. Also, the waveforms trans- mitted by radar systems often have distinctive features, which are sometimes referred to as unintentional modula- tion on pulse (UMOP). Various techniques have been pro- posed for the extraction and processing of waveform features for signal identification. ELECTRONIC ATTACK EA, which is also known as Electronic Countermeasures, involves actions intended to degrade the ability of an adversary to make use of the electromagnetic spectrum. It may be active or passive in nature. EA Against Communication Signals EA against communication signals can be carried out as deception operations or jamming. Deception operations involve the transmission of sig- nals to mislead the enemy intentionally. For example, after a ground formation has been redeployed for opera- tions elsewhere, simulated radio traffic may be main- tained to give the impression that the formation is still in its original location. Another technique involves the transmission of messages that contain misleading infor- mation in the expectation that the message content will be recovered and used by the adversary. Deception opera- tions must be carefully designed and organized to be convincing; the information provided should be consistent with other information that the intended recipient believes to be true. Large-scale deception operations that involve carefully coordinated activities can influence an adversary’s strategic planning with decisive effect. Several accounts of highly successful Allied deception operations in WW2 have been published (5, 28). Jamming is intended to prevent an adversary from reliably receiving his communication signals by the trans- mission of signals that interfere with their reception. In the simplest form, a jammer consists of an antenna, power amplifier, and signal generator programmed to produce a signal with an appropriately chosen waveform. It is also possible to use a conventional transmitter or radio as an improvised jammer. Jamming systems are often deployed with an adjunct ES capability to ascertain the frequencies 6 ELECTRONIC WARFARE
- 48. of signals worth jamming and to assess the effects of the jamming operation. To be effective, jamming requires that the ratio of jam- mer and communication signal powers (J/S ratio) at the victim radio receiver be sufficient to degrade communica- tion activity adequately. High-power transmitters may be used in combination with directional antennas and the judicious positioning of the jammer near the area where jamming coverage is desired. Several distinct types of communication jamming tech- niques are as follows: Narrowband Jamming. Individual communication sig- nals can be attacked by transmitting an appropriately designed narrowband-jamming signal on the frequency used by the target signal. To determine whether the target signal is still being transmitted, the jamming may be periodically stopped and an ES capability used to check for the presence of the signal. This method of attack has several advantages. First, the jamming range is maximized because the full jamming power is focused on a single signal. Second, the likelihood of interference with own side communication is minimized because only a small part of the radio spectrum is affected. If the jamming signal can be switched rapidly between frequencies, then a single transmitter may be able to jam two or more narrowband signals on a time shared basis. A follower jammer is a special case of narrowband jammer used to jam a FH signal. The practical implemen- tation of the concept is challenging; each hop transmission must be detected, its frequency measured by the ES functionality integrated with the jammer and, before more than a fraction of the hop is transmitted, the jamming transmitter must be tuned to the hop frequency (29). One difficulty is that the jammer must discriminate reliably between the hops from the target transmitter and any other frequency hopping communication systems that may be operating in the environment. A more fundamental issue concerns the propagation delays associated with, first, the path from the transmitter to the jammer, and, second, the path from the jammer to the victim receiver. If the end result is that the overall delay, including the jammer response time, approaches the hop duration, then the effectiveness of the jamming will be degraded.2 Barrage Jamming. A wideband jamming signal is used to degrade communication activities over a relatively wide range of frequencies. A high-power jammer may be needed to provide a useful range. A partial-band jammer is a variation on the barrage jammer concept. The aim is to jam a bandwidth that is sufficiently large enough to include a sufficient proportion of the hops transmitted by a FH radio to make it unusable. The idea is that, by not attempt- ing to jam the full bandwidth used by the frequency hopping radio, the jammer power within the hop bandwidth can be kept higher and can provide an increase in the effective range of the jammer. Many issues must be considered with respect to com- munication jamming: 1. Jamming often interferes with own side communica- tion; 2. The value of information that is obtained by ES may be considered to be of greater military value than the effect of disrupting communication; 3. An adversary can infer the presence of enemy forces with EW capabilities from the observation of jam- ming signals and, if given time, may find ways of countering its effects. Consequently, aside from some specialized applications, the decision to carry out communication jamming is usually made at a relatively high level and is closely coordinated with operational plans. The deployment of communications jammers on aircraft provides several advantages. The jammer is mobile and can be positioned quickly to affect the desired area while mini- mizing the effect on friendly forces. Also, the required transmitter power can be reduced because, for a given range, the propagation losses are normally much lower than they would be for the signals from a ground based jammer. Recently, serious interest has been expressed in the idea of using low-power communications jammers on small unmanned air vehicles (UAVs) to provide localized jamming coverage in the direct support of small-unit opera- tions (30). EA Against Radar Signals EA against radar signals is often concerned with degrading the performance of surveillance, target acquisition, and target tracking radars to protect platforms such as aircraft and surface ships. The value of these platforms and the potential effectiveness of radar-guided weapons has led to much emphasis being placed on EA. Active EA techniques are used to create false targets or otherwise degrade the operation of the victim radar: 1. A noise jammer transmits wideband noise in the frequency ranges used by radar systems of potential concern, which makes it difficult for the radar to detect the target and get a range measurement; 2. A range gate pull-off jammer attempts to create a false target that seems to move away fromthe jammer platform. The jammer first creates a false target at the jammer platform by transmitting a pulse timed to coincide with the arrival of each pulse transmitted by the victim radar. The timing of successive pulses is gradually shifted so that the jammer pulses received by the victim radar correspond to a target that is moving away from the jammer platform. The digital radio frequency memory (DRFM) improves the technique by storing and transmitting a replica of the radar-pulse waveform. This method makes it more difficult for the radar to discriminate against the jammer signal. 2 This problem can be avoided if the hop frequency sequence can be predicted using observations of the hop frequencies and a priori knowledge of the algorithm used to generate the hop sequence. ELECTRONIC WARFARE 7
- 49. Several practical problems are noted in the deployment of jammers. The operation of jammers used for the self- protection of platforms, such as aircraft, is usually restricted to the jamming of threat signals as required. This method minimizes several risks, which include the possibility of interference with other systems on the plat- form, and that the presence of the platform can be inferred by the detection and direction finding of signals trans- mitted by the jammer. In this situation, an integrated ES capability for performing the detection, characteriza- tion, and assessment of threat signals is required to provide information needed for the control of the jammer. One way of sidestepping this issue is to deploy jammers on specia- lized platforms, and if possible to perform the jamming outside the defended air space. Other solutions include the towing of jammers behind the platform to be protected, or deploying jammers on UAVs. Passive EA techniques attempt to degrade the effec- tiveness of enemy radars without transmitting signals. A widely used idea is to create false targets by dropping chaff (typically metal coated plastic strips) from aircraft to confuse tracking radars associated with antiaircraft defense systems. Chaff can also be dispersed via rockets or shells fired from platforms such as ships as a counter- measure to radar-guided missiles. Another approach is to tow decoys behind an aircraft or ship. The use of passive EA to confuse the guidance systems of antiaircraft or antiship missiles is often combined with maneuvers designed to position the platform to minimize the like- lihood that the missile-guidance system will reacquire its target or that the missile will fortuitously pass near its target. Another form of passive EA concerns the use of stealth techniques to reduce the reflected energy returned to a radar transmitter by a platform (i.e., reduce the apparent radar cross section of the platform). The effec- tiveness of this technique is increased if combined with active EA from other platforms. Other forms of EA are also important. Radar systems can be destroyed by missiles designed to home in on the signals transmitted by the radar. Conventional military operations against deployed systems identified by EW sen- sors or other intelligence are also possible. Recently, the concept of using directed energy or electromagnetic pulse (EMP) to damage or disrupt the operation of electronic equipment has received attention. ELECTRONIC PROTECTION Electronic protection, also known as electronic-counter- counter measures, concerns techniques and technologies intended to preserve the ability of defense electronic sys- tems to operate in hostile electromagnetic environments. Active EP includes measures taken to enhance the ability of defense electronic equipment to operate without hindrance by enemy EW. Protection against intercept and jamming of communi- cation signals can be provided in various ways: 1. Equipment can be designed to operate over wide frequency ranges, which offers improved opportu- nities for a system to switch to quieter frequencies if interference or jamming is encountered; 2. Directional antennas can be employed to make the interception of a signal difficult for a receiver outside the main beam response of the transmitting antenna. Jamming resistance can be achieved if the direction that the jamming signal is coming from corres- ponds to a null in the receiving antenna directional response. 3. Careful choices of sites may be able to take advantage of terrain masking of areas potentially usable by jammers or ES systems; 4. Power management allows the transmitter power to be set at the minimum level required for reliable communication. Low-power operation is desirable for short-range communication because the range at which the signal can be detected and intercepted is reduced. High power levels can be used to provide reliable operation over longer ranges and/or to over- come jamming; 5. Low probability of intercept techniques can be used to render DF and intercept difficult. FH techniques are widely used by modern tactical radios; 6. Redundancy can be achieved by design and/or tac- tical procedures to limit the damage caused by the effects of enemy EA; for example, different types of communication systems can be networked and man- aged to ensure that the disruption of one system does not prevent the communication of important infor- mation. Similar techniques are applicable to radar systems with several differences: 1. A radar system may be able to search over a restricted range of angles and still perform its mis- sion requirements. An ES system outside the search area will not be illuminated by the mainbeam of the radar antenna and may have difficulty detecting the signals; 2. Radar antennas are generally designed to be highly directive to provide angle resolution. However, antenna designs that also achieve low sidelobe levels are desirable for several reasons. First, sensitive ES systems can usefully detect pulses that correspond to the antenna sidelobes if these are sufficiently large. Second, some jamming techniques make use of signals that are received through sidelobes in the radar antenna response and therefore confuse the radar into showing a target at an angle offset from the jammer; 3. Frequency agility involves changing the transmitter frequency pulse to pulse or between groups of pulses. It has some similarities to the use of FH by commu- nication systems, although the primary ideas are to complicate the task of an ES system in interpreting whether the received pulses are from one or more radars, and to reduce the effectiveness of single fre- quency jammers. 8 ELECTRONIC WARFARE
- 50. 4. LPI radars tend to use continuous wave signals with frequency or phase modulation to provide the desired range resolution. Technical considerations generally restrict the average transmitter power with the result that they are mostsuited to applications in which long range is not required. Against these signals, conven- tional radar ES systems are usually limited to very short detection ranges because of the low transmitter power and the effect of receiver optimizations for the processing of short duration pulse signals.3 Passive EP generally places considerable emphasis on training and operational procedures. Some of the most spectacular EW successes, such as the decryption of mes- sages ciphered by the German Enigma machine in WW2, resulted, at least in part, from the failure of radio operators to follow correct procedures. The security of communication systems can be compromised in many possible ways. Exam- ples include the transmission of unimportant or unneces- sarily long messages; the repeated transmission of the same message with and without encryption; the failure to use code words and available EP capabilities, such as power management, FH, and encryption; and the failure to safeguard encryption equipment and keys. The likelihood of such lapses can be reduced substantially by the institu- tion of suitable procedures followed by training under realistic conditions. Emission Security policy includes defining procedures and techniques for minimizing the possibility of sensitive information being obtained from the intercept of RF signals that are generated unintentionally in the operation of computer or other electronic systems. In field or operational environments, tactical EP strat- egy is set by Emission Control (EMCON) orders, which define specific rules for the management of electromagnetic emissions (12) during a military operation. These rules attempt to strike a balance between various requirements: 1. Maintaining command and control capabilities; 2. Limiting mutual interference between friendly sys- tems; 3. Limiting the useful information that enemy ES can provide; 4. The execution of deception operations. EMCON rules include the following: 1. Restrictions on transmit power times and use of radio black-out policy; 2. Guidelines, such as frequency allocations and approved system configurations; 3. Restrictions on the type of information that can be transmitted (and thus denied to the enemy); ADDITIONAL TOPICS EW and Navigation Systems Before WW2, specialized direction-finding systems were developed for navigation purposes. From measurements of the angles to radio stations or beacons at known locations, position estimates could be computed. Although there were limitations on the achievable accuracy, this capability was extremely important, particularly at night and in bad weather. During WW2, more sophisticated systems were developed and deployed. Examples include Knickebein, X-Gerat, Y-Gerat, Decca Navigator, GEE, G-H, and Oboe. Various efforts were made to jam the signals associated with these systems, particularly those used for bombing navigation.4 Luftwaffe attempts to use the Knickebein, X- Great, and Y-Gerat navigation systems to guide bombers to targets in the U.K. were successfully countered by jam- ming, although a series of damaging raids was conducted using the X-Gerat system before effective jamming techni- ques were devised (5). German attempts to jam allied systems, such as GEE and Oboe, were generally less suc- cessful. For example, by the time successful jamming was initiated against Oboe signals at 200 MHz, the Mark III version had moved to 3 GHz. At this frequency, the tech- nical capabilities of the Germans were inadequate for the implementation of effective countermeasures. In addition, both sides made efforts to interfere with enemy radio beacons, sometimes with the result that air- craft got lost or were even captured after landing in unfriendly territory. After WW2, various navigation systems were developed and deployed. More recently, the global positioning system (GPS) has become very important, particularly in Western countries, because of the availability of worldwide coverage and the high accuracy that can be achieved. This avail- ability has led to the widespread use of GPS for guiding precision weapons and defining target locations. The mili- tary importance of GPS has motivated the development and marketing of GPS jammers. At the same time, recognition of the potential impact of GPS jamming has resulted in serious efforts to develop and implement anti-jam features in military GPSs (31). EW and IFF Systems Identification friend foe (IFF) systems are used to provide a means of quickly and positively identifying friendly air- craft. When an unknown aircraft is observed, the IFF system transmits a specially coded signal and looks for the transmission of an appropriate signal in response from the IFF system in the unknown aircraft. After early IFF systems were deployed in British bom- bers during WW2, the Germans discovered that the bom- bers could be tracked by transmitting signals to trigger their IFF systems and observing the IFF signals trans- 3 An interesting idea is to use commercial FM radio stations as a transmitter in a bistatic radar system. The receivers are located some distance from the transmitter, and the signal processing is designed to measure the relative time shifts between the signal that propagates directly from the transmitter to the receiver and the signal that arrives via a reflection from the target. 4 Investigations in the UK revealed that bombing attacks carried out at night were often ineffective without the use of electronic navigation aides (5). ELECTRONIC WARFARE 9
- 51. Discovering Diverse Content Through Random Scribd Documents
- 52. Japan, trails her purple skirts over the trellis under which the rickshaws go to their abode. The corn-bottles have come up exactly where I asked them to, scattered thick among the leaves of the chrysanthemums which are already tall and bushy. They are exactly the right blue in exactly the right green and they give a little air, not at all a disagreeable little air, of discernment and sophistication to their corner of the garden. I would like to venture to say that they resemble blue stars in a green sky, if I were sure of offending nobody’s sense of humour. It is natural enough to observe this and pass on, but why should one find a subtle pleasure in the comparison, and linger over it? It must be the same throb of joyful activity with which the evolved human intelligence first detected a likeness between any two of the phenomena about it, and triumphed in the perception, attracted to wisdom and stirred to art. Those indeed were days to live in, when everything was mysteriously to copy and inherit and nothing was exploited, explained, laid bare, when the great sweet thoughts were all to think and heroism had not yet received its molecular analysis, and babies equipped with an instinctive perception of the fundamental weakness of socialistic communism were neither born nor thought of. These seem violent reflections to make in a garden, and they may well be obscured behind the long bed of poppies and field-daisies and more bluets that runs along the side of the house under the windows that support the roses. If you can tell me for what primitive reason poppies and field-daisies and corn-flowers go well together I had rather you didn’t. I have clumps and clumps of hollyhocks, and a balustrade of them, pink and white ones, on each side of the steps that run down from the verandah in front of the drawing-room door. It is an unsophisticated thing, the single hollyhock, like a bashful school child in a sun-bonnet. Do what you will you cannot make it feel at home among the beaux and belles of high life in the garden; it never looks really happy except just inside a cottage paling with a bunch of rhubarb on one side and a tangle of “old man” on the other. Still it is a good and grateful flower in whatever station it pleases the sun to
- 53. call it. It gets along on the merest necessities of life when times are bad and water scarce, and flowers, with anything like a chance, twice in the season. One cannot, after all, encourage class feeling in the garden; there every one must stand on his own roots, and take his share of salts and carbon dioxide without precedence, and the hollyhocks in my garden receive as much consideration as anybody. Petunias are up all over the place, purple and white and striped. I knew by experience that we could have too many petunias on this shelf, so whenever a vague, young pushing thing disclosed itself to be a petunia, as it nearly always did, I requested Atma to pull it up. Nevertheless they survive surprisingly everywhere, looking out among the feet of the roses, flaunting over the forget-me-nots, unexpected in a box of seedling asters. Now if I were going to recognize social distinctions in the garden, which I am not, I should call the good petunia a person unmistakably middle-class. Whether it is this incapacity of hers to see a snub, or her very full skirt, or her very high colour, the petunia always seems to me a bourgeoise little lady in her Sunday best, with her hair smooth and her temper well kept under for the occasion. I think she leads her family a nagging life, and goes to church regularly. One should always mass them; a single petunia here and there among the community of flowers is more desolate and ineffective than most maiden ladies. Rather late this spring we discovered a corner of the bed in front of the dining- room window to be quite empty, and what to put in we couldn’t think, and were considering, when Atma told us that he knew of a thousand petunias homeless and roaming the shelf. I quite believed him, and bade him gather them in, with such a resultant blaze of purple as I shall never in future be without. The border just beyond them is simply shouting with yellow coreopsis, and behind that rise the dark branches of the firs on the khud-side, and between these, very often in broken pictures sharp against the blue, the jagged points and peaks of the far snows. All this every morning the person has with her eggs and bacon who sits opposite the dining-room window. I am glad to say that the other members of my family object to the glare.
- 54. Atma has a liberal and progressive mind toward the garden; he is always trying to smuggle some new thing into it. In out-of-the-way corners I constantly come upon perfect strangers, well-rooted and entirely at home, and when I ask him by whose order they were admitted, he smiles apologetically and says that without doubt they will be very beautiful, and that his brother gave them to him. He can never tell me the name. “It will be so high,” he shows me with his hand, stooping, “and the flower will be red, simply red it will arrive.” I look at it without enthusiasm, and weakly let it stay. Generally it “arrives” a common little disappointment, but once a great leggy thing turned out an evening primrose, and I knew, before it was too late, that I had been entertaining an angel unawares. “To grow a little catholic,” writes Stevenson, “is the compensation of years.” Dear shade, is it so? In the spiritual outlook, perhaps, in the moral retrospect,—but in matters of taste, in likes and dislikes? You who wrote nothing lightly must have proved this dispensation, poorer spirits can only wish it more general. I remember youth as curious and enterprising, hospitable to everything, and I begin to find the middle years jealously content with what they have. Who, when he has reached the age of all the world, looks with instinctive favour upon anything new? An acquaintance, who may create the common debt of friendship; you are long since heavily involved. An author, who may insist upon intimately engaging your intelligence,— a thing you feel, after a time, to be a liberty in a new-comer. Or even a flower, offering another sentiment to the little store that holds some pain already. Now this godetia. I suppose it argues a depth of ignorance, but until Mr. Johnson recommended it to me in the spring, I had never heard of godetia. Mr. Johnson is the source of seeds and bulbs for Simla, we all go to him; but I, for one, always come away a little ruffled by his habit of referring to everything by its Latin name, and plainly showing that his respect for you depends upon your understanding him. I have wished to preserve Mr. Johnson’s respect, and things have come up afterward that I did not think I had ordered. However, this is by the way. Mr. Johnson assured me that godetia had a fine fleshy flower of variegated
- 55. colours, would be an abundant bloomer, and with reasonable care should make a good appearance. I planted it with misgivings, and watched its advent with aloofness, I knew I shouldn’t recognize it, and I didn’t. I had never seen it before, I very nearly said so; and at my time of life, with so many old claims pressing, I could not attempt a new affection. And I have taken the present opportunity, when Atma’s back is turned, and pulled it all up. Besides it may have been fleshy, but it wasn’t pretty, and the slugs ate it till its appearance was disgraceful. I suppose our love of flowers is impregnated with our love of life and our immense appreciation of each other. We hand our characteristics up to God to figure in; we look for them in animals with delight and laughter, and it is even our pleasure to find them out here in the garden. Who cares much for lupins, for example; they are dull fellows, they have no faces; yet who does not care for every flower with a heart and eyes, that gives back your glance to you and holds up its head bravely to any day’s luck, as you would like to do. But it is growing late. I can still see a splendid crimson cactus glooming at me from his tub in the verandah; the rest of the garden has drawn away into the twilight. Only the honeysuckle, that nobody notices when the sun is bright and the flowers all talk at once, sends out a timid sweetness to the night and murmurs, “I am here.” If I might have had a seam to do, it would have been finished; but instead there has been this vexatious chapter, which only announces, when all is said and done, that another human being has spent a day in the garden. I intended to write about the applied affections. But it is too late even for the misapplied affections, generally thought, I believe, the more interesting presentment. Happy Thisbe on the verandah, conscious of another bud to her tapestry, glances at the fading west and makes ready to put all away. I will lay down my pen, as she does her needle, and gather up my sheets and scraps, as she does her silks and wools; and humbly, if I can get no one else to do it for me, carry my poor pattern into the house.
- 56. Chapter X THE Princess has a hill almost entirely to herself. She lives there in a castle almost entirely made of stone, with turrets and battlements. Her affectionate subjects cluster about her feet in domiciles walled with mud and principally roofed with kerosene tins, but they cheerfully acknowledge this to be right and proper, and all they can pay for. One of the many advantages of being a princess is that you never have to put down anything for house-rent; there is always a castle waiting for you and a tax-payer happy to paper it. The world will not allow that it is responsible to a beggar for a crust; but it is delighted to admit that it owes every princess a castle. It is a curious world; but it is quite right, for princesses are to be encouraged and beggars aren’t. The Princess is married to the Roy-Regent, who puts his initial upon Resolutions and writes every week to the Secretary of State; but it is the Princess who is generally “at home,” and certainly the Princess who matters. The Roy-Regent may induce his Government to make Resolutions; the Princess could persuade it, I am sure, to break them—if she wanted to. Unfortunately we are not permitted to see that comedy, which would be adorable. She does not want to. She is not what you would call a political princess; I have no doubt she has too much else to do. To begin with, only to begin with, she has to go on being beautiful and kind and unruffled; she has to keep the laughter in her eyes and the gentleness in her heart; she has to be witty without being cynical, and initiated without being hard. She has to see through all our little dodges to win her favour and not entirely despise us, and to accept our rather dull and very daily homage without getting sick and tired of us. To say nothing of the Roy-Regent and the babies who have some claims, I suppose, though we are apt to talk about the Princess as if she were here solely to hold her Majesty’s vice-Drawing-rooms and live up to a
- 57. public ideal. All the virtues, in short, which the rest of us put on of a Sunday, the Princess must wear every day; and that is why it is so difficult and often so tiresome to be a real princess. Fortunately the Simla Princess is not expected to hold her commission for life. Her Majesty knew, I suppose, from her own royal experience, how it got on the nerves, and realized that if she required anything like that it would be impossible to get the right kind of people. So at the end of every four or five years the Roy- Regent goes home to his ordinary place in the Red Book burdened for life with a frontier policy, but never again compelled to drive out in the evenings attended by four cantering Sikhs, each Sikh much larger than himself and shaking a lance. He may go on to greater things, or he may simply return to the family estates; but in any case the Princess can put her crown away in a drawer and do things, if she likes, in the kitchen, which must be a great relief. Of course she can never quite forget that she has been a princess, in commission, once. The thought must have an ennobling effect ever after, and often interpose, as it were, between the word and the blow in domestic differences. For this reason alone, many of us would gladly undertake to find the necessary fortitude for the task; but it is not a thing you can get by merely applying for it. To the state of the Princess belongs that quaint old-fashioned demonstration, the curtsey. The Princess curtseys to the Queen- Empress—how I should like to see her do it!—and we all curtsey to the Princess. This alone would make Simla a school for manners, now that you have to travel so far, unless you are by way of running in and out of Windsor Castle, to find the charming form in ordinary use. How admirable a point of personal contact lies in the curtsey— what deference rendered, what dignity due! “You are a Princess,” it says, “therefore I bend my knee. I am a Person, therefore I straighten it again,” and many things more graceful, more agreeable, more impertinent than that. Indeed, there is a very little that cannot be said in the lines and the sweep of a curtsey. To think there was a time when conversation was an art, and curtseying an accomplishment, is to hate our day of monosyllables and short cuts,
- 58. of sentiments condensed, and opinions taken for granted. One wonders how we came to lose the curtsey, and how much more went with it, how we could ever let it go, to stand instead squarely on our two feet and nod our uncompromising heads, and say what we have to say. I suppose it is one of the things that are quite gone; we can never reaffect it, indeed our behaviour, considered as behaviour, is growing steadily worse. Already you may be asked, by a person whom you have never seen before, whether you prefer Ecclesiastics or Omar Khayyám, or how you would define the ego, or what you think of Mr. Le Gallienne—matters which require confidence, almost a curtain. We have lost the art of the gradual approach; presently we shall hustle each other like kinetic atoms. A kinetic atom, I understand, goes straight to the point. We all love curtseying to the Princess therefore, partly because it is a lost art, and partly because it is a way in which we can say, without being fulsome or troublesome, how happy we are to see her. There is only one circumstance under which it is not entirely a privilege. That is when, dismounted, one meets her in one’s habit. Whether it is the long boots or the short skirt, or the uncompromising cut, I cannot say, but I always feel, performing a curtsey to the Princess in my habit, that I am in a false position. Every true woman loves to stalk about in her habit, and tap her heels with her riding crop; there is a shadow of the privileges of the other sex about it which is alluring, and which, as the costume is sanctioned, one can enjoy comfortably; but it is not arranged for curtseying, and there ought to be a dispensation permitting ladies wearing it to bow from the waist. Then the Princess passes on, leaving you smiling. I have seen people continue to smile in a lower key for twenty minutes after the Princess has gone by, as water will go on reflecting a glow long after the sunlight has left it. The effect is quite involuntary, and of course it looks a little foolish, but it is agreeable to feel, and nobody, positively nobody, can produce it but the Princess. Indeed the power to produce it would be a capital test for princesses.
- 59. If I were in any way in a position to submit princesses to tests, I should offer that of the single pea and the twenty feather beds with confidence to ours. Which is a pride and a pleasure to be able to say in these days, when ladies thus entitled are so apt to disguise themselves in strong minds or blunt noses or irritating clothes. It is delightful to be assured that, in spite of this tendency, the Princess has not yet vanished, the Princess of the fairy tales, the real Princess, from among us, that such a one is sitting at the moment in her castle, not ten minutes’ walk from here, eating marmalade with a golden spoon, or whatever she likes better than marmalade, and bringing to life day after day that delight in living which you must have, or there’s no use in being a princess. It is possible that she may not put on her diadem every morning; there is no necessity for that, since you could not imagine her without it; and if she prefers reading her Browning to watching her gold-fish, it is not in any way my affair. Indeed, although she occupies a public position, there is no one who more readily accedes her right to a private life than I, though, of course, with the rest of her subjects, I would prefer that she had as little of it as possible. It is said that the Roy-Regent, knowing what would be expected of her, was not content until he had found the most beautiful and agreeable Princess there was; and I can well believe that he sailed over seas and seas to find her, though it is probably only a tradition that they met at George Washington’s country seat where the Princess was looking for trailing arbutus,—another lovely thing whose habitat is the banks of the Potomac. And an improbable tradition, as George Washington never encouraged princesses. Last night there was an entertainment at the castle and among the guests a chief of one of those smaller Indias that cluster about the great one. He wore his own splendid trappings, and he was a handsome fellow, well set up; and above his keen dark face, in front of the turban, set round with big irregular pearls, was fastened a miniature of the Queen-Empress who holds his fealty in her hand. To him the Princess, all in filmy lace with her diadem flashing, spoke kindly. They sat upon gold-backed chairs a little way apart, and as
- 60. she leaned to confer her smile and he to receive it, I longed to frame the picture and make perpetual the dramatic moment, the exquisite odd chance. “Surely,” thought I, “the world has never been so graciously bridged before.” Talking of George Washington, if the good man could have seen that, I think he might have melted toward princesses; I do not think, from all we know of him, that he would have had the heart to turn coldly away and disclaim responsibility for this one. I wish he could have seen it; yes, and Martha too, though if anybody thought necessary to make trouble and talk about sacred principles of democracy, it would have been Martha. Martha, she would have been the one. Her great and susceptible husband would have taken a philosophic pinch of snuff and toasted posterity. I see that I have already admitted it, I have slipped in the path of virtuous resolution and lofty indifference; I have gone back, just for a minute, into the world. The reason I have neglected every flower in the garden this morning to write about the Princess is that I have been dining with her. It is so difficult to be unmoved and firm when you know the band will play and there will be silver soup-plates, to say nothing of the Roy-Regent smiling and pleased to see you, and the Roman punch in the middle of the menu. At home, one so seldom has Roman punch in the middle of the menu. Besides, now that I think of it, it was a “command” invitation, and I did not go for any of these reasons, or even to see the Princess, but because I had to; a lofty compulsion of State was upon me, and nobody would place her loyalty in question on account of a possible draught. If there had been a draught and I had taken cold I should have felt an added nobility to-day; somewhat the virtue, I suppose, of the elderly statesman who contracts a fatal influenza at a distinguished interment, and so creates a vicious circle of funerals; but there was no draught. The Princess lives in splendid isolation. If it were not for the Roy- Regent and the babies, and the Commander-in-Chief and his family, she would die of loneliness. And of course the Bishop, though I can’t understand in what way one would depend much upon a bishop,
- 61. except to ask a blessing when he came to dinner. Kind and human as the Princess is she lives in another world, with an A.D.C. always going in front to tell people to get up, “Their Excellencies are coming.” You cannot ask after the Princess’s babies as you would ask after the babies of a person like yourself; you must say, “How are Your Excellency’s babies?” and this at once removes them far beyond the operation of your affectionate criticism. When it is impossible even to take babies for granted the difficulties of the situation may be imagined. The situation is glorious but troubling, your ideas often will not flow freely in it, and is there anything more dreadful at a supreme moment than to have your ideas stick? You find yourself saying the same thing you said the last time you had the honour, which is the most mortifying thing that can happen in any conversation. I often wonder whether the Princess does not look at our little mud houses and wish sometimes that she could come in. The thought is a reckless one but I do entertain it. If you take a kind and friendly interest in people as the Princess does in us all, you cannot be entirely satisfied merely to add them up as population and set them a good example. Nor can it be very interesting to look at the little mud houses and observe only that they have chimneys, and not to know how the mantelpieces are done or whether there is a piano, or if anybody else’s sweet-peas are earlier than yours. In my dreams I sometimes invite the Princess to tea. An A.D.C. always comes behind her carrying the diadem on a red silk cushion, but at my earnest prayer he is made to stay outside on the verandah. We have the best china; and in one dream the Princess broke a cup and we wept together. On another occasion she gave me a recipe for pickled blackberries and told me of a way—I always forget the way—of getting rid of frowns. There is generally something to spoil a dream, and the thing that spoils this one is the A.D.C., who will look in at the window. All the same we have a lovely time, the Princess ignoring all her prerogatives, unless I say something about the state of the country, when she instantly, royally, changes the subject....
- 62. Chapter XI IF you choose to live on the top of one of the Himalayas there are some things you must particularly pay for. One of them is earth. Your mountain, if it is to be depended upon, is mostly made of rock and I have already mentioned how radically it slopes. So a garden is not at all a thing to be taken for granted. Sometimes you have a garden and sometimes only a shaly ledge, or you may have a garden to-day which to-morrow has slid down the hill and superimposed itself upon your neighbour below. That occurs in the rains; it is called a “slip.” It has never been our experience because the shelf is fairly flat; but it has happened to plenty of people. I suppose such a garden is recoverable, if you are willing to take the trouble, but it could never be quite the same thing. The most permanent plot, however, requires all kinds of attention, and one of the difficulties is to keep it up to its own level. Queer sinkings and fallings away are always taking place in the borders. Atma professes to find them quite reasonable; he says the flowers eat the earth and of course it disappears. The more scientific explanation appears to me to be that the gnomes of the mountain who live inside, have been effecting repairs, and naturally the top falls in. It may be said that gnomes are not as a rule so provident; but very little has yet been established about the Himalayan kind; they might be anything; they probably are. This whole morning Atma and I have been patching the garden. At home when you buy a piece of land you expect that enough earth will go with it for ordinary purposes, but here you buy the land first and the earth afterwards, as you want it, in basketfuls. There is plenty in the jungle, beautiful leaf-mould, but it is against the law to collect it there for various reasons, all of them excellent and tiresome; you must buy it instead from the Town Council, and it costs fourpence a basket. Tiglath-Pileser says it is the smallest
- 63. investment in land he ever heard of, but it takes a great many baskets, and when the bill comes in I shall be glad to know if he is still of that opinion. Meanwhile coolie after coolie dumps his load and I have heard of no process that more literally improves the property. You will imagine whether, when anything is pulled up, we do not shake the roots. How far a sharp contrast will carry the mind! I never shake a root in these our limited conditions without thinking of the long loamy stretches of the Canadian woods where there was leaf-mould enough for a continent of gardens, and of the plank “sidewalk” that half-heartedly wandered out to them from the centre of what was a country town in my day, adorned perhaps at some remote and unfenced corner by a small grocery shop where hickory nuts in a half-pint measure were exposed for sale in the window. I am no longer passionately addicted to hickory nuts—you got the meat out with infinite difficulty and a pin, and if it was obstinate you sucked it —but nothing else, except perhaps the smell in the cars of the train- boy’s oranges, will ever typify to me so completely the liberal and stimulating opportunities of a new country. The town when I was there last had grown into a prosperous city, and there were no hickory nuts in its principal stores, but at the furthest point of a suburban sidewalk I found the little grocery still tempting the school children of the neighbourhood with this unsophisticated product and the half-pint measure in the window. I resisted the temptation to buy any, but I stood and looked so long that the proprietress came curious to the door. And along that sidewalk you might have taken a ton of leaf-mould before anybody made it his business to stop you. We must acknowledge our compensations. Over there they certainly get their leaf-mould cheaper than fourpence a basket, but they have nobody to make things grow in it under a dollar a day. Here Atma, the invaluable Atma, labours for ten rupees a month—about fourteen shillings—and cooks his own meal cakes. The man who works for a dollar a day does it in the earnest hope, if we are to believe his later biographer, of a place in ward politics and the easier situation of a local boss. It would be hard to infect Atma with such vulgar
- 64. ambitions. He is so lately from the hands of his Creator that he has not even yet conceived the idea of accumulation. The other day I told him that he might take a quantity of seed and surplus plants, and sell them, and he would not. “I, how shall I sell?” he said, “I am a gardener. This thing is done by Johnson-sahib,” and he looked at me with amusement. I called him by a foolish name and told him that he should surely sell, and get money; but he shook his head still smiling. “By your honour’s favour,” he said, “month by month I find ten rupees. From this there is food twice a day and clothes, and two or three rupees to go by the hand of an old man who comes from my people. It is enough. What more?” I mentioned the future. “Old?” he cried, “God knows if I will be old. At this time I am a work- doing wallah. When I am old and your honours are gone to Belaat, [2] I also will go, and live with my people.” “And they will, without doubt, give you food and clothes?” I asked. “According as there is,” he said, “without doubt they will give it,” and went on with his work. Here, if you like, was a person of short views and unvexed philosophy. A lecture upon the importance of copper coins trembled on my lips, but I held it back. A base aim is a poor exchange for a lesson in content, and I held it back, wondering whether my servant might not be better off than I, in all that he could do without. Alas for the poor people who have to pay at the rate of a dollar a day and mind their own business into the bargain! Never can they know one of the greatest pleasures of life, to be served by a serving people. There is a spark of patriarchal joy, long extinct west of Suez, in the simple old interpretation which still holds here, of the relation of master and servant, scolding and praise, favour and wrath; a lifelong wage and occasionally a little medicine are still the portion of the servant-folk, accepted as a matter of course, and “Thou wilt not hear orders?” ever a serious reproach. To all of us Outlanders of the East, it is one of the consolations of exile, and to some of us a keen and constant pleasure to be the centre and source of prosperity for these others, a simple, graphic, pressing opportunity to do justice
- 65. and love mercy and walk humbly with their God. I, personally, like them for themselves—who could help liking Atma?—and of persons to whom they do not at all appeal I have my own opinion. It is the difference of race, no doubt, which makes this relation possible and enjoyable, the difference, and what we are accustomed to consider the superiority, of ours. At home all generous minds are somewhat tormented by a sense of the unfairness of the menial brand, and in the attitude of the menial mind there is nothing to modify that impression. Servants in this place are regarded as luxuries, and taxed. So much you pay per capita, and whether the capita belongs to a body entirely in your employment, or to one which only serves you in common with several other people, it doesn’t matter; all the same you pay. Delia and I share a dhurjee, or sewing man, for example, and we are both chargeable for him. This I never could reconcile with my sense of justice and of arithmetic,—that the poll-tax of a whole man should be paid on half a tailor; but there is no satisfaction to be got out of Tiglath-Pileser. Some people have more respect for the law than it really deserves. I had the pleasure, however, of bringing him to a sense of his responsibilities when the tax-paper came in, from which he learned that no less than fifteen heads of families looked to him to be their providence. Under the weight of this communication he turned quite pale, and sat down hastily upon the nearest self-sustaining object, which happened to be the fender. But as a matter of fact he liked the idea. Every Englishman does, and this is why a certain measure of success attends not only his domestic but his general experiments in governing the East. He loves the service of an idea, and nothing flatters him so truly as his conception of all that he has to do. The ear sharpens if its owner lives in the garden. It is no longer muffled by the four walls of a house, and remote sounds visit it, bringing with them a meaning which somehow they never have indoors, even when they penetrate there. Up here they principally make one aware of the silence, which is such a valuable function of sounds. I should like to write a chapter about the quiet of Simla, but
- 66. of course if one began like that one would never finish. It is our vast solace, our great advantage; we live without noise. The great ranges forbid it; the only thing they will listen to is a salute from the big gun, and they pass that from one to another, uncertain that is not an insult. And the quenching comment in the silence that follows! It is tremendous, invincible, taken up and rewritten in the lines of all the hills. It stands always before our little colony, with a solemn finger up, so that a cheer from the cricket ground is a pathetic thing, and the sound of the Roy-Regent’s carriage wheels awakens memories of Piccadilly. We are far withdrawn and very high up, fifty- six miles down to the level, and then it is only empty India—and the stillness lies upon us and about us and up and down the khuds, almost palpable and so morne, but with the sweetest melancholy. Consider, you of London and New York, what it must be to live on one mountain-side and hear a crow caw across the valley, on the other. Of course we are a Secretariat people; we have no factory whistles. This afternoon, however, I hear an unlicensed sound. It is the sound of an infant giving tongue, and it comes from the quarters. Now there ought not to be a baby in the quarters; it is against all orders. No form of domestic ménage is permitted there; the place is supposed to be a monastery, and the servants to house their women-folk elsewhere. The sound is as persistent as it is unwarrantable; it is not only a breach of custom, but displeasing. How am I to reckon with it? I may send for Dumboo and complain. In that case the noise will cease at once; they will give opium to the child, which will injure its digestion, and in the future, as a grown-up person, it will enjoy life less because I could not put up with its crying as an infant. I can report the matter to Tiglath-Pileser, which would mean an end to the baby, not illegally, by banishment. But is it so easy? One approves, of course, of all measures to discourage them about the premises, but when in spite of rules and regulations a baby has found its way in, and is already lamenting its worldly prospects at the top of its voice, in honest confidence that at least
- 67. the roof over its head will be permanent, a complication arises. I cannot dislodge such a one. Better deafness and complicity. Far down the khud-side an Imperial bugle. Abroad the spaces the mountains stand in, and purple valleys deepening. Among the deodars a whisper, not of scandal, believe me. A mere announcement that the day is done. On the other side of the hill a pony trotting, farther and fainter receding, but at the farthest and faintest it is plain that he goes short in front. From the bazaar a temple bell, with the tongue of an alien religion....
- 68. Chapter XII TO-DAY I think India, down below there on the other side of the hill, must be at its hottest. A white dust haze hangs over the plains, but we know what is going on under it; nearly all of us have gasped through June more than once in those regions. It is the time when you take medical advice before committing yourself to a railway journey, even with the provision of a cracked-ice pillow,—the favourite time to step out of the train and die of cholera in the waiting room. It is also the very special time for the British private soldier to go out in anger and kick with his foot the punkah-wallah who has fallen asleep with the slack rope in his hand, so that the punkah-wallah, in whom is concealed unknown to the private soldier an enlarged spleen, immediately dies. There is then trouble and high-talking, because of the people who consider that the death of a punkah-wallah demands the life of a private soldier who only meant to admonish him, a contention which cannot be judged without a knowledge of the relative values concerned, and an experience of the temperature in which the rash and negligent act was committed. There is reason in the superstition which associates great heat with the devil. Operating alone, it can do almost as much as he can. The dust haze from the plains hangs all about us, obscuring even the near ranges, impalpable but curiously solid. It has a flavour which it is impossible not to taste if ever one breathes through the mouth, and hour by hour it silently gathers upon the furniture. It has been like this for a week, pressing round us at a measured distance, which just enables us to see our own houses and gardens. Within that space, the sunlight and every circumstance as usual. It is a little like living under a ground-glass bell. Do not choose the present time of year to come to see Simla. You would have to make a house-to- house visitation, and piece it together from memory.
- 69. Even here, in the garden, much too hot the eye of heaven shines. I have abandoned the pencil-cedar, and taken refuge under a trellis covered with a banksia rose, which is thicker, and I have added to my defences a pith hat and an umbrella. Notwithstanding these precautions, we all gasp together to-day in the garden; and I am inclined to agree with the mignonette, which is not as a rule talkative, that this is no longer the summer—exquisite word—that we expect in Simla, but the odious “hot weather” which comes instead in the country down below. The mignonette, by the way, stands to my discernment, immediately under the pencil-cedar. When I sowed it there in the spring, Tiglath-Pileser said, “It will never do anything under a conifer.” When it began to show, he said again, “It may come up, but it will never do anything. Nothing ever does anything under a conifer.” Atma was not of this advice. “Come up?” he said, looking at it sternly, “wherefore should it not come up, if your honour wishes it?” Atma always takes this view; he seems to suppose that the flowers, like himself, are above all things anxious to please, and if any of them fail in their duty, he implies, with indignation, that he will know the reason why. But his opinion is too constant, and I did not trust it about the mignonette. I insisted, instead, that every morning the fallen cedar spines should be picked out of it, and the earth freshly stirred about the roots; and I have a better patch of mignonette under my conifer than can be produced anywhere else in the garden. I am sure that the shade of a conifer is no less beneficial than any other kind of shade, except that there is never enough of it; nor can I accept the theory that there is anything poisonous in the spines. They only pack and only lie very closely together, never blown about like leaves, and so keep away the air and light, and if you happen to have the use of twenty or thirty brown fingers to pick them out, there is no reason why you cannot produce quantities of things beside mignonette under a conifer. Do anything? I do not know a more able-bodied or hard- working flower on the shelf. A thing like that offers one for some time afterwards a valuable handle in arguments.
- 70. However you do it, there is no more delicious experience in life than to put something beautiful where nothing was before, I mean in any suitable empty space. I have done it; I have had the consummation of this pleasure for a fortnight. There was no goldenrod in Simla till I went to America and got it. I make the lofty statement with confidence, but subject to correction. Some one may have thought of it long ago, and may be able to confront me with finer plumes than mine. If this should be so, I shall accept it with reluctance and mortification, and hereby promise to go and admire the other person’s, which is the most anybody can do; but my pride does not expect such a fall. It is the Queen’s goldenrod, not the President’s, though he has a great deal of it and makes, I think, rather more fuss about it. A field flower of generous mind, it ignores the political line, and I gathered the seed one splendid autumn afternoon in Canada; so here on the shelf it may claim its humble part in the Imperial idea. A friend of my youth lent herself to the project; she took me in her father’s buggy, and as we went along the country roads I saw again in the light of a long absence, the quiet of the fields and the broad pebbled stretches of the river, and the bronze and purple of the untrimmed woods that had always been for me the margin of the thought of home. The quiet of after-harvest held it all, nothing was about but a chipmunk that ran along the top of a fence; you could count the apples in the orchards among their scanty leaves; it was time to talk and to remember. And so, not by anything unusual that we did or said, but by the rare and beautiful correspondence that is sometimes to be felt between the sentiment of the hour and the hour itself, this afternoon took its place in the dateless calendar of the heart which is so much more valuable a reference than any other. What a delight it is when old forgotten things construct themselves again and the years gather into an afternoon! And is there any such curious instance of real usefulness for hidden treasure in the attic? We found masses of goldenrod, all dry and scattering, principally along the railway embankment, which we took for a good omen that it would be a travelling flower; and in the fulness of time it was
- 71. given to Atma with instructions. His excitement was even greater than mine, he nursed it tenderly, but it needed no nursing. It came up in thousands delighted with itself and the new climate, overrunning its boxes so that Atma pointed to it like a proud father. Then we planted it out along the paling behind the coreopsis, and it immediately—that is to say in three months’ time—grew to be five feet high, with the most thick and lovely yellow sprays, which have been waving there against the fir-trees, as I said before, for the last fortnight. It has quite lost the way to its proper season; at home it blossoms in September and this is only June,—but it appears to be rather the better than the worse for that, though it does seem to look about, as the Princess said when I sent her some, for the red sumach which is its friend and companion at home. It is itself like a little fir-tree with flat spreading branches of blossom, especially when it stands in groups as they do, and the sun slants upon it giving the sprays an edge of brighter gold so that it is the most luminous thing in the garden. And the warm scent of it, holding something so far beyond itself and India, something essential, impregnated with the solace that one’s youth and its affections are not lost, but only on the other side of the world! Another delightful thing about the goldenrod is the way the bees and butterflies instantly found it out. The sprays are dotted with them all day long, swaying and dipping with the weight of the little greedy bodies; their hum of content stands in the air with the warm and comfortable scent. “This is good fare” they seem to say. “There are some things they make better in America.” I had never before done anything for a bee or a butterfly, it is not really so easy, and I would not have believed there was such pleasure in it. “Le fleur qui vole”— is not that charming of M. Bourget? I suppose it argues a very empty plane of life, but these little creatures have an immense power of entertaining a person who spends day after day in the theatre of their activities. I am reminded that here in India one ought to have marvellous tales to tell of them, only Simla is not really India, but a little bit of England with an Adirondack climate and the “insect belt” of Central Asia; and things
- 72. are not so wonderful here as you would think to look at us on the map. Scorpions and centipedes do come up from the plains and live in the cracks of the wall whence they crawl out to be despatched when the first fires are lighted, but they have not the venom of those below. Scorpions Atma will take hold of by the poison bags at the end of their tails, and hold up in the air dangling and waving their arms; and nobody even screams at a centipede. Millipedes which look much more ferocious but are really quite harmless often run like little express trains across your bath-room walls, and very large, black, garden spiders also come there to enjoy the damp. They enjoy the damp, but what they really like is to get into the muslin curtain over the window and curl up and die. The first time I saw one of them in the folds of the curtain I thought it would be more comfortable in the garden and approached it with caution and a towel, to put it out. Then I perceived from its behaviour—it did not try to run away, but just drew its legs a little closer under it, as you or I would do if we absolutely didn’t care what happened so long as we were left in peace—that it had come there on purpose, being aware of its approaching end. I decided that the last moments of even a spider should be respected, but every day I shook the curtain and he drew his legs together a little more feebly than the day before, until at last he dropped out, the shell of a spider, comfortably and completely dead. I admired his expiring, it was business-like and methodical, the thing he had next to do, and he was so intent upon it, not in any way to be disturbed or distracted, asking no question of the purposes of nature, simply carrying them out. One might moralize. Talking of spiders I have just seen a fly catch one. It was, of course, an ichneumon fly. One has many times heard of his habit of pouncing upon his racial enemy, puncturing and paralyzing him and finally carrying him off, walling him up and laying an egg in him, out of which comes a young ichneumon to feed upon his helpless vitals; but one does not often see the tragedy in the air. He held his fat prey quite firmly in his merciless jaws and he went with entrain, the
- 73. villain! The victim spider and the assassin fly! One might moralize again. It is hotter than ever, and the sunlight under the ground-glass bell has a factitious look, as if we had here a comedy with a scene of summer. A hawk-moth darts like a hummingbird in and out of the honeysuckle, and a very fine rose-chafer all in green and gold paces across this paragraph. I believe there are more rose-chafers this year than there ought to be, and Atma has a heavy bill against them in every stage of their existence, but they are such attractive depredators. When I find one making himself comfortable in the heart of a La France, I know very well that on account of the white grub he was once and the many white grubs he will be again I ought to kill him and think no more about it; but one hesitates to send a creature out of the world who exercises such good taste when he is in it. I know it is quite too foolish to write, but the extent of my vengeance upon such a one is only to put him into a common rose. The birds are silent; the butterflies bask on the gravel like little ships with big sails. Even the lizards have sought temporary retirement between the flower-pots. I am the only person who is denied her natural shelter and compelled to resort to an umbrella. Tiglath- Pileser said the other day that he thought it was quite time I made some acknowledgment of the good it was doing me. It is doing me good—of course. But what strikes me most about it is the wonderful patience and fortitude people can display in having good done to them.
- 74. Chapter XIII I HAVE had a morning of domestic details with the Average Woman. I don’t quite know whether one ought to write about such things, or whether one ought to draw a veil; I have not yet formed a precise opinion as to the function of the commonplace in matter intended for publication. But surely no one should scorn domestic details, which make our universal background and mainstay of existence. Theories and abstractions serve to adorn it and to give us a notion of ourselves: but we keep them mostly for lectures and sermons, the monthly reviews, the original young man who comes to tea. All would be glad to shine at odd times, but the most luminous demonstration may very probably be based upon a hatred of cold potatoes and a preference for cotton sheets. And of course no one would dare to scorn the average woman; she is the backbone of society. Personally I admire her very humbly, and respect her very truly. For many of us, to become an average woman is an ambition. I think I will go on. Besides, Thalia interrupted us, and Thalia will always lend herself to a chapter. The Average Woman is not affectionate but she is solicitous, and there was the consideration of my original situation and my tiresome health. Then she perceived that I had a garden and that it was a pretty garden. I said, indifferently, that people thought so; I knew it was a subject she would not pursue unless she were very much encouraged, and there was no reason at all why she should pursue it; she would always be a visitor in such a place, whereas there were many matters which she could treat with familiar intelligence. I was quite right; she wandered at once into tins of white enamel, where it seemed she had already spent several industrious hours. We sympathized deeply over the extent to which domestic India was
- 75. necessarily enamelled, though I saw a look of criticism cross her face when I announced that I hoped one day to be rich enough not to possess a single article painted in that way—not a chair, not a table. I think she considered my declaration too impassioned, but she did not dissent from it. That is a circumstance one notes about the Average Woman: she never dissents from anything. She never will be drawn into an argument. One could make the most wild and whirling statement to her, if one felt inclined, and it is as likely as not that she would say “Yes indeed,” or “I think so too,” and after a little pause of politeness go on to talk about something else. I can’t imagine why one never does feel inclined. We continued to discuss interior decoration, and I learned that she was preparing a hearth seat for her drawing-room, one of those low square arrangements projecting into the room before the fire, upon which two ladies may sit before dinner and imagine they look picturesque, while the rest of the assembled guests, from whom they quite cut off the cheerful blaze, wonder whether they do. The Average Woman declared that she could no longer live without one. “As time goes on one notices that fewer and fewer average women can,” I observed absently, and hastily added, “I mean, you know, that of course very portly ladies—” “Oh, I see,” said she. “No, of course not.” “So long,” I went on, pursuing the same train of thought, “as one can sit down readily upon a hearth seat, and especially so long as one can clasp one’s knees upon it, one is not even middle-aged. To clasp one’s knees is really to hug one’s youth.” “I had such a pretty one in Calcutta,” said the Average Woman. “So cosy it looked. Everybody admired it.” “But in Calcutta,” I exclaimed with astonishment, “it is always so hot —and there are no fireplaces.” “Oh, that didn’t matter,” replied she triumphantly, “I draped the mantelpiece. It looked just as well.” And yet there are people who say that the Average Woman has no imagination.
- 76. “Talking of age,” she continued, “how old do you suppose Mrs. —— is? Somebody at tiffin yesterday who knew the family declared that she could not be a day under thirty-seven. I should not give her more than thirty-five myself. My husband says thirty-two.” “About a person’s age,” I said, “what can another person’s husband know?” “What should you say?” she insisted. I am sorry to have to underline so much, but you know how the average woman talks in italics. It is as if she wished to make up in emphasis—but I will not finish that good-natured sentence. “Oh,” said I, “you cannot measure Mrs. ——’s age in years! She is as old as Queen Elizabeth and as young as the day before yesterday. Parts of her date from the Restoration and parts from the advent of M. Max Nordau—” At that moment Thalia arrived. “And that is the age of all the world,” I finished. “We were wondering,” said the Average Woman, “how old Mrs. —— is.” “You were wondering,” I corrected her. “What does it matter?” said Thalia, which was precisely what I should have expected her to say. What does it matter? Why should the average woman excite herself so greatly about this particularly small thing? How does it bear upon the interest or the attractiveness or the value of any woman to know precisely how many years she counts between thirty and forty, at all events to another of her sex? Yet to the average woman it seems to be the all-important fact, the first thing she must know. She is enragée to find it out, she will make the most cunning enquiries and take the most subtle means. Much as I appreciate the average woman, I have in this respect no patience with her. It is as if she would measure the pretensions of all others by recognized rule of thumb with a view to discovering the surplus claim and properly scoring it down. It is surely a survival from days when we were much more feminine than we are now; but
- 77. it is still very general, even among married ladies, for whom, really, the question might have an exhausted interest. “What does it matter?” said Thalia. “I see your fuchsias, like me, have taken advantage of a fine day to come out. What a lot you’ve got!” “Yes,” I said, without enthusiasm, “they were here when we came.” “Oh, don’t you like them?” exclaimed the Average Woman, “I think the fuchsia such a graceful, pretty flower.” “It is graceful and it is pretty,” I assented. There are any number of fuchsias, as Thalia said, standing in rows along the paling under the potato-creeper; the last occupant must have adored them. They remain precisely in the pots in which they were originally cherished. Knowing that the first thing I do for a flower I like is to put it in the ground where it has room to move its feet and stir about at night, and take its share in the joys of the community, Tiglath-Pileser says compassionately of the fuchsia, “It is permitted to occupy a pot;” but I notice that he does not select it for his button-hole notwithstanding. Thalia looked at me suspiciously. “What have you got against it?” she demanded, and the Average Woman chorussed, “Now tell us.” I fixed a fuchsia sternly with my eye. “It’s an affected thing,” I said. “Always looking down. I think modesty can be an overrated virtue in a flower. It is also like a ballet-dancer, flaunting short petticoats, which doesn’t go with modesty at all. I like a flower to be sincere; there is no heart, no affection, no sentiment about a fuchsia.” Thalia listened to this diatribe with her head a little on one side. “You are full of prejudices,” said she, “but there is something in this one. Nobody could say ‘My love is like a fuchsia.’” “It depends,” I said; “there are ladies not a hundred miles from here who thrill when they are told that they walk like the partridge and shine like the moon. I shouldn’t care about it myself.”
- 78. “No, indeed,” said the Average Woman. “That bit beyond the mignonette seems rather empty. What are you going to put in there?” “Oh, nothing,” I said. “I don’t know,” remarked Thalia combatively, “when there are so many beautiful things in the world, why you should discriminate in favour of nothing.” “Yes, why?” said the Average Woman. “Well,” I replied defiantly, “that’s my spare bedroom. You’ve got to have somewhere to put people. I don’t like the feeling that every border is fully occupied and not a square inch available for any one coming up late in the season.” You can see that Thalia considers that while we are respected for our virtues our weaknesses enable us to enjoy ourselves. She accepts them as an integral and intentional part of us and from some of them she even extracts a contemplative pleasure. The Average Woman looks down upon such things and I did not dare to encounter her glance of reserved misunderstanding. Thalia smiled. I felt warmed and approved. “Alas!” said she, “my garden is all spare bedrooms.” She lives, poor dear, on the other side of the Jakko and has to wait till September for her summer. “I see you keep it aired and ready.” As a matter of fact Atma had freshly turned the earth. I hold to that in the garden; it seems to me a parallel to good housekeeping. The new-dug mould makes a most enhancing background; and an empty bed, if it is only freshly made, offers the mind as much pleasure as a gay parterre. It is the sense, I suppose, of effort expended and care taken, and above all it is a stretch of the possible, a vista beyond the realized present which is as valuable in a garden as it is in life. Oh no, not as valuable. In life it is the most precious thing, and it is sparingly accorded. Thalia has it, I know, but I looked at the Average Woman in doubt. Thalia, whatever else she does, will have high comedy always for her portion, and who can tell in what scenes
- 79. she will play or at what premières she will assist? But the Average Woman,—can one not guess at the end of ten years what she will be talking about, what she will have experienced, what she will have done? I looked at the Average Woman and wondered. She was explaining to Thalia the qualities of milk tea. I decided that she was probably happier than Thalia, and that there was no need whatever to be sorry for her. She stayed a long time; I think she enjoyed herself; and when she went away of course we talked about her. We spoke in a vein of criticism, and I was surprised to learn that the thing about the Average Woman to which Thalia took most exception was her husband. I had always found the poor patient creatures entirely supportable, and I said so. “Oh, yes,” replied Thalia impatiently, “in themselves they’re well enough. But didn’t you hear her? ‘George adores you in “Lady Thermidore.”’ Now that annoys me.” “Does it?” said I. “Why shouldn’t George adore you in Lady Thermidore if he wants to, especially if he tells his wife?” “That’s exactly it,” said Thalia. “If he really did he wouldn’t tell her. But he doesn’t. She just says so in order to give herself the pleasure of imagining that I am charmed to believe that George—her George —” “I see,” I said, sympathetically. “They are always offering their husbands up to me like that,” continued Thalia, gloomily. “They expect me, I suppose, to blush and simper. As if I hadn’t a very much better one of my own!” “They think it the highest compliment they can pay you.” “Precisely. That’s what is so objectionable. And besides it’s a mistake.” “I shall never tell you that Tiglath-Pileser adores you,” I stated. “My dear, I have known it for ages!” said Thalia, en se sauvant, as they do in French novels.
- 80. Perhaps the Average Woman is a little tiresome about her husband. She is generally charged with quoting him overmuch. I don’t think that; his opinions are often useful and nearly always sensible, but she certainly assumes a far too general interest for him as a subject upon which to dwell for long periods. Average wives of officials are much more distressingly affected in this way than other ladies are; it is quite a local peculiarity of bureaucratic centres. They cherish the delusion, I suppose, that in some degree they advance the interests of these unfortunate men by a perpetual public attitude of adoration, but I cannot believe it is altogether the case. On the contrary, I am convinced that the average official husband himself would find too much zeal in the recounting of his following remarkable traits. His obstinate and absurd devotion to duty. “In my husband the Queen has a good bargain!” His remarkable youth for the post he holds,—I remember a case where my budding affection for the wife of an Assistant Secretary was entirely checked by this circumstance. The compliments paid him by his official superiors, those endless compliments. And more than anything perhaps, his extraordinary and deplorable indifference to society. “I simply cannot get my husband out; I am positively ashamed of making excuses for him.” When her husband is served up to me in this guise I feel my indignation rising out of all proportion to its subject, always an annoying experience. Why should I be expected to accept his foolish idea that he is superior to society, and admire it? Why should I be assumed to observe with interest whether he comes out; why indeed, so far as I am concerned, should he not eternally stay in? It comes to this that one positively admires the woman who has the reticence to let her husband make his own reputation. What offends one, I suppose, is the lack of sincerity. A very different case is that of the simple soul who says, “Tom will not allow me to have it in the house,” or “Jim absolutely refuses to let me know her.” One hears that with the warm thrill of mutual bondage; one has one’s parallel ready—the tyranny I could relate of Tiglath-Pileser! The note of grievance is primitive and natural; but the woman who butters her husband in friendly council, what excuse has she?
- 81. Chapter XIV THE rains have come. They were due on the fifteenth of June and they are late; this is the twentieth. The whole of yesterday afternoon we could see them beating up the valleys, and punctually at midnight they arrived, firing their own salute with a great clap of thunder and a volley on the roof—it is a galvanized roof—that left no room for doubt. You will notice that it is the rains that have come and not the rain; there is more difference than you would imagine between water and water. The rain is a gentle thing and descends in England; the rains are untamed, torrential, and visit parts of the East. They come to stay; for three good months they are with us, pelting the garden, beating at the panes. It would be difficult for persons living in the temperate zone to conceive how wet, during this period, our circumstances are. One always hears them burst with a feeling of apprehension; it is such a violent movement of nature, potential of damage, certain of change; and life is faced next morning at breakfast with a gloom which is not assumed. A dripping dulness varied by deluges, that is the prospect for the next ninety days. The emotions of one who will be expected to support it under an umbrella, with the further protection of a conifer only, are offered, please, to your kind consideration. I dreamed as the night wore on of shipwreck in a sea of mountains on a cane chair, and when I awoke, salvaged in my bed, it was raining as hard as ever. At breakfast Tiglath-Pileser said, uneasily, that it would probably clear up in half an hour. “It simply can’t go on like this,” remarked Thisbe, and I saw that they were thinking of me, under the conifer. When you suspect commiseration the thing to do is to enhance it. “Clear up?” said I with indifference. “Why should it clear up? It has only just begun.”
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