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Mobile
Computing
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Kumkum Garg
Mobile
Computing
Theory and Practice
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Delhi • Chennai • Chandigarh
Department of Electronics and Computer Engineering
Indian Institute of Technology Roorkee

Assistant Acquisitions Editor: Pradeep Banerjee
Assistant Production Editor: Amrita Naskar
Composition: Aptara®, Inc.
Copyright © 2010 Dorling Kindersley (India) Pvt. Ltd
This book is sold subject to the condition that it shall not, by way of trade or otherwise, be
lent, resold, hired out, or otherwise circulated without the publisher’ s prior written consent in
any form of binding or co ver other than that in which it is published and without a similar
condition including this condition being imposed on the subsequent purchaser and without
limiting the rights under cop yright reser ved above, no part of this publication ma y be
reproduced, stored in or introduced into a retrie val system, or transmitted in an y form or
by any means (electronic, mechanical, photocopying, recording or otherwise), without the
prior written per mission of both the cop yright owner and the abo ve-mentioned publisher
of this book.
ISBN 978-81-317-3166-6
10 9 8 7 6 5 4 3 2 1
Published by Dorling Kindersley (India) Pvt. Ltd, licensees of P earson Education in South Asia.
Head Office: 7th Floor , Knowledge Boule vard, A-8(A), Sector – 62, Noida, UP 201309, India.
Registered Office: 11 Community Centre, Panchsheel P ark, New Delhi 110017, India.
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Contents
Preface xiii
1 Introduction to Mobility 1
1.1 Process migration 1
1.2 Mobile computing 2
1.3 Mobile agents 3
1.4 Technical issues for mobility 4
1.5 Personal communication systems 4
1.6 Context-aware computing 5
1.7 Outline of the book 6
1.8 Summary 7
Problems 7
Multiple-choice questions 8
Further reading 9
2 Wireless and Cellular Communication 11
2.1 The electromagnetic spectr um 11
2.1.1 Radio waves 12
2.1.2 Microwaves 12
2.1.3 Infrared waves 12
2.1.4 Lightwaves 13
2.2 Communication satellites 13
2.2.1 Geostationar y satellites 14
2.2.2 Medium ear th orbit satellites 14
2.2.3 Low earth orbit satellites 14
2.3 Multiple-access schemes 15
2.3.1 FDMA—F requency division multiple access 16
2.3.2 TDMA—Time division multiple access 16
2.3.3 CDMA—Code division multiple access 17
2.4 Cellular communication 18
2.4.1 The first generation (1G): 1980 18
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2.4.2 The second generation (2G): 1992 19
2.4.3 The 2.5 generation (2.5G): 1996 20
2.4.4 The third generation (3G): 2000 ⫹ 20
2.4.5 The 3.5 generation (3.5G): 2000 ⫹ 21
2.4.6 The fourth generation (4G): 2002 ⫹ 21
2.5 Summary 22
Problems 22
Further reading 24
3 Wireless Networ ks 25
3.1 The need for ne w wireless standards 26
3.2 IEEE 802.11 WLAN standard 27
3.2.1 Physical la yer 27
3.2.2 MAC layer 29
3.2.3 Frame str ucture 32
3.2.4 Services 32
3.3 Bluetooth 33
3.3.1 Advantages of Bluetooth 35
3.3.2 Bluetooth applications 35
3.3.3 Bluetooth protocol stack 35
3.3.4 Bluetooth tracking ser vices 37
3.3.5 Bluetooth frame str ucture 38
3.4 Infrared systems 39
3.5 HiperLAN 40
3.6 The IEEE 802.16 WiMAX standard 41
3.7 Comparison of wireless technologies 42
3.8 Summary 43
Problems 44
Further reading 45
4 Logical Mobility I— Migrating Processes 47
4.1 What is a process? 47
4.2 Process migration 48
4.3 The steps in process migration 48
4.4 The advantages of process migration 52
4.5 Applications of process migration 53
4.6 Alternatives to process migration 53
4.7 Summary 54
Problems 54
Further reading 56
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5 Physical Mobility 57
5.1 The requirements for ph ysical mobility 57
5.1.1 Wireless communication 57
5.1.2 Mobility 58
5.1.3 Portability 59
5.2 Overview of IPv4 and IPv6 61
5.2.1 IPv4 61
5.2.2 IPv6 62
5.3 Mobile IP 62
5.3.1 Goals of mobile IP 62
5.3.2 Applicability 63
5.3.3 Mobility suppor t in IPv4 63
5.3.4 Mobility suppor t in IPv6 66
5.4 Cellular IP 67
5.4.1 The cellular IP access networ k 68
5.4.2 Routing and paging cache 69
5.5 TCP for mobility 69
5.5.1 Indirect TCP 70
5.5.2 Snooping TCP 71
5.5.3 Mobile TCP 72
5.6 Mobile databases 73
5.6.1 Design issues 73
5.6.2 Problems in mobile databases 74
5.6.3 Commercially a vailable systems 74
5.7 The CODA file system—A case study 74
5.7.1 Cache manager V enus 75
5.7.2 Venus states 75
5.7.3 Design criteria 77
5.8 Summary 78
Problems 78
Further reading 80
6 Mobile Ad Hoc Networ ks 81
6.1 MANET characteristics 81
6.2 Classification of MANETs 82
6.3 Technologies for ad hoc networ ks 83
6.4 Routing in MANETs 83
6.4.1 Traditional routing protocols 83
6.4.2 Requirements for routing protocols 84
6.4.3 Classification of routing protocols 84
6.5 Proactive routing protocols — The DSDV protocol 85
6.5.1 Example of DSD V operation 86
6.6 Reactive routing protocols 88
Contents vii
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6.6.1 Dynamic source routing (DSR) 89
6.6.1.1 Route disco very in DSR 89
6.6.1.2 Route maintenance in DSR 91
6.6.1.3 Route cache in DSR 91
6.6.2 Adaptive on-demand distance vector protocol 92
6.6.2.1 Route disco very in AODV 92
6.6.2.2 Route maintenance in A ODV 93
6.7 Comparison betw een DSR and A ODV 96
6.8 Summary 97
Problems 98
Further reading 100
7 Wireless Sensor Networ ks 101
7.1 Applications of wireless sensor networ ks 101
7.2 Differences from mobile ad hoc networ ks 103
7.3 Design issues 104
7.4 WSN architecture 104
7.4.1 Sensor hard ware components 105
7.4.2 WSN communications architecture 105
7.5 Routing protocols for WSN 106
7.5.1 Data-centric protocols 106
7.5.1.1 Flooding and gossiping 107
7.5.1.2 Sensor protocols for infor mation via
negotiation (SPIN) 107
7.5.2 Hierarchical protocols 108
7.5.2.1 Low-energy adaptive clustering hierarch y 108
7.5.2.2 PEGASIS 109
7.5.2.3 TEEN and APTEEN 109
7.5.3 Location-based protocols 110
7.6 Case study 110
7.6.1 The MICA mote 110
7.6.2 TinyOS 111
7.7 Development wor k in WSN 112
7.8 Summary 112
Problems 113
Further reading 115
8 Mobile Handheld Devices 117
8.1 Characteristics of PD As 117
8.1.1 The ARM processor 119
8.1.2 Network connectivity 119
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8.2 Palm handhelds 120
8.3 The Palm OS operating system 121
8.3.1 Memory management 121
8.3.2 Communication and networ king 122
8.4 HP handhelds 122
8.5 Windows CE 123
8.5.1 Memory architecture 124
8.5.2 Memory management 124
8.5.3 Processes and threads 124
8.5.4 Scheduling 125
8.5.5 Real-time perfor mance 125
8.6 The Windows Mobile operating system 125
8.7 Nokia handhelds 127
8.7.1 Specifications of Nokia 9210 127
8.7.2 Features 128
8.8 Symbian operating system 129
8.8.1 Design 129
8.8.2 Symbian str ucture 130
8.9 Summary 130
Problems 131
Further reading 132
9 The Mobile Inter net and Wireless W eb 133
9.1 The Web programming model 133
9.2 The WAP programming model 134
9.3 WAP protocol stack 135
9.4 Information-mode (I-mode) 136
9.5 WAP 2.0 136
9.6 WAP gateway 137
9.6.1 Push operation 138
9.6.2 Push message for mat
(using PAP) 140
9.6.3 Pull operation 141
9.7 Summary 141
Problems 142
Further reading 144
10 Logical Mobility II — Mobile Agents 145
10.1 Mobile agents 146
10.2 Characteristics of mobile agents 146
10.2.1 Architecture 147
10.2.2 Mobile code and agents 147
Contents ix
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10.2.3 Mobile agents and process migration 147
10.2.4 Client/ser ver and mobile agent architectures 147
10.3 Requirements for mobile agent systems 148
10.3.1 Portability 148
10.3.2 Ubiquity 148
10.3.3 Network communication 148
10.3.4 Server security 148
10.3.5 Agent security 149
10.3.6 Resource accounting 149
10.4 Mobile agent platfor ms 149
10.4.1 Aglets 150
10.4.1.1 The aglet object model 150
10.4.1.2 Aglet communication 151
10.4.1.3 The aglet e vent model 152
10.4.2 Agent Tcl 152
10.4.2.1 Agent Tcl architecture 152
10.4.2.2 Agent Tcl applications 155
10.4.3 PMADE 155
10.4.3.1 Agent submitter 156
10.4.3.2 Agent host 158
10.4.3.3 Communication manager s 158
10.4.3.4 State manager s 159
10.4.3.5 Persistence manager 160
10.4.3.6 Security manager 160
10.5 Java and mobile agents 161
10.5.1 Advantages of Ja va 161
10.5.2 Shortcomings of Ja va 161
10.6 Summary 162
Problems 162
Further reading 164
11 Security Issues in Mobile Computing 167
11.1 Security threats to wireless networ ks 168
11.2 IEEE 802.11 security through WEP 169
11.2.1 WEP security features of 802.11 wireless LANs 169
11.2.1.1 Authentication 169
11.2.1.2 Confidentiality 170
11.2.1.3 Integrity 171
11.3 Bluetooth security 172
11.4 WAP 2.0 security 174
11.5 Summary 174
Problems 175
Further reading 177
x Mobile Computing
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12 Design and Programming Projects 179
12.1 Implementation of mobile IP 179
12.2 Comparison betw een AODV and DSR protocols 182
12.3 Bluetooth application 184
12.4 Design of a W AP gateway 189
12.5 Mobile agents for networ k monitoring 190
12.6 An IEEE 802.11 LAN for a typical student hostel 194
12.7 An application using wireless sensor networ ks 196
12.8 Summary 198
Problems 198
Further reading 200
Appendix A—Ja va Networ k Programming 201
A.1 Java programming language 201
A.2 Socket programming 203
A.3 Remote procedure call (RPC) 205
A.4 Remote method in vocation (RMI) 207
Appendix B—Comparison Betw een Qualnet and NS2 211
Index 213
Contents xi
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Preface
M
obile computing or computing-on-the-go is proving to be one of the most promising
technological advances in computer science and engineering to date. With the advent
and proliferation of portable, handheld hardware devices, equipped with wireless com-
munication interfaces and carrying innovative applications and systems software, computing
has now become truly ‘pervasive’ or ‘ubiquitous’. It is now commonplace to see people sitting in
airport and hotel lounges, meeting rooms and even open spaces, keying away at their PDAs or
laptops, checking e-mails and appointments, making to-do lists or just chatting with their
friends. We are also looking at ‘smart dust’, in which thousands of miniature processing devices
can be literally scattered in a battlefield or natural calamity areas to form a network and monitor
the various activities therein, like movement of the enemy, management of bushfires, relief sup-
plies and rehabilitation work, etc.
Technological advances create newer and more innovative applications everyday, which in
turn fuel the demand for new technology. This has become a not-so-vicious circle, keeping
researchers and developers on their toes all the time. The beneficiary is of course the layman on
the street, literally so in the case of mobile computing.
It is important to note that mobile computing is not just mobile or wireless communica-
tion, as some would believe. There is much more to mobile computing, and it is to remove this
confusion that this book has been written. Of course, provision of higher and more wireless
bandwidth is the driving force for mobile computing. But what is more important and challeng-
ing is the design of various application protocols and algorithms, the small-footprint operating
systems, efficient usage of the small-sized user interfaces and, above all, providing security of
systems and applications.
This book provides a focussed look at all the issues mentioned above and gives an insight
into the large number of technologies available in these areas to the user today. Apart from the
theory, which is presented in an easy-to-understand form, we have provided many examples and
suggestions for hands-on programming to help understand better the underlying technologies.
These have been actually undertaken by senior undergraduate and postgraduate students of com-
puter science at IIT Roorkee. To assist the reader in programming applications, an appendix has
been included which deals with some important aspects of Java network programming.
This book is intended for both professionals and students of senior undergraduate- and
postgraduate-level engineering courses in electrical, electronics and computer science who
have a background in computer networks and Java programming. It can be used for a one-semester
or a one-quarter course. It can also be adopted for short-term training courses for new employees or
trainees. To make the new concepts easy to understand, each chapter ends with multiple-choice
xiii
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review questions. Other research-oriented and programming-type questions which exercise the
readers’ mind are also included.
Book organization
This book has been organized into 12 chapters, covering the entire gamut of technologies rele-
vant to mobile computing. These include wireless and cellular communication, wireless local
area networks (WLANs), logical mobility consisting of process migration and mobile agents,
handheld devices and their operating systems, physical mobility, mobile ad hoc networks,
wireless sensor networks, wireless application protocol and the mobile Internet, security issues in
mobile applications, etc. The last chapter gives a brief idea of some design projects that can be
undertaken to better understand the theory. An appendix is also included for explaining the ba-
sics of Java network programming. The material is just right for a four-month, one-semester
course.
For a short-term course for students who are familiar with the basics of wireless communica-
tion, Chapters 2, 4, 10 and 11, which deal with wireless communication, migrating processes,
mobile agents and security, respectively, can be omitted.
The concepts discussed in this book can also be used for research in this fast-growing field,
since most of the technologies that are used and are applicable today may not be relevant tomor-
row as requirements for newer applications arise.
Acknowledgements
Over the entire duration of the writing and compiling of this book, many people have helped
me; without them, this book would not have been possible.
First and foremost, I would like to thank the many experts who reviewed drafts of this book.
Their suggestions have certainly helped to improve the content and presentation of the book.
I am grateful to my Ph.D. student R.B. Patel, who first suggested that I introduce a course on
mobile computing at IIT Roorkee in 2003 in the postgraduate curriculum and write a book on
this important topic.
My heartfelt thanks to all my postgraduate and senior undergraduate students at IIT Roor-
kee, who designed and developed various projects related to mobile computing. These projects
provided the content for the last chapter of the book and helped tremendously in adding to the
‘practice’ part of the title of the book.
I also thank IIT Roorkee and MIT Manipal for providing the working environment that made
this book possible.
Last but not the least, I thank my family members and friends whose support and constant
encouragement during the three years of writing the book made this effort worthwhile and without
their support, this book could never have been finished.
KUMKUM GARG
xiv Mobile Computing
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M
obility has been the hallmark of all animate and living entities in nature. Animals move
from place to place, migrating to find food and shelter. Similarly, early humans migrated
from their natural habitats in search for food. Today, humans move in search of better
employment, entertainment, travel, etc. Thus, mobility stems from a desire to move towards
resources and away from scarcity.
As in nature, so also in the field of computer science, mobility is becoming important and
necessary. Today, both physical and logical entities can move. Physical entities are small,
mobile computers that can change their actual location, unlike early systems, which were bulky
in size and therefore immobile. Logical entities may be either the running user applications
(processes) that migrate within a local cluster of computers or mobile agents, which are net-
work applications that migrate in a network and execute on behalf of their owners anywhere
in the Internet.
The concept of mobility in the field of computer science has thus been chronologically
provided in process migration since the 1970s, in mobile computers since the 1980s and in
mobile agents from the 1990s. In this chapter, we shall briefly discuss these concepts and their
benefits and challenges for deployment. We shall come back to visit them in detail in subse-
quent chapters.
1.1 Process migration
Process migration is the act of transferring a process between two computers connected through
a wired or wireless medium. A process is an operating system abstraction and has code, data and
state, besides a unique identity in the system. Traditionally, process migration was used to
achieve load distribution in a multiprocessor system like a cluster or network of computers, or it
was resorted to for providing fault tolerance in such systems.
Many research operating systems have implemented full-blown process migration mecha-
nisms, as shown in Accent (Zayas 1987), Chorus (Rozier and Legatheaux 1986), Mach (Acetta
et al. 1986) and VKernel (Cheriton 1984). On the other hand, commercial migration-related
products provide a higher-level, checkpoint-like restart version of migration, as seen in Condor
(Litzkow, Linvy, and Mutka 1988).
The main benefits of process migration are that a process might move towards an under-
loaded computer, a specific database, or some rare hardware device. Furthermore, it enables
movement of the programming environment and application to a desired location. For example,
Introduction to Mobility 1
1
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if a computer has a partial failure or is about to shut down, a running process can migrate to
another computer and continue execution there. The resulting flexibility and reliability are
important and necessary.
1.2 Mobile computing
Mobile computing is computing that allows continuous access to remote resources, even to small
computing devices such as laptops, palmtops and other handheld devices like personal digital
assistants (PDAs) and digital cell phones. Mobile computing has become possible with the rapid
advances in very-large-scale integration (VLSI) and digital/wireless communication technologies.
There are basically three issues of concern in physical mobility. These are given below and have
been dealt with in various ways by various researchers. We shall introduce these issues in this
chapter but discuss details in subsequent chapters.
1. Weak connectivity: It is a well-known fact that wireless communication suffers from fre-
quent disconnection and slow speeds, as compared with wired communication. The challenge is
how a computer can operate when disconnected from the network or intermittently connected
or connected over very slow communication links. This issue has been taken up in the CODA file
system, which will be discussed in detail in Chapter 5.
2. Wireless connectivity: When a computer moves between cells in a wireless network or
from one computer network to another, it is required to continue operating without having to
re-register in the new location. In other words, the handoff should be smooth. This issue has
been dealt with admirably by the development of two protocols—mobile Internet protocol (IP)
and cellular IP, both of which are discussed in detail in Chapter 5.
3. Ubiquitous computing: This is the term coined by Mark Weiser and refers to the scenario
when computers are present everywhere around us but have been rendered so small and cheap
that they fade into the background. This is also called pervasive computing. Wireless sensor net-
works (WSNs) are examples of such ubiquitous or pervasive computing, and are discussed in de-
tail in Chapter 7.
Thus, mobility of physical devices can be viewed at three different levels of granularity.
These are as follows:
1. Macro-mobility: This is mobility through a global network. While moving in such a
network, it should be possible to communicate without breaking the existing access. In
Chapter 5, we shall read about mobile IP, which is the protocol that takes care of macro-
mobility.
2. Micro-mobility: This is mobility of a device in one single administrative domain of the
global network. For cellular networks, this is the lowest level of mobility. Cellular IP is the
protocol designed to take care of micro-mobility, and this will also be discussed in
Chapter 5.
3. Ad hoc mobility: This is mobility within a mobile ad hoc network (MANET), caused by device
mobility constantly changing the network topology. We shall study MANETs in Chapter 7
and visit several ad hoc routing protocols therein.
Whatever the type of mobility, the benefits of mobile computing are obvious, since there is
physical movement towards a desired resource. Here, both the owner and the computer move to
2 Mobile Computing

provide both qualitative and quantitative benefits. Since it is possible to use computer resources
while moving, users can take the computer away from its usual workplace and still be productive.
Thus, mobile computing, like process migration, enables movement of the programming envi-
ronment and application. If a wireless phone cannot connect from a specific area, moving to a
new area can overcome natural obstacles.
A major benefit with mobile computing is that the use of computers is increased, not only
for computer professionals, but also for the lay person. This is very important, because in this
information age, having continuous access is imperative for everyone on the go.
1.3 Mobile agents
A mobile agent is a program that can move through a network and autonomously execute tasks
on behalf of the users. An agent is different from a user application, as it represents and acts on
the owner’s behalf by inheriting the owner’s authority. Unlike mobile code (applets), mobile
agents carry data and thread of control. They require agent environments, acting like docking
stations, to execute and are supported on top of a programming environment like a Java virtual
machine (JVM).
Mobile agents are used to great advantage in applications like e-commerce, software
distribution, information retrieval, system administration, network management, etc. They
are well suited for slow and unreliable links and also provide fault tolerance. Many mobile
agent systems have been developed and reported in the literature. Some of the more well-
known systems are Aglets, Agent Tcl and PMADE (platform for mobile agent development and
execution).
Since mobile agents also migrate towards a source of information or towards a computer that
they manage, they provide great flexibility and can mean easier reconfiguration or improved
reliability. Mobile agents may not have sufficient resources or connectivity from one host and
may move to another host.
It can be seen from the above that there is much commonality between the three kinds of
mobility discussed above. Researchers have, over the years, developed various means and mech-
anisms to deploy the above concepts to real-life situations. As a result, we have numerous tech-
nologies that can be used to advantage. We discuss some of these briefly below. Detailed
discussions are given in subsequent chapters.
1. Java as a language offers many concepts that are directly useful for mobile systems. For
example, remote method invocation (RMI), object serialization and mobile code are all very
useful for process migration and mobile agents.
2. Similarly, wireless technologies provide support for mobile computing, with the develop-
ment of many wireless protocols like Bluetooth, the Infrared Data Association (IrDA) stan-
dards, wireless access protocol (WAP), etc.
3. Infrastructure support for transparent movement of entities from one location to another
on the Internet and for issues of performance, scalability and reliability have been pro-
vided by the presence of numerous mobile agent systems that have been developed in re-
cent years.
4. Standardization has been provided in the form of CORBA (common object request broker
architecture) and the MASIF (mobile agent system interoperability facility) standard, which
allow for interoperable systems to be built and used worldwide.

1.4 Technical issues for mobility
Mobile systems, as we have seen from the above discussion, are being increasingly deployed world-
wide. But there are many challenges and technical issues of concern here. These are as follows:
1. Security is the biggest challenge for mobility. Security includes user authentication, data
integrity and privacy, prevention of denial of service and non-repudiation. It may be appre-
ciated that it is easier to provide security for a stationary system as compared to a mobile
one, since the former can be guarded by intrusion detection systems and firewalls. The latter
provides more security holes that have to be plugged. These include problems like unautho-
rized access, data corruption, denial of access/service, spoofing, Trojan horses, replaying and
eavesdropping, among others.
The existing security infrastructure is designed only to protect stationary systems and
thus needs to be adapted or modified for use in mobile systems. Security of mobile systems
is the subject of Chapter 11.
2. Reliability, in terms of availability of resources, in the presence of disconnection, is also a
major issue for mobile systems. In fact, it is both a technical issue and a benefit for mobility.
Reliability can be improved by mobility but needs additional support in the form of caching
and loading of state. Similarly, replication and check-pointing, file-hoarding, message-
queuing and fault-tolerance tools need to be provided.
3. Naming and locating are common issues for all forms of mobility. Without locating a mobile
object, communication with it or its control is not possible. Communication channels must
be reconstructed after every movement. Naming is to be associated with authentication, and
all recycling is to be done with great care. Controlling a mobile entity is necessary to check
its status or to suspend, kill or recall it.
All three of the above issues and their implementation will be discussed in detail in sub-
sequent chapters.
1.5 Personal communication sy stems
A personal communication system (PCS) is a generic name for a commercial system that offers
several kinds of personal communication services and extended mobility. The Federal Communi-
cations Commission (FCC) defines PCS as a mobile and wireless service that can be integrated
with different networks to provide a wide variety of mobile and wireless services to individuals
and business organizations. It was deployed in the USA in 1996. A PCS employs a mobile station
(MS), an inexpensive, lightweight and portable handset, to communicate with a PCS base station
(BS). The common features of these systems are as follows:
1. They are based on a second-generation technology like GSM (global system for mobile
communication), IS-136 or IS-95.
2. Since they use the higher 1900-MHz band, an MS needs more power. This is because higher
frequencies have a shorter range than lower ones. Alternatively, it can be said that the BS and
the MS need to be closer to each other; that is, use smaller cell sizes.
3. They offer a whole spectrum of communication services ranging from an ordinary cell
phone, short message service (SMS), to cable TV and limited Internet access.
A typical PCS architecture is shown in Figure 1.1.
4 Mobile Computing

1.6 Context-aware computing
A context-aware computing system is one which has user, device and application interfaces
which enable it to remain aware of various parameters like its surroundings, circumstances or
actions. These parameters can be thought of as the present mobile network, surrounding devices
or systems, changes in the state of the connecting network, etc. These could also mean physical
parameters such as the present time of the day, presently remaining memory and battery power,
presently available nearest connectivity, past sequence of actions, cached data records, etc.
The context of a mobile device represents the circumstances, situations, applications or
physical environment under which it is being used. For example, the context is student when
the device is used to download faculty lectures.
Context-aware computing leads to application-aware computing. This is because the appli-
cation programming interfaces (APIs) are part of the context. For example, when using an e-mail
ID, a mail-receiving or mail-sending application software is used for computing. An application
can adapt itself to the context. For example, if context is a contact, the phone-talk application
will adapt itself to use of the telephone number from the ‘contact’ and to the use of GSM or code
division multiple access (CDMA) communication.
Context-aware computing also leads to pervasive or ubiquitous computing. In mobile device
data-communication, context includes the existence of the service discovery protocol, radio
interface and corresponding protocol. If the service discovery protocol senses the context and
finds Bluetooth, then the device uses Bluetooth to communicate. Use of context in computing
helps in reducing the possibility of errors and ambiguity in the actions. It also helps in deciding
the expected system response on computation.
The five types of contexts that are important in context-aware computing are as follows:
Physical context: The context can be that of the physical environment. The parameters for
defining a physical context are service disconnection, light level, noise level and signal
strength. Assume a mobile phone is operating in a busy, congested area. If the device is
aware of the surrounding noises, it can raise the speaker volume. If there is intermittent loss
of connectivity during the conversation, the device can introduce background noises so that
the user does not feel discomfort due to intermittent periods of silence.
Computing context: Computing context is defined by interrelationships and conditions of
the network connectivity protocol in use. Examples of the latter could be Bluetooth, ZigBee,
GSM, general packet radio service (GPRS) or CDMA. Computing context may also be bandwidth
Specialized
mobile
network
MS Cellular
network
Cable
TV
PCS base
station
PCS
node
PCS base
station
PCS
node
MS
Figure 1.1 PCS Architecture

and available resources. Examples of resources in a mobile device are keypad, display unit,
printer and device cradle.
User context: The user context is defined as user location, user profiles, and persons near the
user. It is based on the condition of the user, the primary intent of the systems and all other
elements that allow users and computing systems to communicate.
Temporal context: Temporal context defines the interrelation between time and the occur-
rence of an event or action. A group of interface components has an intrinsic or extrinsic
temporal context. For example, when a user presses a key to add a contact in his mobile
device, the device should prompt him to enter a number as an input.
Structural context: It defines a sequence and structure formed by the elements or records.
Graphical user interface (GUI) elements have structural context. Interrelation among the
GUI elements depends on the structural positions on the display screen. For example, in a
date, the hours are displayed on the left of the minutes.
1.7 Outline of the book
This book discusses both the theory and practice of mobile computing, so that the reader gets a
complete idea of not only the techniques available to facilitate mobile computing, but also how
to program and implement applications based on them.
Chapter 2 deals with the basics of wireless and cellular communication. The various wireless
frequencies present in the electromagnetic spectrum, like radio, microwave, infrared and light,
and their characteristic features and applications are presented. Satellite communication is dis-
cussed with reference to geostationary, medium-orbit and low-orbit satellites. The various gener-
ations of cellular phone communication are given in detail, as they form the basis of all
communication for the handheld devices used in mobile computing.
Chapter 3 discusses wireless local area networks (WLAN). The most popular WLAN is the
IEEE Standard 802.11. Its various extensions and modifications are dealt with in detail. The Blue-
tooth and infrared LANs are also presented. Both the versions of the European Standard HiperLAN
are discussed. A comparison of all the above standards is presented to bring out their essential
features and applications.
Chapter 4 deals with logical mobility. It discusses in detail the concept of process migration,
which is the forerunner of mobile computing. The need for process migration and its various
steps are presented.
Chapter 5 deals with physical mobility, its requirements and the challenges associated
with it. It discusses the limitations of IP in providing for physical mobility. It shows how
mobile IP and cellular IP overcome these problems in micro- and macro-mobility scenarios.
The chapter also introduces mobile databases and their design issues. Finally, it looks in detail
at the CODA file system developed to take into account disconnected operation in mobile
computing.
Chapter 6 is on MANETs. The characteristics and classification of MANET are discussed in
detail, along with their application. The proactive and reactive routing strategies for MANET are
introduced. Three popular MANET routing protocols, namely, destination sequenced distance
vector (DSDV), dynamic source routing (DSR) and adaptive on demand distance vector (AODV)
are discussed in detail, with examples of their routing mechanisms. A performance comparison
of DSR and AODV is also given.
6 Mobile Computing

Chapter 7 deals with WSNs. It shows how these are different from MANETs and gives their
characteristics, architecture and some popular routing techniques developed for them. Case stud-
ies of the Mica mote sensor node and the TinyOS operating system used for it are also presented.
Chapter 8 discusses the handheld devices like PDAs and pocket computers used in mobile
computing. It discusses the characteristics of various such devices, including Palm and HP devices.
The operating systems used with such devices have certain special features. These are presented
with respect to the Palm OS, the Windows CE and Windows Mobile operating systems.
Wide area mobile computing is the subject of Chapter 9, which presents what is now called
the mobile Internet and the WAP, used to access the Internet on the move. The traditional Web
programming model is compared with the wireless Web programming model. The WAP protocol
stack is introduced and the WAP Gateway is discussed in detail, together with its design.
Chapter 10 revisits logical mobility in the form of mobile agents, their characteristics and
architecture and highlights their differences with process migration, mobile codes and mobile
objects. The two earliest and basic mobile agent platforms, namely, Aglets and Agent Tcl, are pre-
sented in detail. PMADE, a mobile agent platform developed at IIT Roorkee, is also presented. A
discussion on the advantages of Java as a programming language for mobile agents is also given.
Chapter 11 discusses the most important and crucial issue of security in mobile computing
systems. It highlights the security threats present in wireless systems. The security mechanisms
present in IEEE 802.11, Bluetooth and WAP2.0 to take cognizance of and counter these threats
are also discussed in this chapter.
Since this book is about mobile computing practice, the last chapter, Chapter 12, presents in
detail as many as seven programming projects that can be designed and implemented by readers
in different aspects of mobile computing. It thus provides an opportunity to have hands-on
experience in designing and coding such systems.
The appendix gives some details of Java as a network programming language, and covers
topics like socket programming, remote procedure call (RPC) and the Java RMI. Some examples
are given to provide a clear understanding of these concepts.
1.8 Summary
Mobility is the hallmark of all animate beings and represents the movement from scarcity to
resource-rich locations. In computing, mobility is characterized by logical or physical mobility
and is represented by process migration, mobile agents or handheld-device communication.
Mobile computing includes all these concepts, and it gives rises to a number of benefits, together
with many technical issues and challenges. These have been dealt with in various ways, as discussed
in this chapter, and the details are the subjects of the ensuing chapters.
In the next chapter, we shall concentrate on the various communication technologies that
facilitate mobile computing.
Problems
1. What are the most important challenges facing mobile computing today? Discuss each of
them in detail.
2. Distinguish mobile computing from distributed computing.
3. Go on to the Web and find about the state-of-the-art in mobile computing.

4. Do you have a pocket computer or PDA? If so, list the facilities it provides that can be listed
under mobile computing applications.
5. The computer-networking architecture consists of seven layers, as given in the ISO OSI
reference model. In your opinion, in which layer(s) should mobility be incorporated
and why?
6. Recent conferences on mobile computing, such as ACM Mobicom and MobiSys, have pub-
lished many articles on the subject. Read them and identify some of the current research
challenges being addressed by researchers.
7. Do you think Java is suited for programming mobile computing systems? Explain your
answer. (Do not look ahead into the later chapters of the book!)
8. What is your idea of a ubiquitous computing scenario for the home? Elaborate on this.
9. Discuss why security concerns in traditional systems are simpler than those in mobile
systems. Give one example of a security threat that is present in the latter but not in the former.
10. Give one example where ‘disconnected operation’ may become imperative in a mobile com-
puting scenario.
Multiple-choice questions
1. Which one of the following is ‘computing that allows continuous access to remote resources
even with the physical mobility of small computing devices such as laptops’?
(a) Soft computing
(b) Mobile computing
(c) Remote computing
(d) Ubiquitous computing
2. Pervasive computing is also called by which one of the following names?
(a) Soft computing
3. Wireless sensor networks are examples of which one of the following?
(a) Soft computing
4. Which one of the following can be characterized as ‘mobility through a global network’?
(a) Macro-mobility
(b) Micro-mobility
(c) Ad hoc mobility
(d) None of the above
5. Mobility of a device in one single administrative domain of the global network is known as
which one of the following?
(a) Macro-mobility
(b) Micro-mobility
8 Mobile Computing

(c) Ad hoc mobility
6. Which of the following is true for statements X and Y?
X: A mobile agent is a program that can move through a network and autonomously execute
tasks on behalf of the users.
Y: Process migration is the act of transferring a process between two computers connected
through a wired or wireless medium.
(a) X is true but Y is false
(b) X is false but Y is true
(c) Both X and Y are true
(d) Both X and Y are false
7. What is an Aglet?
(a) A wireless protocol
(b) A mobile agent
(c) A pervasive computing technique
8. Which one of the following is false for mobile agents?
(a) They are well suited for slow and unreliable links
(b) They cannot provide fault tolerance
(c) Unlike mobile code (applets), mobile agents carry data and thread of control
(d) They require agent environments
9. Which one of the following is not a wireless protocol?
(a) Bluetooth
(b) IrDA
(c) WAP
(d) CSMA/CD
10. Which one of the following is true for statements X and Y?
X: It is easier to provide security for a mobile system as compared to a stationary system.
Y: Security includes user authentication, data integrity and privacy, prevention of denial of
service and non-repudiation.
(a) X is true but Y is false
(b) X is false but Y is true
(c) Both X and Y are true
(d) Both X and Y are false
Further reading
A.S. Tanenbaum (2005), Computer Networks, 4th ed. (Prentice Hall India).
A.T. Campbell (2000), ‘Design, Implementation and Evaluation of Cellular IP’, IEEE Personal
Communications, 7 (August): 42–49.
C. Perkins (1998), Mobile IP: Design Principles and Practice (Addison-Wesley Longman).
D. Kotz et al. (1997), ‘Agent Tcl: Targeting the Needs of Mobile Computing’, IEEE Internet Computing,
1(4): 58–67.

D. Lange and M. Oshima (1998), ‘Mobile Agents with Java: The Aglet API’, World Wide
Web, 1(3).
D. Milojicic et al. (1998), ‘MASIF: The OMG Mobile Agent System Interoperability Facility’, in
Proceedings of the International Workshop on Mobile Agents (MA ’98), Stuttgart.
D. Milojicic, F. Douglis and R. Wheeler (eds) (2000), Mobility: Processes, Computers and Agents
(Addison-Wesley).
D.B. Lange and M. Oshima (1998), Programming and Deploying Java Mobile Agents with Aglets
(Addison-Wesley).
D.P. Agrawal and Q.A. Zeng (2003), Introduction to Wireless and Mobile Systems (Thomson).
D.R. Cheriton (1984), ‘The V-kernel: A Software Base for Distributed Systems’, IEEE Software, 1(2):
19–42.
E. Pitoura and G. Samaras (1998), Data Management for Mobile Computing (Norwell, MA: Kluwer
Academic Publishers).
E.R. Zayas (1987), ‘Attacking the Process Migration Bottleneck’, in Proceedings of the 11th ACM
on Operating Systems Principles, pp. 13–24.
F. Adelstein et al. (eds) (2005), Fundamentals of Mobile and Pervasive Computing (Tata McGraw-
Hill).
J. Kistler and M. Satyanarayan (1992), ‘Disconnected Operation in the CODA Distributed
System’, ACM Transactions on Computer Systems, 10(1): 3–25.
M. Rozier and J.M. Legatheaux (1986), ‘The Chorus Distributed Operating System: Some Design
Issues’, Y. Parker et al (eds.), in Proceedings of the NATO Advanced Study Institute on
Distributed Operating Systems: Theory and Practice, Springer-Verlag, New York, August
1986, pp. 261–289.
M. Weiser (1991), ‘The Computer of 21st Century’, Scientific American, 265(3): 94–104.
M.C. Powell and B.P. Miller (1983), ‘Process Migration in DEMOS/MP’, ACM SIGOPS OS Review,
17(5): 110–119.
M.J. Acetta et al. (1986), ‘Mach, a New Kernel Foundation for UNIX Development’, in Proceed-
ings of the Summer USENIX Conference, June 1986, pp. 93–113.
M.J. Litzkow, M. Livny, and M.W. Mutka (1988), ‘Condor—A Hunter of Idle Workstations’, in
Proceedings of the 8th International Conference on Distributed Systems, June 1988,
pp. 104–111.
R. Kamal (2007), Mobile Computing (Oxford University Press).
R.B. Patel (2002), ‘Manual of PMADE’ (Internal Report, Department of E&CE, IIT Roorkee,
Uttarakhand, India).
Reza B’Far (2005), Mobile Computing Principles: Designing and Developing Mobile Applications with
UML and XML (Cambridge University Press).
T. Imielinski and H.F. Korth (eds) (1996), Mobile Computing (Norwell, MA: Kluwer Academic
Publishers).
U. Hausmann et al. (2003), Principles of Mobile Computing, 2nd ed. (Springer).
V. Kumar (2006), Mobile Database Systems (John Wiley).
W.R. Cockayne and M. Zyda (1998), Mobile Agents (Manning Publications).
10 Mobile Computing

I
n this chapter, we will discuss the transmission technologies that form the basis of all mobile
computing. In particular, we study in detail mobile or wireless communication and the differ-
ent protocols that have been developed to physically or logically connect two mobile devices.
Thus, this chapter looks at the physical layer technologies used in mobile computing, using what
is called unguided media, as opposed to guided media, which consist of copper fibres, twisted
pairs and optical fibres, and which are used for wired communication. The basis for all wireless
transmission is the electromagnetic spectrum, in which lie the different frequency bands that are
used for wireless communication. We will discuss in detail the characteristics of each of these
frequency bands and the wireless and cellular communication systems enabled by them.
It is assumed here that the reader is familiar with the theoretical basis for data communica-
tion, that is, the terms frequency, wavelength, channel, speed of light, bandwidth, the maximum data
rate of a channel, etc., and the relation between them.
For the sake of completeness, and because it is an important and relevant relation for this
book, we must state here that the amount of information that a noisy channel can carry is gov-
erned by its bandwidth. According to Shannon, the maximum data rate of a noisy channel
whose bandwidth is H Hz and whose signal-to-noise ratio is S/N is given by
Maximum data rate (bits/sec) H log2 (1 S/N)
For example, a channel of 3,000 Hz bandwidth and signal-to-thermal noise ratio of 30 dB can
never transmit more than 30,000 bps.
For other related information, the uninitiated reader is referred to Tanenbaum (2003).
2.1 The electromagnetic spectr um
The electromagnetic spectrum is shown in Figure 2.1. The radio, microwave, infrared and visible
light portions of the spectrum can all be used for transmitting information by modulating the
wave’s amplitude, frequency or phase. The higher frequencies, that is, ultraviolet light, X-rays
and gamma rays, would give better results, but are normally not used because they are difficult to
produce and modulate, do not propagate well through buildings and are harmful to humans.
The various frequency bands have official International Telecommunication Union (ITU) names,
as given in Figure 2.1, and are based on their wavelengths.
In this section, we give, very briefly, the characteristics, advantages and disadvantages of
each of the above wave bands and see how they are used for wireless transmission.
Wireless and Cellular
Communication 2
11

12 Mobile Computing
F(Hz)
Band L MF H VH UH SHF EHF THF
Twisted pair
Coa
Maritime
Radio Microwave Infrared
Visible
UV X-ray Gamma
AM
radio
FM
radio
T
Terrestrial
microwave
Satellite Fiber
optics
104
105
106
107
108
109
1010
1011
1012
1013
1014
1015
1016
F(Hz) 100
102
104
106
108
1010
1012
1014
1016
1018
1020
1022
1024
Figure 2.1 The Electromagnetic Spectr um
2.1.1 Radio waves
Radio waves are present at the lower end of the spectrum and are widely used for both indoor and
outdoor communication. They have the advantage that they are omnidirectional and are able to
travel long distances, penetrating easily through buildings. Their disadvantages are that they suffer
from interference between users and from electrical equipment. They also exhibit frequency-
dependent properties; that is, at low frequencies, they pass through objects, but attenuation in power
occurs as distance from the source increases. On the other hand, high-frequency radio waves travel in
straight lines and cannot penetrate through obstacles. Furthermore, rain and sleet absorb such waves.
2.1.2 Microwaves
Frequencies above 100 MHz are called microwaves. These have the advantage that they can be
narrowly focused because they travel in straight lines. Thus, by properly aligning the sending
and receiving antennae, they are able to give much higher signal-to-noise ratio. For the same rea-
son, they are affected by the curvature of the earth if long-distance communication is to be used,
making it necessary to build repeater towers for the transmitting antennae. Microwaves are less
expensive to use than optical fibres and are therefore popular in mountainous and urban areas.
Microwaves have the disadvantage that they suffer from multipath fading. This is because
they do not pass easily through buildings and obstacles and are refracted by the atmospheric
layer; some waves therefore arrive out-of-phase with the direct ones, resulting in cancellation of
the signal. The effect of this type of fading changes with weather and frequency.
2.1.3 Infrared waves
Unguided infrared and millimeter waves offer an alternative to the standard radio frequency
communication for short ranges. However, they are subject to the following restrictions:
• Transmission distance of less then 2 miles
• Line-of-sight limitations
• Restricted to 16 Mbps throughput
• Presence of environmental disturbances, such as fog, dust and heavy rain

Wireless and Cellular Communication 13
However, the advantages of this technology are as follows:
• Reasonable high bandwidth
• No government license required for operation
• Cost-effective
• Capable of traversing multiple paths without interferences
• More secure than radio
• Immune to radio frequency interference and electromagnetic interference
Infrared communication has very little use on the desktop. For example, it can be used for
connecting notebook computers and printers, but is not used in computer-to-computer commu-
nication. The Infrared Data Association (IrDA) has defined a number of standards governing
infrared wireless communication. These include the IrDA-data and IrDA-control standards. These
will be discussed in detail in the next chapter.
2.1.4 Lightwaves
Unguided optical signalling has been around for many years. In recent years, coherent optical
signalling using lasers mounted on rooftops has been used to connect the local area networks
(LANs) in two buildings. The signals are inherently unidirectional, so each building requires a
laser and photodetector. This scheme is very inexpensive and offers very high bandwidth. It is
easy to install and does not require a license to operate. A major disadvantage is that laser beams
cannot penetrate rain or thick fog. However, they work well on sunny days and can be effectively
used for ‘wireless outdoors’.
2.2 Communication satellites
Communication satellites have provided a very powerful wireless communication system since
the first artificial satellite was put into orbit in 1962. A communication satellite is like a big
microwave repeater in the sky. It consists of many transponders, each of which listens to some
frequency spectrum, amplifies the incoming signal and rebroadcasts it at another frequency
(to avoid interference with the incoming signal).
Mobile satellite services allow global coverage, because in these systems satellites play the
role of mobile base stations (BSs). Satellite-based systems are categorized according to the orbital
altitude of the satellite. This is shown in Figure 2.2.
Satellite-based systems
Medium earth orbit satellites (MEOS)
(widely varying altitudes between those
of GEOS and LEOS)
Geostationary satellites (GEOS)
(altitude of 35,786 km)
Low earth orbit satellites (LEOS)
(altitude of the order of 1,000 km)
Figure 2.2 Satellite Systems

14 Mobile Computing
The major advantage of GEOS systems is that contiguous global coverage up to 75 degrees
latitude can be provided with just three satellites. Their main drawback is that they have a large
240–270 ms round-trip propagation delay and need higher radio frequency (RF) power. On the
other hand, LEOS require less power but frequent handoffs. We shall discuss the characteristics of
each of these in some detail below.
2.2.1 Geostationary satellites
Satellites at the altitude of 35,800 km in a circular equatorial orbit appear motionless in the sky.
Such satellites are called geostationary satellites. With current technology, it is unwise to have
geostationary satellites spaced much closer than 2 degrees in the 360-degree equatorial plane, to
avoid interference.
ITU has allocated certain frequencies to satellite users. The main ones are listed in Table 2.1.
The C band was the first to be designed for commercial satellite traffic. This band is already over-
crowded because it is also used by the common carriers for terrestrial microwave link. The L and S
bands were added by an international agreement in 2000. However, they are narrow and
crowded. The next higher band available to commercial communication carriers is Ku
(K under) band. This band is not (yet) congested, and at these frequencies, satellites can be placed
as close as 1 degree. However, another problem exists: rain. Water is an excellent absorber of
these short microwaves. Bandwidth has also been allocated in the Ka (K above) band for com-
mercial satellite traffic, but the equipment needed to use it is still expensive.
A new development in the communication satellite world is the development of low-cost
microstations, also called VSATs (very small aperture terminals). These tiny terminals have
1 m or smaller antennas (versus 10 m for a standard GEO antenna) and can put out about 1 watt
of power. In many VSAT systems, the microstations do not have enough power to communicate
directly with one another (via the satellite of course). Instead, a special ground station called the
hub, with a large, high-gain antenna, is needed to relay traffic between VSATs. See Figure 2.3.
2.2.2 Medium ear th orbit satellites
MEOs are deployed much lower than the GEOs, and must be tracked as they move through the
sky, as they drift slowly in longitude, taking about 6 hours to circle the earth. They have a smaller
footprint on the ground and require less powerful transmitters to reach them. The 24 GPS (global
positioning system) satellites orbiting at about 18,000 km above the earth are an example of
MEO satellites.
2.2.3 Low ear th orbit satellites
Moving down in altitude, we come to the LEO satellites. Due to their rapid motion, large numbers of
them are needed for a complete system. On the other hand, because the satellites are so close to earth,
Band Downlink Uplink Bandwidth Problems
L 1.5 GHz 1.6 GHz 15 MHz Low Bandwidth; crowded
S 2.2 GHz 2.2 GHz 70 MHz Low Bandwidth; crowded
C 4.0 GHz 6.0 GHz 500 MHz Terrestrial interference
Ku 11 GHz 14 GHz 500 MHz Rain
Ka 20 GHz 30 GHz 3,500 MHz Rain, equipment cost
Table 2.1 The Main Satellite Bands

the ground does not need much power, and the round-trip delay is only a few milliseconds. The
examples are Iridium, Globalstar and Teledisc, of which only the last one is briefly discussed here.
Teledesic is targeted at bandwidth-hungry Internet users all over the world. The goal of the
Teledesic system is to provide millions of concurrent Internet users with an uplink of as much as
100 Mbps and a downlink of up to 720 Mbps using a small, fixed, VSAT-type antenna, com-
pletely bypassing the telephone systems. It uses 30 satellites with large footprints, using the
high-bandwidth Ka band, and packet-switching in space, with each satellite capable of routing
packets to its neighbours. Users who want to send packets request and get assigned bandwidth
dynamically, in about 50 ms.
2.3 Multiple-access schemes
In a wireless environment, there is a need to address the issue of simultaneous multiple access by
many users or mobile stations (MSs) in the transmission range between the BS and themselves.
Users are able to receive signals transmitted by others in the system. To accommodate a number of
users, many traffic channels need to be made available. To provide simultaneous two-way com-
munications (duplex communication), a forward (downlink) channel from BS to MS and a reverse
(uplink) channel from MS to BS are necessary. Two types of duplex systems are used: frequency
Hub
Communication
satellite
VSAT
1 3 2 4
Figure 2.3 Hub and VSA Ts

16 Mobile Computing
division duplexing (FDD) divides the frequency used, and time division duplexing (TDD) divides
the same frequency by time.
There are three basic ways in which many channels can be allocated within a given band-
width. These are with respect to frequency, time and code division multiplexing, using three
multiple-access techniques. These are frequency division multiple access, (FDMA), time division
multiple access (TDMA) and code division multiple access (CDMA). FDMA mainly uses FDD,
while TDMA and CDMA systems use either FDD or TDD. We will discuss these three techniques
in this section along with their advantages and disadvantages.
A multiple-access technique is important in mobile cellular systems, so that an MS can
distinguish a signal from the serving BS, and also discriminate the signals from an adjacent BS.
Multiple-access techniques are based on the orthogonalization of signals.
An FDMA system is one which uses different carrier frequencies to transmit the signal
for each user. If a system uses distinct time to transmit the signal for different users, it is a
TDMA system. If a system uses different codes to transmit the signal for each user, it is a
CDMA system.
2.3.1 FDMA—F requency division multiple access
In FDMA, the allocation of frequencies to channels can either be fixed (as in radio stations) or
dynamic (demand-driven). Furthermore, channels can be assigned to the same frequency at all
times, that is, pure FDMA, or change frequencies according to a certain pattern, that is, FDMA
combined with TDMA. The latter is done in many wireless systems to circumvent narrowband
interference at some frequencies, known as frequency hopping. The sender and the receiver
agree on a hopping pattern, so that the receiver can tune to the right frequency. Hopping patterns
are normally fixed for a long period.
As an example of FDMA, let us consider a mobile phone network based on the global system
for mobile communication (GSM) standard for 900 MHz. There are 124 multiple-access channels
per direction available at 900 MHz. The basic frequency allocation scheme is fixed and regulated
by a national authority. All uplinks use the band between 890.2 and 915 MHz; all downlinks use
935.2 to 960 MHz. The BS allocates a certain frequency for uplink and downlink to establish a
duplex channel with a mobile phone. Each channel (uplink and downlink) has a bandwidth of
200 KHz. Uplinks and downlinks have a fixed relation.
For a certain channel n,
if the uplink frequency is fu 890 MHz n 0.2 MHz,
the downlink frequency is fd fu 45 MHz, i.e., fd 935 MHz n 0.2 MHz.
2.3.2 TDMA—T ime division multiple access
TDMA offers a much more flexible scheme as compared with FDMA. Tuning to a certain fre-
quency is not required, and the receiver can stay at the same frequency all the time. Very simple
receivers and transmitters can thus be designed, since listening to many channels separated in
time is easier than listening to different frequencies at the same time. Many different algorithms
exist to control medium access using only one frequency. Almost all MAC schemes for wired
networks like Ethernet, token ring, Asynchronous transfer mode (ATM), etc., work according
to this principle.
In TDMA, synchronization between receiver and sender has to be achieved in the time
domain. This can be done either by using a fixed pattern or by using a dynamic allocation. Fixed
allocation is not efficient in cases where bandwidth requirement is variable.

Many systems like IS-54, IS-136, GSM and digital European cordless telecommunications
(DECT) use TDMA with fixed allocation. For example, for the DECT cordless phone system, the
BS uses 1 out of 12 slots for the downlink, whereas the MS uses 1 out of 12 different slots for the
uplink. Uplink and downlink are separated in time. Up to 12 different MS can use the same fre-
quency without interference. Each connection is allotted its own uplink and downlink pair. The
pattern is repeated every 10 ms; that is, each slot has a duration of 417 µsec. This repetition guar-
antees access to the medium every 10 ms, independent of any other connection. A guard band is
also used at the beginning and end of each slot to avoid collisions due to drifts in receiver and
transmitter clock frequency or computational delays in placing the data in a slot.
Fixed access patterns are efficient for connections with a constant data rate, as in classical
voice transmission with 32 or 64 Kbps duplex. But they are inefficient for bursty data or asym-
metric connections, as in Web browsing, where no data transmission occurs while the page is
being read, whereas clicking on a hyperlink triggers data transfer from the MS to the BS, followed
by a large volume of data returned from the Web server. In such cases, demand-oriented TDMA
schemes are used. In demand-oriented TDMA, the allocation is traffic dependent, and the BS
can reserve time slots for an MS on demand.
2.3.3 CDMA—Code division multiple access
CDMA is the best technical solution available today and is the basis for the 3G mobile systems. It
is also widely used in the United States in 2G mobile systems, competing with Digital advanced
mobile phone system (D-AMPS). For example, Sprint personal communication services (PCS)
uses CDMA, whereas ATT Wireless uses D-AMPS. CDMA is also known as International Stan-
dard IS-95 or cdmaOne.
CDMA is completely different from FDMA and TDMA. Instead of dividing the allowed fre-
quency range into a few hundred narrow channels, CDMA allows each station to transmit over the
entire frequency spectrum all the time. Multiple simultaneous transmissions are separated using
codes. Codes used by users should have a good autocorrelation and should be orthogonal to
other codes. For details of these two terms, the reader is referred to Tanenbaum (2003). Autocorre-
lation helps a receiver to reconstruct the original data precisely even in the presence of distortion
by noise, and orthogonality is necessary for two stations to share the medium without interference.
Tanenbaum (2003) has given a very good analogy to explain the concept of CDMA:
An airport lounge has many pairs of people conversing. TDMA is comparable to all the people being in
the middle of the room but taking turns speaking. FDMA is comparable to the people being in widely
separated clumps, each clump holding its own conversation at the same time as, but still independent
of, the others. CDMA is comparable to everybody being in the middle of the room talking at once, but
with each pair in a different language. The French-speaking couple just hones in on the French, reject-
ing everything that is not French as noise.
In CDMA we extract only the desired signal and reject everything else as random noise.
Here, each bit time is subdivided into m short intervals called chips. There are normally 64 or
128 chips per bit or longer. Each station is assigned a unique m-bit code called a chip sequence.
Chip sequences in IS-95, for example, are 242
1 chips long, and the chipping sequence is
1228800 chips/s; that is, the code repeats after 41.425 days.
To transmit a 1 bit, a station sends its chip sequence. To transmit a 0 bit, it sends the 1-bit’s
complement of its chip sequence. No other patterns are permitted. Thus, for m 6, if station A
is assigned the chip sequence 010011, it sends a 1 bit by sending 010011 and a 0 bit by sending

18 Mobile Computing
101100. For pedagogical purposes, it is more convenient to use a bipolar notation, with binary
0 being ⫺1 and binary 1 being ⫹1.
To synchronize the sender and the receiver, the sender transmits a long predefined chip
sequence so that the receiver can lock onto it. Transmissions that are not synchronized are treated
as noise. The longer the chip sequence, the higher the probability of detecting it correctly in the
presence of noise.
Implementation of the chip sequences and codes is complicated, and this is a major draw-
back with the CDMA scheme. But it is used for wireless mobile communication, as it operates in
a much higher (1.25 MHz) band than D-AMPS and GSM, where it can support many more users
than either of these systems.
For a good comparison of the above techniques, the reader is referred to Schiller (2006).
2.4 Cellular communication
Wireless communication using unguided media, that is, radio and microwave frequencies or
satellites, has found widespread use in mobile phones. These are currently being used for voice
communication, but soon they will find use in data communication. Cellular communication
has undergone many generations, in which the communication bandwidths and data speeds
have continuously increased. This has given rise to many applications that have benefited mobility.
In this section, we discuss briefly these generations and how they have revolutionized not only
mobile phone communication, but also mobile computing.
2.4.1 The first generation (1G): 1980
Analog cellular systems were the first generation of mobile telephone communication systems.
They used analog frequency modulation for only voice (speech) transmission. The various sys-
tems that fall in this category are AMPS (Advanced Mobile Phone Service) (USA), Nordic Mobile
Telephone (NMT)-900 (Sweden) and Cellular Digital Packet Data (CDPD), which is designed to
provide packet data services on the top of existing AMPS.
The system architecture is such that a geographic region is divided into cells. The size of the
cells in AMPS is about 10–20 km across, but is lesser in digital systems. Each cell uses some set of
frequencies not used by its neighbours. Transmission frequencies are reused in nearby but not
adjacent cells. Figure 2.4a illustrates the concept of frequency reuse. The cells are normally circu-
lar but are shown as hexagonal for ease of drawing.
F6
F7
F1
F5
F2
F3
F4
F6
F7
F7
F6
F5
F1
F2
F1
F5
F2
F4
F3
F4
F3
Figure 2.4(a) Adjacent cells use different
frequencies
Figure 2.4(b) Microcells add more
users
(a) (b)

If the area is overloaded, the power is reduced and the overloaded cells are split into smaller
microcells. This allows for more frequency reuse and is shown in Figure 2.4b. At the centre of
each cell is a BS to which all the telephones in the cell transmit. In a small system, all the BSs are
connected to a single device which is called an MTSO (mobile telephone switching office) or
MSC (mobile switching centre). In a larger system, several MTSOs may be used in a hierarchical
manner.
Handoff: At any time instant, a mobile phone logically belongs to one cell and is under con-
trol of its BS. When it moves physically from the cell, the BS notices the phone’s fading signal
and finds out from other neighbouring BSs as to which one is getting the strongest signal. It then
transfers ownership of the mobile to the BS of that cell. If a call is in progress, the mobile is asked
to switch to the new channel used in that adjacent cell. This process is called handoff or handover.
A BS is only a radio relay; the channel assignment is done by the MTSO.
Handoffs can be either soft or hard. In a soft handoff, the mobile is acquired by the new BS
before the old one signs off. Thus, there is no loss of continuity. But it requires the mobile to be
able to tune to two frequencies at the same time. Neither first- nor second-generation devices can
do this. 3G CDMA systems provide soft handover, resulting in seamless connectivity to the
mobile. In a hard handoff, the old BS drops the mobile before the new one acquires it. The call
is disconnected abruptly if there is no available frequency with the new BS, or there is a call drop
till the new frequency is received. This is noticeable by the user but is typically of very short
duration of about 60 ms in GSM systems.
Different kinds of handover are possible when a mobile moves from one cell to another or when
traffic through a specific stage becomes very high. Readers are referred to (Kamal, 2007) for details.
The AMPS system uses 832 full-duplex channels, each consisting of a pair of simplex chan-
nels (824–849 MHz for transmission and 869–894 MHz for reception). The individual cells use
different frequencies, using a system referred to as FDMA. Here the maximum supported bit rate
is 19.2 Kb/s.
Although 1G communication provided a good start, its main disadvantage was low speed
due to low available frequencies, interference due to frequency reuse and poor security.
2.4.2 The second generation (2G): 1992
The first generation of mobile phones was analog; the second generation was digital. The term
PCS is sometimes used in the marketing literature to indicate the second-generation systems.
Sometimes PCS is classified as a 2.5-generation (2.5G) system separately. The various advantages
of digital cellular are as follows:
1. It is more robust as it displays resistance to noise and crosstalk and has efficient error correction.
2. It exhibits the intelligence of the digital network.
3. It is more flexible and can be integrated with the wired digital network.
4. Reduced RF transmission power is needed.
5. Encryption can be provided for communication privacy.
6. System complexity is reduced.
7. User capacity is increased.
There are two basic technologies for managing shared access in digital cellular systems,
which are further classified as shown in Figure 2.5.
The IS-54 standard is a North American standard based on TDMA. It contains the 30 KHz
spacing of AMPS to make the evolution from analog to digital easier. Each channel provides a
raw bit rate of 48.6 Kb/s.

20 Mobile Computing
The Pan-European GSM is based on TDMA with eight slots per radio channel. Each user
transmits periodically in each of the slots with duration of 0.57 seconds. In the present version,
GSM supports full-rate 22.8 Kb/s transmission.
In spite of many improvements over 1G, 2G still has many shortcomings. First, it still focuses
only on low data rate speech service. Second, the capacity still does not satisfy the ever-growing
demand, and finally multimedia service is still not provided.
2.4.3 The 2.5 generation (2.5G): 1996
As an interim step towards the higher data rates of 3G, whose technology differs considerably
from the current cellular technology, some new techniques are being deployed as stopgap meas-
ures. They use incremental advances in cellular technology to increase the capacity of the currently
deployed infrastructure.
Enhanced data rates for GSM evolution (EDGE) is a 2.5G system, with a data rate of 384 kbps,
which is higher than GSM. The errors introduced by the high speeds necessitate the use of nine
different schemes for modulation and error correction.
GPRS (general packet radio service) is another 2.5G scheme which is actually an overlay
packet network over D-AMPS or GSM. In this scheme, voice Internet protocol (IP) packets can be
exchanged between mobile senders and receivers, with speeds of 115 kbps. This is done by reserv-
ing some time slots on some frequencies for packet traffic. The BS can dynamically vary the number
and location of the time slots, based on how much voice traffic is to be sent in the cell. The BS
sends the packet received from the mobile unit, to the Internet, through a wired connection.
2.4.4 The third generation (3G): 2000 ⴙ
ⴙ
In 1992, 3G was envisaged by ITU as International Mobile Telecommunication (IMT2000), but it
still has not seen the light of day. Its aim is to implement true ‘anybody at any place’ communi-
cation with ‘anyone at any time’. IMT2000 is defined as a system aimed at ‘the provision of
worldwide mobile service through a limited number of wireless access points by combining vari-
ous services and different systems’. It promises to connect up to 2 billion people worldwide by
2010 and offer data rates of up to 2 Mbps.
The frequency bands identified for IMT2000 are 1885–2025 MHz and 2110–2200 MHz. Its
goals are to
• Support high mobile velocity (300–500 km/hour), compared with less than 100 km/hour in
GSM.
• Support global wandering, as opposed to district and country in GSM.
• Support multimedia service, especially Internet service, 144 Kb/s (outdoor and higher velocity),
384 Kb/s (from outdoor to indoor, lower velocity), 2 Mb/s (indoor); speech with quality of
service (QoS) and other services 4–100–200 Kbs/s (GSM, lower velocity).
2G Technologies
TDMA
European GSM IS-54 IS-95
CDMA
Figure 2.5 The 2G Technologies

• Convenience for transition and evolvement or innovation, compatibility of services with
various fixed/mobile networks. High quality and security comparable to the fixed network.
• Highest spectrum availability, higher QoS, speech recognition technology, lower cost, higher
security.
• Use the advantages of technologies such as adversity transmitting and receiving, multi-
path combining, turbo code, channel estimation, signal-to-interference power ratio (SIR)
measurement and Transmit power control (TPC), space-time technology, multi-user de-
tection and interference cancellation, beam forming and smart antennas, and soft hand-
off, etc.
Service targets for IMT-2000 are worldwide roaming, software radio and user identity mod-
ule (smart card). Various services for users include multirate multimedia, that is, voice, image and
high-speed data up to 2 Mbps.
2.4.5 The 3.5 generation (3.5G): 2000 ⴙ
ⴙ
The technical breakthrough towards 3.5G provides for an open architecture for service based on
multimedia, and application of technologies such as smart antenna, software defined radio and
TD-CDMA. The standardization focus has been moved from radio to network side, giving rise to
the following advantages:
• Increased possibility to accommodate different types of radio in one system.
• A shift to the networking paradigm causes the problem of migrating legacy systems, so the
effort for maintaining interoperability has been increased.
• Rapid upgrade of the standard for 4G, giving rise to more attention on the system evolution
scenario.
2.4.6 The four th generation (4G): 2002 ⴙ
ⴙ
Between 1992 and 1995, there was a project in the European Community that was called Mobile
Broadband System (MBS), which targeted future outdoor, cellular scenarios with high mobility
and high data rates, to provide mobile multimedia communications. These systems will be the
fourth mobile generation.
The European Radio communications Office (ERO) has proposed some features for 4G sys-
tems which include high bandwidth, ubiquity (connectivity everywhere), seamless integration
with wired networks (especially IP), adaptive resource and spectrum management, software
radio, besides high quality of multimedia service.
To implement the above features, innovative concepts are needed. The approach taken at
the Mobile Multimedia Communication (MMC) project of the Delft University of Technology is
to form a multi-disciplinary team in which user aspects get as much attention as the technologi-
cal challenges. This MMC project has the following research goals:
• User interface and transparency
• Compression: Research has been carried out in two areas for source coding and two tech-
niques have been proposed:
1. H.263 for mobile video communication
2. Compression of the shapes of video objects
• Transmission protocols: The MMC project uses a hierarchical protocol structure that pro-
vides different QoS to the various traffic streams in mobile multimedia communication. The
hierarchy is realized with a hybrid TDM/FDM (time division multiplexing/frequency division

22 Mobile Computing
multiplexing) technique in which frames (the largest unit of data) are composed of packets,
fragments and radio data units (RDUs).
• Broadband radio transmission: MMC transmission is located at the V band (from 40–75 GHz),
centred at 60 GHz. New techniques for measurements have been proposed and carried out.
The chosen modulation scheme is orthogonal frequency division multiplex (OFDM), which
is specifically able to cope with the problems of the multipath reception.
However, 4G is still a distant dream, since as of today even 3.5G systems are yet to take off.
2.5 Summary
The electromagnetic spectrum contains all the frequencies that can be used in wireless commu-
nication and is the basis of all mobile computing. The different portions of the spectrum com-
prise radio waves, microwaves, infrared, and lightwaves, and their characteristics determine the
data rates and applications in which each of these ‘unguided’ media can be used.
Communication satellites are an upcoming and useful long-range transmission system.
Depending on their height of deployment, these can classified as geostationary orbit, medium
earth orbit and low earth orbit and can be used in different applications.
Cellular communication has revolutionized the way mobile handhelds and phones are used.
These handhelds are currently being used more for voice communication, but soon they will find
widespread use for data. The first-generation systems were analog, and second-generation ones
were digital with many options, like GSM, FDMA, TDMA and CDMA. There is a lot of talk about
3G, 3.5G and 4G systems, all of which are yet to take shape in reality. Each generation has
improved on the capabilities of the older generation, with many new features added for broad-
band applications. Handover is an important aspect of all mobile systems and must be handled
with proper care to provide seamless connectivity to mobile devices.
In the next chapter, we discuss wireless LAN (WLAN) standards, which are based on the
short-range wireless communication technologies discussed in this chapter.
Problems
1. If a binary signal is sent over a 4 KHz channel whose signal-to-noise ratio is 20 dB, what is
the maximum data rate achievable?
2. In a tabular form, compare radiowaves, microwaves and infrared waves, with respect to their
data rates, transmission distance, interference and cost.
3. Repeat Question 2 by comparing the three satellite communication types, namely, GEOS,
MEOS and LEOS.
4. Give typical applications for each of the three satellite systems.
5. Discuss how digital communication is better than analog communication.
6. Compare and contrast FDMA, TDMA and CDMA techniques.
7. Elaborate on the goals of IMT2000.
8. Identify the generation of your own mobile phone. Do you think it has the functionality
discussed in this chapter for the relevant generation?
9. Differentiate between the two types of handoffs.

10. What are the features proposed for 4G systems?
11. Assume that 4G mobile technology handhelds are already available. Give some ideas for new
features that can be added in 4G systems of tomorrow.
Multiple-choice questions
1. The higher frequencies, that is, ultraviolet light, X-rays and gamma rays, are normally not
used for wireless transmission, because of which one of the following reasons?
(a) They are difficult to produce and modulate
(b) Do not propagate well through buildings
(c) They are harmful to humans
(d) All of the above
2. Which of the following is false for microwaves?
(a) They travel in straight lines and are thus affected by the earth’s curvature
(b) They are relatively inexpensive to use
(c) They can propagate well through buildings
(d) They are preferred over optic fiber, especially in harsh terrain or urban areas
3. According to Shannon’s theorem, the maximum data rate D of a noisy channel whose band-
width is H Hz, and whose signal-to-noise ratio is S/N, is given by which one of the following
formulae?
(a) D H log2 (1 S/N)
(b) D H (1 log2 S/N)
(c) D 2H log2 (1 S/N)
4. Which of the following is the correct sequence of waves in increasing order of frequencies?
(a) Radio, microwaves, infrared, ultraviolet light, X-rays, gamma rays
(b) Microwaves, radio, visible light, X-ray, ultraviolet light, gamma rays
(c) Radio, microwaves, infrared, ultraviolet light, gamma rays, X-rays
(d) Microwaves, radio, infrared, visible light, X-rays, gamma rays
5. Which one of the following is not true for infrared waves?
(a) They are capable of traversing multiple paths without interferences
(b) They are less secure than radio
(c) They have reasonably high bandwidth
(d) No government license is required for their operation
6. Globalstar satellites, which are close to the earth, do not need much power, and their round-
trip delay is only a few milliseconds, are examples of which one of the following?
(a) Geostationary satellites (GEOS)
(b) Medium earth orbit satellites (MEOS)
(c) Low earth orbit satellites (LEOS)
7. To which one of the following generations does CDMA belong?
(a) First generation
(b) Second generation
(c) Third generation
(d) Fourth generation

24 Mobile Computing
8. Which one of the following is the multiple-access scheme used in GSM?
(a) Time division multiple access (TDMA)
(b) Frequency division multiple access (FDMA)
(c) Code division multiple access (CDMA)
(d) A combination of TDMA and FDMA
9. Which one of the following best characterizes IS 95?
(a) a standard for cellular CDMA
(b) a standard for cellular TDMA
(c) a standard procedure for measuring indoor multipath propagation characteristics
(d) a standard interconnecting base stations
10. The efficiency of a wireless system is given in which of the following units?
(a) bits per second
(b) bits per second per Hertz
(c) bits per second per Hertz per km2
Further reading
A.S. Tanenbaum (2003), Computer Networks, 4th ed. (New Delhi, India: Pearson Education).
C. Shannon (1948), ‘A Mathematical Theory of Communication’, Bell System Technical Journal, 27
(July, October): 379–423, 623–656.
C.R. Casal, F. Schoute and R. Prasad, ‘A Novel Concept for Fourth Generation Mobile Multimedia
Communication’, www.ubicom.tudelft.nl/MMC/Docs/VTC99.pdf (accessed November 2005)
———, ‘Evolution towards Fourth Generation Mobile Multimedia Communication’, www.ubicom.
tudelft.nl/MMC/Docs/paper38.pdf (accessed March 2005)
D.P. Agrawal and Q.A. Zeng (2003), Introduction to Wireless and Mobile Systems (Thomson, Singapore).
J.F. Huber, D. Weiler and H. Brand (2000), ‘UMTS, the Mobile Multimedia Vision for IMT-2000:
A Focus on Standardization’, IEEE Communications Magazine, 38 (September): 129–136.
J.H. Schiller (2006), Mobile Communications, 2nd ed. (Pearson Education, USA).
J.S. Lee and L.E. Miller (1998), CDMA Systems Engineering Handbook (London: Artech House).
R. Kamal (2007), Mobile Computing (Oxford University Press).
X. Zhou, ‘Overview of the Third Generation Mobile Communications’, www.meru.cecs.missouri.
edu/workshop/zxb_pres1.ppt. (accessed January 2005)

W
ith the advent and recent proliferation of handheld devices, wireless local area net-
works (WLANs) have become very popular. One can see them in offices, campus build-
ings, airports, hotels, restaurants, etc., facilitating continuous access to the Internet,
through what has come to be known as the wireless indoors. Recently, the concept of the
wireless outdoors has also emerged, which is concerned with the so-called last mile technology
or wireless local loop (WLL) or fixed wireless access. To provide connectivity to millions of homes
and businesses one has to lay fibre, coax, or category 5 twisted pair, which is a very daunting and
costly affair. The provider uses a directed antenna and a transmitter of predefined power to ensure
stable reception of high-frequency signals within a limited coverage area, such as an individual
building.
WLL can be narrowband or wideband. Broadband wireless or wireless metropolitan area
networks (WMANs) simply require erecting a big antenna on a hill just outside the town and
installing antennas directed at it on customers’ rooftops. We shall study both WLAN and WMAN
standards in this chapter.
WLANs can operate in two configurations—with base stations or access points that are con-
nected to the wired network, or without base stations, that is, mobile ad hoc networks (MANETs).
MANETs are the subject of discussion in Chapter 6. Both configurations, however, use the short-
range radio-wave transmission discussed in Section 2.2. See Figure 3.1.
Wireless Networ ks 3
Base station
To wired network
(a) (b)
Figure 3.1 Wireless Networ ks (a) With Base Station (b) Without Base Station
25

26 Mobile Computing
When wireless networks were first developed, there were many challenges that had to be
met. Some of them have been mentioned in Chapter 2. These challenges included finding an
available worldwide frequency band, dealing with the finite range of signals, maintaining user
privacy, taking limited battery life into account, understanding the implications of mobility,
making the system economically viable, etc. We shall be dealing with all these issues in the book.
3.1 The need for ne w wireless standards
The main standard developed for WLANs is called the IEEE 802.11. So the question that arises
here is: what is the need for a new standard, that is, why can’t the universal Ethernet be used for
WLANs? The answer lies in the many ways in which wireless operation differs from the tradi-
tional wired one. Some of these are discussed below.
1. Ethernet uses carrier sense multiple access with collision detection (CSMA/CD). An
Ethernet station just waits until the ether is idle and starts transmitting. If it does not receive a
noise burst back within the first 64 bytes, it assumes that the frame has been delivered correctly.
But carrier sensing is not possible in the wireless environment. Also, not all stations are within
the radio range of each other. Transmissions going on in one part of a cell may not be received
elsewhere in the same cell. There are two problems encountered in this scenario—the problem of
the hidden station and the problem of the exposed station.
a. The hidden station problem: Shown in Figure 3.2 is a WLAN containing stations A, B
and C. C, which is not in the radio range of A, is transmitting to station B. If station A
senses the channel, it will not hear anything because it is hidden from C. It falsely con-
cludes that it may now start transmitting to B, resulting in a collision.
b. The exposed station problem: Consider the same WLAN, but now the scenario is as
shown in Figure 3.3. A is transmitting to some station D not shown in the diagram. B is
near A and can hear A sending. It falsely concludes that it cannot transmit to C, even
though it can do so simultaneously. Thus, because of B’s exposed location to A, it defers
its transmission even when it need not.
2. Multipath fading (interference). This is due to reflection of radio signals by solid objects,
which results in signals being received along multiple paths. This may cause interference, leading
to data becoming error-prone in the wireless environment.
A B C
C is transmitting
Range of
C’s radio
A wants to send
to B but cannot
hear that B is busy
Figure 3.2 The Hidden Station Problem

3. No handoff in Ethernet. Handoff is the mechanism of allowing a mobile device contin-
ued access even when it moves from one cell (network) to another. This is a major requirement
in wireless networks, but is not needed in the wired Ethernet.
4. Half-duplex transmission. Most radios are half duplex. They cannot transmit and
listen for noise bursts at the same time in a single frequency. Thus, carrier sensing is not possible.
5. Absence of mobility-aware software. Software that is mobility-based or mobility-aware
is yet to be made universally available. Until that happens, WLANs of mobile, handheld comput-
ers cannot be deployed as universally and simply as the standard Ethernet.
The above limitations of the standard Ethernet necessitated the development of a new stan-
dard for WLANs.
3.2 IEEE 802.11 WLAN standard
The IEEE 802.11 WLAN Standard is popularly known as the Wi-Fi standard. We shall now study
in detail its protocol stack, frame structure and services. The physical layer radio-transmission
techniques are beyond the scope of this book, but we shall mention them briefly here. Figure 3.6
shows the lower two layers of the IEEE 802.11 protocol stack. Here the data link layer consists of
two sublayers, called the logical link control (LLC) layer and the medium access control (MAC)
layer. The IEEE 802.11 protocol stack is discussed in detail below.
3.2.1 Physical layer
The 802.11 standard was developed in 1997 with data rates of 1 to 2 Mbps for WLANs. Initially,
it had three possible modulation techniques for sending MAC frames from a sender station to a
receiver station. Only some highlights of these techniques are given below and are as follows. For
details, please refer to Tanenbaum (2003).
• 802.11: Infrared, which uses diffused transmission at 0.85 or 0.95 microns. Two speeds are
permitted, those of 1 Mbps and 2 Mbps. The advantage of infrared transmission, as seen in
Chapter 2, is that infrared signals do not penetrate walls, so cells in adjacent rooms are well
insulated from each other. But it is not good in sunlight, as sunlight swamps infrared signals.
Further, bandwidth is limited.
C
A B
A is transmitting
Range of
A’s radio
B wants to send to C
but mistakenly thinks the
transmission will fail
Figure 3.3 The Exposed Station Problem

28 Mobile Computing
• 802.11: FHSS (frequency hopping spread spectrum), in which the transmitter hops from
frequency to frequency hundreds of times per second. It uses 79 channels, each 1 MHz wide,
starting at the low end of the 2.4 GHz ISM (industrial, scientific, medical applications) band.
A pseudorandom generator is used to produce the sequence of hopped frequencies. Figure 3.4
shows the concept of FHSS. Stations need to use the same seed for the pseudorandom gener-
ator and stay synchronized in time to hop to the same frequencies. The dwell time, which is
the amount of time spent at each frequency, is adjustable, but must be less than 400 msec.
Since the hopping sequence and dwell time are not known, FHSS provides security against
eavesdropping. It is resistant to multipath fading and is relatively insensitive to radio inter-
ference, which makes it popular for building-to-building links, that is, for wireless outdoors.
Its main disadvantage is its low bandwidth and low power.
• 802.11: DSSS (direct sequence spread spectrum) is like CDMA, but has some differences. It
is also restricted to 1 or 2 Mbps. Each bit is transmitted as 11 chips in what is called a Barker
sequence. Phase shift modulation is used at 1 or 2 Mbaud to transmit 1 or 2 bits per baud,
when operating at 1 or 2 Mbps, respectively. The concept of DSSS is shown in Figure 3.5.
Subsequently, these speeds were considered too slow, and in 1999, two new standards were
proposed. These are as follows:
• 802.11a: OFDM (orthogonal frequency division multiplexing), which uses the wider 5 GHz
ISM frequency band to deliver up to 54 Mbps. In OFDM, which is a form of spread spectrum,
but different from CDMA and FHSS, 52 different frequencies are used: four for synchroniza-
tion and 48 for data. Splitting the signal into many narrow bands offers key advantages like
better immunity to narrowband interference and the possibility of using non-contiguous
bands. It also has good spectrum efficiency in terms of bits/Hz and good immunity to multi-
path fading.
• 802.11b: HR-DSSS (high-rate DSSS) is another spread-spectrum technique, which uses
11 million chips/second to deliver data rates up to 11 Mbps in the 2.4 GHz band. Data rates
Spreading
Transmitter Receiver
Digital signal Digital signal
Spreading signal
Hopping
pattern
Power
Frequency
Power
Frequency
Power
Frequency
Despread
Hopping
pattern
Figure 3.4 Concept of F requency Hopping Spread Spectr um (FHSS)

supported are 1, 2, 5.5 and 11 Mbps. These rates may be dynamically adapted during opera-
tion to achieve the optimum speed possible under current conditions of load and noise.
Although it is incompatible with 802.11a and is much slower, its range is 7 times greater.
In 2001, another standard was proposed, which is
• 802.11g: This uses the modulation technique of 802.11a, that is, OFDM, and the frequency
band of 802.11b, so it theoretically delivers up to 54 Mbps data rates.
3.2.2 MAC layer
To overcome the hidden and exposed terminal problems of the CSMA/CD-based Ethernet, the
MAC sublayer of 802.11 supports two modes of operation. These are the DCF and the PCF (which
is optional) and are discussed below.
1. Distributed Coordination Function (DCF): As the name suggests, this mode does not use
any central control like the Ethernet. But it uses CSMA/CA, that is, CSMA with collision avoid-
ance, which itself supports two methods of operation.
Spreading
Transmitter Receiver
Digital signal
s(t)
Digital signal
s(t)
Spreading signal
m(t)
Code
c(t)
Power
Frequency
Power
Frequency
Power
Frequency
Despread
Code
c(t)
Figure 3.5 Direct Sequence Spread Spectr um (DSSS)
Upper layers
Logical link control
MAC sublayer
802.11
Infrared
802.11
FHSS
802.11
DSSS
802.11a
OFDM
802.11b
HR-DSSS
802.11g
OFDM
Physical
layer
Data
link
layer
Figure 3.6 The Lower Layers of the IEEE 802.11 Protocol Stack

30 Mobile Computing
The first method uses physical channel sensing. When a station wants to transmit, it senses
the channel. If it is idle, it starts transmitting. It does not continue to sense the channel while
transmitting, but sends the complete frame, which may be destroyed at the receiver due to inter-
ference there. If a collision occurs, the colliding stations wait for a random time, using the Ether-
net binary exponential backoff (BEB) algorithm, and then try again later. If the receiver does not
send an acknowledgement, the transmitter knows that a collision has occurred. There is no colli-
sion detection at the transmitter.
The second method is based on multiple access with collision avoidance for wireless
(MACAW) and uses virtual channel sensing. It works as shown in Figure 3.7. Suppose there are
four stations A, B, C and D in a network, such that B and C are within the range of A. D is not
within A’s range but is within the range of B.
Suppose A decides to send data to B. The protocol works as follows:
1. A sends a small 30 byte RTS (request to send) frame to B.
2. If B is ready to receive data, it responds with a CTS (clear to send) frame.
3. When A receives the CTS, it sends its data frame and starts an acknowledgement (ACK)
timer.
4. If B correctly receives the data frame, it responds with an ACK frame and terminates the
exchange.
5. In case A’s ACK timer expires before it receives the ACK, the whole protocol is repeated.
6. C also receives the RTS frame, as it is in the range of A. It realizes that someone else wants
to send data, so it stops transmitting till the data exchange is done.
7. D receives the CTS frame as it is in the range of B. Thus, it also maintains the same
state as C.
Note that the signals shown in Figure 3.7 for C and D, called network allocation vector
(NAV), are not transmitted. They are internal reminders to indicate that no data can be transmit-
ted during that time. This is a kind of virtual channel busy signal, asserted by the stations them-
selves, using the NAV. The time for which they must wait can be calculated using the
information present in the RTS and CTS frames.
Because of the noisy, wireless channel, the probability of the frame reaching the destination
successfully decreases with frame length. For noisy channels, 802.11 allows frames to be frag-
mented into smaller pieces, each with its own checksum. Once the channel has been acquired
using RTS and CTS, multiple fragments can be sent in a row (see Figure 3.8). The sequence of
fragments is called a fragment burst. Fragmentation increases the throughput by allowing only
bad fragments to be retransmitted, not the whole frame.
A
B
C
D
RTS
CTS ACK
Data
NAV
NAV
Time
Figure 3.7 CSMA/CA V irtual Channel Sensing

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The lyrical faculty is evinced by such poems. But other singers of our
day might have produced them—singers of the white race. Not so, I think,
of “The Band of Gideon.” Upon that poem is the stamp, not of genius only,
but of Negro genius. In it is re-incarnated, by a cultured, creative mind, the
very spirit of the old plantation songs and sermons. The reader who has in
his possession that background will respond to the unique and powerful
appeal of this poem.
THE BAND OF GIDEON

The band of Gideon roam the sky,
The howling wind is their war-cry,
The thunder’s roll is their trumpet’s peal
And the lightning’s flash their vengeful steel.
Each black cloud
Is a fiery steed.
And they cry aloud
With each strong deed,
“The Sword of the Lord and Gideon.”
And men below rear temples high
And mock their God with reasons why,
And live in arrogance, sin, and shame,
And rape their souls for the world’s good name.
Each black cloud
Is a fiery steed.
And they cry aloud
The band of Gideon roam the sky
And view the earth with baleful eye;
In holy wrath they scourge the land
With earthquake, storm, and burning brand.
Each black cloud
Is a fiery steed.
And they cry aloud
The lightnings flash and the thunders roll,
And “Lord have mercy on my soul,”
Cry men as they fall on the stricken sod,
In agony searching for their God.
Each black cloud
Is a fiery steed.
And they cry aloud
And men repent and then forget
That heavenly wrath they ever met.
The band of Gideon yet will come

And strike their tongues of blasphemy dumb.
Each black cloud
Is a fiery steed.
And they cry aloud
The reader, I predict, will be drawn again and again to this mysterious
poem. It will continue to haunt his imagination, and tease his thought. The
stamp of the African mind is upon it. Closely allied, on the one hand by its
august refrain to the Spirituals, on the other hand it touches the most refined
and perfected art; such, for example, as Rossetti’s ballads or Vachel
Lindsay’s cantatas. It can scarcely be wondered at that the people of his
race should call this untimely dead singer their Negro Lycidas.
II. James David Corrothers
THE DREAM AND THE SONG

So oft our hearts, beloved lute,
In blossomy haunts of song are mute;
So long we pore, ’mid murmurings dull,
O’er loveliness unutterable;
So vain is all our passion strong!
The dream is lovelier than the song.
The rose thought, touched by words, doth turn
Wan ashes. Still, from memory’s urn,
The lingering blossoms tenderly
Refute our wilding minstrelsy.
Alas! we work but beauty’s wrong!
Yearned Shelley o’er the golden flame?
Left Keats, for beauty’s lure, a name
But “writ in water”? Woe is me!
To grieve o’er floral faëry.
My Phasian doves are flown so long—
The dream is lovelier than the song!
Ah, though we build a bower of dawn,
The golden-winged bird is gone,
And morn may gild, through shimmering leaves,
Only the swallow-twittering eaves.
What art may house or gold prolong
A dream far lovelier than a song?
The lilting witchery, the unrest
Of wingèd dreams, is in our breast;
But ever dear Fulfilment’s eyes
Gaze otherward. The long-sought prize,
My lute, must to the gods belong.
Cherokee-Indian, Scotch-Irish, French, and African blood in James
David Corrothers, the author of this poem, makes his complexion, he
supposed, “about that of the original man.” The reader has already had, at
the beginning of the discussion of Dunbar, a sonnet from this poet. The
sonnet, the above poem, and the others given here were published in The

J. D. Corrothers
Century Magazine. Not unworthy of The Century’s standards, the reader
must say.
James David Corrothers was born in
Michigan, July 2, 1869. His mother in giving
him life surrendered her own. His father never
cared for him. Sheltered for a few years by
maternal relatives, he was out on the world in
early boyhood, dependent on his own resources.
Soon, because he was a Negro, he was a
wanderer for work through several states. Often
without money, friends, or food, he slept out of
doors, sometimes in zero weather. At nineteen
years of age, as before stated, he was shining
shoes in a Chicago barber shop. There he was
“discovered.”
Henry D. Lloyd was having his boots shined
by young Corrothers when the two fell into book talk. The distinguished
writer was astonished at the knowledge possessed by one engaged in such a
menial occupation. Out of this circumstance, it seems, the Negro boot-black
became a student in Northwestern University at Evanston, Illinois. By
mowing lawns and doing whatever odd jobs he could find he worked his
way for three years in the university. Then, by the kindness of Frances E.
Willard, he had a year in Bennett College, Greensboro, North Carolina.
Prior to his entrance at Northwestern there had been but one brief
opportunity in his life for attending school. But the wandering youth,
battling against the adverse fates, or, concretely stated, the disadvantage of
being a Negro, had managed somehow to make great books his
companions. Hence, he had entered what Carlyle calls “the true modern
university.” Hence, his literary conversation with Mr. Lloyd.
Out of those early struggles, and perhaps also out of later bitter
experiences, came such poems as the following:
AT THE CLOSED GATE OF JUSTICE

To be a Negro in a day like this
Demands forgiveness. Bruised with blow on blow,
Betrayed, like him whose woe-dimmed eyes gave bliss,
Still must one succor those who brought one low,
To be a Negro in a day like this.
Demands rare patience—patience that can wait
In utter darkness. ’Tis the path to miss,
And knock, unheeded, at an iron gate,
Demands strange loyalty. We serve a flag
Which is to us white freedom’s emphasis.
Ah! one must love when truth and justice lag,
To be a Negro in a day like this—
Alas! Lord God, what evil have we done?
Still shines the gate, all gold and amethyst
But I pass by, the glorious goal unwon,
“Merely a Negro”—in a day like this!
Even though his face be “red like Adam’s,” and even though his art be
noble like that of the masters of song, yet had Mr. Corrothers, even in the
republic of letters, felt the handicap of his complexion, as this sonnet bears
witness:
THE NEGRO SINGER

O’er all my song the image of a face
Lieth, like shadow on the wild, sweet flowers.
The dream, the ecstasy that prompts my powers,
The golden lyre’s delights, bring little grace
To bless the singer of a lowly race.
Long hath this mocked me: aye, in marvelous hours,
When Hera’s gardens gleamed, or Cynthia’s bowers,
Or Hope’s red pylons, in their far, hushed place!
But I shall dig me deeper to the gold;
Fetch water, dripping, over desert miles
From clear Nyanzas and mysterious Niles
Of love; and sing, nor one kind act withhold.
So shall men know me, and remember long,
Nor my dark face dishonor any song.
Death has silenced the muse of this dark singer,
one of the best hitherto. That his endowment was
uncommon and that his achievement, as evinced by
these poems, is one of distinction, to use Mr.
Howells’s word, every reader equipped to judge
of poetry must admit.
III. A Group of Singing Johnsons
In all rosters the name Johnson claims liberal space. Five verse-smiths
with that cognomen will be presented in this book, and there is a sixth.
These many Johnsons are no further related to one another, so far as I know,
than that they are all Adam’s offspring, and poets. Only three of them will
be presented in this chapter: James Weldon Johnson, of Florida, author of
Fifty Years and Other Poems (1917); Charles Bertram Johnson, of Missouri,
author of Songs of My People (1918); Fenton Johnson, of Chicago, author
of A Little Dreaming (1914); Unions of the Dusk (1915), and Songs of the
Soil (1916). The fourth and fifth are women, and will find a place in another
group; the sixth is Adolphus Johnson, author of The Silver Chord,
Philadelphia, 1915. The three mentioned above will be treated in the order
in which they have been named.
1. James Weldon Johnson

James Weldon Johnson
Now of New York, but born in Florida and reared in the South, James
Weldon Johnson is a man of various abilities, accomplishments, and
activities. He was graduated with the degrees of A. B. and A. M. from
Atlanta University and later studied for three years in Columbia University.
First a school-principal, then a practitioner of the law, he followed at last
the strongest propensity and turned author. His literary work includes light
operas, for which his brother, J. Rosamond Johnson, composed the music,
and a novel entitled The Autobiography of an Ex-Colored Man. Having
been United States consul in two Latin-American countries, he is a master
of Spanish and has made translations of Spanish plays and poems. The
English libretto of Goyescas was made by him for the Metropolitan Opera
Company in 1915. He is also one of the ablest editorial writers in the
country. In the Public Ledger’s contest of 1916 he won the third prize. His
editorials are widely syndicated in the Negro weekly press. Poems of his
have appeared in The Century, The Crisis, and The Independent.
Professor Brander Matthews in his
Introduction to Fifty Years and Other Poems
speaks of “the superb and soaring stanzas” of
the title-poem and describes it as “a poem
sonorous in its diction, vigorous in its
workmanship, elevated in its imagination, and
sincere in its emotion.” Doubtless this will seem
like the language of exaggeration. The sceptic,
however, must withhold judgment until he has
read the poem, too long for presentation here.
Mr. Johnson’s poetical qualities can be
represented in this place only by briefer though
inferior productions. A poem of special
significance, and characterized by the qualities
noted by Professor Matthews in “Fifty Years,” is
the following:
O SOUTHLAND!

O Southland! O Southland!
Have you not heard the call,
The trumpet blown, the word made known
To the nations, one and all?
The watchword, the hope-word,
Salvation’s present plan?
A gospel new, for all—for you:
Man shall be saved by man.
O Southland! O Southland!
Do you not hear to-day
The mighty beat of onward feet,
And know you not their way?
’Tis forward, ’tis upward,
On to the fair white arch
Of Freedom’s dome, and there is room
For each man who would march.
O Southland, fair Southland!
Then why do you still cling
To an idle age and a musty page,
To a dead and useless thing?
’Tis springtime! ’Tis work-time!
The world is young again!
And God’s above, and God is love,
And men are only men.
O Southland! my Southland!
O birthland! do not shirk
The toilsome task, nor respite ask,
But gird you for the work.
Remember, remember
That weakness stalks in pride;
That he is strong who helps along
The faint one at his side.
For pure lyric beauty and exquisite pathos, Wordsworthian in both
respects, but no hint of imitation, the following stanzas may be set, without
disadvantage to them, by the side of any in our literature:

The glory of the day was in her face,
The beauty of the night was in her eyes,
And over all her loveliness, the grace
Of Morning blushing in the early skies.
And in her voice, the calling of the dove;
Like music of a sweet, melodious part.
And in her smile, the breaking light of love;
And all the gentle virtues in her heart.
And now the glorious day, the beauteous night,
The birds that signal to their mates at dawn,
To my dull ears, to my tear-blinded sight
Are one with all the dead, since she is gone.
Yet one other poem of this fine singer’s I will give, selecting from not a
few that press for the restricted space. The easy flow of the verse and the
ready rhyme will be remarked—and that supreme quality of good lyric
poetry, austere simplicity.
THE YOUNG WARRIOR

Mother, shed no mournful tears,
But gird me on my sword;
And give no utterance to thy fears,
But bless me with thy word.
The lines are drawn! The fight is on!
A cause is to be won!
Mother, look not so white and wan;
Give Godspeed to thy son.
Now let thine eyes my way pursue
Where’er my footsteps fare;
And when they lead beyond thy view,
Send after me a prayer.
But pray not to defend from harm,
Nor danger to dispel;
Pray, rather, that with steadfast arm
I fight the battle well.
Pray, mother of mine, that I always keep
My heart and purpose strong,
My sword unsullied and ready to leap
Unsheathed against the wrong.
Arduous labors in other fields than poetry threaten to silence Mr.
Johnson’s muse, and that is to be regretted.
2. Charles Bertram Johnson
School-teacher, preacher, poet—this is Charles Bertram Johnson of
Missouri. And in Missouri there is no voice more tuneful, no artistry in song
any finer, than his. Nor in so bold an assertion am I forgetting the sweet
voice and exquisite artistry of Sarah Teasdale. Mr. Johnson’s art is not
unlike hers in all that makes hers most charming. Only there is not so much
of his that attains to perfection of form. On pages 52 and 63 were given two
of his quatrain poems. These were of his people. But a lyric poet should
sing himself. That is of the essence of lyric poetry. In so singing, however,
the poet reveals not only his individual life, but that of his race to the view
of the world. Another quatrain poem, personal in form, may be accepted as
of racial interpretation:

Charles Bertram Johnson
SOUL AND STAR
So oft from out the verge afar
The dear dreams throng and throng,
Sometimes I think my soul a star,
And life a pulséd song.
Born at Callao, Missouri, October 5, 1880, of
a Kentucky mother and a Virginia father, Charles
Bertram Johnson attended a one-room school
“across the railroad track,” where—who can
explain this?—he was “Introduced to Bacon,
Shakespeare, and the art of rhyming.” It reads
like an old story. Some freak of a schoolmaster
whose head is filled with “useless” lore—poetry,
tales, and “such stuff”—nurturing a child of
genius into song. But it was Johnson’s mother who was the great influence
in his life. She was an “adept at rhyming” and “she initiated me into the
world of color and melody”—so writes our poet. It is always the mother.
Then, by chance—but how marvelously chance comes to the aid of the
predestined!—by chance, he learns of Dunbar and his poetry. The ambition
to be a poet of his people like Dunbar possesses him. He knows the path to
that goal is education. He therefore makes his way to a little college at
Macon, Missouri, from which, after five years, he is graduated—without
having received any help in the art of poetry, however. Two terms at a
summer school and special instruction by correspondence seem to have
aided him here, or to have induced the belief that he had been aided. For
twenty-odd years he followed the profession of teaching. For ten years of
that period he also preached. The ministry now claims his entire energies,
and the muse knocks less and less frequently at his door.
Yet he still sings. In a recent number of The Crisis I find a poem of his
that in suggesting a life of toil growing to a peaceful close is filled with
soothing melody:
OLD FRIENDS

Sit here before my grate,
Until it’s ashen gray,
Or till the night grows late,
And talk the time away.
I cannot think to sleep,
And miss your golden speech,
My bed of dreams will keep—
You here within my reach.
I have so much to say,
The time is short at best,
A bit of toil and play,
And after that comes rest.
But you and I know now
The wisdom of the soul,
The years that seamed the brow
Have made our visions whole.
Sit here before my grate
Until the ash is cold;
The things you say of late
Are fine as shriven gold.
Even though one be born to sing, if circumstances have made him a
preacher he may be expected to moralize his song. Whether we shall be
reconciled to this will depend on the art with which it is done. If the moral
idea be a sweet human one, and if the verse still be melifluous, we will
submit, and our delight will be twofold—ethical and esthetical. We will put
our preacher-poet of Missouri to the test:
SO MUCH

So much of love I need,
And tender passioned care,
Of human fault and greed
To make me unaware:
So much of love I owe,
That, ere my life be done,
How shall I keep His will
To owe not any one?
Truth is, Mr. Johnson is not given to preaching in verse any more than
other poets. His sole aim is beauty. He assures me it is truth. Instead of
admitting disagreement I only assert that, being a poet, he must find all
truth beautiful. It is only for relative thinking we need the three terms, truth,
goodness, and beauty.
I will conclude this presentation of the Missouri singer with a lyrical
sermonette:
A RAIN SONG
Chill the rain falls, chill!
Dull gray the world; the vale
Rain-swept; wind-swept the hill;
“But gloom and doubt prevail,”
My heart breaks forth to say.
Ere thus its sorrow-note,
“Cheer up! Cheer up, to-day!
To-morrow is to be!”
Babbled from a joyous throat,
A robin’s in a mist-gray tree.
Then off to keep a tryst—
He preened his drabbled cloak—
Doughty little optimist!—
As if in answer, broke
The sunlight through that oak.
3. Fenton Johnson

Dreams and visions—such are the treasures of suffering loyal hearts:
dreams, visions, and song. Happy even in their sorrows the people to whom
God has given poets to be their spokesmen to the world. Else their hearts
should stifle with woe. As the prophet was of old so in these times the poet.
As a prophet speaks Fenton Johnson, his heart yearning toward the black
folk of our land:
THESE ARE MY PEOPLE
These are my people, I have built for them
A castle in the cloister of my heart;
And I shall fight that they may dwell therein.
The God that gave Sojourner tongue of fire
Has made with me a righteous covenant
That these, my brothers of the dusk, shall rise
To Sinai and thence in purple walk
A newer Canaan, vineyards of the West.
The rods that chasten us shall break as straw
And fire consume the godless in the South;
The hand that struck the helpless of my race
Shall wither as a leaf in drear November,
And liberty, the nectar God has blest,
Shall flow as free as wine in Babylon.
O God of Covenants, forget us not!
Fenton Johnson seems to be more deeply rooted in the song-traditions of
his people than are most of his fellow-poets. To him the classic Spirituals
afford inspiration and pattern. Whoever is familiar with those “canticles of
love and woe” will recognize their influence throughout Mr. Johnson’s three
volumes of song. I shall make no attempt here to illustrate this truth but
shall rather select a piece or two that will represent the poet’s general
qualities. Other poems more typical of him as a melodist could be found but
these have special traits that commend them for this place.
THE PLAINT OF THE FACTORY CHILD

Mother, must I work all day?
All the day? Ay, all the day?
Must my little hands be torn?
And my heart bleed, all forlorn?
I am but a child of five,
And the street is all alive
With the tops and balls and toys,—
Pretty tops and balls and toys.
Day in, day out, I toil—toil!
And all that I know is toil;
Never laugh as others do,
Never cry as others do,
Never see the stars at night,
Nor the golden glow of sunlight,—
And all for but a silver coin,—
Just a worthless silver coin.
Would that death might come to me!
That blessed death might come to me,
And lead me to waters cool,
Lying in a tranquil pool,
Up there where the angels sing,
And the ivy tendrils cling
To the land of play and song,—
Fairy land of play and song.
THE MULATTO’S SONG

Die, you vain but sweet desires!
Die, you living, burning fires!
I am like a Prince of France,—
Like a prince whose noble sires
Have been robbed of heritage;
I am phantom derelict,
Drifting on a flaming sea.
Everywhere I go, I strive,
Vainly strive for greater things;
Daisies die, and stars are cold,
And canary never sings;
Where I go they mock my name,
Never grant me liberty,
Chance to breathe and chance to do.
The Vision of Lazarus, contained in A Little Dreaming, is a blank-verse
poem of about three-hundred lines, original, well-sustained, imaginative,
and deeply impressive.
In one of the newer methods of verse, and yet with a splendid suggestion
of the old Spirituals, I will take from a recent magazine a poem by Mr.
Johnson that will show how the vision of his people is turned toward the
future, from the welter of struggling forces in the World War:
THE NEW DAY

From a vision red with war I awoke and saw the Prince of Peace hovering over No Man’s
Land.
Loud the whistles blew and thunder of cannon was drowned by the happy shouting of the
people.
From the Sinai that faces Armageddon I heard this chant from the throats of white-robed
angels:
Blow your trumpets, little children!
From the East and from the West,
From the cities in the valley,
From God’s dwelling on the mountain,
Blow your blast that Peace might know
She is Queen of God’s great army.
With the crying blood of millions
We have written deep her name
In the Book of all the Ages;
With the lilies in the valley,
With the roses by the Mersey,
With the golden flower of Jersey,
We have crowned her smooth young temples.
Where her footsteps cease to falter
Golden grain will greet the morning,
Where her chariot descends
Shall be broken down the altar
Of the gods of dark disturbance.
Nevermore shall men know suffering,
Nevermore shall women wailing
Shake to grief the God of Heaven.
From the East and from the West,
From the cities in the valley,
From God’s dwelling on the mountain,
Little children, blow your trumpets!
From Ethiopia, groaning ’neath her heavy burdens I heard the music of the old slave songs.
I heard the wail of warriors, dusk brown, who grimly fought the fight of others in the
trenches of Mars.
I heard the plea of blood-stained men of dusk and the crimson in my veins leapt furiously:
Forget not, O my brothers, how we fought
In No Man’s Land that peace might come again!
Forget not, O my brothers, how we gave
Red blood to save the freedom of the world!
We were not free, our tawny hands were tied;

, y ;
But Belgium’s plight and Serbia’s woes we shared
Each rise of sun or setting of the moon.
So when the bugle blast had called us forth
We went not like the surly brute of yore,
But, as the Spartan, proud to give the world
The freedom that we never knew nor shared.
These chains, O brothers mine, have weighed us down
As Samson in the temple of the gods;
Unloosen them and let us breathe the air
That makes the goldenrod the flower of Christ;
For we have been with thee in No Man’s Land,
Through lake of fire and down to Hell itself;
And now we ask of thee our liberty,
Our freedom in the land of Stars and Stripes.
I am glad that the Prince of Peace is hovering over No Man’s Land.
4. Adolphus Johnson
From the Preface of Adolphus Johnson’s The Silver Chord I will take a
paragraph that is more poetic and perfect in expression than any stanza in
his book. Poetry, I think, is in him, but when he wrote these rhymes he was
not yet sufficiently disciplined in expression. But this is how he can say a
thing in prose:
“As the Goddess of Music takes down her lute, touches its silver chords,
and sets the summer melodies of nature to words, so an inspiration comes to
me in my profoundest slumbers and gently awakens my highest faculties to
the finest thought and serenest contemplation herein expressed. Always
remember that a book is your best friend when it compels you to think,
disenthralls your reason, enkindles your hopes, vivifies your imagination,
and makes easier all the burdens of your daily life.”
IV. William Stanley Braithwaite
The critical and the creative faculties rarely dwell together in harmony.
One or the other finally predominates. In the case of Mr. Braithwaite it
seems to be the critical faculty. He has preferred, it seems, to be America’s
chief anthologist, encouraging others up rugged Parnassus, rather than
himself to stand on the heights of song. Since 1913 he has edited a series of
annual anthologies of American magazine verse, which he has provided

with critical reviews of the verse output of the respective year. Of several
anthologies of English verse also he is the editor. Three books of original
verse stand to his credit: Lyrics of Life and Love (1904), The House of
Falling Leaves (1908), and Sandy Star and Willie Gee (1922). These dates
seem to prove that the creative impulse has waned.
Verse artistry, in simple forms, reaches a degree of excellence in Mr.
Braithwaite’s lyrics that has rarely been surpassed in our times. Graceful
and esthetically satisfying expression is given to elusive or mystical and
rare fancies. I will give one of his brief lyrics as an example of the qualities
to which I allude:
SANDY STAR
No more from out the sunset,
No more across the foam,
No more across the windy hills
Will Sandy Star come home.
He went away to search it,
With a curse upon his tongue,
And in his hands the staff of life
Made music as it swung.
I wonder if he found it,
And knows the mystery now:
Our Sandy Star who went away
With the secret on his brow.
In a number of Mr. Braithwaite’s lyrics, as in this one, there is an
atmosphere of mystery that, with the charming simplicity of manner,
strongly suggests Blake. There is a strangeness in all beauty, it has been
said. There is commonly something of Faëryland in the finest lyric poetry.
Another lyric illustrating this quality in Mr. Braithwaite is the following:
IT’S A LONG WAY

It’s a long way the sea-winds blow
Over the sea-plains blue,—
But longer far has my heart to go
Before its dreams come true.
It’s work we must, and love we must,
And do the best we may,
And take the hope of dreams in trust
To keep us day by day.
It’s a long way the sea-winds blow—
But somewhere lies a shore—
Thus down the tide of Time shall flow
My dreams forevermore.
Mr. Braithwaite’s art rises above race. He seems not to be race-conscious
in his writing, whether prose or verse. Yet no man can say but that race has
given his poetry the distinctive quality I have indicated. In this connection a
most interesting poem is his “A New England Spinster.” The detachment is
perfect, the analysis is done in the spirit of absolute art. I will quote but two
of its dozen or so stanzas:
She dwells alone, and never heeds
How strange may sound her own footfall,
And yet is prompt to others’ needs,
Or ready at a neighbor’s call.
But still her world is one apart,
Serene above desire and change;
There are no hills beyond her heart,
Beyond her gate, no winds that range.
Here is the true artist’s imagination that penetrates to the secrets of life.
No poet’s lyrics, with their deceptive simplicity, better reward study for a
full appreciation of their idea. So much of suggestion to the reader of the
poems which follow:
FOSCATI

Blest be Foscati! You’ve heard tell
How—spirit and flesh of him—blown to flame,
Leaped the stars for heaven, dropped back to hell,
And felt no shame.
I here indite this record of his journey:
The splendor of his epical will to perform
Life’s best, with the lance of Truth at Tourney—
Till caught in the storm.
Of a woman’s face and hair like scented clover,
Te Deums, Lauds, and Magnificat, he
Praised with tongue of saint, heart of lover—
Missed all, but found Foscati!
AUTUMN SADNESS
The warm October rain fell upon his dream,
When once again the autumn sadness stirred,
And murmured through his blood, like a hidden stream
In a forest, unheard.
The drowsy rain battered against his delight
Of the half forgotten poignancies,
That settle in the dusk of an autumn night
On a world one hears and sees.
One was, he thought, an echo merely,
A glow enshadowed of truths untraced;
But the autumn sadness, brought him yearly,
Was a joy embraced.
THANKING GOD

The way folks had of thanking God
He found annoying, till he thought
Of flame and coolness in the sod—
Of balms and blessings that they wrought.
And so the habit grew, and then—
Of when and how he did not care—
He found his God as other men
The mystic verb in a grammar of prayer.
He never knelt, nor uttered words—
His laughter felt no chastening rod;
“My being,” he said, “is a choir of birds,
And all my senses are thanking God.”
Mr. Braithwaite is thoroughly conversant, as these selections indicate,
with the subtleties and finest effects of the art poetic, and his impulses to
write spring from the deepest human speculations, the purest motives of art.
Hence in his work he takes his place among the few.
V. George Reginald Margetson
Under tropical suns, amid the tropical luxuriance of nature, developed
the many-hued imagination of the subject of this sketch. His nature is
tropical, for Mr. Margetson is a prolific bard: Songs of Life, The Fledgling
Bard and the Poetry Society, Ethiopia’s Flight, England in the West Indies
—four published books, and more yet unpublished—are proof. No excerpts
can fully reveal the distinctive quality of Mr. Margetson’s poetry—its
sonorous and ever-varying flow, like a mountain stream, its descriptive
richness in which it resembles his native islands. For he was born in the
British West Indies, and there lived the first twenty years of his life. Coming
to America in 1897, his home has been in Boston or its environment since
that time. Educated in the Moravian School at St. Kitts, he has lived with
and in the English poets from Spenser to Byron—Byron seems to have been
his favorite—and so has cultivated his native talent. I can give here but one
brief lyric from his pen.
THE LIGHT OF VICTORY

George Reginald Margetson
In the East a star is rising,
Breaking through the clouds of war,
With a light old arts revising
Shattering steel and iron bar.
Freedom’s heirs with banners blazing,
Emblems of Democracy,
At the magic light are gazing
Battling with Autocracy.
Through the night brave souls are marching
With the armies of the Free;
Where the Stars and Stripes o’er-arching
Form a sheltering canopy.
Allies! hold a front united!
Shaping well our destiny;
Let each brutal wrong be righted
In the drive for Liberty!
VI. William Moore
The productions I have seen in the Negro magazines and newspapers
from William Moore’s pen give me the idea of a poet distinctly original and
distinctly endowed with imagination. If there appears some obscurity in his
poems let it not be too hastily set down against him as a fault. Some ideas
are intrinsically obscure. The expression of them that should be lucid would
be false, inadequate. Some poets there needs must be who, escaping from
the inevitable, the commonplace, will transport us out into infinity to
confront the eternal mysteries. Mr. Moore does this in two sonnets which I
will give to represent his poetic work:
EXPECTANCY

I do not care for sleep, I’ll wait awhile
For Love to come out of the darkness, wait
For laughter, gifted with the frequent fate
Of dusk-lit hope, to touch me with the smile
Of moon and star and joy of that last mile
Before I reach the sea. The ships are late
And mayhap laden with the precious freight
Dawn brings from Life’s eternal summer isle.
And should I find the sweeter fruits of dream—
The oranges of love and mating song—
I’ll laugh so true the morn will gayly seem
Endless and ships full laden with a throng
Of beauty, dreams and loves will come to me
Out of the surge of yonder silver sea.
AS THE OLD YEAR PASSED
I stood with dear friend Death awhile last night,
Out where the stars shone with a lustre true
In sacred dreams and all the old and new
Of love and life winged in a silver flight
Off to the sea of peace that waits where white,
Pale silences melt in the tranquil blue
Of skies so tender beauty doth imbue
The time with holiness and singing light.
My heart is Life, my soul, O Death, is thine!
Is thine to kiss with yearning life again,
Is thine to strengthen and to sweet incline
To peace and mellowed dream of joy’s refrain.
I’ll stand with Death again to-night, I think,
Out where the stars reveal life’s deeper brink.
VII. Joshua Henry Jones, Jr.
Poets are born and nurtured in all conditions of life: Joseph Cotter the
elder was a slave-woman’s child; Dunbar wrote his first book between the
runs of the elevator he tended; Leon R. Harris was left in infancy to the
dreary shelter of an orphanage, then indentured to a brutal farmer;

Joshua Henry Jones, Jr.
Carmichael came from the cabin of an unlettered
farmer in the Black Belt of Alabama; of a dozen
others the story is similar. Born in poverty, up
through adversities they struggled, with little
human help save perhaps from the croons and
caresses of a singing mother, and a few terms at
a wretched school, they toiled into the kingdom
of knowledge and entered the world of poetry.
Some, however, have had the advantages
afforded by parents of culture and of means.
Among these is the subject of this sketch, the son
of Bishop J. H. Jones, of the African Methodist
Episcopal Church. He has had the best
educational opportunity offered by American
colleges. He is a graduate of Brown University.
Writing has been his employment since graduation, and he has been on the
staffs of several New England papers. His first book of poems, entitled The
Heart of the World (1919), now in the second edition, reveals at once a
student of poetry and an independent artist in verse. His second book,
Poems of the Four Seas (1921), shows that his vein is still rich in ore.
In Chapter VIII I give his “Goodbye, Old Year.” Another poem of similar
technique takes for its title the last words of Colonel Roosevelt: “Turn out
the light, please.” The reader cannot but note the sense of proper effect
exhibited in the short sentences, the very manner of a dying man. But more
than this will be perceived in this poem. It will seem to have sprung out of
the world-weary soul of the young poet himself. Struggle, grief, weariness
in the strife, have been his also. Hence:
TURN OUT THE LIGHT

Turn out the light. Now would I slumber,
I’m weary with the toil of day.
Let me forget my pains to number.
Turn out the light. Dreams come to play.
Turn out the light. The hours were dreary.
Clouds of despair long hid the sun.
I’ve battled hard and now I’m weary.
Turn out the light. My day is done.
I’ve done life’s best gloom’s ways to brighten—
I’ve scattered cheer from heart to heart,
And where I could I’ve sought to righten
The wrongs of men ere day depart.
This morn ’twas bright with hope—and cheery.
This noon gave courage—made me brave.
But as the sun sank I grew weary
Till now my soul for rest doth crave.
Turn out the light. I’ve done my duty
To friend and enemy as well.
I go to sleep where things of beauty
In glitt’ring chambers ever dwell.
Turn out the light. Now would I slumber.
To rest—to dream—soon go we all.
Let’s hope we wake soul free of cumber.
Turn out the light. Dream comrades call.
The next piece I select from Mr. Jones’s first book will represent his
talent in another sphere. I suggest that comparison might be made between
this song in literary English and Mr. Johnson’s Negro love song in dialect,
page 226.
A SOUTHERN LOVE SONG

Dogwoods all a-bloom
Perfume earth’s big room,
White full moon is gliding o’er the sky serene.
Quiet reigns about,
In the house and out;
Hoot owl in the hollow mopes with solemn mien.
Birds have gone to rest
In each tree-top nest;
Cotton fields a-shimmer flash forth silver-green.
O’er the wild cane brake,
Whip-poor-wills awake,
And they speak in tender voicings, Heart, of You.
Answering my call,
Through the leafy hall,
Telling how I’m waiting for your tripping, Sue.
All the world is glad,
Just because I’m mad.
Sense-bereft am I through my great love for you.
Night is all a-smile,
Happy all the while.
That is why my heart so filled with song o’erflows.
I have tarried long,
Lilting here my song.
And I’ll ever waiting be till life’s step slows.
Come to me, my girl,
Precious more than pearl,
I’ll be waiting for you where the grapevine grows.
How my heart doth yearn,
And with anguish burn,
Hungry for sweet pains awaked with your embrace.
Starward goes my cry.
Echo hears my sigh.
Heaven itself its pity at my plight shows trace.
Parson waits to wed.
Soon the nuptials said.
I’ve a rose-clad cottage reared for you to grace.
The title-piece of Mr. Jones’s first volume reveals his mastery of
effective form and his command of the language of passionate appeal. The

World War, in which the Negroes of the country gave liberally and
heroically, both of blood and treasure, for democracy, quickened failing
hopes in them and kindled anew their aspirations. In this poem the writer
speaks for his entire race:
THE HEART OF THE WORLD

In the heart of the world is the call for peace—
Up-surging, symphonic roar.
’Tis ill of all clashings; it seeks release
From fetters of greed and gore.
The winds of the battlefields echo the sigh
Of heroes slumbering deep,
Who gave all they had and now dreamlessly lie
Where the bayonets sent them to sleep.
Peace for the wealthy; peace for the poor;
Peace on the hillside, and peace on the moor.
In the heart of the world is the call for right:
For fingers to bind up the wound,
Slashed deep by the ruthless, harsh hand of might,
When Justice is crushed to the ground.
’Tis ill of the fevers of fear of the strong—
Of jealousies—prejudice—pride.
“Is there no ideal that’s proof against wrong?”
Man asks of the man at his side.
Right for the lowly; right for the great;
Right all to pilot to happiness’gate.
In the heart of the world is the call for love:
White heart—Red—Yellow—and Black.
Each face turns to Bethlehem’s bright star above,
Though wolves of self howl at each back.
The whole earth is lifting its voice in a prayer
That nations may learn to endure,
Without killing and maiming, but doing what’s fair
With a soul that is noble and pure.
Love in weak peoples; love in the strong;
Love that will banish all hatred and wrong.
In the heart of the world is the call of God;
East—West—and North—and South.
Stirring, deep-yearning, breast-heaving call for God
A-tremble behind each mouth.
The heart’s ill of torments that rend men’s souls.
Skyward lift all faiths and hopes;
Across all the oceans the evidence rolls,
R f hi ll lif ’ id l

Refreshing all life’s arid slopes.
God in the highborn; God in the low;
God calls us, world-brothers. Hark ye! and know.
From Poems of the Four Seas I will take a piece that gives the Negro
background for the yearning expressed in the foregoing poem:
BROTHERS
They bind his feet; they thong his hands
With hard hemp rope and iron bands.
They scourge his back in ghoulish glee;
And bleed his flesh;—men, mark ye—free.
They still his groans with fiendish shout,
Where flesh streams red they ply the knout.
Thus sons of men feed lust to kill
And yet, oh God! they’re brothers still.
They build a pyre of torch and flame
While Justice weeps in deepest shame.
E’en Death in pity bows its head,
Yet ’midst these men no prayer is said.
They gather up charred flesh and bone—
Mementos—boasting brave deed done.
They sip of gore their souls to fill;
Drink deep of blood their hands did spill.
Go tell the world what men have done
Who prate of God and yet have none;
Think of themselves as wholly good,
Blaspheme the name of brotherhood;
Who hearken not as brothers cry
For brother’s chance to live and die.
To keep a demon’s murder tryst
They’d rend the sepulcher of Christ.
VIII. Walter Everette Hawkins
CREDO

I am an Iconoclast.
I break the limbs of idols
And smash the traditions of men.
I am an Anarchist.
I believe in war and destruction—
Not in the killing of men,
But the killing of creed and custom.
I am an Agnostic.
I accept nothing without questioning.
It is my inherent right and duty
To ask the reason why.
To accept without a reason
Is to debase one’s humanity
And destroy the fundamental process
In the ascertainment of Truth.
I believe in Justice and Freedom.
To me Liberty is priestly and kingly;
Freedom is my Bride,
Liberty my Angel of Light,
Justice my God.
I oppose all laws of state or country,
All creeds of church and social orders,
All conventionalities of society and system
Which cross the path of the light of Freedom
Or obstruct the reign of Right.
This is a faithful self-characterization—such a man in reality is Walter
Everette Hawkins. A fearless and independent and challenging spirit. He is
the rare kind of man that must put everything to the severe test of absolute
principles. He hates shams, hypocrisies, compromises, chicaneries,
injustices. His poems are the bold and faithful expressions of his
personality. Free he has ever been, free he will be ever, striking right out for
freedom and truth. Such a personality is refreshing to meet, whether you
encounter it in the flesh or in a book.
Born about thirty-five years ago, on a little farm in North Carolina, the
thirteenth child of ex-slave parents, young Hawkins, one may imagine, was
not opulent in this world’s goods. Nor were his opportunities such as are

Walter Everette Hawkins
usually considered thrilling. A few terms of
miserable schooling in the village of Warrenton,
the fragments of a few more terms in a school
maintained by the African Methodist Church,
then—“the University of Hard Knocks.” In the
two first-named schools the independent-spirited
lad seems not to have gotten along well with his
teachers, hence a few dismissals. Always too
prone to ask troublesome, challenging questions,
too prone to doubts and reflections, he was
thought incorrigible. In his “University” he
chose his own masters—the great free spirits of
the ages—and at the feet of these he was
teachable, even while the knocks were hardest.
A lover of wild nature and able to commune
with nature’s spirit, deeply fond also of communing with the world’s master
minds in books, Mr. Hawkins is by necessity—while his spirit soars—the
slave of routine toil, being, until recently, a mail clerk in the post office of
the City of Washington. “My only recreation,” he writes me, “is in stealing
away to be with the masters, the intellectual dynamos, of the world, who
converse with me without wincing and deliver me the key to life’s riddle.”
A true expression of himself I said Mr. Hawkins’s poems are. In no
degree are they fictions. As a companion to Credo, quoted to introduce him,
I will give the last poem in his book, which will again set him before us as
he is:
HERO OF THE ROAD

Let me seek no statesman’s mantle,
Let me seek no victor’s wreath,
Let my sword unstained in battle
Still lie rusting in its sheath;
Let my garments be unsullied,
Let no man’s blood to me cling;
Life is love and earth is heaven,
If I may but soar and sing.
This then is my sternest struggle,
Ease the load and sing my song,
Lift the lame and cheer the cheerless
As they plod the road along;
And we see ourselves transfigured
In a new and bigger plan;
Man transformed, his own Messiah,
God embodied into man.
For the whining craven class of men Mr. Hawkins has little respect:
The man who complains
When the world is all song,
Or dares to sit mute
When the world is all wrong;
Who barters his freedom
Vile honors to win,
Deserves but to die
With the vilest of men.
Upon the times in which we live his judgment is severe. His
condemnation, however, bears witness to that earnestness of soul and that
idealism of spirit which will not let the world repose in its wickedness.
From a list of several poems attesting this I select the following as perhaps
the most complete in form:
THE DEATH OF JUSTICE

These the dread days which the seers have foretold,
These the fell years which the prophets have dreamed;
Visions they saw in those full days of old,
The fathers have sinned and the children blasphemed.
Hurt is the world, and its heart is unhealed,
Wrong sways the sceptre and Justice must yield.
We have come to the travail of troublous times,
Justice must bow before Moloch and Baal;
Blasphemous prayers for the triumph of crimes,
High sounds the cry of the children who wail.
In the brute strength of the sword men rely,
They count not Justice in reckoning things;
Whom their lips worship their hearts crucify,
This the oblation the votary brings.
Locked in death-struggle humanity’s host,
Seeking revenge with the dagger and sword;
This is the pride which the Pharisees boast,
Man damns his brother in the name of his Lord.
Time dims the glare of the pomp and applause,
Vainglorious monarchs and proud princes fall;
Until the death of Time revokes his laws,
His awful mandate shall reign over all.
A number of Mr. Hawkins’s productions reveal possibilities of beauty
and effectiveness, which he had not the patience or the skill to realize. One
imagines that he has never been able to bring his spirit to a submissive
study of the minutiæ of metrical composition. A poet in esse—or in posse—
is all that nature ever makes. And even the most free spirit must know well
the traditions. Whether this iconoclast knows the Cavalier traditions of

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