![Originally published in the German language by B. G. Teubner GmbH as ``Jo
Èrg Eberspa
Ècher/Hans-Jo
Èrg Vo
Ègel/Christian
Bettstetter: GSM Global System for Mobile Communication. 3. Au¯age (3rd edition)''.
q B. G. Teubner Stuttgart/Leipzig/Wiesbaden, 2001
Copyright q 2001 by John Wiley & Sons, Ltd
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Library of Congress Cataloging-in-Publication Data
Eberspa
Ècher, I. (Jo
Èrg)
[GSM, Global System for Mobile Communication. English]
GSM switching, services, and protocols / Jo
Èrg Eberspa
Ècher, Hans-Jo
Èrg Vo
Ègel,
Christian Bettstetter.± 2nd ed.
p. cm.
Includes bibliographical references and index.
Prey. ed.: GSM switching, services, and protocol. 1999.
ISBN 0-471-49903-X (alk. paper)
1. Global system for mobile communications. I. Vo
Ègel, Hans-Jo
Èrg. II. Bettstetter,
Christian. III Title.
TK5103.483 .E2413 1999
621.3820
2±dc2l
00-054550
Use the Internet and eliminate mail time and postage costs http://cip.loc.gov/cip
British Library Cataloguing in Publication Data
A catalogue record for this book is available from the British Library
ISBN 0471 49903 X
Typeset by Deerpark Publishing Services Ltd, Shannon, Ireland
Printed and bound in Great Britain by Biddles Ltd, Guildford, U.K.
This book is printed on acid-free paper responsibly manufactured from sustainable forestry, in which at least two trees are
planted for each one used for paper production.](https://image.slidesharecdn.com/2152637-250604032623-93567e51/85/Gsm-Switching-Services-And-Protocols-Second-Edition-Jorg-Eberspacher-7-320.jpg)
![ments and airports, as supplement or alternative to wired LANs, and they are also consid-
ered to be a good supplement to UMTS access technologies. The efforts to ``mobilize'' the
Internet are also worth mentioning in this context. A new routing protocol called Mobile IP
[48,49] has been developed, which allows a mobile computer to change its point of
attachment to the Internet. A further strong innovation impulse for mobile data and multi-
media communication is the development of wireless Mobile ATM systems based on the
exchange technology Asynchronous Transfer Mode (ATM).
Another emerging class of wireless networks is used for short-range communication.
Bluetooth, for example, replaces cables by enabling direct wireless information exchange
between electronic devices (e.g. between cellular phones, Personal Digital Assistants
(PDAs), computers, and peripherals). These networks are also called Body Area Networks
or Personal Area Networks. Unlike the mobile technologies mentioned above, they are not
based on a ®xed network infrastructure (e.g. base stations). The possibility of building up
1.2 Classi®cation of Mobile Communication Systems 3
Figure 1.1: Overview of contemporary and future mobile communication systems](https://image.slidesharecdn.com/2152637-250604032623-93567e51/85/Gsm-Switching-Services-And-Protocols-Second-Edition-Jorg-Eberspacher-31-320.jpg)
![1.3 Some GSM History and Statistics
In 1982 the development of a pan-European standard for digital cellular mobile radio was
started by the Groupe Spe
Âcial Mobile of the CEPT (Confe
Ârence Europe
Âenne des Admin-
istrations des Postes et des Te
Âle
Âcommunications). Initially, the acronym GSM was derived
from the name of this group. After the founding of the European standardization institute
ETSI (European Telecommunication Standards Institute), the GSM group became a Tech-
nical Committee of ETSI in 1989. After the rapid worldwide proliferation of GSM
networks, the name has been reinterpreted as Global System for Mobile Communication.
After a series of incompatible analog networks had been introduced in parallel in Europe,
e.g. Total Access Communication System (TACS) in the UK, NMT in Scandinavia, and the
C-Netz in Germany, work on the de®nition of a Europe-wide standard for digital mobile
radio was started in the late 1980s. The GSM was founded, which developed a set of
technical recommendations and presented them to ETSI for approval. These proposals
were produced by the Special Mobile Group (SMG) in working groups called Sub Tech-
nical Committees (STCs), with the following division of tasks: service aspects (SMG 01),
radio aspects (SMG 02), network aspects (SMG 03), data services (SMG 04), and network
operation and maintenance (SMG 06). Further working groups were mobile station testing
(SMG 07), IC card aspects (SGM 09), security (SGM 10), speech aspects (SMG 11), and
system architecture (SMG 12) [18]. SGM 05 dealt with future networks and was respon-
sible for the initial standardization phase of the next generation of the European mobile
radio system, the UMTS. Later, SMG 05 was closed, and UMTS became an independent
project and Technical Body of ETSI. In the meantime, the Third Generation Partnership
Project (3GPP) has been founded in cooperation with other standardization committees
worldwide. Its goal is the composition of the Technical Speci®cations for UMTS. Finally,
in July 2000, ETSI announced the closure of the SMG which has been responsible for
setting GSM standards for the last 18 years. Their remaining and further work has been
transferred to groups inside and outside ETSI; most of the ongoing work has been handed
over to the 3GPP.
After the of®cial start of the GSM networks during the summer of 1992 (Table 1.1), the
number of subscribers has increased rapidly, such that during the fall of 1993 already far
more than one million subscribers made calls in GSM networks, more than 80% of them in
Germany. On a global scale, the GSM standard also received very fast recognition, as
evident from the fact that at the end of 1993 several commercial GSM networks started
operation outside Europe, in Australia, Hong Kong, and New Zealand. Afterward, GSM
has also been introduced in Brunei, Cameroon, Iran, South Africa, Syria, Thailand, USA
and United Arab Emirates. Whereas the majority of the GSM networks operate in the 900
MHz band (GSM900), there are also networks operating in the 1800 MHz band
(GSM1800) ± Personal Communication Network (PCN), Digital Communication System
(DCS1800) ± and in the United States in the 1900 MHz band (GSM1900) ± Personal
Communication System (PCS). These networks use almost completely identical technol-
ogy and architecture; they differ essentially only in the radio frequencies used and the
pertinent high-frequency technology, such that synergy effects can be taken advantage of,
and the mobile exchanges can be constructed with standard components.
In parallel to the standardization efforts of ETSI, already in 1987 the then existing prospec-
1.3 Some GSM History and Statistics 5](https://image.slidesharecdn.com/2152637-250604032623-93567e51/85/Gsm-Switching-Services-And-Protocols-Second-Edition-Jorg-Eberspacher-33-320.jpg)
![The Mobile Radio Channel
and the Cellular Principle
Many measures, functions and protocols in digital mobile radio networks are based on the
properties of the radio channel and its speci®c qualities in contrast to information trans-
mission through guided media. For the understanding of digital mobile radio networks it is
therefore absolutely necessary to know a few related basic principles. For this reason, the
most important fundamentals of the radio channel and of cellular and transmission tech-
nology will be presented and brie¯y explained in the following. For a more detailed
treatment, see the extensive literature [4,42,50,64].
2.1 Characteristics of the Mobile Radio Channel
The electromagnetic wave of the radio signal propagates under ideal conditions in free
space in a radial-symmetric pattern, i.e. the received power PEf, decreases with the square
of the distance L from the transmitter:
PEf ,
1
L2
These idealized conditions do not apply in terrestrial mobile radio. The signal is scattered
and re¯ected, for example, at natural obstacles like mountains, vegetation, or water
surfaces. The direct and re¯ected signal components are then superimposed at the receiver.
This multipath propagation can already be explained quite well with a simple two-path
model (Figure 2.1). With this model, one can show that the received power decreases much
2
Figure 2.1: Simpli®ed two-path model of radio propagation
GSM Switching, Services and Protocols: Second Edition. Jo
È rg Eberspa
È cher,
Hans-Jo
È rg Vo
È gel and Christian Bettstetter
Copyright q 2001 John Wiley & Sons Ltd
Print ISBN 0-471-49903-X Online ISBN 0-470-84174-5](https://image.slidesharecdn.com/2152637-250604032623-93567e51/85/Gsm-Switching-Services-And-Protocols-Second-Edition-Jorg-Eberspacher-37-320.jpg)
![more than with the square of the distance from the transmitter. We can approximate the
received power by considering the direct path and only one re¯ected path (two-path
propagation) [42]:
PE ˆ P0
4
4pL=l†2
2ph1h2
lL
2
ˆ P0
h1h2
L2
2
and we obtain, under the simpli®ed assumptions of the two-path propagation model, from
Figure 2.1, a propagation loss of 40 dB per decade:
aE ˆ
PE2
PE1
ˆ
L1
L2
4
; aE ˆ 40 log
L1
L2
in dB
In reality, the propagation loss depends on the propagation coef®cient g, which is deter-
mined by environmental conditions:
PE , L2g
; 2 # g # 5
In addition, propagation losses are also frequency dependent, i.e. in a simpli®ed way,
propagation attenuation increases disproportionately with the frequency.
However, multipath propagation not only incurs a disproportionately high path propaga-
tion loss. The different signal components reaching the receiver have traveled different
distances by virtue of dispersion, infraction, and multiple re¯ections, hence they show
different phase shifts. On the one hand, there is the advantage of multipath propagation,
that a partial signal can be received even if there is no direct path, i.e. there is no line of
sight between mobile and base station. On the other hand, there is a serious disadvantage:
the superpositions of the individual signal components having different phase shifts with
regard to the direct path can lead, in the worst cases, to cancellations, i.e. the received
signal level shows severe disruptions. This phenomenon is called fading. In contrast to this
fast fading caused by multipath propagation, there is slow fading caused by shadowing.
Along the way traveled by a mobile station, multipath fading can cause signi®cant varia-
tions of the received signal level (Figure 2.2). Periodically occurring signal breaks at a
distance of about half a wavelength are typically 30±40 dB. The smaller the transmission
bandwidth of the mobile radio system, the stronger the signal breaks ± at a bandwidth of
about 200 kHz per channel this effect is still very visible [8].
Furthermore, the fading dips become ¯atter as one of the multipath components becomes
stronger and more pronounced. Such a dominant signal component arises, for example, in
the case of a direct line of sight between mobile and base station, but it can also occur
under other conditions. If such a dominant signal component exists, we talk of a Rice
channel and Ricean fading, respectively. (S. O. Rice was an American scientist and
mathematician.) Otherwise, if all multipath components suffer from approximately
equal propagation conditions, we talk of Rayleigh fading. (J. W. Strutt, 3rd Baron
Rayleigh, was a British physicist, Nobel prize winner.)
During certain time periods or time slots, the transmission can be heavily impacted
because of fading or can be entirely impossible, whereas other time slots may be undis-
turbed. The results of this effect within the user data are alternating phases, which show
either a high or low bit error rate, which is leading to error bursts. The channel thus has
2 The Mobile Radio Channel and the Cellular Principle
10](https://image.slidesharecdn.com/2152637-250604032623-93567e51/85/Gsm-Switching-Services-And-Protocols-Second-Edition-Jorg-Eberspacher-38-320.jpg)
![memory in contrast to the statistically independent bit errors in memoryless symmetric
binary channels.
The signal level observed at a speci®c location is also determined by the phase shift of the
multipath signal components. This phase shift depends on the wavelength of the signal,
and thus the signal level at a ®xed location is also dependent on the transmission
frequency. Therefore the fading phenomena in radio communication are also frequency
speci®c. If the bandwidth of the mobile radio channel is small (narrowband signal), then
the whole frequency band of this channel is subject to the same propagation conditions,
and the mobile radio channel is considered frequency-nonselective. Depending on location
(Figure 2.2) and the spectral range (Figure 2.3), the received signal level of the channel,
however, can vary considerably. On the other hand, if the bandwidth of a channel is large
(broadband signal), the individual frequencies suffer from different degrees of fading
(Figure 2.3) and this is called a frequency-selective channel [15,54]. Signal breaks because
of frequency-selective fading along a signal path are much less frequent for a broadband
signal than for a narrowband signal, because the fading holes only shift within the band and
the received total signal energy remains relatively constant [8].
Besides frequency-selective fading, the different propagation times of the individual multi-
path components also cause time dispersion on their propagation paths. Therefore, signal
distortions can occur due to interference of one symbol with its neighboring symbols
(``intersymbol interference''). These distortions depend ®rst on the spread experienced
by a pulse on the mobile channel, and second on the duration of the symbol or of the
interval between symbols. Typical multipath channel delays have a range from half a
microsecond in urban areas to about 16±20 ms in mountainous terrain, i.e. a transmitted
pulse generates several echoes which reach the receiver with delays of up to 20 ms. In
digital mobile radio systems with typical symbol durations of a few microseconds, this can
lead to smearing of individual pulses over several symbol durations.
In contrast to wireline transmission, the mobile radio channel is a very bad transmission
medium of highly variable quality. This can go so far that the channel cuts out for short
periods (deep fading holes) or that single sections in the data stream are so much interfered
2.1 Characteristics of the Mobile Radio Channel 11
Figure 2.2: Typical signal in a channel with Rayleigh fading](https://image.slidesharecdn.com/2152637-250604032623-93567e51/85/Gsm-Switching-Services-And-Protocols-Second-Edition-Jorg-Eberspacher-39-320.jpg)
![with (bit error rate typically 1022
or 1021
), that unprotected transmission without further
protection or correction measures is hardly possible. Therefore, mobile information trans-
port requires additional, often very extensive measures, which compensate for the effects
of multipath propagation. First, an equalizer is necessary, which attempts to eliminate the
signal distortions caused by intersymbol interference. The operational principle of such an
equalizer for mobile radio is based on the estimation of the channel pulse response to
periodically transmitted, well-known bit patterns, known as the training sequences [4,64].
This allows the determination of the time dispersion of the channel and its compensation.
The performance of the equalizer has a signi®cant effect on the quality of the digital
transmission. On the other hand, for ef®cient transmission in digital mobile radio, channel
coding measures are indispensable, such as forward error correction with error-correcting
codes, which allows reduction of the effective bit error rate to a tolerable value (about 1025
to 1026
). Further important measures are control of the transmitter power and algorithms
for the compensation of signal interruptions in fading, which may be of such a short
duration that a disconnection of the call would not be appropriate.
2.2 Separation of Directions and Duplex Transmission
The most frequent form of communication is the bidirectional communication which
allows simultaneous transmitting and receiving. A system capable of doing this is called
full-duplex. One can also achieve full-duplex capability, if sending and receiving do not
occur simultaneously but switching between both phases is done so fast that it is not
noticed by the user, i.e. both directions can be used quasi-simultaneously. Modern digital
mobile radio systems are always full-duplex capable.
Essentially, two basic duplex procedures are employed: Frequency Division Duplex
(FDD) using different frequency bands in each direction, and Time Division Duplex
(TDD) which periodically switches the direction of transmission.
2 The Mobile Radio Channel and the Cellular Principle
12
Figure 2.3: Frequency selectivity of a mobile radio channel](https://image.slidesharecdn.com/2152637-250604032623-93567e51/85/Gsm-Switching-Services-And-Protocols-Second-Edition-Jorg-Eberspacher-40-320.jpg)
![2.2.1 Frequency Division Duplex (FDD)
The frequency duplex procedure has been used already in analog mobile radio systems and
is also used in digital systems. For the communication between mobile and base station,
the available frequency band is split into two partial bands, to enable simultaneous sending
and receiving. One partial band is assigned as uplink (from mobile to base station) and the
other partial band is assigned as downlink (from base to mobile station):
² Uplink: transmission band of mobile station ˆ receiving band of base station
² Downlink: receiving band of mobile station ˆ transmission band of base station
To achieve good separation between both directions, the partial bands must be a suf®cient
frequency distance apart, i.e. the frequency pairs of a connection assigned to uplink and
downlink must have this distance band between them. Usually, the same antenna is used
for sending and receiving. A duplexing unit is then used for the directional separation,
consisting essentially of two narrowband ®lters with steep ¯anks (Figure 2.4). These ®lters,
however, cannot be integrated, so pure frequency duplexing is not appropriate for systems
with small compact equipment [15].
2.2.2 Time Division Duplex (TDD)
Time duplexing is therefore a good alternative, especially in digital systems with time
division multiple access. Transmitter and receiver operate in this case only quasi-simulta-
neously at different points in time; i.e. the directional separation is achieved by switching
in time between transmission and reception, and thus no duplexing unit is required.
Switching occurs frequently enough that the communication appears to be over a quasi-
simultaneous full-duplex connection. However, out of the periodic interval T available for
the transmission of a time slot only a small part can be used, so that a time duplex system
requires more than twice the bit rate of a frequency duplex system.
2.2 Separation of Directions and Duplex Transmission 13
Figure 2.4: Frequency and time duplex (schematic)](https://image.slidesharecdn.com/2152637-250604032623-93567e51/85/Gsm-Switching-Services-And-Protocols-Second-Edition-Jorg-Eberspacher-41-320.jpg)
![2.3 Multiple Access Procedures
The radio channel is a communication medium shared by many subscribers in one cell.
Mobile stations compete with one another for the frequency resource to transmit their
information streams. Without any other measures to control simultaneous access of several
users, collisions can occur (multiple access problem). Since collisions are very undesirable
for a connection-oriented communication like mobile telephony, the individual subscri-
bers/mobile stations must be assigned dedicated channels on demand. In order to divide the
available physical resources of a mobile system, i.e. the frequency bands, into voice
channels, special multiple access procedures are used which are presented in the following
(Figure 2.5).
2.3.1 Frequency Division Multiple Access (FDMA)
Frequency Division Multiple Access (FDMA) is one of the most common multiple access
procedures. The frequency band is divided into channels of equal bandwidth such that each
conversation is carried on a different frequency (Figure 2.6). Best suited to analog mobile
radio, FDMA systems include the C-Netz in Germany, TACS in the UK, and AMPS in the
USA. In the C-Netz, two frequency bands of 4.44 MHz each are subdivided into 222
individual communication channels at 20 kHz bandwidth. The effort in the base station to
realize a frequency division multiple access system is very high. Even though the required
hardware components are relatively simple, each channel needs its own transceiving unit.
Furthermore, the tolerance requirements for the high-frequency networks and the linearity
of the ampli®ers in the transmitter stages of the base station are quite high, since a large
number of channels need to be ampli®ed and transmitted together [15,54]. One also needs
a duplexing unit with ®lters for the transmitter and receiver units to enable full-duplex
operation, which makes it nearly impossible to build small, compact mobile stations, since
the required narrowband ®lters can hardly be realized with integrated circuits.
2 The Mobile Radio Channel and the Cellular Principle
14
Figure 2.5: Multiple access procedures](https://image.slidesharecdn.com/2152637-250604032623-93567e51/85/Gsm-Switching-Services-And-Protocols-Second-Edition-Jorg-Eberspacher-42-320.jpg)
![2.3.3 Code Division Multiple Access (CDMA)
Systems with Code Division Multiple Access (CDMA) are broadband systems, in which
each subscriber uses the whole system bandwidth (similar to TDMA) for the complete
duration of the connection (similar to FDMA). Furthermore, usage is not exclusive, i.e. all
the subscribers in a cell use the same frequency band simultaneously. To separate the
signals, the subscribers are assigned orthogonal codes. The basis of CDMA is a band-
spreading or spread spectrum technique. The signal of one subscriber is spread spectrally
over a multiple of its original bandwidth. Typically, spreading factors are between 10 and
1000; they generate a broadband signal for transmission from the narrowband signal, and
this is less sensitive to frequency-selective interference and disturbances. Furthermore, the
spectral power density is decreased by band spreading, and communication is even possi-
ble below the noise threshold [15].
2.3.3.1. Direct Sequence CDMA
A common spread-spectrum procedure is the direct sequence technique (Figure 2.10). In it
the data sequence is multiplied directly ± before modulation ± with a spreading sequence to
generate the band-spread signal. The bit rate of the spreading signal, the so-called chip
rate, is obtained by multiplying the bit rate of the data signal by the spreading factor, which
generates the desired broadening of the signal spectrum. Ideally, the spreading sequences
are completely orthogonal bit sequences (``codes'') with disappearing cross-correlation
functions. Since such completely orthogonal sequences cannot be realized, practical
systems use bit sequences from pseudo noise (PN) generators to spread the band
[15,54]. For despreading, the signal is again multiplied with the spreading sequence at
the receiver, which ideally recovers the data sequence in its original form.
2 The Mobile Radio Channel and the Cellular Principle
18
Figure 2.10: Principle of spread spectrum technique for direct sequence CDMA](https://image.slidesharecdn.com/2152637-250604032623-93567e51/85/Gsm-Switching-Services-And-Protocols-Second-Edition-Jorg-Eberspacher-46-320.jpg)
![this array antenna, then the received signals at the antenna elements differ mainly in their
phase ± each shifted by Dw (Figure 2.12) ± and amplitude.
In this way, the response of the antenna to a signal incident at angle ui can be characterized
by the complex response vector ~
a ui† which de®nes amplitude gain and phase of each
antenna element relative to the ®rst antenna element (a1 ˆ 1):
~
a ui† ˆ
a1 ui†
a2 ui†
¼
aM ui†
2
6
6
6
6
6
6
4
3
7
7
7
7
7
7
5
ˆ
1
a2 ui†
¼
aM ui†
2
6
6
6
6
6
6
4
3
7
7
7
7
7
7
5
The Nm multipath components (Nm ˆ 3 in Figure 2.12) of a signal s1(t) generate, depend-
ing on the incidence angle ui, a received signal vector ~
x1 t† which can be written with the
respective antenna response vector and the signal of the ith multipath bis1(t) shifted in
amplitude and phase against the direct path s1(t) as
~
x1 t† ˆ ~
a u1†s1 t† 1
X
Nm
iˆ2
~
a ui†bis1 t† ˆ ~
a1s1 t†
In this case, the vector ~
a1 is also designated the spatial signature of the signal s1(t), which
remains constant as long as the source of the signal does not move and the propagation
conditions do not change [65]. In a multi-access situation, there are typically several
sources (Nq); this yields the following result for the total signal at the array antenna:
neglecting noise and interferences,
~
x t† ˆ
X
Nq
jˆ1
~
ajsj t†
From this summation signal, the signals of the individual sources are separated by weight-
ing the received signals of the individual antenna elements with a complex factor (weight
vector ~
wi), which yields
~
wH
i ~
aj ˆ
0; i ± j
1; i ˆ j
(
For the weighted summation signal [65] one gets
~
wH
i ~
x t† ˆ
X
Nq
jˆ1
~
wH
i ~
ajsj t† ˆ si t†
Under ideal conditions, i.e. neglecting noise and interference, the signal si(t) of a single
source i can be separated from the summation signal of the array antenna by using an
appropriate weight vector during signal processing. The determination of the respectively
optimal weight vector, however, is a nontrivial and computation-intensive task. Because of
the considerable processing effort and also because of the mechanical dimensions of the
antenna ®eld, array antennas are predominantly used in base stations.
2.3 Multiple Access Procedures 21](https://image.slidesharecdn.com/2152637-250604032623-93567e51/85/Gsm-Switching-Services-And-Protocols-Second-Edition-Jorg-Eberspacher-49-320.jpg)
![So far only the receiving direction has been considered. The corresponding principles,
however, can also be used for constructing the directional characteristics of the transmitter.
Assume symmetric propagation conditions in the sending and receiving directions, and
assume the transmitted signals si(t) are weighted with the same weight vector ~
wi as the
received signal, before they are transmitted through the array antenna; then one obtains the
following summation signal radiated by the array antenna:
~
y t† ˆ
X
Nq
jˆ1
~
wjsj t†
and for the signal received on the ith opposite side, respectively:
^
si t† ˆ ~
aH
i ~
y t† ˆ
X
Nq
jˆ1
~
aH
i ~
wjsj t† ˆ si t†
Thus, by using array antennas, one can separate the simultaneously received signals of
spatially separated subscribers by exploiting the directional selectivity of the mobile radio
channel. Because of the use of intelligent signal processing and corresponding control
algorithms, such systems are also known as systems with intelligent antennas.
The directional characteristics of the array antenna can be controlled adaptively such that a
signal is only received or transmitted in exactly the spatial segment where a certain mobile
station is currently staying. On the one hand, one can thus reduce co-channel interference
in other cells, and on the other hand, the sensitivity against interference can be reduced in
the current cell. Furthermore, because of the spatial separation, physical channels in a cell
can be reused, and the lobes of the antenna diagram can adaptively follow the movement of
mobile stations. In this case, yet another multiple access technique (Figure 2.13) is de®ned
and known as Space Division Multiple Access (SDMA).
SDMA systems are currently the subject of intensive research. The SDMA technique can
be combined with each of the other multiple access techniques (FDMA, TDMA, CDMA).
This enables intracellular spatial channel reuse, which again increases the network capa-
city [29]. This is especially attractive for existing networks which can use an intelligent
implementation of SDMA by selectively upgrading base stations with array antennas,
appropriate signal processing, and respective control protocols.
2 The Mobile Radio Channel and the Cellular Principle
22
Figure 2.13: Schematic representation of spatial multiple access (uplink)](https://image.slidesharecdn.com/2152637-250604032623-93567e51/85/Gsm-Switching-Services-And-Protocols-Second-Edition-Jorg-Eberspacher-50-320.jpg)
![This ratio of the useful signal to the interfering signal is usually measured in decibels (dB)
and called the Signal-to-Noise Ratio (SNR). The intensity of the interference is essentially
a function of co-channel interference depending on the frequency reuse distance D. From
the viewpoint of a mobile station, the co-channel interference is caused by base stations at
distance D from the current base station. A worst-case estimate for the signal-to-noise ratio
W of a mobile station at the border of the covered area at distance R from the base station
can be obtained, subject to propagation losses, by assuming that all six neighboring inter-
fering transmitters operate at the same power and are approximately equally far apart
(distance D large against cell radius R) [42]:
W ˆ
P0R2g
X
6
iˆ1
Pi 1 N
P0R2g
X
6
iˆ1
P0D2g
1 N
ˆ
P0R2g
6P0D2g 1 N
By neglecting the noise N we obtain the following approximation for the Carrier-to-
Interference Ratio C/I (CIR):
W
C
I
ˆ
R2g
6D2g ˆ
1
6
R
D
2g
Therefore the signal-to-noise ratio depends essentially on the ratio of the cell radius R to
the frequency reuse distance D. From these considerations it follows that for a desired or
needed signal-to-noise ratio W at a given cell radius, one must choose a minimum distance
for the frequency reuse, above which the co-channel interference fall below the required
threshold.
2.4.3 Formation of Clusters
The regular repetition of frequencies results in a clustering of cells. The clusters generated
2 The Mobile Radio Channel and the Cellular Principle
24
Figure 2.14: Model of a cellular network with frequency reuse](https://image.slidesharecdn.com/2152637-250604032623-93567e51/85/Gsm-Switching-Services-And-Protocols-Second-Edition-Jorg-Eberspacher-52-320.jpg)
![in this way can comprise the whole frequency band. In this case all of the frequencies in the
available spectrum are used within a cluster. The size of a cluster is characterized by the
number of cells per cluster k, which determines the frequency reuse distance D. Figure 2.15
shows some examples of clusters. The numbers designate the respective frequency sets fbi
used within the single cells.
For each cluster the following holds:
² A cluster can contain all the frequencies of the mobile radio system.
² Within a cluster, no frequency can be reused. The frequencies of a set fbi may be reused
at the earliest in the neighboring cluster.
² The larger a cluster, the larger the frequency reuse distance and the larger the signal-to-
noise ratio. However, the larger the values of k, the smaller the number of channels and
the number of active subscribers per cell.
The frequency reuse distance D can be derived geometrically from the hexagon model
depending on k and the cell radius R:
D ˆ R
3k
p
The signal-to-noise ratio W [42] is then
W ˆ
R2g
6D2g ˆ
R2g
6 R
3k
p
2g ˆ
1
6
3k†g=2
According to measurements one can assume that, for good speech understandability, a
carrier-to-interference ratio (CIR) of about 18 dB is suf®cient. Assuming an approximate
propagation coef®cient of g ˆ 4, this yields the minimum cluster size
10 logW $ 18 dB; W $ 63:1 ) D 4:4R
2.4 Cellular Technology 25
Figure 2.15: Frequency reuse and cluster formation](https://image.slidesharecdn.com/2152637-250604032623-93567e51/85/Gsm-Switching-Services-And-Protocols-Second-Edition-Jorg-Eberspacher-53-320.jpg)
![1
6
3k†g=2
ˆ W $ 63:1 ) k $ 6:5 ) k ˆ 7
These values are also con®rmed by computer simulations, which have shown that for W ˆ
18 dB a reuse distance D ˆ 4:6R is needed [42]. In practically implemented networks, one
can ®nd other cluster sizes, e.g. k ˆ 3 and k ˆ 12. A CIR of 15 dB is considered a
conservative value for network engineering.
The cellular models mentioned so far are very idealized for illustration and analysis. In
reality, cells are neither circular nor hexagonal; rather they possess very irregular forms
and sizes because of variable propagation conditions. An example of a possible cellular
plan for a real network is shown in Figure 2.16, where one can easily recognize the
individual cells with the assigned channels and the frequency reuse. Especially obvious
are the different cell sizes, which depend on whether it is an urban, suburban, or rural area.
Figure 2.16 gives an impression of the approximate contours of equal signal power around
the individual base stations. In spite of this representation, the precise ®tting of signal
power contours remains an idealization. The cell boundaries are after all blurred and
de®ned by local thresholds, beyond which the neighboring base station's signal is received
stronger than the current one.
2 The Mobile Radio Channel and the Cellular Principle
26
Figure 2.16: Cell structure of a real network](https://image.slidesharecdn.com/2152637-250604032623-93567e51/85/Gsm-Switching-Services-And-Protocols-Second-Edition-Jorg-Eberspacher-54-320.jpg)
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- 5. GSM Switching, Services and Protocols Second Edition GSM Switching, Services and Protocols: Second Edition. Jo È rg Eberspa È cher, Hans-Jo È rg Vo È gel and Christian Bettstetter Copyright q 2001 John Wiley & Sons Ltd Print ISBN 0-471-49903-X Online ISBN 0-470-84174-5
- 6. GSM Switching, Services and Protocols Second Edition Jo Èrg Eberspa Ècher Technische Universita È t Mu È nchen, Germany Hans-Jo Èrg Vo Ègel The Fantastic Corporation, Switzerland and Christian Bettstetter Technische Universita È t Mu È nchen, Germany JOHN WILEY & SONS, LTD Chichester . New York . Weinheim . Brisbane . Singapore . Toronto
- 7. Originally published in the German language by B. G. Teubner GmbH as ``Jo Èrg Eberspa Ècher/Hans-Jo Èrg Vo Ègel/Christian Bettstetter: GSM Global System for Mobile Communication. 3. Au¯age (3rd edition)''. q B. G. Teubner Stuttgart/Leipzig/Wiesbaden, 2001 Copyright q 2001 by John Wiley & Sons, Ltd Baf®ns Lane, Chichester, West Sussex, PO19 1UD, England National 01243 779777 International (+44) 1243 779777 e-mail (for orders and customer service enquiries): cs-books@wiley.co.uk Visit our Home Page on http://www.wiley.co.uk or http://www.wiley.com All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except under the terms of the Copyright Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency, 90 Tottenham Court Road, London, W1P 9HE, UK, without the permission in writing of the Publisher, with the exception of any material supplied speci®cally for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the publication. Neither the author(s) nor John Wiley & Sons Ltd accept any responsibility or liability for loss or damage occasioned to any person or property through using the material, instructions, methods or ideas contained herein, or acting or refraining from acting as a result of such use. The author(s) and Publisher expressly disclaim all implied warranties, including merchantability of ®tness for any particular purpose. Designations used by companies to distinguish their products are often claimed as trademarks. In all instances where John Wiley & Sons is aware of a claim, the product names appear in initial capital or capital letters. Readers, however, should contact the appropriate companies for more complete information regarding trademarks and registration. Other Wiley Editorial Of®ces John Wiley & Sons, Inc., 605 Third Avenue, New York, NY 10158-0012, USA WILEY-VCH Verlag GmbH Pappelallee 3, D-69469 Weinheim, Germany John Wiley & Sons Australia, Ltd, 33 Park Road, Milton, Queensland 4064, Australia John Wiley & Sons (Canada) Ltd, 22 Worcester Road Rexdale, Ontario, M9W 1L1, Canada John Wiley & Sons (Asia) Pte Ltd, 2 Clementi Loop #02-01, Jin Xing Distripark, Singapore 129809 Library of Congress Cataloging-in-Publication Data Eberspa Ècher, I. (Jo Èrg) [GSM, Global System for Mobile Communication. English] GSM switching, services, and protocols / Jo Èrg Eberspa Ècher, Hans-Jo Èrg Vo Ègel, Christian Bettstetter.± 2nd ed. p. cm. Includes bibliographical references and index. Prey. ed.: GSM switching, services, and protocol. 1999. ISBN 0-471-49903-X (alk. paper) 1. Global system for mobile communications. I. Vo Ègel, Hans-Jo Èrg. II. Bettstetter, Christian. III Title. TK5103.483 .E2413 1999 621.3820 2±dc2l 00-054550 Use the Internet and eliminate mail time and postage costs http://cip.loc.gov/cip British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0471 49903 X Typeset by Deerpark Publishing Services Ltd, Shannon, Ireland Printed and bound in Great Britain by Biddles Ltd, Guildford, U.K. This book is printed on acid-free paper responsibly manufactured from sustainable forestry, in which at least two trees are planted for each one used for paper production.
- 8. Contents Preface for Second Edition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii 1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Digital, Mobile, Global: Evolution of Networks . . . . . . . . . . . . . . . . . . . . . 1 1.2 Classi®cation of Mobile Communication Systems. . . . . . . . . . . . . . . . . . . . 2 1.3 Some GSM History and Statistics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.4 Overview of the Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2 The Mobile Radio Channel and the Cellular Principle. . . . . . . . . . . . . . . . . . . . . . 9 2.1 Characteristics of the Mobile Radio Channel. . . . . . . . . . . . . . . . . . . . . . . 9 2.2 Separation of Directions and Duplex Transmission . . . . . . . . . . . . . . . . . . . 12 2.2.1 Frequency Division Duplex (FDD). . . . . . . . . . . . . . . . . . . . . . . . 13 2.2.2 Time Division Duplex (TDD). . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.3 Multiple Access Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.3.1 Frequency Division Multiple Access (FDMA) . . . . . . . . . . . . . . . . . 14 2.3.2 Time Division Multiple Access (TDMA) . . . . . . . . . . . . . . . . . . . . 15 2.3.3 Code Division Multiple Access (CDMA) . . . . . . . . . . . . . . . . . . . . 18 2.3.3.1 Direct Sequence CDMA . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.3.3.2 Frequency Hopping CDMA . . . . . . . . . . . . . . . . . . . . . . . 19 2.3.4 Space Division Multiple Access (SDMA). . . . . . . . . . . . . . . . . . . . 20 2.4 Cellular Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.4.1 Fundamental De®nitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.4.2 Signal-to-Noise Ratio. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.4.3 Formation of Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.4.4 Traf®c Capacity and Traf®c Engineering . . . . . . . . . . . . . . . . . . . . 27 3 System Architecture and Addressing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 3.1 General Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 3.2 Addresses and Identi®ers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3.2.1 International Mobile Station Equipment Identity (IMEI) . . . . . . . . . . . 31 3.2.2 International Mobile Subscriber Identity (IMSI) . . . . . . . . . . . . . . . . 32 3.2.3 Mobile Subscriber ISDN Number (MSISDN) . . . . . . . . . . . . . . . . . 32 3.2.4 Mobile Station Roaming Number (MSRN) . . . . . . . . . . . . . . . . . . . 33 3.2.5 Location Area Identity (LAI) . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 3.2.6 Temporary Mobile Subscriber Identity (TMSI) . . . . . . . . . . . . . . . . 34 3.2.7 Local Mobile Subscriber Identity (LMSI). . . . . . . . . . . . . . . . . . . . 34 3.2.8 Cell Identi®er (CI) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 3.2.9 Base Transceiver Station Identity Code (BSIC) . . . . . . . . . . . . . . . . 35 3.2.10 Identi®cation of MSCs and Location Registers . . . . . . . . . . . . . . . . 35 GSM Switching, Services and Protocols: Second Edition. Jo È rg Eberspa È cher, Hans-Jo È rg Vo È gel and Christian Bettstetter Copyright q 2001 John Wiley & Sons Ltd Print ISBN 0-471-49903-X Online ISBN 0-470-84174-5
- 9. Contents vi 3.3 System Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 3.3.1 Mobile Station (MS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 3.3.2 Radio Network ± Base Station Subsystem (BSS) . . . . . . . . . . . . . . . 36 3.3.3 Mobile Switching Network (MSS) . . . . . . . . . . . . . . . . . . . . . . . . 37 3.3.3.1 Mobile Switching Center (MSC) . . . . . . . . . . . . . . . . . . . . 37 3.3.3.2 Home and Visitor Registers (HLR and VLR) . . . . . . . . . . . . 38 3.3.4 Operation and Maintenance (OMSS) . . . . . . . . . . . . . . . . . . . . . . 39 3.3.4.1 Network Monitoring and Maintenance . . . . . . . . . . . . . . . . 39 3.3.4.2 User Authentication and Equipment Registration . . . . . . . . . . 40 3.4 Subscriber Data in GSM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 3.5 PLMN Con®gurations and Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 3.5.1 Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.5.2 Con®gurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 4 Services. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 4.1 Bearer Services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 4.2 Teleservices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 4.2.1 Voice 50 4.2.2 Fax Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 4.2.3 Short Message Service (SMS) . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4.3 Supplementary Services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4.3.1 Supplementary Services of Phase 1 . . . . . . . . . . . . . . . . . . . . . . . 53 4.3.2 Supplementary Services of Phase 2 . . . . . . . . . . . . . . . . . . . . . . . 53 4.4 GSM Services of Phase 2+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 5 Air Interface ± Physical Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 5.1 Logical Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 5.1.1 Traf®c Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 5.1.2 Signaling Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 5.1.3 Example: Connection Setup for Incoming Call . . . . . . . . . . . . . . . . 61 5.1.4 Bit Rates, Block Lengths, and Block Distances . . . . . . . . . . . . . . . . 61 5.1.5 Combinations of Logical Channels. . . . . . . . . . . . . . . . . . . . . . . . 62 5.2 Physical Channels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 5.2.1 Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 5.2.2 Multiple Access, Duplexing, and Bursts. . . . . . . . . . . . . . . . . . . . . 65 5.2.3 Optional Frequency Hopping . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 5.2.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 5.3 Synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 5.3.1 Frequency and Clock Synchronization. . . . . . . . . . . . . . . . . . . . . . 70 5.3.2 Adaptive Frame Synchronization 74 5.4 Mapping of Logical Channels onto Physical Channels 75 5.4.1 26-Frame Multiframe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 5.4.2 51-Frame Multiframe. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 5.5 Radio Subsystem Link Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 5.5.1 Channel Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 5.5.1.1 Channel Measurement during Idle Mode . . . . . . . . . . . . . . . 83 5.5.1.2 Channel Measurement during a Connection . . . . . . . . . . . . . 84 5.5.2 Transmission Power Control . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 5.5.3 Disconnection due to Radio Channel Failure. . . . . . . . . . . . . . . . . . 88 5.5.4 Cell Selection and Operation in Power Conservation Mode . . . . . . . . . 90 5.5.4.1 Cell Selection and Cell Reselection . . . . . . . . . . . . . . . . . . 90 5.5.4.2 Discontinuous Reception. . . . . . . . . . . . . . . . . . . . . . . . . 91 5.6 Power-up Scenario . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
- 10. Contents vii 6 Coding, Authentication, and Ciphering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 6.1 Source Coding and Speech Processing . . . . . . . . . . . . . . . . . . . . . . . . . . 96 6.2 Channel Coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 6.2.1 External Error Protection: Block Coding . . . . . . . . . . . . . . . . . . . . 103 6.2.1.1 Block Coding for Speech Traf®c Channels . . . . . . . . . . . . . . 104 6.2.1.2 Block Coding for Data Traf®c Channels . . . . . . . . . . . . . . . 105 6.2.1.3 Block Coding for Signaling Channels . . . . . . . . . . . . . . . . . 106 6.2.2 Internal Error Protection: Convolutional Coding. . . . . . . . . . . . . . . . 107 6.2.3 Interleaving. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 6.2.4 Mapping onto the Burst Plane. . . . . . . . . . . . . . . . . . . . . . . . . . . 117 6.3 Security-Related Network Functions and Encryption . . . . . . . . . . . . . . . . . . 118 6.3.1 Protection of Subscriber Identity . . . . . . . . . . . . . . . . . . . . . . . . . 119 6.3.2 Veri®cation of Subscriber Identity . . . . . . . . . . . . . . . . . . . . . . . . 120 6.3.3 Generating Security Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 6.3.4 Encryption of Signaling and Payload Data . . . . . . . . . . . . . . . . . . . 122 7 Protocol Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 7.1 Protocol Architecture Planes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 7.2 Protocol Architecture of the User Plane. . . . . . . . . . . . . . . . . . . . . . . . . . 127 7.2.1 Speech Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 7.2.2 Transparent Data Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . 130 7.2.3 Nontransparent Data Transmission . . . . . . . . . . . . . . . . . . . . . . . . 131 7.3 Protocol Architecture of the Signaling Plane . . . . . . . . . . . . . . . . . . . . . . . 134 7.3.1 Overview of the Signaling Architecture . . . . . . . . . . . . . . . . . . . . . 134 7.3.2 Transport of User Data in the Signaling Plane . . . . . . . . . . . . . . . . . 142 7.4 Signaling at the Air Interface (Um) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 7.4.1 Layer 1 of the MS-BTS Interface . . . . . . . . . . . . . . . . . . . . . . . . 144 7.4.1.1 Layer 1 Services. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 7.4.1.2 Layer 1: Procedures and Peer-to-Peer Signaling . . . . . . . . . . . 146 7.4.2 Layer 2 Signaling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 7.4.3 Radio Resource Management . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 7.4.4 Mobility Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 7.4.4.1 Common MM Procedures . . . . . . . . . . . . . . . . . . . . . . . . 157 7.4.4.2 Speci®c MM Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 159 7.4.4.3 MM Connection Management . . . . . . . . . . . . . . . . . . . . . 159 7.4.5 Connection Management. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162 7.4.6 Structured Signaling Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 166 7.4.7 Signaling Procedures for Supplementary Services. . . . . . . . . . . . . . . 167 7.4.8 Realization of Short Message Services . . . . . . . . . . . . . . . . . . . . . 171 7.5 Signaling at the A and Abis Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 172 7.6 Signaling at the User Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 8 Roaming and Switching 181 8.1 Mobile Application Part Interfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 8.2 Location Registration and Location Update . . . . . . . . . . . . . . . . . . . . . . . 182 8.3 Connection Establishment and Termination . . . . . . . . . . . . . . . . . . . . . . . 186 8.3.1 Routing Calls to Mobile Stations . . . . . . . . . . . . . . . . . . . . . . . . . 186 8.3.1.1 Effect of the MSRN Assignment on Routing. . . . . . . . . . . . . 186 8.3.1.2 Placement of the Protocol Entities for HLR Interrogation . . . . . 187 8.3.2 Call Establishment and Corresponding MAP Procedures. . . . . . . . . . . 189 8.3.2.1 Outgoing Connection Setup . . . . . . . . . . . . . . . . . . . . . . . 189 8.3.2.2 Incoming Connection Setup . . . . . . . . . . . . . . . . . . . . . . . 191 8.3.3 Call Termination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 8.3.4 MAP Procedures and Routing for Short Messages . . . . . . . . . . . . . . 193
- 11. Contents viii 8.4 Handover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 8.4.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 8.4.2 Intra-MSC Handover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 8.4.3 Decision Algorithm for Handover Timing . . . . . . . . . . . . . . . . . . . 197 8.4.4 MAP and Inter-MSC Handover. . . . . . . . . . . . . . . . . . . . . . . . . . 204 8.4.4.1 Basic Handover between two MSCs . . . . . . . . . . . . . . . . . . 204 8.4.4.2 Subsequent Handover. . . . . . . . . . . . . . . . . . . . . . . . . . . 205 9 Data Communication and Networking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 9.1 Reference Con®guration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 9.2 Overview of Data Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 9.3 Service Selection at Transitions between Networks . . . . . . . . . . . . . . . . . . . 212 9.4 Bit Rate Adaptation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213 9.5 Asynchronous Data Services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216 9.5.1 Transparent Transmission in the Mobile Network. . . . . . . . . . . . . . . 216 9.5.2 Nontransparent Data Transmission . . . . . . . . . . . . . . . . . . . . . . . . 219 9.5.3 PAD Access to Public Packet-Switched Data Networks . . . . . . . . . . . 222 9.5.3.1 Asynchronous Connection to PSPDN PADs . . . . . . . . . . . . . 222 9.5.3.2 Dedicated PAD Access in GSM . . . . . . . . . . . . . . . . . . . . 223 9.6 Synchronous Data Services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 9.6.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 9.6.2 Synchronous X.25 Packet Data Network Access . . . . . . . . . . . . . . . 224 9.6.2.1 Basic Packet Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 9.6.2.2 Dedicated Packet Mode . . . . . . . . . . . . . . . . . . . . . . . . . 225 9.7 Teleservices: Fax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 10 Aspects of Network Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 10.1 Objectives of GSM Network Management . . . . . . . . . . . . . . . . . . . . . . . . 231 10.2 Telecommunication Management Network (TMN) . . . . . . . . . . . . . . . . . . . 233 10.3 TMN Realization in GSM Networks. . . . . . . . . . . . . . . . . . . . . . . . . . . . 236 11 General Packet Radio Service (GPRS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 11.1 System Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242 11.2 Services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244 11.2.1 Bearer Services and Supplementary Services. . . . . . . . . . . . . . . . . . 244 11.2.2 Quality of Service. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245 11.2.3 Simultaneous Usage of Packet Switched and Circuit Switched Services . 247 11.3 Session Management, Mobility Management, and Routing . . . . . . . . . . . . . . 247 11.3.1 Attachment and Detachment Procedure . . . . . . . . . . . . . . . . . . . . . 247 11.3.2 Session Management and PDP Context . . . . . . . . . . . . . . . . . . . . . 247 11.3.3 Routing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 11.3.4 Location Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 11.4 Protocol Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252 11.4.1 Transmission Plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252 11.4.1.1 GPRS Backbone: SGSN±GGSN . . . . . . . . . . . . . . . . . . . 252 11.4.1.2 Air Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253 11.4.1.3 BSS ± SGSN Interface . . . . . . . . . . . . . . . . . . . . . . . . . 255 11.4.2 Routing and Conversion of Addresses. . . . . . . . . . . . . . . . . . . . . . 255 11.4.3 Signaling Plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 11.5 Interworking with IP Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 11.6 Air Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258 11.6.1 Multiple Access and Radio Resource Management . . . . . . . . . . . . . . 258 11.6.2 Logical Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 11.6.3 Mapping of Packet Data Logical Channels onto Physical Channels . . . . 263
- 12. Contents ix 11.6.4 Channel Coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264 11.7 Authentication and Ciphering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 11.7.1 User Authentication. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 11.7.2 Ciphering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 11.7.3 Subscriber Identity Con®dentiality . . . . . . . . . . . . . . . . . . . . . . . . 267 11.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 12 GSM ± The Story Goes On . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271 12.1 Globalization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271 12.2 Overview of GSM Services in Phase 2+ . . . . . . . . . . . . . . . . . . . . . . . . . 272 12.3 Bearer and Teleservices of GSM Phase 2+ . . . . . . . . . . . . . . . . . . . . . . . . 273 12.3.1 Improved Codecs for Speech Services: Half- Rate Codec, EFR Codec, and AMR Codec. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 12.3.2 Advanced Speech Call Items (ASCI) . . . . . . . . . . . . . . . . . . . . . . 276 12.3.2.1 Voice Broadcast Service (VBS). . . . . . . . . . . . . . . . . . . . 277 12.3.2.2 Voice Group Call Service (VGCS). . . . . . . . . . . . . . . . . . 279 12.3.2.3 Enhanced Multi-Level Precedence and Pre-emption (eMLPP) . 280 12.3.3 New Data Services and Higher Data Rates: HSCSD, GPRS, and EDGE . 281 12.4 Supplementary Services in GSM Phase 2+ . . . . . . . . . . . . . . . . . . . . . . . . 282 12.4.1 Supplementary Services for Speech . . . . . . . . . . . . . . . . . . . . . . . 282 12.4.2 Location Service (LCS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 12.5 Service Platforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 12.5.1 CAMEL ± GSM and Intelligent Networks . . . . . . . . . . . . . . . . . . . 284 12.5.2 Service Platforms on the Terminal Side. . . . . . . . . . . . . . . . . . . . . 286 12.5.2.1 SIM Application Toolkit (SAT). . . . . . . . . . . . . . . . . . . . 286 12.5.2.2 Mobile Station Application Execution Environment (MExE) . . 287 12.6 Wireless Application Protocol (WAP). . . . . . . . . . . . . . . . . . . . . . . . . . . 287 12.6.1 Wireless Markup Language (WML) . . . . . . . . . . . . . . . . . . . . . . . 288 12.6.2 Protocol Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289 12.6.3 System Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 12.6.4 Services and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292 12.7 Beyond GSM: On the Road to UMTS. . . . . . . . . . . . . . . . . . . . . . . . . . . 293 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297 Appendix A: GSM Standards. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301 Appendix B: GSM Addresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311 Appendix C: Acronyms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321
- 13. Preface for Second Edition ``GSM ± the story goes on'' is the new title of the last chapter of this book ± and GSM is indeed an ongoing success story. Since the release of the ®rst edition of this book (2 years ago), the number of GSM subscribers has grown from 100 to 380 million worldwide. Nobody expected such an enormous number when the ®rst GSM networks started their operation in 1991! In some countries the number of cellular phones is already higher than the number of ®xed phones. Not only are the subscriber numbers experiencing a tremendous growth, but the tech- nological evolution of GSM is also continuing. Many new services and applications have been developed and standardized during the last few years and are now being implemented in GSM networks and terminals. Substantial progress has been achieved, for example, by improving the voice services. Enhanced speech codecs, such as the Enhanced Full-Rate (EFR) and the Adaptive Multi- Rate (AMR) codecs, provide better speech quality. Moreover, services for group commu- nication have been developed, which are especially useful for closed user groups. Service platforms (e.g. CAMEL and the SIM Application Toolkit) allow network operators to quickly introduce new services. In addition to speech communication, the mobile data traf®c is growing. Several billion text messages are being exchanged between mobile users each month with the GSM Short Message Service (SMS). Indeed, the ®eld for GSM data applications and products is huge: news services, mobile payment with cellular phones, telemetry, ¯eet management, loca- tion-based information services, and automatic emergency call systems are just some examples of the broad range of services that became possible with GSM. In the future, mobile access to the Internet will be of particular importance. The Wire- less Application Protocol (WAP) has been developed to create an ``information Web'' for cellular phones. WAP applications, such as stock broking and online auctions, enjoy an increasing popularity. The introduction of the General Packet Radio Service (GPRS) ± with its packet switched transmission technology at the air interface ± enables more ef®cient, faster, and easier access to the worldwide Internet. GPRS will contribute to the soft migration from GSM toward third generation mobile systems (UMTS, IMT-2000). The world of mobile communications remains exciting! This second edition of our book gave us the opportunity to include the new GSM technologies. They are treated in Chapters 11 and 12. Chapter 11 is completely new and explains in detail the General Packet Radio Service (GPRS). Chapter 12 gives an overview of services recently introduced in GSM Phase 2+. It covers new speech and data services, supplementary services, location services, service platforms, WAP, Advanced Speech Call GSM Switching, Services and Protocols: Second Edition. Jo È rg Eberspa È cher, Hans-Jo È rg Vo È gel and Christian Bettstetter Copyright q 2001 John Wiley & Sons Ltd Print ISBN 0-471-49903-X Online ISBN 0-470-84174-5
- 14. Preface for Second Edition xii Items (ASCI), and gives an outlook toward UMTS. Some other chapters have been updated and slightly modi®ed. We are grateful to Professor Gottfried R. Luderer and Christoph Schmelz for the proof- reading of some chapters as well as to Sarah Hinton and the other people from Wiley for the good cooperation. Last but not least, we would like to thank our readers for many comments and sugges- tions that have reached us. Their feedback greatly helped us to re®ne and enhance the book and to correct some errors. We are looking forward to staying in contact with you! Munich, March 2001 Jo Èrg Eberspa Ècher joerg.eberspaecher@ei.tum.de Hans-Jo Èrg Vo Ègel h.voegel@fantastic.com Christian Bettstetter christian.bettstetter@ei.tum.de PS: Please visit our book's Web page at http://www.lkn.ei.tum.de/gsm_buch with comments, news, and errata.
- 15. Preface GSM is much more than the acronym of Global System for Mobile Communication; it stands for an extraordinarily successful stage of development in modern information technology. GSM means a new dimension for more than 50 million users ± and there are more and more every day ± a dimension of personal communication. Today GSM is deployed in more than 100 countries and by over 220 network operators, many of them outside Europe. The mobile telephone has advanced from status symbol to useful appli- ance, not only in business but also in private everyday life. Its principal use is for wireless telephony, but GSM data communication is increasingly gaining importance. This modern digital system for mobile communication is based on a set of standards, which were worked out in Europe and can now be considered truly global. Many of the new standardization initiatives of GSM Phase 2+ are in fact coming from outside of Europe. Depending on locally available frequency bands, different GSM air interfaces are de®ned (e.g. for 900 MHz, 1800 MHz, and 1900 MHz). However, architecture and protocols, in particular for user±network signaling and global roaming are identical in all networks. Thus, GSM enables worldwide development, manufacturing and marketing of innovative products, that stand up well under competition. GSM also stands for complexity. Whether in the terminals or the exchange equipment, whether in hardware or software, GSM technology is extraordinarily involved and exten- sive; certainly the most complex communication systems by themselves comprise the standards published by the European Telecommunication Standards Institute (ETSI). This book arose from an effort to explain and illustrate the essential technical principles of GSM in spite of this complexity, and to show the interrelations between the different subfunctions in a better way than is possible in the framework of standards. Points of crystallization were provided by our course ``Communication Networks 2'' at the Munich University of Technology as well as our GSM lab course, which requires the students to prepare by studying an extensive GSM manuscript. This lab course is also part of the English graduate program in ``communications engineering'' at our university which is leading to an MSc degree. The foundation of this book is, however, in the ETSI standards themselves (besides some scienti®c publications), which were, on one hand, ``boiled down'' in this book and, on the other hand, augmented by explanations and interpretations. The book is intended for all those who want to acquire a deeper knowledge of the complex GSM system without losing their way in the detail and wording of the standards. Addressed are the students of electrical engineering, computer science, and information technology at universities and technical institutes, those in industry or network operations GSM Switching, Services and Protocols: Second Edition. Jo È rg Eberspa È cher, Hans-Jo È rg Vo È gel and Christian Bettstetter Copyright q 2001 John Wiley & Sons Ltd Print ISBN 0-471-49903-X Online ISBN 0-470-84174-5
- 16. xiv Preface who use and apply the technology, but also researchers who want to gain insight into the architecture and functional operation of the GSM system. In accordance with the publisher and editors, our book presents the entire architecture of GSM with concentration on the communication protocols, the exchange technology, and the realization of services. The most important principles of the GSM transmission tech- nology are also included in order to give a rounded treatment. Those who are involved with the implementation of GSM systems should ®nd the book to be a useful start and they should ®nd adequate guidance on the standards. The study of the standards is also recom- mended when there are doubts about the latest issues of the ETSI standards, for with this book we had to consider the standards to be ``frozen'' in their state as of summer 1997. The authors especially thank Professor Martin Bossert (Ulm University) for many helpful hints and clarifying discussions. We are very grateful to Professor Gottfried R. Luderer (Arizona State University, Tempe, AZ) for the translation of the German version of the book as well as for the critical technical review of the manuscript and numerous proposals for improvement. It was his strong commitment and determined translation work, which made this book possible. We also give our cordial thanks to the people at Wiley for initiating this book and for the smooth cooperation. Their support in every phase of the project was critical to its speedy production and publication. The authors are grateful in advance for any kind of response to this book. Readers should address us (wireless or over guided media), preferably via email. Munich, July 1998 Jo Èrg Eberspa Ècher Joerg.Eberspaecher@ei.tum.de Hans-Jo Èrg Vo Ègel Hans-Joerg.Voegel@ei.tum.de
- 17. Index A A3 algorithm, 120 A5 algorithm, 123, 266 A8 algorithm, 121, 267 Access burst, see Bursts Access grant channel, see AGCH ACELP (Algebraic code excitation ± linear prediction), 274 ACSE (Association control service element), 239 Ad hoc networking, 3 Adaptive frame alignment, 74, 80 Address assignment dynamic IP address, 257 TMSI, see TMSI Addresses, 30 BCC, 35 BSIC, 35 CC, 32 CI, 35 FAC, 31 GCI, 35 IMEI, 31 IMSI, 32 IP address, 243, 248, 255 LAC, 34 LAI, 33, 119 LMSI, 34 MNC, 32 MSIN, 32 MSISDN, 32 MSRN, 33, 182, 186 NCC, 35 NDC, 32 NMSI, 32 NSAPI, 252, 255 PDP address, 243 P-TMSI, 247 SN, 32 SNR, 31 TAC, 31 TID, 252, 255 TLLI, 254, 255 TMSI, 34, 119 ADPCM, 98 AGCH (Access grant channel), 58 Air interface, 35, 43, 57, 63, 95 GPRS, 253, 258 signaling, 134, 144 UMTS, 294 A-law, 127 Aloha, 153, 254 AMPS (Advanced Mobile Phone System), 4 AMR (Adaptive multirate) codec, 273 Antenna array, 20 intelligent antenna, 22 response vector, 21 Applications, 4, 283, 292 ARQ (Automatic repeat request), 131, 147, 211, 220 GPRS, 254 ASCI (Advanced speech call items), 272, 276 ATM mobile ATM, 3 Attach GPRS, 247 IMSI, 159, 181 AUC (Authentication center), 30, 40, 120 GSM Switching, Services and Protocols: Second Edition. Jo È rg Eberspa È cher, Hans-Jo È rg Vo È gel and Christian Bettstetter Copyright q 2001 John Wiley & Sons Ltd Print ISBN 0-471-49903-X Online ISBN 0-470-84174-5
- 18. Authentication, 40, 118, 120, 156, 166, 182 center, see AUC GPRS, 266 Automatic repeat request (ARQ), see ARQ B Barring, 53 Base station controller, see BSC Base station subsystem, see BSS Base transceiver station, see BTS Battery life, 97 BCCH (Broadcast control channel), 58 see also Logical channels allocation (BA), 82 Bearer capability, 212 Bearer service, see Services BHCA (Busy hour call attempts), 27 Billing GPRS, 241, 246 Bit number (BN), 72 Bit rate adaptation, 211, 213 Bit stealing, 77 Black list, 31 Block, 102, 110, 263 distance, 61, 262 error ratio, 131 length, 61, 262 Block coding, 95, 100, 103 GPRS, 264 Bluetooth, 3 Bm (mobile B channel), 58 Border gateway (BG), 244 Broadcast control channel, see BCCH Browser, 288 BS_xx_xx parameter, 79 BSC (Base station controller), 29, 37 signaling functions, 139 BSIC, see Addresses BSS, 35, 36 application part (BSSAP), 43, 138, 172 application part + (BSSAP+), 257 management application part (BSSMAP), 138, 172 operation and maintenance application part (BSSOMAP), 172, 240 BTS (Base transceiver station), 29, 36 color code (BCC), see Addresses identity code (BSIC), see Addresses management (BTSM), 141 signaling functions, 139 Bursts, 65 access burst (AB), 68 burst errors, 111 dummy burst (DB), 68 frequency correction burst (FB), 68 mapping, 95, 117 normal burst (NB), 67, 117 synchronization burst (SB), 68 C Call arrival rate, 27 barring, 53 blocking probability, 27 conference, 54 forwarding, 53 group call, 245 hold, 54, 168 incoming, 61, 191 mean holding time, 27 outgoing, 190 priorities, 280 queuing, 163 reestablishment, 89 release/termination, 151, 162, 193 restriction, 53 routing, 186 setup, 61, 151, 161, 186, 189 transfer, 54 waiting, 54 Call control, 137, 162 messages, 160 CAMEL, 272, 284 application part (CAP), 285 Camping, 90 Capacity on demand, 258 Card, 289 Carrier ~ -to-interference ratio, 24 BCCH carrier, 59, 65 frequency, 65 CBCH (Cell broadcast channel), 60 CCBS (Completion of call to busy subscriber), 54, 282 Index 322
- 19. CCCH (Common control channel), 58 CDMA, see Multiple access cdma2000, 294 DS-CDMA, 18 FH-CDMA, 17, 19 TD-CDMA, 294 wideband (W-CDMA), 294 Cell, 23 allocation (CA), 37, 65 identi®er (CI), 35 assignment, 82 global identi®er (GCI), 35 maximum diameter, 75 selection, 80, 90, 137, 151 Cell broadcast channel, see CBCH Cellular principle, 9, 23 CELP (Code exited linear predictive coding), 98 CEP (Connection end point), 147 Channel, 14, 65 allocation, 23, 150 GPRS, 258, 261 assignment, 59, 150, 166 change, 154 channels per cell, 27 combinations, 62, 262 control channel, 57 logical channel, 57, 213, see also Logical channels measurement, 82, 154, 194 mode adaptation, 275 physical channel, 15, 57, 63 GPRS, 259 release, 166 request, 166 signaling channel, 57 spatial reuse, 23 traf®c channel, 57 Channel coding, 12, 49, 95, 100, 211 see also Block coding, Convolutional coding AMR, 275 GPRS, 264 packet data, 264 unequal error protection, 103 Chip rate, 18 CI, see Addresses Ciphering, see Encryption cipher key Kc, see Kc key Closed user group, 54, 245, 279 Cluster, 24 CMI (Common management information) protocol (CMIP), 236 service (CMIS), 236 service element (CMISE), 239 C-Netz, 4, 5 Code block code, see Block coding CDMA, 18 convolutional code, see Convolutional coding Codec, 96, 273 adaptive multirate (AMR), 273 enhanced full rate (EFR), 273 half-rate, 273 mode adaptation, 275 Collision, 14, 166 Comfort noise, 97 Common control channel, see CCCH Compression, 96 Conference call, 54 Con®dentiality, 118 Con®guration, 44 Connection control, 43 Connection management, 137, 138, 162 Connection setup, see Call setup Constraint length, 109 Control channel, 57 Control plane, 125 Conversion of addresses, 255 Convolutional coding, 95, 100, 107 GPRS, 264 Country code (CC), see Addresses CRC (Cyclic redundancy check) code, 103, 274 CS1-4 (coding schemes), 264 D DAB (Digital Audio Broadcast), 4 Data burst, 66 Data link layer GPRS, 253 Data rate AMR, 275 bearer services, 49 Index 323
- 20. bit rate adaptation, 214 EDGE, 281 EFR, 273 GPRS, 241, 258, 265 gross data rate, 15, 63, 66, 77, 276 HSCSD, 281 net data rate, 61, 262 Data transmission, 209 see also Protocol architecture, Services GPRS, see GPRS HSCSD, see HSCSD in signaling plane, 142 nontransparent, 131 packet switched, 241 transparent, 130 Databases, 30 distributed, 45 DCCH (Dedicated control channel), 58 DCS1800, 5, 271 Deck, 289 DECT (Digital Enhanced Cordless Telecommunication), 2 Dedicated control channel, see DCCH Detach GPRS, 247 IMSI, 159, 181 DHCP (Dynamic host con®guration protocol), 257 Differential encoding, 63 Disconnection, 88 Discontinuous reception, 91 transmission, 97 Dispersion, 11 DL_RXLEV, 200 DL_RXQUAL, 200 Dm (mobile D channel), 58 DNS (Domain name service), 258 Downlink, 13 DRX, see Discontinuous reception DTAP (Direct transfer application part), 138, 172 DTMF (Dual-tone multifrequency), 138, 164 Dualband, 271 Dummy burst, see Bursts Duplex, 12, 65 FDD (Frequency division duplex), 12 TDD (Time division duplex), 12 DVB (Digital Video Broadcast), 4 E Early assignment, 163 Eavesdropping, 119 ECSD (Enhanced circuit switched data), 282 EDGE (Enhanced Data Rates for GSM Evolution), 272, 281, 295 8-PSK, see Modulation EFR (Enhanced full-rate) codec, 273 EGPRS (Enhanced GPRS), 282 EIR (Equipment identity register), 30, 40 Emergency call, 50, 90, 138, 154, 163, 280, 283, 290 EMLPP (Enhanced multi-level precedence and pre-emption), 276, 280 Encryption, 95, 118, 122 activation, 155, 166 GPRS, 266 Engset model, 27 E-OTD (Enhanced observed time difference), 283 Equalization, 12 Equipment identity register, see EIR Erlang blocking formula, 27 Error concealment, 98 Error correction, see Channel coding Error detection, see ARQ ETSI (European Telecommunication Standards Institute), 5, 294 Evolution, 272 F FACCH (Fast associated control channel), 58 Fading Rayleigh, 10 Rice, 10 Fast associated control channel, see FACCH Fax, 226 see also Services adaptation protocol, 211 FCAPS (Fault, con®guration, accounting, performance, security) management, 233 FCCH (Frequency correction channel), 58, 68 FDD, see Duplex Index 324
- 21. FDMA, see Multiple access Fill bits, 101 Final assembly code (FAC), 31 Fire code, 103, 265 Flow control, 147, 254 Forward error correction, see Channel coding Frame hyperframe, 76 multiframe, 76, 263 number (FN), 72 search frame, 84 superframe, 76 TDMA frame, 15 Frame check sequence, 132 Freephone service, 54 Frequency band, 14, 15 UMTS, 294 carrier frequency, 15 distance, 13 reuse distance, 23 Frequency correction burst, see Bursts channel, see FCCH Frequency hopping, 16, 19, 68, 80 G Gateway mobile switching center, see GMSC GCR (Group call register), 278 GEA (GPRS encryption algorithm), 267 General Packet Radio Service, see GPRS Generator polynomial, 104, 107 GGSN (Gateway GPRS support node), 243 Global cell identi®er (GCI), 35 GMLC (Gateway mobile location center), 283 GMM/SM (GPRS mobility management and session management) protocol, 256 GMSC (Gateway mobile switching center), 30, 38 GMSK (Gaussian minimum shift keying), see Modulation GPRS (General Packet Radio Service), 2, 55, 241, 272 GPS (Global Positioning System), 283 Grey list, 31 Group call, 276 area, 277 GPRS, 245 GSM Global System for Mobile Communica- tion, 2, 5 Groupe Spe Âcial Mobile, 5 GSN (GPRS support node), 242 GTP (GPRS tunneling protocol), 244, 252 Guard band, 23, 65 period, 67, 74 H Handback, 205 Handover, 23, 80, 82, 194 causes, 200 decision, 86 decision algorithm, 197 external, 195 hysteresis, 200 intercell, 194 inter-MSC, 204 internal, 195 intracell, 194 intra-MSC, 197 ping-pong handover, 86, 203 radio resource management, 137, 155 subsequent, 205 threshold values, 200 HDLC (High level data link control), 132, 136 HIPERLAN, 2 HLR (Home location register), 30, 38 HO_MARGIN, 200 Hold, 54 Home location register, see HLR Hopping assignment, 68 HSCSD (High Speed Circuit Switched Data), 272, 281 HTML (Hypertext markup language), 288 I ID hopping, 34 Identi®cation calling line, 54 connected line, 54 Index 325
- 22. Identi®ers, see Addresses IEEE 802.11, 2 IMEI, see Addresses IMSI, see Addresses attach, 159, 181 detach, 156, 159, 181 IMT-2000, 2, 272, 293 IN (Intelligent network), 284 application part (INAP), 285 Incall modi®cation, 164 Infrastructure, 35 Insert subscriber data, 182 Interfaces GPRS, 243 GSM, 42, 44 Interference, 11, 23 Interleaving, 100, 111 GPRS, 264 International mobile station equipment identity, see Addresses International mobile subscriber identity, see Addresses International switching center, see ISC Internet, 1, 4, 241, 257, 273, 287, 293 Interworking function, 38 GPRS-IP, 257 GSM-ISDN, 38 transparent data services, 212 IP (Internet Protocol), 241, 288 ISC (International switching center), 38 ISDN, 209 interworking, 42 services, 47 user part (ISUP), 42, 142 ITU-T E. series, 33 G. series, 128, 134 M. series, 39, 233 Q. series, 138, 163 T. series, 51, 212 V. series, 132, 209, 211, 213, 226 X. series, 132, 224, 241 J Java, 287 JavaScript, 289 K Kc key, 41, 122, 266 Ki key, 41, 120, 266 L L_RXLEV threshold, 87, 200 L_RXQUAL threshold, 87, 200 LAI, see Addresses LAPB, 224 LAPDm, 135, 147 Late assignment, 163 Layer 2 relay (L2R), 211, 220 LCS (Location service), 283 LEO (Low earth orbiting satellite), 4 Link access procedure on Dm channels, see LAPDm Link control, 80 LLC (Logical link control) GPRS, 254 LMSI, see Addresses Local mobile subscriber identity, see Addresses Location area, 29, 33, 39 code (LAC), see Addresses identity (LAI), see Addresses Location registration, 182 Location service (LCS), 283 Location update, 34, 159, 182 GPRS, 249 strategy, 249 Log area ratio (LAR), 99 Logical channels, 57 channel coding, 102 GPRS, 259 group call, 278 GSM, 57 mapping to physical channels, 75, 263 LPC, 98 M MAC (Medium access control) see also Random access GPRS, 254 MAIO (Mobile allocation index offset), 69 Management layer business (BML), 235 Index 326
- 23. element (EML), 235 network (NML), 235 service (SML), 235 Management, 35, see also Network manage- ment Man-machine interface, 176 MAP (Mobile application part), 43, 141, 181, 189, 257 Markov process, 27 Maximum likelihood decoding, 111 Measurement report, 82, 154 Mediation device, 235 function, 237 Memory, 108 MEO (Medium earth orbiting satellite), 4 Message transfer part, see MTP MExE (Mobile station application execution environment), 272, 287 Microbrowser, 288 Midamble, 67 MNAP (Management network access point), 238 Mobile access hunting, 54 Mobile allocation (MA), 65 Mobile application part (MAP), see MAP Mobile Internet, 241 Mobile IP, 3 Mobile network code (MNC), see Addresses Mobile station, 35 dedicated mode, 146 GPRS, 250 idle mode, 146 serial number, 31 stolen, 31 Mobile station roaming number, see Addresses Mobile subscriber identi®cation number (MSIN), see Addresses Mobile subscriber ISDN number, see Addresses Mobile switching center, see MSC Mobile switching network, 35, 37 Mobile termination (MT), 209 Mobility, 1, 31, 36, 53, 137, 282 Mobility management, 43, 137, 156, 181 connection management, 159 GPRS, 249, 256 messages, 156 MOC (Managed object class), 237 Modem, 211 Modulation, 63 8-PSK, 281 GMSK, 63 MSK (Minimum shift keying), 65 Monitoring, 137 MOS (Mean opinion score), 100 MoU (Memorandum of Understanding), 6 MS, see Mobile station MS_RANGE, 199 MSC (Mobile switching center), 29, 37 anchor MSC, 196, 278 relay MSC, 278 signaling functions, 139 MSISDN, see Addresses MSK (Minimum shift keying), see Modulation MSRN, see Addresses MTP (Message transfer part), 138, 257 Multiband, 271 Multicarrier system, 15, 65, 294 Multicast, 276 GPRS, 245 Multiple access, 14, 65 CDMA (Code division multiple access), 14, 18 FDMA (Frequency division multiple access), 14 in GPRS, 258 SDMA (Space division multiple access), 14, 20 TDMA (Time division multiple access), 14, 15 Multiplex frequency, 14 statistical, 241, 254 time, 15 Multislot, 62, 254, 259, 281 N National destination code (NDC), see Addresses National mobile subscriber identity (NMSI), see Addresses Index 327
- 24. NCH (Noti®cation channel), 58 NEF (Network element function), 237 Network color code, 35 Network management, 39, 231 center (NMC), 240 TMN, 232 Network operation, see Operation NMT (Nordic Mobile Telephone), 4 Non-transparent service, see Services Noti®cation channel, see NCH NSAPI (Network service access point identi®er), see Addresses Numbering multinumbering, 213 single numbering, 213 O OACSU, 163 OHG (Operators harmonization group), 294 Operation and maintenance, 239 see also Network management BSS, 172 BSSOMAP, 240 OMAP (OM and administration part), 239 OMC (OM center), 30, 172, 240 OMSS (OM subsystem), 35, 39 Operation system, 234 OSF (Operating system function), 237 P PACCH (Packet associated control channel), 261 Packet assembler, 48 Packet data network, see PDN Packet temporary mobile subscriber identity, see Addresses PAD access, 222 PAGCH (Packet access grant channel), 260 Paging, 34, 59, 151, 166, 192 channel, see PCH Paging systems, 4 Parity, 101, 264 PBCH (Packet broadcast channel), 259 PCCCH (Packet common control channel), 260 PCH (Paging channel), 58 PCM, 98 PCN, 5, 271 PCS, 5, 271 PDCH (Packet data channel), 259 PDN (Packet data network), 42, 242 PDP (Packet data protocol), 243 context, 247 PDTCH (Packet data traf®c channel), 259 Phase 2+, 272 Physical channel, 63 mapping from logical channels, 75, 263 Physical layer, 57, 63, 95 GPRS, 254 signaling, 134, 144 PIN, 36 PLL (Physical link layer), 254 PLMN, 29 home ~, 188 visited ~, 188 PNCH (Packet noti®cation channel), 260 Poisson process, 27 Power budget, 199 conservation mode, 90 consumption, 97 control, 80, 82, 86 PWD_CTRL_FAIL, 200 MS maximal (MS_TXPR_MAX), 199 power-up scenario, 92 spectrum, 71 PPCH (Packet paging channel), 260 PRACH (Packet random access channel), 260 Precedence, 280 Pre-emption, 280 Priorities, 280 Propagation loss, 10 multipath, 9 Protocol architecture, 125 GPRS, 252 nontransparent data, 131 signaling, 134 speech, 127 transparent data, 130 user plane, 127 WAP, 289 Pseudo noise, 18 Index 328
- 25. PSPDN, 222 PSTN (Public switched telephone network), 42, 211 Psycho-acoustics, 210 PTCCH (Packet timing advance control channel), 261 PTM service (in GPRS), 245 P-TMSI (Packet temporary mobile subscriber identity), see Addresses PTP service (in GPRS), 245 Puncturing, 101 PWR_CTRL_FAIL, 87 Q QoS (Quality of service), 232, 241, 245 Quality monitoring, 80, 82, 194 Quantization, 96 Quarter bit number (QN), 72 R RACH (Random access channel), 58 Radio channel, 9 dispersion, 11 frequency-selective, 11 interference, 11 Radio interface, see Air interface Radio link failure, 88 Radio link protocol (RLP), see RLP Radio network, 35 Radio resource management, 79, 137, 150 GPRS, 258 messages, 152 Radio subsystem link control, 80 cell selection, 90 channel measurement, 82 disconnection, 88 power conservation, 90 power control, 86 RAND, 120 Random access AGCH (Access grant channel), 58 burst, 68 RACH (Random access channel), see RACH Rate bit rate, see Data rate code rate, 101, 108 data rate, see Data rate Reduced TDMA frame number, see RFN Reference con®guration, 209 Re¯ection coef®cient, 99 Registers, 30 Registration, 40 Releases, 273 Reverse charging, 54 RFL (Physical RF layer), 254 RFN (Reduced TDMA frame number), 68, 71 RLC (Radio link control) GPRS, 254 RLP (Radio link protocol), 49, 131, 220 Roaming, 181 SIM card roaming, 271 ROSE (Remote operations service element), 239 Routing, 44, 186 GPRS, 249, 255 SMS, 193 Routing area (RA), 250 RPE (Regular pulse excitation), 98, 274 RXLEV, 82, 87, 154, 198 RXQUAL, 82, 87, 154, 198 S SACCH (Slow associated control channel), 58, 80 Sampling, 96 SAP (Service access point), 147 SAT (SIM application toolkit), 272, 286 Satellite communication, 4 SCCP (Signaling connection control part), 138, 141, 257 SCH (Synchronization channel), 58, 68 SCP (Service control point), 285 SDCCH (Stand-alone dedicated control channel), 58 SDMA, see Multiple access Security, 118 Serial number, 31 Service platforms, 284 Services, 47 additional, 48 bearer services, 47, 48 3.1 kHz, 50 Index 329
- 26. asynchronous data, 48, 216 GPRS, see GPRS HSCSD, 281 nontransparent, 48, 219 synchronous data, 48, 224 transparent, 48, 216 UDI, 50 data services, 48, 209, 281 asynchronous, 216 GPRS, see GPRS HSCSD, 281 nontransparent, 219 synchronous data, 224 transparent, 216 WAP, 292 EDGE, 281 essential, 48 GPRS, see GPRS HSCSD, 281 phase 1, 272 phase 2, 272 phase 2+, 55, 272 service platforms, 284 supplementary services, 47, 52 connection management, 137 phase 1, 53 phase 2, 53 phase 2+, 282 signaling, 167 teleservices, 47, 50 fax, 51, 226 MHS (message handling system), 50 SMS, 52, see also SMS SMSCB, 52 teletext, 50 videotex, 50 voice, 50 transport services, 48 WAP, 292 Session management GPRS, 247, 256 SGSN (Serving GPRS support node), 242 Shift register, 104, 107 Signal level (RXLEV), see RXLEV quality (RXQUAL), see RXQUAL Signaling, 42 A and Abis interface, 172 Air interface, 144 architecture, 134 channel, 57 DTMF, 138, 164 GPRS, 256 point, 44 SS#7, 42, 134, 285 structured overview of phases, 166 supplementary services, 167 user interface, 176 Signal-to-noise ratio, 23 Silence descriptor, 97 SIM (Subscriber identity module), 31, 36 SIM application toolkit, 272, 286 data download, 286 proactive SIM, 286 Slow associated control channel, see SACCH SMG (Special Mobile Group), 5 SMLC (Serving mobile location center), 283 SMS (Short Message Service), 2, 4, 143 cell broadcast (SMSCB), 60 connection management, 137 gateway MSC (SMS-GMSC), 143 interworking MSC (SMS-IWMSC), 143 over GPRS, 244 protocols (SM-TP, SM-RP, SM-CP), 143 routing, 193 service center (SMS-SC), 143 SMSS (Switching and management subsys- tem), 35 SNDCP (Subnetwork dependent conver- gence protocol), 253 SOSS (Support of operator-speci®c services), 284 Source coding, 95 Spatial reuse, 23 Spatial signature, 21 Spectral ef®ciency, 273 Speech coder, 98 pause, 96 processing, 95 protocols, 127 quality, 100, 102, 273 Spread spectrum, 18 spreading factor, 18 spreading sequence, 18 SRES (Signature response), 120 Index 330
- 27. SSP (Service switching point), 285 Stand-alone dedicated control channel, see SDCCH Statistics networks, 7, 271 subscribers, 7, 293 Stealing ¯ag, 67 Subscriber, 40 authentication, see Authentication, 118 privacy, see Security, 118 Subscriber identity protection, 119 veri®cation, see Authentication, 120 Subscriber Identity Module, see SIM Subscriber Number (SN), see Addresses Supplementary service, see Services Switching, 181 Switching and management subsystem, see SMSS Synchronization, 15, 17, 70 adaptive frame synchronization, 74 burst, see Bursts channel, see SCH frequency and clock, 70 System architecture GPRS, 242 GSM, 29, 35 WAP, 291 System information messages, 79 T TACS (Total Access Communication System), 5 Tail bits, 67, 103 Tandem free operation (TFO), 273 TBF (Temporary block ¯ow), 261 TCAP (Transaction capabilities application part), 141, 257 TCH (Traf®c channel), 57 TCP, 253 TD-CDMA, 294 TDD, see Duplex TDMA, see Multiple access TD-SCDMA, 294 Telecommunication service, 47 Telephone book, 36 Teleservice, see Services Temporary mobile subscriber identity, see Addresses Terminal adapter (TA), 209 Terminal equipment (TE), 209 TETRA (Trans European Trunked Radio), 4 3GPP (Third Generation Partnership Project), 5, 294 TID (Tunnel identi®er), see Addresses Time slot, 15, 66 multislot, 62 number (TN), 72 Timing advance (TA), 74, 199, 261 TLLI (Temporary logical link identi®er), see Addresses TMN (Telecommunication management network), 39, 232 logical layered architecture, 235 management layers, see Management layer mediation device, 235 TMSI see Addresses allocation, 156, 182 TOA (Time of arrival), 283 Traf®c capacity, 27 channel (TCH), 57 engineering, 27 load, 27 Training sequence, 67 Transceiver, 36 Transparent service, see Services TRAU (Transcoding and rate adaptation unit), 127 Triband, 271 Trouble tickets, 231 Tunneling, 244 Type approval code (TAC), 31 Type code (TC), 79 U U_RXLEV threshold, 87, 200 U_RXQUAL threshold, 87, 200 UDI (Unrestricted digital information), see Services UDP, 253 Index 331
- 28. UEP (Unequal error protection), 275, see also Channel coding UL_RXLEV, 200 UL_RXQUAL, 200 Um interface, see Air interface UMTS (Universal Mobile Telecommunica- tion System), 2, 272, 293 Uplink, 13 UPT (Universal personal telecommunica- tion), 4 User interface, 176 User plane, 125 USF (Uplink state ¯ag), 259, 261 UTRA (UMTS terrestrial radio access), 294 network (UTRAN), 295 UWC-136, 294 V VBS (Voice broadcast service), 276 vCalendar, 289 vCard, 289 VGCS (Voice group call service), 276, 279 Visited location register, see VLR Viterbi decoding, 111 VLR (Visited location register), 30, 38 Vocoder, 98 Voice activity detection (VAD), 96 Voicebox, 53, 284 W WAE (Wireless application environment), 289 WAP (Wireless Application Protocol), 2, 272, 287 WBMP (Wireless bitmap) format, 288 WDP (Wireless datagram protocol), 290 White list, 31 Wireless LAN, 2 WML (Wireless markup language), 288 browser, 288, 289 WSP (Wireless session protocol), 290 WTA (Wireless telephony application) inter- face, 289 WTLS (Wireless transport layer security), 290 WTP (Wireless transaction protocol), 290 WWW (World Wide Web), 245, 287 X X.25, 224, 241 XML (Extensible markup language), 288 XSL (Extensible style language), 289 Z Zero-termination, 108 Index 332
- 29. Introduction 1.1 Digital, Mobile, Global: Evolution of Networks Communication everywhere, with everybody, and at any time ± we have come much closer to this goal during the last few years. Digitalization of communication systems, enormous progress in microelectronics, computers, and software technology, inventions of ef®cient algorithms and procedures for compression, security, and processing of all kinds of signals, as well as the development of ¯exible communication protocols have been important prerequisites for this progress. Today, technologies are available that enable the realization of high-performance and cost-effective communication systems for many application areas. In the ®eld of ®xed networks ± where the end systems (user equipment) are connected to the network over a line (two-wire copper line, coaxial cable, glass ®ber) ± new network technologies (such as xDSL and cable modem) have been introduced, providing broadband access to the Internet. The largest technological and organizational challenge is, however, the support of subscri- ber mobility. It can be distinguished between two kinds of mobility: terminal mobility and personal mobility. In the case of terminal mobility, the subscriber is connected to the network in a wireless way ± via radio or light waves ± and can move with his or her terminal freely, even during a communication connection. The degree of mobility depends on the type of mobile radio network. The requirements for a cordless in-house telephone are much less critical than for a mobile telephone that can be used in a car or train. If mobility is to be supported across the whole network (or country) or even beyond the network (or national) boundaries, additional switching technology and administrative functions are required, to enable the subscribers to communicate in wireless mode outside of their home areas. Such extended network functions are also needed to realize personal mobility and univer- sal reachability. This is understood to comprise the possibility of location-independent use of all kinds of telecommunication services ± including and especially in ®xed networks. The user identi®es himself or herself (the person), e.g. by using a chip card, at the place where he or she is currently staying and has access to the network. There, the same communication services can be used as at home, limited only by the properties of the 1 GSM Switching, Services and Protocols: Second Edition. Jo È rg Eberspa È cher, Hans-Jo È rg Vo È gel and Christian Bettstetter Copyright q 2001 John Wiley & Sons Ltd Print ISBN 0-471-49903-X Online ISBN 0-470-84174-5
- 30. local network or terminal used. A worldwide unique and uniform addressing is an impor- tant requirement. In the digital mobile communication system GSM (Global System for Mobile Commu- nication), which is the subject of this book, terminal mobility is the predominant issue. Wireless communication has become possible with GSM in any town, any country, and even on any continent. GSM technology contains the essential ``intelligent'' functions for the support of personal mobility, especially with regard to user identi®cation and authentication, and for the localization and administration of mobile users. Here it is often overlooked that in mobile communication networks by far the largest part of the communication occurs over the ®xed network part, which interconnects the radio stations (base stations). Therefore it is no surprise that in the course of further development and evolution of the telecommunication networks, a lot of thought is given to the convergence of ®xed and mobile networks. Today, GSM is used mainly for speech communication, but its use for mobile data communication is growing steadily. The GSM Short Message Service (SMS) is a great success story: several billion text messages are being exchanged between mobile users each month. The driving factor for new (and higher bandwidth) data services is the wire- less access to the Internet. The key technologies that have been introduced in GSM, the General Packet Radio Service (GPRS) and the Wireless Application Protocol (WAP), are also explained in this book. The next generation of mobile communications is known as Universal Mobile Telecom- munication System (UMTS) in Europe and as International Mobile Telecommunication System 2000 (IMT-2000) worldwide. The standardization has already progressed quite far, such that the ®rst networks are expected to start operation in 2002. Despite the differences to GSM (in particular with regard to transmission technique and capacity), it is a clear goal of this future network technology to keep the newly introduced GSM technologies and make them essential components of UMTS/IMT-2000. 1.2 Classi®cation of Mobile Communication Systems This book deals almost exclusively with GSM; however, GSM is only one of many facets of modern mobile communication. Figure 1.1 shows the whole spectrum of today's and ± as far as can be seen ± future mobile communication systems. For the bidirectional ± and hence genuine ± communication systems, the simplest variant is the cordless telephone with very limited mobility (in Europe especially the DECT stan- dard). This technology is also employed for the expansion of digital PBXs with mobile extensions. A related concept is Radio in the Local Loop (RLL) or Wireless Local Loop (WLL). Both concepts require only limited mobility. Local Area Networks (LANs) have also been augmented with mobility functions: Wireless LANs have been standardized and are now offered by several companies. WLANs offer IP- based, wireless data communication with very high bit rates but limited mobility. IEEE 802.11 systems transmit up to 11 Mbit/s, and HIPERLAN will offer up to 25 Mbit/s. Both systems form pico-cellular networks. They are installed, for example, in of®ce environ- 1 Introduction 2
- 31. ments and airports, as supplement or alternative to wired LANs, and they are also consid- ered to be a good supplement to UMTS access technologies. The efforts to ``mobilize'' the Internet are also worth mentioning in this context. A new routing protocol called Mobile IP [48,49] has been developed, which allows a mobile computer to change its point of attachment to the Internet. A further strong innovation impulse for mobile data and multi- media communication is the development of wireless Mobile ATM systems based on the exchange technology Asynchronous Transfer Mode (ATM). Another emerging class of wireless networks is used for short-range communication. Bluetooth, for example, replaces cables by enabling direct wireless information exchange between electronic devices (e.g. between cellular phones, Personal Digital Assistants (PDAs), computers, and peripherals). These networks are also called Body Area Networks or Personal Area Networks. Unlike the mobile technologies mentioned above, they are not based on a ®xed network infrastructure (e.g. base stations). The possibility of building up 1.2 Classi®cation of Mobile Communication Systems 3 Figure 1.1: Overview of contemporary and future mobile communication systems
- 32. such networks in a spontaneous and fast way gave them the name ad hoc networks. WLAN technologies also include the capability for peer-to-peer ad hoc communication (besides the classical client-to-base station transmission modus). GSM belongs to the class of cellular networks, which are used predominantly for public mass communication. They had an early success with analog systems like the Advance Mobile Phone System (AMPS) in America, the Nordic Mobile Telephone (NMT) in Scan- dinavia, or the C-Netz in Germany. Founded on the digital system GSM (with its variants for 900 MHz, 1800 MHz, and 1900 MHz), a market with millions of subscribers world- wide was generated, and it represents an important economic force. A strongly contribut- ing factor to this rapid development of markets and technologies has been the deregulation of the telecommunication markets, which allowed the establishment of new network operators. Another competing or supplementing technology is satellite communication based on Low Earth Orbiting (LEO) or Medium Earth Orbiting (MEO) satellites, which also offers global, and in the long term even broadband, communication services. Trunked radio systems ± in digital form with the European standard Trans European Trunked Radio (TETRA) ± are used for business applications like ¯eet control. They offer private services that are only accessible by closed user groups. Besides bidirectional communication systems, there also exists a variety of unidirectional systems, where subscribers can only receive but not send data. With unidirectional message systems (paging systems) users may receive short text messages. A couple of years ago, paging systems were very popular, since they offered a cost-effective reach- ability with wide-area coverage. Today, the SMS in GSM has replaced the function of paging systems. Some billion SMS messages are being exchanged between mobile GSM users each month. Digital broadcast systems, such as Digital Audio Broadcast (DAB) and Digital Video Broadcast (DVB), are very interesting for wireless transmission of radio and television stations as well as for audio- and video-on-demand and broadband transmission of Internet pages. The path to the future universal telecommunication networks (UMTS/IMT-2000) has been opened with the realization of the personal communication services, Universal Personal Telecommunication (UPT), based on intelligent networks. During the last few years, the huge success of GSM as well as the exploding number of Internet users gave the design and development of third generation mobile systems a new orientation: One of the most important goals in the evolution from GSM to UMTS is to offer an ef®cient and powerful mobile access to the Internet. GSM and its enhancements, however, will remain for many years the technological base for mobile communication, and it continues to open up new application areas. At the moment, the area of mobile e-commerce (e.g. mobile payment with cellular phones, mobile banking) is particularly attractive. Also text-based news services, locating, ¯eet management, telemetry applications, and automatic emergency call systems are of great interest. The techniques and procedures presented in this book are the foundation for such innovative applications. 1 Introduction 4
- 33. 1.3 Some GSM History and Statistics In 1982 the development of a pan-European standard for digital cellular mobile radio was started by the Groupe Spe Âcial Mobile of the CEPT (Confe Ârence Europe Âenne des Admin- istrations des Postes et des Te Âle Âcommunications). Initially, the acronym GSM was derived from the name of this group. After the founding of the European standardization institute ETSI (European Telecommunication Standards Institute), the GSM group became a Tech- nical Committee of ETSI in 1989. After the rapid worldwide proliferation of GSM networks, the name has been reinterpreted as Global System for Mobile Communication. After a series of incompatible analog networks had been introduced in parallel in Europe, e.g. Total Access Communication System (TACS) in the UK, NMT in Scandinavia, and the C-Netz in Germany, work on the de®nition of a Europe-wide standard for digital mobile radio was started in the late 1980s. The GSM was founded, which developed a set of technical recommendations and presented them to ETSI for approval. These proposals were produced by the Special Mobile Group (SMG) in working groups called Sub Tech- nical Committees (STCs), with the following division of tasks: service aspects (SMG 01), radio aspects (SMG 02), network aspects (SMG 03), data services (SMG 04), and network operation and maintenance (SMG 06). Further working groups were mobile station testing (SMG 07), IC card aspects (SGM 09), security (SGM 10), speech aspects (SMG 11), and system architecture (SMG 12) [18]. SGM 05 dealt with future networks and was respon- sible for the initial standardization phase of the next generation of the European mobile radio system, the UMTS. Later, SMG 05 was closed, and UMTS became an independent project and Technical Body of ETSI. In the meantime, the Third Generation Partnership Project (3GPP) has been founded in cooperation with other standardization committees worldwide. Its goal is the composition of the Technical Speci®cations for UMTS. Finally, in July 2000, ETSI announced the closure of the SMG which has been responsible for setting GSM standards for the last 18 years. Their remaining and further work has been transferred to groups inside and outside ETSI; most of the ongoing work has been handed over to the 3GPP. After the of®cial start of the GSM networks during the summer of 1992 (Table 1.1), the number of subscribers has increased rapidly, such that during the fall of 1993 already far more than one million subscribers made calls in GSM networks, more than 80% of them in Germany. On a global scale, the GSM standard also received very fast recognition, as evident from the fact that at the end of 1993 several commercial GSM networks started operation outside Europe, in Australia, Hong Kong, and New Zealand. Afterward, GSM has also been introduced in Brunei, Cameroon, Iran, South Africa, Syria, Thailand, USA and United Arab Emirates. Whereas the majority of the GSM networks operate in the 900 MHz band (GSM900), there are also networks operating in the 1800 MHz band (GSM1800) ± Personal Communication Network (PCN), Digital Communication System (DCS1800) ± and in the United States in the 1900 MHz band (GSM1900) ± Personal Communication System (PCS). These networks use almost completely identical technol- ogy and architecture; they differ essentially only in the radio frequencies used and the pertinent high-frequency technology, such that synergy effects can be taken advantage of, and the mobile exchanges can be constructed with standard components. In parallel to the standardization efforts of ETSI, already in 1987 the then existing prospec- 1.3 Some GSM History and Statistics 5
- 34. tive GSM network operators and the national administrations joined in a group whose members signed a common Memorandum of Understanding (MoU). The MoU Associa- tion was supposed to form a base for allowing the transnational operation of mobile stations using internationally standardized interfaces. In August 2000, the GSM MoU had 394 members which operated GSM networks in 150 countries (see Figure 1.2). Figure 1.2 illustrates the impressive growth in the number of GSM networks and GSM subscribers. In 1997, 6 years after the commercial start of the ®rst GSM networks, GSM 1 Introduction 6 Table 1.1: Time history ± milestones in the evolution of GSM Year Event 1982 Groupe Spe Âcial Mobile established by the CEPT. 1987 Essential elements of wireless transmission are speci®ed, based on prototype evaluation (1986). Memorandum of Understanding (MoU) Association founded in September with 13 members from 12 countries. 1989 GSM becomes an ETSI Technical Committee (TC). 1990 The Phase 1 GSM900 speci®cations (designed 1987±1990) are frozen. Adaptation to DCS1800 commences. 1991 First GSM networks launched. The DCS1800 speci®cations are ®nalized. 1992 Most European GSM networks turn commercial by offering voice communication services. Some 13 networks in 7 countries are ``on air'' by the end of the year. 1993 First roaming agreements in effect. By the end of 1993, 32 networks in 18 countries are operational. 1994 Data transmission capabilities launched. The number of networks rises to 69 in 43 different countries by the end of 1994. 1995 MoU counts 156 members from 86 countries. After the GSM standardization Phase 2 including adaptations and modi®cations for the PCS1900 (Personal Communication System) is passed, the ®rst PCS1900 Network is launched in the USA. Facsimile, data and SMS roaming starts. Video signals are transmitted via GSM for demonstration purposes. An estimated 50 000 GSM base stations are in use all over the world. 1996 January: 120 networks in 71 countries operational. June: 133 networks in 81 countries operational. 1997 July: 200 GSM networks from 109 countries operational, amounting to 44 million subscribers worldwide. 1998 January: 268 GSM networks with 70 million subscribers worldwide. End of 1998: 320 GSM networks in 118 countries with 135 million subscribers worldwide. 1999 Wireless Application Protocol (WAP). End of 1999: 130 countries, 260 million subscribers. 2000 August: 362 million users. General Packet Radio Service (GPRS).
- 35. had 68 million users and thus a share of approx. 28% of the worldwide mobile market. In the following year, the subscriber number almost doubled, and it doubled again by the beginning of 2000. At the time of writing, in September 2000, there were about 380 million subscribers in all three frequency bands (900 MHz, 1800 MHz, 1900 MHz). In total, there were 373 networks in 142 countries in operation. The share of GSM in the worldwide radio communication market has thus grown up to 60% (of 635 million users) and is still rising. If we consider only digital systems, GSM is even more successful; its market share was over 68% in the middle of 2000. The largest market is Europe with 64% of all subscribers, followed by the Asian Paci®c region with 28%. Moreover, China and many African and South-American countries are operating GSM networks, which opens up a market with substantial growth possibilities. It is expected that in the year 2003 over 600 million people will be using GSM. Relevant numbers can be obtained from the Web page of the GSM Association at http://www.gsmworld.com. All of these networks have implemented Phase 1 of the GSM standard, or the later de®ned PCN/PCS version of it. In many places, additional services and service characteristics of GSM Phase 2 have also been realized. Phase 1 is essentially the basis for this book, but we will also go into important developments of Phase 2 and Phase 21. 1.4 Overview of the Book The remainder of this book is as follows. In Chapter 2, we give an introduction to radio channel characteristics and the cellular principle. The understanding of duplex and multi- ple access schemes serves as the basis for understanding GSM technology. Chapter 3 introduces the GSM system architecture and addressing. It explains the basic structure and elements of a GSM system and their interfaces as well as the identi®ers of users, equipment, and system areas. The GSM services are covered in Chapter 4. Next, Chapter 5 deals with the physical layer at the air interface (How is speech and data transmitted over the radio channel?). Among other things, it describes GSM modulation, multiple access, duplexing, frequency hopping, the logical channels, and synchronization. In Chapter 6, we discuss GSM coding (source coding, speech processing, and channel coding) and mechan- isms for authentication and encryption. Chapter 7 covers the entire protocol architecture of GSM (payload transport and signaling). For example, communication protocols for radio 1.4 Overview of the Book 7 Figure 1.2: GSM network and subscriber statistics. Source: GSM Association, EMC World Cellular Database
- 36. resource management, mobility management, connection management at the air interface are explained. Chapter 8 describes in detail three main principles that are needed for roaming and switching: location registration and update (i.e. How does the network keep track of the user and ®nd him or her when there is an incoming call?), connection establishment and termination, and handover. In Chapter 9 we give an overview of data communication and networking, and Chapter 10 deals with some aspect of network opera- tion. Finally, Chapters 11 and 12 present the latest developments in GSM technology. Chapter 11 explains in detail GPRS which can be used for wireless Internet access. Chapter 12 gives an overview of some more services recently introduced in GSM Phase 21. It covers new speech services, high-rate data services, supplementary services for speech and location services, service platforms, WAP, and Advanced Speech Call Items (ASCI). We conclude this book with an outlook to UMTS. 1 Introduction 8
- 37. The Mobile Radio Channel and the Cellular Principle Many measures, functions and protocols in digital mobile radio networks are based on the properties of the radio channel and its speci®c qualities in contrast to information trans- mission through guided media. For the understanding of digital mobile radio networks it is therefore absolutely necessary to know a few related basic principles. For this reason, the most important fundamentals of the radio channel and of cellular and transmission tech- nology will be presented and brie¯y explained in the following. For a more detailed treatment, see the extensive literature [4,42,50,64]. 2.1 Characteristics of the Mobile Radio Channel The electromagnetic wave of the radio signal propagates under ideal conditions in free space in a radial-symmetric pattern, i.e. the received power PEf, decreases with the square of the distance L from the transmitter: PEf , 1 L2 These idealized conditions do not apply in terrestrial mobile radio. The signal is scattered and re¯ected, for example, at natural obstacles like mountains, vegetation, or water surfaces. The direct and re¯ected signal components are then superimposed at the receiver. This multipath propagation can already be explained quite well with a simple two-path model (Figure 2.1). With this model, one can show that the received power decreases much 2 Figure 2.1: Simpli®ed two-path model of radio propagation GSM Switching, Services and Protocols: Second Edition. Jo È rg Eberspa È cher, Hans-Jo È rg Vo È gel and Christian Bettstetter Copyright q 2001 John Wiley & Sons Ltd Print ISBN 0-471-49903-X Online ISBN 0-470-84174-5
- 38. more than with the square of the distance from the transmitter. We can approximate the received power by considering the direct path and only one re¯ected path (two-path propagation) [42]: PE ˆ P0 4 4pL=l†2 2ph1h2 lL 2 ˆ P0 h1h2 L2 2 and we obtain, under the simpli®ed assumptions of the two-path propagation model, from Figure 2.1, a propagation loss of 40 dB per decade: aE ˆ PE2 PE1 ˆ L1 L2 4 ; aE ˆ 40 log L1 L2 in dB In reality, the propagation loss depends on the propagation coef®cient g, which is deter- mined by environmental conditions: PE , L2g ; 2 # g # 5 In addition, propagation losses are also frequency dependent, i.e. in a simpli®ed way, propagation attenuation increases disproportionately with the frequency. However, multipath propagation not only incurs a disproportionately high path propaga- tion loss. The different signal components reaching the receiver have traveled different distances by virtue of dispersion, infraction, and multiple re¯ections, hence they show different phase shifts. On the one hand, there is the advantage of multipath propagation, that a partial signal can be received even if there is no direct path, i.e. there is no line of sight between mobile and base station. On the other hand, there is a serious disadvantage: the superpositions of the individual signal components having different phase shifts with regard to the direct path can lead, in the worst cases, to cancellations, i.e. the received signal level shows severe disruptions. This phenomenon is called fading. In contrast to this fast fading caused by multipath propagation, there is slow fading caused by shadowing. Along the way traveled by a mobile station, multipath fading can cause signi®cant varia- tions of the received signal level (Figure 2.2). Periodically occurring signal breaks at a distance of about half a wavelength are typically 30±40 dB. The smaller the transmission bandwidth of the mobile radio system, the stronger the signal breaks ± at a bandwidth of about 200 kHz per channel this effect is still very visible [8]. Furthermore, the fading dips become ¯atter as one of the multipath components becomes stronger and more pronounced. Such a dominant signal component arises, for example, in the case of a direct line of sight between mobile and base station, but it can also occur under other conditions. If such a dominant signal component exists, we talk of a Rice channel and Ricean fading, respectively. (S. O. Rice was an American scientist and mathematician.) Otherwise, if all multipath components suffer from approximately equal propagation conditions, we talk of Rayleigh fading. (J. W. Strutt, 3rd Baron Rayleigh, was a British physicist, Nobel prize winner.) During certain time periods or time slots, the transmission can be heavily impacted because of fading or can be entirely impossible, whereas other time slots may be undis- turbed. The results of this effect within the user data are alternating phases, which show either a high or low bit error rate, which is leading to error bursts. The channel thus has 2 The Mobile Radio Channel and the Cellular Principle 10
- 39. memory in contrast to the statistically independent bit errors in memoryless symmetric binary channels. The signal level observed at a speci®c location is also determined by the phase shift of the multipath signal components. This phase shift depends on the wavelength of the signal, and thus the signal level at a ®xed location is also dependent on the transmission frequency. Therefore the fading phenomena in radio communication are also frequency speci®c. If the bandwidth of the mobile radio channel is small (narrowband signal), then the whole frequency band of this channel is subject to the same propagation conditions, and the mobile radio channel is considered frequency-nonselective. Depending on location (Figure 2.2) and the spectral range (Figure 2.3), the received signal level of the channel, however, can vary considerably. On the other hand, if the bandwidth of a channel is large (broadband signal), the individual frequencies suffer from different degrees of fading (Figure 2.3) and this is called a frequency-selective channel [15,54]. Signal breaks because of frequency-selective fading along a signal path are much less frequent for a broadband signal than for a narrowband signal, because the fading holes only shift within the band and the received total signal energy remains relatively constant [8]. Besides frequency-selective fading, the different propagation times of the individual multi- path components also cause time dispersion on their propagation paths. Therefore, signal distortions can occur due to interference of one symbol with its neighboring symbols (``intersymbol interference''). These distortions depend ®rst on the spread experienced by a pulse on the mobile channel, and second on the duration of the symbol or of the interval between symbols. Typical multipath channel delays have a range from half a microsecond in urban areas to about 16±20 ms in mountainous terrain, i.e. a transmitted pulse generates several echoes which reach the receiver with delays of up to 20 ms. In digital mobile radio systems with typical symbol durations of a few microseconds, this can lead to smearing of individual pulses over several symbol durations. In contrast to wireline transmission, the mobile radio channel is a very bad transmission medium of highly variable quality. This can go so far that the channel cuts out for short periods (deep fading holes) or that single sections in the data stream are so much interfered 2.1 Characteristics of the Mobile Radio Channel 11 Figure 2.2: Typical signal in a channel with Rayleigh fading
- 40. with (bit error rate typically 1022 or 1021 ), that unprotected transmission without further protection or correction measures is hardly possible. Therefore, mobile information trans- port requires additional, often very extensive measures, which compensate for the effects of multipath propagation. First, an equalizer is necessary, which attempts to eliminate the signal distortions caused by intersymbol interference. The operational principle of such an equalizer for mobile radio is based on the estimation of the channel pulse response to periodically transmitted, well-known bit patterns, known as the training sequences [4,64]. This allows the determination of the time dispersion of the channel and its compensation. The performance of the equalizer has a signi®cant effect on the quality of the digital transmission. On the other hand, for ef®cient transmission in digital mobile radio, channel coding measures are indispensable, such as forward error correction with error-correcting codes, which allows reduction of the effective bit error rate to a tolerable value (about 1025 to 1026 ). Further important measures are control of the transmitter power and algorithms for the compensation of signal interruptions in fading, which may be of such a short duration that a disconnection of the call would not be appropriate. 2.2 Separation of Directions and Duplex Transmission The most frequent form of communication is the bidirectional communication which allows simultaneous transmitting and receiving. A system capable of doing this is called full-duplex. One can also achieve full-duplex capability, if sending and receiving do not occur simultaneously but switching between both phases is done so fast that it is not noticed by the user, i.e. both directions can be used quasi-simultaneously. Modern digital mobile radio systems are always full-duplex capable. Essentially, two basic duplex procedures are employed: Frequency Division Duplex (FDD) using different frequency bands in each direction, and Time Division Duplex (TDD) which periodically switches the direction of transmission. 2 The Mobile Radio Channel and the Cellular Principle 12 Figure 2.3: Frequency selectivity of a mobile radio channel
- 41. 2.2.1 Frequency Division Duplex (FDD) The frequency duplex procedure has been used already in analog mobile radio systems and is also used in digital systems. For the communication between mobile and base station, the available frequency band is split into two partial bands, to enable simultaneous sending and receiving. One partial band is assigned as uplink (from mobile to base station) and the other partial band is assigned as downlink (from base to mobile station): ² Uplink: transmission band of mobile station ˆ receiving band of base station ² Downlink: receiving band of mobile station ˆ transmission band of base station To achieve good separation between both directions, the partial bands must be a suf®cient frequency distance apart, i.e. the frequency pairs of a connection assigned to uplink and downlink must have this distance band between them. Usually, the same antenna is used for sending and receiving. A duplexing unit is then used for the directional separation, consisting essentially of two narrowband ®lters with steep ¯anks (Figure 2.4). These ®lters, however, cannot be integrated, so pure frequency duplexing is not appropriate for systems with small compact equipment [15]. 2.2.2 Time Division Duplex (TDD) Time duplexing is therefore a good alternative, especially in digital systems with time division multiple access. Transmitter and receiver operate in this case only quasi-simulta- neously at different points in time; i.e. the directional separation is achieved by switching in time between transmission and reception, and thus no duplexing unit is required. Switching occurs frequently enough that the communication appears to be over a quasi- simultaneous full-duplex connection. However, out of the periodic interval T available for the transmission of a time slot only a small part can be used, so that a time duplex system requires more than twice the bit rate of a frequency duplex system. 2.2 Separation of Directions and Duplex Transmission 13 Figure 2.4: Frequency and time duplex (schematic)
- 42. 2.3 Multiple Access Procedures The radio channel is a communication medium shared by many subscribers in one cell. Mobile stations compete with one another for the frequency resource to transmit their information streams. Without any other measures to control simultaneous access of several users, collisions can occur (multiple access problem). Since collisions are very undesirable for a connection-oriented communication like mobile telephony, the individual subscri- bers/mobile stations must be assigned dedicated channels on demand. In order to divide the available physical resources of a mobile system, i.e. the frequency bands, into voice channels, special multiple access procedures are used which are presented in the following (Figure 2.5). 2.3.1 Frequency Division Multiple Access (FDMA) Frequency Division Multiple Access (FDMA) is one of the most common multiple access procedures. The frequency band is divided into channels of equal bandwidth such that each conversation is carried on a different frequency (Figure 2.6). Best suited to analog mobile radio, FDMA systems include the C-Netz in Germany, TACS in the UK, and AMPS in the USA. In the C-Netz, two frequency bands of 4.44 MHz each are subdivided into 222 individual communication channels at 20 kHz bandwidth. The effort in the base station to realize a frequency division multiple access system is very high. Even though the required hardware components are relatively simple, each channel needs its own transceiving unit. Furthermore, the tolerance requirements for the high-frequency networks and the linearity of the ampli®ers in the transmitter stages of the base station are quite high, since a large number of channels need to be ampli®ed and transmitted together [15,54]. One also needs a duplexing unit with ®lters for the transmitter and receiver units to enable full-duplex operation, which makes it nearly impossible to build small, compact mobile stations, since the required narrowband ®lters can hardly be realized with integrated circuits. 2 The Mobile Radio Channel and the Cellular Principle 14 Figure 2.5: Multiple access procedures
- 43. 2.3.2 Time Division Multiple Access (TDMA) Time Division Multiple Access (TDMA) is a more expensive technique, for it needs a highly accurate synchronization between transmitter and receiver. The TDMA technique is used in digital mobile radio systems. The individual mobile stations are cyclically assigned a frequency for exclusive use only for the duration of a time slot. Furthermore, in most cases the whole system bandwidth for a time slot is not assigned to one station, but the system frequency range is subdivided into subbands, and TDMA is used for multiple access to each subband. The subbands are known as carrier frequencies, and the mobile systems using this technique are designated as multicarrier systems (not to be confused with multicarrier modulation). The pan-European digital system GSM employs such a combination of FDMA and TDMA; it is a multicarrier TDMA system. A frequency range of 25 MHz holds 124 single channels (carrier frequencies) of 200 kHz bandwidth each, with each of these frequency channels containing again 8 TDMA conversation channels. Thus the sequence of time slots assigned to a mobile station represents the physical channels of a TDMA system. In each time slot, the mobile station transmits a data burst. The period assigned to a time slot for a mobile station thus also determines the number of TDMA channels on a carrier frequency. The time slots of one period are combined into a so-called TDMA frame. Figure 2.7 shows ®ve channels in a TDMA system with a period of four time slots and three carrier frequencies. The TDMA signal transmitted on a carrier frequency in general requires more bandwidth than an FDMA signal, since because of multiple time use, the gross data rate has to be correspondingly higher. For example, GSM systems employ a gross data rate (modulation data rate) of 271 kbit/ s on a subband of 200 kHz, which amounts to 33.9 kbit/ s for each of the eight time slots. Especially narrowband systems suffer from time- and frequency-selective fading (Figures 2.2 and 2.3) as already mentioned. In addition, there are also frequency-selective co- channel interferences, which can contribute to the deterioration of the transmission quality. In a TDMA system, this leads to the phenomenon that the channel can be very good during one time slot, and very bad during the next time slot when some bursts are strongly interfered with. On the other hand, a TDMA system offers very good opportunities to 2.3 Multiple Access Procedures 15 Figure 2.6: Channels of an FDMA system (schematic)
- 44. attack and drastically reduce such frequency-selective interference by introducing a frequency hopping technique. With this technique, each burst of a TDMA channel is transmitted on a different frequency (Figure 2.8). In this technique, selective interference on one frequency at worst hits only every ith time slot, if there are i frequencies available for hopping. Thus the signal transmitted by a frequency hopping technique uses frequency diversity. Of course, the hopping sequences 2 The Mobile Radio Channel and the Cellular Principle 16 Figure 2.7: TDMA channels on multiple carrier frequencies Figure 2.8: TDMA with use of frequency hopping technique
- 45. must be orthogonal, i.e. one must ascertain that two stations transmitting in the same time slot do not use the same frequency. Since the duration of a hopping period is long compared to the duration of a symbol, this technique is called slow frequency hopping. With fast frequency hopping, the hopping period is shorter than a time slot and is of the order of a single symbol duration or even less. This technique then belongs already to the spread spectrum techniques of the family of code division multiple access techniques, Frequency Hopping CDMA (FH-CDMA) (see Section 2.3.3). As mentioned above, for TDM access, a precise synchronization between mobile and base station is necessary. This synchronization becomes even more complex through the mobility of the subscribers, because they can stay at varying distances from the base station and their signals thus incur varying propagation times. First, the basic problem is to determine the exact moment when to transmit. This is typically achieved by using one of the signals as a time reference, like the signal from the base station (downlink, Figure 2.9). On receiving the TDMA frame from the base station, the mobile can synchronize and transmit time slot synchronously with an additional time offset (e.g. three time slots in Figure 2.9). Another problem is the propagation time of the signals, so far ignored. It also depends on the variable distance of the mobile station from the base. These propagation times are the reason why the signals on the uplink arrive not frame-synchronized at the base, but with variable delays. If these delays are not compensated, collisions of adjacent time slots can occur (Figure 2.9). In principle, the mobile stations must therefore advance the time-offset between reception and transmission, i.e. the start of sending, so much that the signals arrive frame-synchronous at the base station. 2.3 Multiple Access Procedures 17 Figure 2.9: Differences in propagation delays and synchronization in TDMA systems
- 46. 2.3.3 Code Division Multiple Access (CDMA) Systems with Code Division Multiple Access (CDMA) are broadband systems, in which each subscriber uses the whole system bandwidth (similar to TDMA) for the complete duration of the connection (similar to FDMA). Furthermore, usage is not exclusive, i.e. all the subscribers in a cell use the same frequency band simultaneously. To separate the signals, the subscribers are assigned orthogonal codes. The basis of CDMA is a band- spreading or spread spectrum technique. The signal of one subscriber is spread spectrally over a multiple of its original bandwidth. Typically, spreading factors are between 10 and 1000; they generate a broadband signal for transmission from the narrowband signal, and this is less sensitive to frequency-selective interference and disturbances. Furthermore, the spectral power density is decreased by band spreading, and communication is even possi- ble below the noise threshold [15]. 2.3.3.1. Direct Sequence CDMA A common spread-spectrum procedure is the direct sequence technique (Figure 2.10). In it the data sequence is multiplied directly ± before modulation ± with a spreading sequence to generate the band-spread signal. The bit rate of the spreading signal, the so-called chip rate, is obtained by multiplying the bit rate of the data signal by the spreading factor, which generates the desired broadening of the signal spectrum. Ideally, the spreading sequences are completely orthogonal bit sequences (``codes'') with disappearing cross-correlation functions. Since such completely orthogonal sequences cannot be realized, practical systems use bit sequences from pseudo noise (PN) generators to spread the band [15,54]. For despreading, the signal is again multiplied with the spreading sequence at the receiver, which ideally recovers the data sequence in its original form. 2 The Mobile Radio Channel and the Cellular Principle 18 Figure 2.10: Principle of spread spectrum technique for direct sequence CDMA
- 47. Thus one can realize a code-based multiple access system. If an orthogonal family of spreading sequences is available, each subscriber can be assigned his or her own unique spreading sequence. Because of the disappearing cross-correlation of the spreading sequences, the signals of the individual subscribers can be separated in spite of being transmitted in the same frequency band at the same time. In a simpli®ed way, this is done by multiplying the received summation signal with the respective code sequence (Figure 2.11): s t†cj t† ˆ cj t† X n iˆ1 di t†ci t† ˆ dj t† with cj t†ci t† ˆ 0; i ± j 1; i ˆ j ( Thus, if direct sequence spreading is used, the procedure is called Direct Sequence Code Division Multiple Access (DS-CDMA). 2.3.3.2. Frequency Hopping CDMA Another possibility for spreading the band is the use of a fast frequency hopping technique. If one changes the frequency several times during one transmitted data symbol, a similar spreading effect occurs as in case of the direct sequence procedure. If the frequency hopping sequence is again controlled by orthogonal code sequences, another multiple access system can be realized, the Frequency Hopping CDMA (FH-CDMA). 2.3 Multiple Access Procedures 19 Figure 2.11: Simpli®ed scheme of code division multiple access (uplink)
- 48. 2.3.4 Space Division Multiple Access (SDMA) An essential property of the mobile radio channel is multipath propagation, which leads to frequency-selective fading phenomena. Furthermore, multipath propagation is the cause of another signi®cant property of the mobile radio channel, the spatial fanning out of signals. This causes the received signal to be a summation signal, which is not only determined by the Line of Sight (LOS) connection but also by an undetermined number of individual paths caused by refractions, infractions, and re¯ections. In principle, the directions of incidence of these multipath components could therefore be distributed arbitrarily at the receiver. Especially on the uplink from the mobile station to the base station, there is, however, in most cases a main direction of incidence (usually LOS), about which the angles of inci- dence of the individual signal components are scattered in a relatively narrow range. Frequently, the essential signal portion at the receiver is distributed only over an angle of a few tens of degrees. This is because base stations are installed wherever possible as free-standing units, and there are no interference centers in the immediate neighborhood. This directional selectivity of the mobile radio channel, which exists in spite of multipath propagation, can be exploited by using array antennas. Antenna arrays generate a direc- tional characteristic by controlling the phases of the signals from the individual antenna elements. This allows the receiver to adjust the antenna selectively to the main direction of incidence of the received signal, and conversely to transmit selectively in one direction. This principle can be illustrated easily with a simple model (Figure 2.12). The individual multipath components bis1(t) of a transmitted signal s1(t) propagate on different paths such that the multipath components incident at an antenna under the angle ui differ in amplitude and phase. If one considers an array antenna with M elements (M ˆ 4 in Figure 2.12) and a wave front of a multipath component incident at angle ui on 2 The Mobile Radio Channel and the Cellular Principle 20 Figure 2.12: Multipath signal at an antenna array
- 49. this array antenna, then the received signals at the antenna elements differ mainly in their phase ± each shifted by Dw (Figure 2.12) ± and amplitude. In this way, the response of the antenna to a signal incident at angle ui can be characterized by the complex response vector ~ a ui† which de®nes amplitude gain and phase of each antenna element relative to the ®rst antenna element (a1 ˆ 1): ~ a ui† ˆ a1 ui† a2 ui† ¼ aM ui† 2 6 6 6 6 6 6 4 3 7 7 7 7 7 7 5 ˆ 1 a2 ui† ¼ aM ui† 2 6 6 6 6 6 6 4 3 7 7 7 7 7 7 5 The Nm multipath components (Nm ˆ 3 in Figure 2.12) of a signal s1(t) generate, depend- ing on the incidence angle ui, a received signal vector ~ x1 t† which can be written with the respective antenna response vector and the signal of the ith multipath bis1(t) shifted in amplitude and phase against the direct path s1(t) as ~ x1 t† ˆ ~ a u1†s1 t† 1 X Nm iˆ2 ~ a ui†bis1 t† ˆ ~ a1s1 t† In this case, the vector ~ a1 is also designated the spatial signature of the signal s1(t), which remains constant as long as the source of the signal does not move and the propagation conditions do not change [65]. In a multi-access situation, there are typically several sources (Nq); this yields the following result for the total signal at the array antenna: neglecting noise and interferences, ~ x t† ˆ X Nq jˆ1 ~ ajsj t† From this summation signal, the signals of the individual sources are separated by weight- ing the received signals of the individual antenna elements with a complex factor (weight vector ~ wi), which yields ~ wH i ~ aj ˆ 0; i ± j 1; i ˆ j ( For the weighted summation signal [65] one gets ~ wH i ~ x t† ˆ X Nq jˆ1 ~ wH i ~ ajsj t† ˆ si t† Under ideal conditions, i.e. neglecting noise and interference, the signal si(t) of a single source i can be separated from the summation signal of the array antenna by using an appropriate weight vector during signal processing. The determination of the respectively optimal weight vector, however, is a nontrivial and computation-intensive task. Because of the considerable processing effort and also because of the mechanical dimensions of the antenna ®eld, array antennas are predominantly used in base stations. 2.3 Multiple Access Procedures 21
- 50. So far only the receiving direction has been considered. The corresponding principles, however, can also be used for constructing the directional characteristics of the transmitter. Assume symmetric propagation conditions in the sending and receiving directions, and assume the transmitted signals si(t) are weighted with the same weight vector ~ wi as the received signal, before they are transmitted through the array antenna; then one obtains the following summation signal radiated by the array antenna: ~ y t† ˆ X Nq jˆ1 ~ wjsj t† and for the signal received on the ith opposite side, respectively: ^ si t† ˆ ~ aH i ~ y t† ˆ X Nq jˆ1 ~ aH i ~ wjsj t† ˆ si t† Thus, by using array antennas, one can separate the simultaneously received signals of spatially separated subscribers by exploiting the directional selectivity of the mobile radio channel. Because of the use of intelligent signal processing and corresponding control algorithms, such systems are also known as systems with intelligent antennas. The directional characteristics of the array antenna can be controlled adaptively such that a signal is only received or transmitted in exactly the spatial segment where a certain mobile station is currently staying. On the one hand, one can thus reduce co-channel interference in other cells, and on the other hand, the sensitivity against interference can be reduced in the current cell. Furthermore, because of the spatial separation, physical channels in a cell can be reused, and the lobes of the antenna diagram can adaptively follow the movement of mobile stations. In this case, yet another multiple access technique (Figure 2.13) is de®ned and known as Space Division Multiple Access (SDMA). SDMA systems are currently the subject of intensive research. The SDMA technique can be combined with each of the other multiple access techniques (FDMA, TDMA, CDMA). This enables intracellular spatial channel reuse, which again increases the network capa- city [29]. This is especially attractive for existing networks which can use an intelligent implementation of SDMA by selectively upgrading base stations with array antennas, appropriate signal processing, and respective control protocols. 2 The Mobile Radio Channel and the Cellular Principle 22 Figure 2.13: Schematic representation of spatial multiple access (uplink)
- 51. 2.4 Cellular Technology Because of the very limited frequency bands, a mobile radio network has only a relatively small number of speech channels available. For example, the GSM system has an alloca- tion of 25 MHz bandwidth in the 900 MHz frequency range, which amounts to a maximum of 125 frequency channels each with a carrier bandwidth of 200 kHz. Within an eightfold time multiplex for each carrier, a maximum of 1000 channels can be realized. This number is further reduced by guardbands in the frequency spectrum and the overhead required for signaling (Chapter 5). In order to be able to serve several 100 000 or millions of subscri- bers in spite of this limitation, frequencies must be spatially reused, i.e. deployed repeat- edly in a geographic area. In this way, services can be offered with a cost-effective subscriber density and acceptable blocking probability. 2.4.1 Fundamental De®nitions This spatial frequency reuse concept led to the development of cellular technology, which allowed a signi®cant improvement in the economic use of frequencies. The essential characteristics of the cellular network principle are as follows: ² The area to be covered is subdivided into cells (radio zones). For easier manipulation, these cells are modeled in a simpli®ed way as hexagons (Figure 2.14). Most models show the base station in the middle of the cell. ² To each cell i a subset of the frequencies fbi is assigned from the total set (bundle) assigned to the respective mobile radio network. Two neighboring cells must never use the same frequencies, since this would lead to severe co-channel interference from the adjacent cells. ² Only at distance D (the frequency reuse distance) can a frequency from the set fbi be reused (Figure 2.4), i.e. cells with distance D to cell i are assigned one or all of the frequencies from the set fb1 belonging to cell i. If D is chosen suf®ciently large, the co- channel interference remains small enough not to affect speech quality. ² When a mobile station moves from one cell to another during an ongoing conversation, an automatic channel/frequency change occurs (handover), which maintains an active speech connection over cell boundaries. The spatial repetition of frequencies is done in a regular systematic way, i.e. each cell with the frequency allocation fbi (or one of its frequencies) sees its neighbors with the same frequencies again at a distance D (Figure 2.14). Therefore there exist exactly six such next neighbor cells. Independent of form and size of the cells ± not only in the hexagon model ± the ®rst ring in the frequency set contains six co-channel cells (see also Figure 2.15). 2.4.2 Signal-to-Noise Ratio The interference caused by neighboring cells is measured as the signal-to-noise ratio: W ˆ useful signal disturbing signal ˆ useful signal neighbor cell interference 1 noise 2.4 Cellular Technology 23
- 52. This ratio of the useful signal to the interfering signal is usually measured in decibels (dB) and called the Signal-to-Noise Ratio (SNR). The intensity of the interference is essentially a function of co-channel interference depending on the frequency reuse distance D. From the viewpoint of a mobile station, the co-channel interference is caused by base stations at distance D from the current base station. A worst-case estimate for the signal-to-noise ratio W of a mobile station at the border of the covered area at distance R from the base station can be obtained, subject to propagation losses, by assuming that all six neighboring inter- fering transmitters operate at the same power and are approximately equally far apart (distance D large against cell radius R) [42]: W ˆ P0R2g X 6 iˆ1 Pi 1 N P0R2g X 6 iˆ1 P0D2g 1 N ˆ P0R2g 6P0D2g 1 N By neglecting the noise N we obtain the following approximation for the Carrier-to- Interference Ratio C/I (CIR): W C I ˆ R2g 6D2g ˆ 1 6 R D 2g Therefore the signal-to-noise ratio depends essentially on the ratio of the cell radius R to the frequency reuse distance D. From these considerations it follows that for a desired or needed signal-to-noise ratio W at a given cell radius, one must choose a minimum distance for the frequency reuse, above which the co-channel interference fall below the required threshold. 2.4.3 Formation of Clusters The regular repetition of frequencies results in a clustering of cells. The clusters generated 2 The Mobile Radio Channel and the Cellular Principle 24 Figure 2.14: Model of a cellular network with frequency reuse
- 53. in this way can comprise the whole frequency band. In this case all of the frequencies in the available spectrum are used within a cluster. The size of a cluster is characterized by the number of cells per cluster k, which determines the frequency reuse distance D. Figure 2.15 shows some examples of clusters. The numbers designate the respective frequency sets fbi used within the single cells. For each cluster the following holds: ² A cluster can contain all the frequencies of the mobile radio system. ² Within a cluster, no frequency can be reused. The frequencies of a set fbi may be reused at the earliest in the neighboring cluster. ² The larger a cluster, the larger the frequency reuse distance and the larger the signal-to- noise ratio. However, the larger the values of k, the smaller the number of channels and the number of active subscribers per cell. The frequency reuse distance D can be derived geometrically from the hexagon model depending on k and the cell radius R: D ˆ R 3k p The signal-to-noise ratio W [42] is then W ˆ R2g 6D2g ˆ R2g 6 R 3k p 2g ˆ 1 6 3k†g=2 According to measurements one can assume that, for good speech understandability, a carrier-to-interference ratio (CIR) of about 18 dB is suf®cient. Assuming an approximate propagation coef®cient of g ˆ 4, this yields the minimum cluster size 10 logW $ 18 dB; W $ 63:1 ) D 4:4R 2.4 Cellular Technology 25 Figure 2.15: Frequency reuse and cluster formation
- 54. 1 6 3k†g=2 ˆ W $ 63:1 ) k $ 6:5 ) k ˆ 7 These values are also con®rmed by computer simulations, which have shown that for W ˆ 18 dB a reuse distance D ˆ 4:6R is needed [42]. In practically implemented networks, one can ®nd other cluster sizes, e.g. k ˆ 3 and k ˆ 12. A CIR of 15 dB is considered a conservative value for network engineering. The cellular models mentioned so far are very idealized for illustration and analysis. In reality, cells are neither circular nor hexagonal; rather they possess very irregular forms and sizes because of variable propagation conditions. An example of a possible cellular plan for a real network is shown in Figure 2.16, where one can easily recognize the individual cells with the assigned channels and the frequency reuse. Especially obvious are the different cell sizes, which depend on whether it is an urban, suburban, or rural area. Figure 2.16 gives an impression of the approximate contours of equal signal power around the individual base stations. In spite of this representation, the precise ®tting of signal power contours remains an idealization. The cell boundaries are after all blurred and de®ned by local thresholds, beyond which the neighboring base station's signal is received stronger than the current one. 2 The Mobile Radio Channel and the Cellular Principle 26 Figure 2.16: Cell structure of a real network
- 55. Another Random Document on Scribd Without Any Related Topics
- 59. The Project Gutenberg eBook of Yester und Li: Die Geschichte einer Sehnsucht
- 60. This ebook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online at www.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook. Title: Yester und Li: Die Geschichte einer Sehnsucht Author: Bernhard Kellermann Release date: July 24, 2012 [eBook #40314] Most recently updated: October 23, 2024 Language: German Credits: Produced by Jens Sadowski *** START OF THE PROJECT GUTENBERG EBOOK YESTER UND LI: DIE GESCHICHTE EINER SEHNSUCHT ***
- 62. Bernhard Kellermann Yester und Li. Roman
- 64. Bernhard Kellermann YESTER und LI Die Geschichte einer Sehnsucht • 3. Auflage • BERLIN und LEIPZIG • 1905 Magazin-Verlag Jaques Hegner
- 65. Alle Rechte vom Verleger vorbehalten Gedruckt in der Spamerschen Buchdruckerei zu Leipzig
- 66. I. Ginstermann kam spät in der Nacht nach Hause. Es mochte zwei Uhr sein. Vielleicht auch drei Uhr. Vielleicht noch später. Er wußte es nicht. Langsam, ganz langsam war er durch die Straßen gewandert. Über den Boden seines Zimmers war ein Schleier von Licht ausgebreitet, der leise zitterte, als er die Türe schloß. Der Mond schien durch die Vorhänge. Auf den Blechgesimsen pochte es, dumpf, in unregelmäßigen Zwischenräumen, wie ein Finger. Es sickerte, rieselte, die Tiefe schluckte. Der Schnee ging weg. Ginstermann machte Licht. Es war ihm, als sei noch eben jemand im Zimmer gewesen, als sei er jetzt noch nicht allein. Auf dem Tische lagen seine Manuskripte verstreut, wie er sie am Abend verlassen hatte, die Kleidungsstücke auf den Stühlen, das Kissen auf der Ottomane in der gleichen Lage. Er blickte zum Fenster hinaus, in den dunklen Hof hinab, er übersah den Kram seines Zimmers, die Skizzen an den Wänden. Alles erschien ihm sonderbar, rätselhaft, wie von einem Finger berührt, der es veränderte. Draußen klopften die Tropfen, und es schien, als ob sie eine seltsame Sprache redeten. Ein leiser Hauch drang durch die Vorhänge, und auch der Hauch schien geheimnisvolle Worte mit sich zu führen. Wer spricht zu mir? dachte Ginstermann. Will mir diese Nacht alle Wunder der Welt und meiner Seele zeigen, um mich zu verwirren? Alles schwankt und fällt, was eben noch feststand. Alle Begriffe sind verworren. Ist es nicht, als sei ich aus langem Schlafe erwacht, und folgten mir wunderbare Träume in mein Erwachen?
- 67. Wer bin ich? Ich habe vergessen, wer ich bin, und weiß nur, daß ich ein anderer bin, als der ich zu sein glaubte. Und welch geringen Anlasses bedurfte es, um meine Seele zu verwandeln? Wer aber bist du? daß du solche Macht über mich hast? Wer aber bist du, daß ich nicht an dir vorübergehen kann wie an anderen Menschen . . . . . . Er sann und sann. Da wurde es Morgen.
- 68. II. Diesen Abend ereignete sich etwas Außergewöhnliches: Ginstermann ging mit zwei Damen über die Straße. Mit zwei jungen Damen in eleganten Abendmänteln. Ginstermann, der wochenlang seine vier Wände nicht verließ, den man nie in Begleitung sah, den noch niemand mit einer Dame hatte gehen sehen. Sie kamen von einer Abendunterhaltung, die Kapelli, der Bildhauer, seinen Bekannten anläßlich seiner Hochzeit gab. Kapelli, der seit Jahren mit seiner Geliebten zusammenlebte, war schließlich, da sie ein Kind erwarteten, auf den Gedanken gekommen, sich trauen zu lassen. Ginstermann wohnte im gleichen Hause und war mit den Bildhauersleuten befreundet. Die Damen gehörten zu Kapellis Kundschaft und waren aus irgend einem Grunde eingeladen worden. Kurz nach zehn Uhr brachen die Mädchen wieder auf. Sie waren kaum eine Stunde dagewesen. Fräulein Martha Scholl hätte noch große Lust gehabt, länger zu bleiben. Sie äußerte das in Worten und Mienen. Aber Fräulein Bianka Schuhmacher war nicht dazu zu bewegen, trotzdem Kapelli und seine Frau alles aufboten. Sie gab vor, sie werde zu Hause erwartet. Vielleicht langweilte sie die Gesellschaft auch. Zur allgemeinen Verwunderung hatte sich Ginstermann erboten, die Damen nach Hause zu begleiten. Sie gingen alle drei langsam, wie vornehme Leute. Die Mädchen dicht nebeneinander, er links von ihnen. In gemessenem Abstand, als sei noch eine vierte Person da, die unsichtbar zwischen ihm und den Mädchen schreite. Es sei nicht einmal kalt. Nein, sehr angenehm sogar.
- 69. Und man habe doch erst März. Im März sei es für gewöhnlich noch sehr unfreundlich. Ginstermann erwiderte nichts mehr darauf, und sie schwiegen wieder. Eine eigentümliche Unruhe erfüllte ihn. Die Ereignisse des Abends hatten ihn verwirrt. Noch immer hörte er die Worte, mit denen er den Mädchen seine Begleitung angeboten, in sich klingen. Das war gar nicht seine Stimme gewesen. Wieder und wieder sah er sich aufstehen, den Stuhl unter den Tisch schieben und Fräulein Bianka Schuhmacher in ihre klugen, durchsichtigen Augen hinein fragen, ob es ihnen nicht unangenehm wäre, wenn er mit ihnen ginge. Das war alles so unerklärlich rasch und ohne eigenen Willen geschehen. Er erinnerte sich, daß seine Hand zitterte, als er ihr beim Anlegen des Abendmantels behilflich war: der Stoff dieses Mantels hatte sich so sanft angefühlt wie Schnee. Und dann dieses zufällige Wiedersehen . . . Da war wiederum Kapellis Atelier, ein Saal nahezu infolge des Meeres von Zigarettenrauch und der drei feierlich verschleierten Lampen, mit den abgetretenen Teppichen an den Wänden, die wie kostbare Gobelins aussahen, den Oleanderstöcken und der Menge Gesichter, deren Augen glänzten. Und er trat ein. Verwirrt durch den ungewöhnlichen Anblick, den Kopf noch erfüllt von der Arbeit des Tages. Und all die glänzenden Augen richteten sich auf ihn, Hände winkten, und man rief seinen Namen. „Bravo, der Einsiedler!“ Da war Kapelli, im schwarzen Festrock, der ihn veränderte, mit dem gutmütigen Philistergesicht und den genialen Augen; Frau Trud, lachend wie immer, das goldblonde Köpfchen wiegend, eine zinnoberrote Schleife vorgebunden; die Faunsmaske des Malers Ritt, das verschwimmende bleiche Gesicht der Malerin von Sacken, ganz in Schwarz, eine Tragödie in ihrem Lächeln; Knut Moderson, der Karikaturenzeichner, Maler Maurer, der Lyriker Glimm, der blonde Goldschmitt und eine Menge anderer noch. Und da waren zwei junge Damen, die er nicht kannte, und bei denen man ihm seinen Platz anwies.
- 70. Zwei verdutzte, erstaunte, ihn anstaunende braune Augen, mit Goldflitterchen darin, ein Puppengesichtchen, frisch, glänzend wie eine Kirsche, Grübchen in den Wangen. Und daneben zwei kühle, fragende Augen, blaßgrün wie Wasser, die jeden Zug seines Gesichtes mit einem Blick aufnahmen, ein feines, nervöses Antlitz, gleichsam durchsichtig, wie es Brustleidende haben. Ein Legendenantlitz. Und dieses Antlitz hatte er schon gesehen. Hatte er schon gesehen. Ah — Kapelli hatte es modelliert. Es war die Büste die er „Seherin“ genannt hatte. Das waren diese schmalen, halbgeöffneten Lippen, die zögernd den Duft von Blüten einzuschlürfen schienen. Und die markierten Schläfen, die bebenden, elfenbeinernen Nasenflügel. Wenn sich dieses schmale Antlitz zurückneigte, und die großen Augen sich auf ein Ziel in der Ferne hefteten, so war es ganz genau die „Seherin“. Kapelli hatte nicht umsonst seine prächtigen Augen. Aber dieses Legendenantlitz hatte er früher schon gesehen. Irgendwo, vor Jahren vielleicht. Er täuschte sich unmöglich. Und während sie rings von Siry sprachen, dem Dichter Siry, der sich vor einigen Wochen erschoß, sann er darüber nach, wo er dieses Gesicht schon gesehen hatte. Und da fiel es ihm ein. Wie ein Blitz durchfuhr es ihn. Welch ein Zufall! Nun wußte er es. Das war im Hoftheater, vorigen Winter. Und er sann . . . . . Der blonde Goldschmitt, der ewig Lebendige, erzählte irgend etwas. Von seinen Fußwanderungen. Vorigen Sommer. Von mittelalterlichen Städtchen, die in der Dämmerung versanken und von Kornfeldern, die in der Sonne kochten, und vom Meer, das er in einer Sommernacht hatte leuchten sehen. Und vom Walde — ah, vom Walde. Goldschmitt, der Malerdichter. Er sprach nur in Superlativen, ebenso seine Mienen. Und fortwährend strich er sich mit den Fingern über das strähnige Haar, das von der Stirne bis in den Nacken lief, eine einzige Welle. Und Dichter Glimm saß, ohne eine Silbe zu sprechen, die Zigarette zwischen den Lippen, durch die
- 71. Wimpern ins Licht blickend, und ließ sich durch Goldschmitts Schilderungen Stimmungen suggerieren. Dieser Goldschmitt erzählte in der Tat gut. Er sah impressionistisch, immer Licht, immer Farbe, ein roter Klecks auf dem Kirchturmdach, und das Bild war fertig. Dazwischen kam Kapelli mit der Zigarettenschachtel und beugte sich über den Tisch, so daß ein Büschel grauer Haare über seine Stirne fiel. Wenn er sprach, so funkelten die Vokale gleich leuchtenden Steinen, und man verspürte Lust, ihn zum Singen aufzufordern. An den Tischen lärmten und lachten sie, und ewig war Ritts nasale Stimme zu hören. Und Fräulein Scholl hing mit den Blicken an Goldschmitts Lippen und hielt die Zigarette mit steifen, ungewohnten Fingern, hier und da Tabak von den Lippen nehmend. Sie schüttelte den Kopf, wenn sie lachte, und die Wellen ihrer Haare wippten. Diese Haare waren von genau der gleichen Farbe wie ihre Augen. Ihre Zähne waren schneeweiß, klein, Puppenzähne, und zuweilen blitzte eine goldene Plombe auf. Manchmal unterbrach sie den Erzählenden und begann eine ähnliche Schilderung, um mitten darin abzubrechen, da ihr der Ausdruck fehlte. Dann blies sie stets eine dünne Rauchwolke in die Luft. Daneben ihre Freundin, reserviert im Wesen. Sie lächelte liebenswürdig. Sie rauchte nicht. Sie hielt die Augen auf Goldschmitt gerichtet und brachte ihn einigemal in Verwirrung, als er sich ungeschickt ausdrückte. Es war, als beobachte sie genau, was um sie vorging, und bilde sich über alles ein Urteil. Dazwischen wieder lachte sie herzlich, wie ein Kind, als sei sie für einen Augenblick eine andere geworden. Wenn sie sprach, so sprach sie schön und ohne Hast. Ihre Stimme erinnerte an die Töne einer Geige, sie war weich und gedämpft. Diese Stimme drang tiefer als in die Ohren und erweckte das Bedürfnis, sie bei geschlossenen Lidern zu hören. Gleichzeitig klang der kühle Stolz einer sich abschließenden Seele aus ihr. Und er saß und sann.
- 72. Wie seltsam es doch ist, dachte er, das Schicksal hat die Menschen an Fäden und führt sie zusammen und auseinander und wieder zusammen, je nach seiner Laune. Hier also traf er sie wieder. Schon angesichts der Büste hatten seine Gedanken hartnäckig eine Erinnerung in ihm auszulösen gesucht. Er entsann sich dessen noch deutlich. Aber nun stand sie klar vor seinen Augen, wie an jenem Abend. In leuchtend weißem Kleide sah er sie vor sich, auf Marmorstufen stehend, mitten im Licht. Und sie hielt die großen Augen auf ihn geheftet, gleichsam erstarrt vor Freude. Als sei er ihr Geliebter und nach langer Fahrt über ferne Meere unerwartet zurückgekehrt. Er stieg die Stufen zum Foyer hinauf und hielt unwillkürlich den Schritt an, betroffen durch den Ausdruck dieses Blickes. Und sah sie an. Das alles währte nicht länger als eine Sekunde. Es war sonderbar, wie ein Rätsel. Sie hatte ihn heute nicht einmal wieder erkannt. Trotzdem war es ihm, als ob ihr Blick zuweilen über seine Züge tastete und etwas suchte. Dann erhoben sich die Damen, und auch er stand auf. Und ohne eigentlich daran gedacht zu haben, bot er ihnen seine Begleitung an. Und nun ging er neben ihnen her. Und war noch so verwirrt durch die Eindrücke des Abends, daß er kein Wort zu sprechen vermochte. All die vielen Gesichter schwebten ihm noch vor Augen, lächelnd, lachend, mit den Augen zwinkernd, er hörte immer noch das Gewirr von Stimmen, und da war wieder die verschleierte Lampe, das mit Zigarettenasche bestreute Tischtuch, Goldschmitt, Glimm, Fräulein Scholl und daneben Fräulein Schuhmacher. Er sah sie ganz deutlich vor sich. Ihre hellen Augen, ihre schmalen Lippen, die leise und vornehm lächelten, ihre Hand. Er hatte noch nie eine solche Hand gesehen. Sie erschien ihm wie ein denkendes, selbständiges Wesen. Und wieder empfand er jenen undefinierbaren Schrecken wie in jenem Moment, da er in seinem Gegenüber jene Dame vom Hoftheater entdeckte.
- 73. Ah — das war auch zu sonderbar. Das mochte jetzt über ein Jahr her sein. Wiederum aber war es ihm unerklärlich, wie ihn dieser alltägliche Zufall in derartige Aufregung versetzen konnte. War ihm diese Spannung rätselhaft, mit der er jeder Bewegung dieses Mädchens gefolgt war, jeder noch so unmerklichen Veränderung dieses durchsichtigen Antlitzes. Das war absolut nicht mehr die Objektivität, mit der er sonst seine Modelle studierte. Wurde er nicht komisch vor sich selbst, daß er mit den jungen Damen lange Straßen entlang ging? Wenn er aber ehrlich sein wollte, so mußte er sich gestehen, daß es ihm auf der anderen Seite unangenehm gewesen wäre, hätte ein anderer diese Rolle übernommen. Daß es ihm gleichzeitig eine physische Befriedigung bereitete, neben dem schlanken Mädchen einherzugehen. Er dachte an sein verlassenes, dunkles Zimmer, das er liebte nahezu wie einen Menschen. Er sah sich bei der Lampe sitzen und schreiben, wie er es Tag für Tag, seit zwei Jahren gewohnt war. Er sah seine Manuskripte auf dem Tische liegen, mit der großen Rede Rammahs, die er in der Mitte abgebrochen hatte, um zu Kapelli hinunterzusteigen. Es erschien ihm töricht, daß er seine Arbeit im Stiche gelassen hatte. Kapelli hätte es ihm gewiß nicht übel genommen, wenn ihm auch Frau Trud einige Zeit böse gewesen wäre. Nun würde er die große Rede, die Rammah, der Gefangene, an die Königin Lehéhe zu richten hatte, beendigt haben. Rammah, der seinen Kopf aufs Spiel setzte, um noch einmal das Antlitz seiner Geliebten zu sehen. Und er dachte an Rammah und Lehéhe, die Königin. Und wiederholte sich im Geiste die Szene und die Worte, die der Gefangene zuletzt sprach. Rammah sagte: Gib dem Gefangenen eine Hand voll Ton, er wird das Bildnis seines Weibes formen, bei Tag, bei Nacht, in jeder Miene — so formt ich Euer Bildnis, Königin, bei Tag, bei Nacht, aus Wolken, Steinen, Wasser, Bäumen, Wind, in jeder Mime, stolz und milde, lächelnd, strahlend, wie ich es sah.
- 74. Und nun sollte er erzählen, daß ihn seine Qual zu den Mönchen getrieben. Aber seine Rede verwirrte sich. Eine unerklärliche Erregung erschütterte Ginstermanns Wesen. Während er sich diese Worte wiederholte, erschien es ihm, als empfände er sie inniger als am Abend, als kämen sie aus dem Tiefsten seines Wesens. Und Lehéhe, die Königin, hatte sich verändert. Nicht mehr die orientalischen Züge, die schmale gebogene Nase, das blauschwarze glatte Haar, nun trug sie die Züge des Mädchens, das ihm zur Seite schritt . . . . . Ginstermann hüllte sich dichter in den Mantel und gab sich Mühe, auf andere Gedanken zu kommen. Die Gewänder der Mädchen rauschten sanft. Es war ihm, als gingen sie sehr rasch. Diese Vorstellung wurde dadurch verstärkt, daß man ihre Schritte nicht hörte. Es war frischer Schnee gefallen. Die Straßen erschienen breiter und öder. Dunkle, unnatürlich große Fußspuren liefen über die Trottoire. Die Bogenlampen leuchteten trüb, umflimmert von feinem Schneestaub, den ein großes Sieb über sie zu schütteln schien. Dunkle Gestalten tauchten lautlos auf, verschwanden lautlos. Irgendwohin. Schatten gleich, die die Straßen einer toten Stadt durchwandern. Und sie selbst glichen solchen Schatten. Ginstermann hatte das peinliche Gefühl, daß die Mädchen auf eine Anrede seinerseits warteten. Ja, vielleicht belustigten sie sich über ihn, der nichts wußte, als vor sich hinzugrübeln. Es war nicht ausgeschlossen, daß Fräulein Scholl ihre Freundin in den Arm kniff und in sich hineinkicherte. Aber ein Seitenblick überzeugte ihn, daß sie beide in Gedanken versunken waren, die nicht in direktem Zusammenhang mit dieser Wanderung standen. Beide lächelten. Aber dieses Lächeln war grundverschieden. Bei Fräulein Schuhmacher hauchte es aus den halbgeöffneten Lippen, bei Fräulein Scholl sprühte es in den Wangengrübchen. Es schien, als denke die eine über etwas Hübsches nach, das in der Vergangenheit ruhte, die andere über etwas Hübsches, das aus der Zukunft schimmerte.
- 75. Fräulein Schuhmacher ging mit geöffneten Augen und blickte zu Boden, als beobachte sie das Spiel ihres Schattens, der bald vorauseilte, bald unter ihren Schritten durchschlüpfte. Ihr Profil war von vornehmer, reiner Linie. Die Stirne gedrückt und eigensinnig. Der Mund der eines Menschen, der wenig gelacht und viel gelitten hat. Fräulein Scholl hielt die Augen geschlossen, und diese geschlossenen Augen lächelten. Während ihre Freundin leicht vornübergebeugt schritt, das Wippen der Libelle im Gang, ging sie aufrecht, mit steifem Stolze. Den Kopf etwas auf die Brust gesenkt. Man konnte sie sich gut als würdevolle Dame vorstellen. Ginstermann sann darüber nach, was er den Damen sagen könne. Der Wunsch erwachte in ihm, ihnen durch irgend eine Bemerkung aufzufallen. Er war oftmals nahe daran zu beginnen, aber stets fand er die Bemerkung deplaziert oder banal. Die einleitende Bemerkung, einleitende Frage forderten sein Lächeln heraus infolge ihrer Ähnlichkeit mit den Ballgesprächen in den Witzblättern. Mit nervöser Hast suchte er in seinem Kopfe nach einem Gedanken, den er hätte anbringen können. Er hätte sich gern geistreich, witzig gezeigt. Er hätte den Mädchen gern etwas mit nach Hause gegeben, ein kleines souvenir de Ginstermann, etwas, das sie noch beschäftigte, während sie sich entkleideten. Etwas Frappierendes, das sie kopfschüttelnd zu fassen suchten, ein schönes Wort, das noch auf der Schwelle ihres Schlafes vor ihnen schimmerte. Aber seine Gedanken schleppten altes Zeug herbei, das einem jeder von den Lippen ablas, wenn man es aussprechen wollte. Oder Einfälle, die er früher irgendwo geäußert, und suchten ihn zur Kolportage seiner eigenen Gedanken zu verführen. Was sollte er diesen Mädchen sagen? Sollte er ihnen einen Vortrag halten über die Schuld im modernen Drama, über die Phonetik des Dialogs? Über die seelische Armut eines Mädchens aus guter Familie? Über Bücher, Theater, Musik?
- 76. Sollte er ihnen die Grimasse der modernen Gesellschaft mit höhnenden Strichen skizzieren? Sollte er ihnen sagen: Meine Damen, so kahl wie dieser Baum hier ist unsere Zeit an Schönheit und dem Wunsche nach ihr. Aber es werden Generationen kommen, deren Schönheitsdurst so gewaltig sein wird, daß man das herrlichste Weib des Landes, nackt, auf geschmücktem Wagen durch die Stadt führen wird. Was sollte er sagen? Sollte er sagen —? So sehr er sich bemühte, er fand nichts. Er hatte es verlernt, mit Menschen zu verkehren, mit jungen Damen angenehm zu plaudern. Die Jahre seiner Einsamkeit hatten ihm die Lippen verschlossen. Wußte er, was diese Mädchen interessieren konnte? „Ach, wie entzückend!“ tief Fräulein Scholl plötzlich aus und blieb stehen. „Ist es nicht herrlich?“ Der Marmorpalast der Akademie lag vor ihnen. Vom bleichen Lichte des Mondes durchstrahlt, umgeben von dunklen Häusermassen, stieg er empor aus wipfelkahlen Bäumen wie ein heiliges Denkmal, durch eine Luftspiegelung aus einer herrlichen Welt herübergetragen. In seiner mehr denn totenhaften Stille, die nicht mehr das Ohr, nur die Phantasie faßte, in seiner sanften Schönheit stand er außerhalb alles Irdischen, außerhalb der Zeit, bereit, jeden Augenblick zu versinken und trivial-praktische Häuserklumpen zu enthüllen. Ginstermann wußte: Das ist der Palast eines gewaltigen Königs. Der König ist gestorben und liegt aufgebahrt auf dunklem Sarkophage inmitten des Palastes. Zu seinen Füßen kauert sein Weib. Pechpfannen umflammen das Lager. Und morgen wird der Palast in Flammen stehen, und den Platz werden Menschen erfüllen, tränenlos in ihrer Trauer, als ein starkes Volk. Und Priester werden das Blut von tausend Kriegern in die rauchenden Trümmer gießen, dem Geliebten zu opfern. „Ist es nicht überwältigend?“ flüsterte Fräulein Scholl. „Es ist schön,“ sagte Ginstermann. Fräulein Schuhmacher streifte ihn mit einem Blicke, wie um die Gedanken zu erraten, die er ihnen vorenthielt.
- 77. Fräulein Scholl wohnte in der Schackstraße. Sie begleiteten sie bis zur Türe, dann gingen sie weiter. Die Leopoldstraße hinunter. Sie gingen nun allein. Mit der Entfernung der Freundin war die Last auf Ginstermanns Seele um das Doppelte gewachsen. Seine Verwirrung steigerte sich, und er fühlte, wie er die Herrschaft über seine Gedanken verlor. Vergebens strengte er sich an, seine Gefühle zu entwirren. Er empfand wiederum den schwindelartigen Zustand, der ihn ergriff, als er aufstand, um den Damen seine Begleitung anzubieten. Gewohnt, immer Herr der Situation und seiner selbst zu sein, empfand er ihn als eine demütigende Peinigung. Es war ihm, als habe man ihn in eine Narkose versetzt, gegen die sich seine halbbetäubten Sinne erfolglos sträubten. Gleichsam ohne selbständigen Willen schritt er neben diesem Weibe einher. Einem Trabanten ähnlich, der in die Bahn eines mächtigen Sternes geriet. Die Seele dieses Weibes hatte sich der seinigen bemächtigt und lockte ihn mit der Gewalt ihres Rätsels. Diese Situation, das Schweigen, aus dem man heraushören konnte, was man wollte, wurde ihm unerträglich. Er richtete sich auf, steckte die Hände in die Manteltaschen, bemüht, sich vor sich selbst das Aussehen eines gleichgültigen Menschen zu geben. Er hörte ihre Schritte über den Boden gleiten, ihre Kleider rauschen, er bemerkte jede Bewegung ihres Kopfes, ihrer Hand, ohne jedoch sein volles Bewußtsein zurückfinden. Die Straße war schnurgerade, wie ein Lineal. Blendend weiß in der Nähe, von düsterem Rauch erfüllt in der Ferne. Beschneite Pappeln flankierten sie, die ihnen in langsamem Zuge entgegenpilgerten. Dann und wann krauchte ein Schatten heran. Die Helmspitze eines Schutzmannes blitzte auf. Eine Katze überschritt geschmeidig die Straße, behutsam Pfote um Pfote in den Schnee setzend. Jeder, der an ihnen vorüberkam, blickte sie an. War es ein Herr, so musterte er zuerst seine Begleiterin, dann ihn; war es eine Dame, so
- 78. galt ihm der erste Blick. Alle dachten sich etwas. Sie dachten, es sind Liebesleute, die sich gezankt haben und nun still, voneinander entfernt ihre Straße gehen. Oder sie dachten, es sind Leute, denen die aufkeimende Liebe die Lippen verschließt und schwermütige Gedanken eingibt. Während seine Sinne dies mechanisch beobachteten, rang seine Seele mit der fremden Gewalt, die auf ihn eindrang. Er wollte froh sein, wenn er wieder allein war. Auf der andern Seite jedoch fürchtete er diesen Moment und suchte er nach Möglichkeiten, ihn hinauszuschieben. Mit ärgerlichem Schrecken dachte er daran, daß er zum ersten und voraussichtlich zum letzten Male neben diesem Weibe ging, das seiner Seele nicht gleichgültig war. Und daß er es nicht verstanden hatte, diese günstige Lage auszunützen, das Wesen dieses Mädchens zu ergründen, und dadurch seine Gedanken vor der peinigenden Gier zu behüten, mit der sie ein ungelöstes Rätsel zu umkreisen pflegten. Da vernahm er plötzlich ihre Stimme. Er verstand ihre Worte nicht und mußte sich erst ihren Klang ins Gedächtnis zurückrufen, bevor er sie erfaßte. „Kennen Sie denn meine Gedichte?“ antwortete er lächelnd, erfreut, daß das Stillschweigen gebrochen war. Sie hatte gesagt: Ich kenne ein Gedicht von Ihnen, Herr Ginstermann, das sehr schön ist. „Ja,“ erwiderte sie, „ich habe sie gelesen. Ein Herr machte mich darauf aufmerksam. Viele sind mir zu herb, zu bitter, aber dieses eine ist sehr schön, und ich empfand das Bedürfnis, Ihnen das zu sagen, bevor wir uns trennen. Es heißt: Martyrium.“ „Das war mein erstes, Fräulein Schuhmacher.“ „Ihr erstes?“ „Ja. Ich trottete meine Straße. Da kam es. Ganz von selbst, ich hatte früher nie Verse geschrieben.“ Sie schwieg und blickte sinnend zu Boden. Da erschrak Ginstermann. Diese wenigen Worte erlaubten ihr, eine Menge Schlüsse auf sein damaliges Innenleben zu ziehen. „Der Gedanke ist schön, und das Bild ist schön,“ fuhr sie leise fort, „es hat einen tiefen Sinn. Ich kenne kein Gedicht, das einen so tiefen
- 79. Eindruck in mir hinterlassen hätte.“ Er wußte, daß dieses Gedicht gut war, zu seinen besten gehörte. Aber keine einzige Besprechung hatte es besonders hervorgehoben. Um so seltsamer erschien es ihm, daß sie darauf gekommen war. Das Gedicht war sehr einfach. Ein Mann, der vor einem Weibe in unverhüllter Schönheit kniet, bittet es, ihm den Dornenkranz der Liebe, mit dem es ihn krönt, tief, tief ins Haupt zu drücken. „Hier bin ich nun zu Hause,“ sagte Fräulein Schuhmacher und blieb stehen. Sie standen vor einer Villa in modernem Stile, deren originelle Architektur Ginstermann schon früher aufgefallen war. Zwei Fenster der ersten Etage waren matt erhellt, als läge ein Kranker im Zimmer. Ginstermann griff an den Hut, da es sich nicht schickt, eine Dame vor der Türe noch zu verhalten. Aber sie schien es nicht zu bemerken. Ihr Blick ruhte auf seinem Antlitz, und wieder gewann er die Vorstellung, als suche sie nach irgend etwas. „Wir sahen uns übrigens schon einmal,“ begann sie von neuem, und ihr Blick traf voll den seinigen. An diesem Blicke erkannte er sie. Hier ist ein Mensch! dachte er, freudig erschreckend. Er fühlte, wie die Erregung in langer Welle durch seinen Körper lief. Diese Augen waren hell und durchsichtig, als brenne ein Licht hinter ihnen. Er wußte, hinter diesen Augen wohnt jemand. „Ja, im Hoftheater,“ erwiderte er, und er lächelte und blickte ihr in die Augen. Es erschien ihm, als seien sie langjährige Bekannte. „Ich verwechselte Sie damals mit jemandem,“ fuhr sie fort, und ihre Lippen zuckten sonderbar, als unterdrückte sie ein Lächeln. Er habe das sofort bemerkt. Fräulein Schuhmacher blickte zum Himmel empor, aus dem große nasse Flocken fielen. „Es taut,“ sagte sie, „ich glaube, es wird nun wirklich Frühling.“ Das klang einfach, aber eine krankhafte Sehnsucht nach dem Frühling lag in dem Tone ihrer Stimme und den Blicken, mit denen sie die großen Flocken verfolgte.
- 80. Dann bot sie ihm die Hand, indem sie ihm für die Begleitung dankte. Sie sah ihn dabei an, aber es schien, als blickte sie durch ihn hindurch. Ginstermann entgegnete: „Ich danke, Fräulein Schuhmacher.“ Das „Ich“ betonend. Sie blickte ihn mit leichter Verwunderung an. Er aber wiederholte: „Ich danke.“ In der gleichen Betonung. Da drückte sie ihm die Hand, jedoch ohne eine andere Sprache als die der Höflichkeit einer modern denkenden Dame. „Adieu,“ sagte sie, „auf Wiedersehn.“ „Adieu,“ sagte er. Sie nickte und ging. Im Augenblick war sie verschwunden. Ein dunkles, schweres Tor glitt lautlos hinter ihr ins Schloß, lautlos, unaufhaltsam. Ginstermann stand allein auf der Straße. Plötzlich fühlte er, daß es düster und kalt war. Er stand noch eine Weile, dann wandte er sich und machte einige zögernde Schritte. Etwas hielt ihn zurück. Und nun blitzte es auf. Sie hatte gesagt: auf Wiedersehen. Sie hatte gesagt: auf Wiedersehen. Er hörte ganz deutlich ihre geschmeidige, leicht verschleierte Stimme. Aber das allein war es nicht. Er ging wieder auf die Stelle zurück, wo er sich von ihr verabschiedet hatte, gleichsam als höre er hier ihre Stimme mit größerer Deutlichkeit in seinem Gedächtnis wiederklingen. Sie hatte das „Wieder“ betont. Das war es. Es war keine Höflichkeitsformel, mechanisch gesprochen. In dieser Betonung lag der Wunsch, ihn wiederzusehen und zugleich eine gewisse Freude, ihn kennen gelernt zu haben. Nun erst ging er seiner Wege. Nach geraumer Zeit bemerkte er, daß er die verkehrte Richtung eingeschlagen hatte. Er machte Kehrt und überschritt, als er sich der Villa näherte, die Straße, um nicht gesehen zu werden. Im Eckzimmer der ersten Etage war Licht. Rötliches, sanftes Licht, das durch das geöffnete Fenster wie feiner Dunst in die Straße hauchte.
- 81. Er erschrack, ohne zu wissen weshalb, als er es bemerkte. Da wanderte die Flamme einer Kerze an den dunklen Fenstern der anstoßenden Zimmer vorbei und verschwand in dem Zimmer, das matt erleuchtet war. Ginstermann stand, verborgen im Schatten einer Pappel, und wartete. Er wartete lange und in sonderbarer Erregung, als spiele sich in dem Zimmer da droben etwas ab, was entscheidend für sein Leben sei. Und doch war es nur der Besuch eines Kindes bei seiner Mutter, vor dem Schlafengehen. Die großen, weißen Flocken fielen langsam auf ihn herab, ihn gleichsam durch ihr geheimnisvolles, sanftes Abwärtsgleiten in einen Zustand der Betäubung versetzend. Das Licht erschien wieder und wanderte an den Gardinen vorüber. Aus seinem Auf und Ab erkannte er ihren Schritt. Er bildete sich ein, das Schließen einer Türe zu vernehmen. Und nun erschrak er, daß er unwillkürlich tiefer in den Schatten zurücktrat. Sie war ans Fenster gekommen. Und sie blickte genau auf den Baum, der ihn verbarg. Etwas wie eine tödliche Angst packte ihn, sie könne ihn durch den dicken Baum hindurch bemerken. Zum ersten Male sah er, wie schlank sie war! Endlich wandte sie den Kopf, und er atmete auf. Sie trat zurück und schloß das Fenster. Er hörte es, als stände er dicht darunter, über ihre Hand, die den Knopf drehte, flossen die Vorhänge zusammen, und fingen den Schatten ihrer Gestalt auf. Das Verlangen erfaßte ihn, irgend etwas zu unternehmen, zu rufen, irgend etwas zu rufen, nur um sie noch eine Sekunde zurückzuhalten. Da wurden die Vorhänge licht. Er ging nach Hause.
- 82. III. Ginstermann verlebte die folgenden Wochen in gewohnter Zurückgezogenheit. Wie früher ließ er sich des Mittags seine Mahlzeit auf das Zimmer bringen, um nicht genötigt zu sein, in einem lärmenden Lokal zu speisen und mit gleichgiltigen Leuten ein Gespräch führen zu müssen. Nur des Abends, wenn die Dämmerung herabsank, und es dunkler war, als wenn alle Lampen in den Straßen brannten, verließ er zuweilen das Haus, um einen kurzen Spaziergang zu unternehmen. Diese Spaziergänge benutzte er dazu, sich in Gedanken auf die Arbeit des Abends vorzubereiten. Die Ereignisse jenes Abends hatten ihm zu denken gegeben. Zu nüchterner Vernunft zurückgekehrt, hatte er mit Erstaunen wahrgenommen, mit welcher Schnelligkeit er die Herrschaft über seine Seele verloren. Wenn er sich daran erinnerte, wie er hinter der Pappel stand und auf das schlanke Mädchen am Fenster blickte, so sah er gleichsam einen Fremden vor sich, dessen Gebaren er kopfschüttelnd und mitleidig lächelnd beobachtete. Er erklärte sich diese Erregung als eine Reaktion seines Gehirns, das sich seit Jahren in rastloser Tätigkeit befand, immer auf der Flucht vor alten und der Jagd nach neuen Gedanken, sich kaum die notdürftigste Ruhe und Zerstreuung gönnend. Jenes unscheinbare Erlebnis war für ihn das gewesen, was für den Nüchternen ein Schluck Wein ist, es hatte ihn berauscht. — Ginstermann hatte früher ein Leben ohne Maß und Ziel gelebt, teils von seinen lebendigen Sinnen getrieben, teils von dem Wunsche, den Hunger seiner Seele an möglichst vielen Eindrücken zu stillen. Erst seine reisende Erkenntnis gebot ihm eine Regulierung
- 83. seiner Lebensweise, wenn er seine Seele nicht durch Erinnerungen überlasten wollte. Sie riet ihm zur Vorsicht angesichts der Empfindsamkeit seiner Seele, die eine Leidenschaft in jungen Jahren noch gesteigert hatte. Jahre der Einsamkeit und Verinnerlichung ließen Erkenntnisse in ihm reifen, die ihm Welt und Menschen in neuem Lichte zeigten. Er erkannte, daß das, was man im allgemeinen Leben nannte, ärmlich und nüchtern war gegen ein Leben in der Phantasie, gegen die Beschäftigung mit den ewigen Ideen, die geheimnisvoll die Jahrtausende regieren, das Tun der Menschen bestimmen. Nach und nach war er zur gänzlichen Unfähigkeit gelangt, mit den Menschen zu verkehren. Er verachtete, er bemitleidete sie. Sie waren ihm zu wenig Luxuswesen, zu wenig Dichter, ohne freie Gefühle, ohne den Wunsch nach Flügeln. Ihre Ziele waren klein und kläglich und reichten nicht über den Tag hinaus. Die gesicherte Existenz im Himmel hatte sie vergessen lassen, daß der Mensch auch auf der Erde etwas zu vollbringen hatte. Seine Geschlechtsgenossen waren ihm nicht sympathisch. Ihre rohen Sinne, ihre Lüsternheit, ihre vergiftete Phantasie stießen ihn ab. Die Widerstandslosigkeit, mit der sie sich den von der Masse diktierten Gesetzen und ihren Trieben unterwarfen, machte sie ihm erbärmlich. Das Weib schien ihm erst auf einer Durchgangsstufe zum Menschen angelangt zu sein. Das Unklare, Vorurteilsvolle, das Spekulierende, das wenig Schöpferische, seine Freude an glitzernden Dingen ließen es ihm als ein Wesen erscheinen, das um tausend Jahre hinter dem Manne zurück war und sich nicht Mühe gab, diesen Vorsprung einzuholen. Es lebte von den Erkenntnissen des Mannes, ohne dies einzugestehen und ihm Dank zu wissen, es lebte von seiner Seele, ohne ihm etwas dagegen zu geben. Auf die Suche zu gehen nach einem Gefährten, einer Gefährtin, hatte er schon lange aufgegeben, da ihn die Erfahrung lehrte, daß in jedem neuen Menschen wieder der alte steckte, dem er mißmutig und gelangweilt den Rücken gedreht hatte.
- 84. Nicht als ob er in Zeiten geistiger Ebbe nicht unter seiner Vereinsamung gelitten hätte. Es geschah manchmal, daß er des Nachts mit fiebernden Augen in die wogenden Visionen seiner Phantasie starrte, und gleichzeitig sein Herz in ihm vor Hunger und Sehnsucht pochte. Er war entstanden aus Mann und Weib und deshalb zerklüftet. Er hatte das empfindsame, lebensfrohe Gemüt seiner Mutter geerbt und den hochmütigen Verstand seines Vaters. Diese beiden, Gemüt und Verstand, lebten in ungleicher Ehe. Er pflegte über seine weichen Empfindungen spöttisch zu lächeln. Er stand skeptisch jeder Erscheinung gegenüber und entkleidete sie des Tandes, mit dem gutmütige Dummköpfe sie geschmückt. Im Grunde seiner Natur aber lebte das Bestreben, alle Dinge wiederum zu verklären und mit einem Schmucke zu versehen, wie ihn seine Seele liebte. In den folgenden einsamen Abenden, die ihm eine ruhige Sammlung seiner Gedanken erlaubten, gelang es ihm, die Fremdkörper wiederum auszuscheiden, die seiner Seele gefährlich zu werden gedroht hatten. Er machte Nachträge in sein Tagebuch, revidierte seine Aufzeichnungen, blätterte in alten Manuskripten, ließ wieder und wieder die ewigen Fragen Revue passieren, nach neuen Gesichtspunkten, neuen Perspektiven suchend. Indem er die Entwicklung seines inneren Menschen überblickte, erkannte er mit Deutlichkeit, daß sein Weg in die Höhe führte. Abgründe lagen zwischen ihm und der Welt. Und alle Brücken waren gefallen. Er hatte ihre Irrtümer und Götzen überwunden. Mit Genugtuung bemerkte er, daß er gewachsen war, seit er sich das letzte Mal sah, daß seine Seele fortfuhr, ihr Licht in die Finsternis zu schleudern. Und mit dieser Erkenntnis kam frischer Mut über ihn und neuer Stolz. Ein ungestümer Schaffensdrang erfüllte sein Wesen. Fiebernd vor Schaffensfreude und Finderglück verbrachte er seine Tage und Nächte. Draußen schneite und stürmte es. Es war ihm gleichgültig, ob das Jahr vorwärts oder rückwärts ging.
- 85. Der Vorfall von neulich entwich in weite Fernen und verlor an Leben und Bedeutung. Das schlanke Mädchen tauchte nur dazwischen in seinen Gedanken auf und versuchte ihn mit großen, schimmernden Augen zu bannen. Aber sie brachten ihm keine Gefahr mehr. Blick und Farbe erloschen, sobald er es wollte. Und nur, wenn sein Gehirn müde war von langer Arbeit, stieg der Wunsch in ihm auf, das Mädchen wiederzusehen, sich zu erfreuen am Klange dieser Stimme, der Klarheit dieser Augen. Aber des Morgens erwachte er stets heiter, sorglos und ohne Wünsche. Der Wert jenes Weibes verringerte sich keineswegs in seiner Vorstellung. Er war überzeugt, daß sie einen reiferen, höheren Typus repräsentierte, als ihre Schwestern, die er kannte. „In seinem Herzen jedoch wohnte die Sehnsucht nach einem Weibe hinter den Sternen. Singe hieß sie, das ist: ich bin nicht.“ Seine Gefühle gehörten den Gestalten, die er schuf, seine Gedanken gehörten ihnen. Seine Seele gehörte seiner Arbeit, seinem Ziele.
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