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Computer Networks and Internets
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FIFTH EDITION
Computer Networks
and Internets
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DOUGLAS E. COMER
Cisco Research
Cisco, Inc.
San Jose, CA 95138
and
Department of Computer Sciences
Purdue University
West Lafayette, IN 47907
FIFTH EDITION
Computer Networks
and Internets
Upper Saddle River, New Jersey 07458
Library of Congress Cataloging-in Publication Data on File.
Editorial Director, Computer Science and Engineering: Marcia J. Horton
Editorial Assistant: Melinda Haggerty
Director of Marketing: Margaret Whaples
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Senior Managing Editor: Scott Disanno
Production Editor: Irwin Zucker
Art Director: Kenny Beck
Cover Art: © Sebastian Kaulitzki/Shutterstock
Art Editor: Gregory Dulles
Media Editor: David Alick
Manufacturing Manager: Alan Fischer
Manufacturing Buyer: Lisa McDowell
© 2009, 2004, 2001, 1999, 1997 by Pearson Education, Inc. Upper Saddle River, New Jersey, 07458. All rights reserved.
Printed in the United States of America. This publication is protected by Copyright and permission should be obtained
from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by
any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write
to: Rights and Permissions Department, Pearson Education, 1 Lake Street, Upper Saddle River, NJ 07458
The author and publisher of this book have used their best efforts in preparing this book. These efforts include the development, research, and testing of the theories and programs to determine their effectiveness. The author and publisher make no
warranty of any kind, expressed or implied, with regard to these programs or the documentation contained in this book. The
author and publisher shall not be liable in any event for incidental or consequential damages in connection with, or arising
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10 9 8 7 6 5 4 3 2 1
ISBN 10: 0-13-606127-3
ISBN 13: 978-0-13-606127-4
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UNIX is a registered trademark of The Open Group in the U.S. and other countries. Microsoft Windows is a trademark of
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Incorporated. Alpha is a trademark of Digital Equipment Corporation. Pentium is a trademark of Intel Corporation
X Window System is a trademark of X Consortium, Incorporated.Smartjack is a trademark of Westell, Incorporated.
To Packets Everywhere
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Contents
Preface xxiii
PART I Introduction And Internet Applications 39
Chapter 1 Introduction And Overview 1
1.1 Growth Of Computer Networking 1
1.2 Why Networking Seems Complex 2
1.3 The Five Key Aspects Of Networking 2
1.4 Public And Private Parts Of The Internet 6
1.5 Networks, Interoperability, And Standards 8
1.6 Protocol Suites And Layering Models 9
1.7 How Data Passes Through Layers 11
1.8 Headers And Layers 12
1.9 ISO and the OSI Seven Layer Reference Model 13
1.10 The Inside Scoop 13
1.11 Remainder Of The Text 14
1.12 Summary 15
Chapter 2 Internet Trends 17
2.1 Introduction 17
2.2 Resource Sharing 17
2.3 Growth Of The Internet 18
2.4 From Resource Sharing To Communication 21
2.5 From Text To Multimedia 21
2.6 Recent Trends 22
2.7 Summary 23
viii Contents
Chapter 3 Internet Applications And Network Programming 27
3.1 Introduction 27
3.2 Two Basic Internet Communication Paradigms 28
3.3 Connection-oriented Communication 29
3.4 The Client-Server Model Of Interaction 30
3.5 Characteristics Of Clients And Servers 31
3.6 Server Programs And Server-Class Computers 31
3.7 Requests, Responses, And Direction Of Data Flow 32
3.8 Multiple Clients And Multiple Servers 32
3.9 Server Identification And Demultiplexing 33
3.10 Concurrent Servers 34
3.11 Circular Dependencies Among Servers 35
3.12 Peer-To-Peer Interactions 35
3.13 Network Programming And The Socket API 36
3.14 Sockets, Descriptors, And Network I/O 36
3.15 Parameters And The Socket API 37
3.16 Socket Calls In A Client And Server 38
3.17 Socket Functions Used By Both Client And Server 38
3.18 The Connection Function Used Only By A Client 40
3.19 Socket Functions Used Only By A Server 40
3.20 Socket Functions Used With The Message Paradigm 43
3.21 Other Socket Functions 44
3.22 Sockets, Threads, And Inheritance 45
3.23 Summary 45
Chapter 4 Traditional Internet Applications 49
4.1 Introduction 49
4.2 Application-Layer Protocols 49
4.3 Representation And Transfer 50
4.4 Web Protocols 51
4.5 Document Representation With HTML 52
4.6 Uniform Resource Locators And Hyperlinks 54
4.7 Web Document Transfer With HTTP 55
4.8 Caching In Browsers 57
4.9 Browser Architecture 59
4.10 File Transfer Protocol (FTP) 59
4.11 FTP Communication Paradigm 60
4.12 Electronic Mail 63
4.13 The Simple Mail Transfer Protocol (SMTP) 64
4.14 ISPs, Mail Servers, And Mail Access 66
4.15 Mail Access Protocols (POP, IMAP) 67
Contents ix
4.16 Email Representation Standards (RFC2822, MIME) 67
4.17 Domain Name System (DNS) 69
4.18 Domain Names That Begin With www 71
4.19 The DNS Hierarchy And Server Model 72
4.20 Name Resolution 72
4.21 Caching In DNS Servers 74
4.22 Types Of DNS Entries 75
4.23 Aliases And CNAME Resource Records 76
4.24 Abbreviations And The DNS 76
4.25 Internationalized Domain Names 77
4.26 Extensible Representations (XML) 78
4.27 Summary 79
PART II Data Communication Basics 83
Chapter 5 Overview Of Data Communications 85
5.1 Introduction 85
5.2 The Essence Of Data Communications 86
5.3 Motivation And Scope Of The Subject 87
5.4 The Conceptual Pieces Of A Communication System 87
5.5 The Subtopics Of Data Communications 90
5.6 Summary 91
Chapter 6 Information Sources And Signals 93
6.1 Introduction 93
6.2 Information Sources 93
6.3 Analog And Digital Signals 94
6.4 Periodic And Aperiodic Signals 94
6.5 Sine Waves And Signal Characteristics 95
6.6 Composite Signals 97
6.7 The Importance Of Composite Signals And Sine Functions 97
6.8 Time And Frequency Domain Representations 98
6.9 Bandwidth Of An Analog Signal 99
6.10 Digital Signals And Signal Levels 100
6.11 Baud And Bits Per Second 101
6.12 Converting A Digital Signal To Analog 102
6.13 The Bandwidth Of A Digital Signal 103
6.14 Synchronization And Agreement About Signals 103
6.15 Line Coding 104
x Contents
6.16 Manchester Encoding Used In Computer Networks 106
6.17 Converting An Analog Signal To Digital 107
6.18 The Nyquist Theorem And Sampling Rate 108
6.19 Nyquist Theorem And Telephone System Transmission 108
6.20 Encoding And Data Compression 109
6.21 Summary 110
Chapter 7 Transmission Media 113
7.1 Introduction 113
7.2 Guided And Unguided Transmission 113
7.3 A Taxonomy By Forms Of Energy 114
7.4 Background Radiation And Electrical Noise 115
7.5 Twisted Pair Copper Wiring 115
7.6 Shielding: Coaxial Cable And Shielded Twisted Pair 117
7.7 Categories Of Twisted Pair Cable 118
7.8 Media Using Light Energy And Optical Fibers 119
7.9 Types Of Fiber And Light Transmission 120
7.10 Optical Fiber Compared To Copper Wiring 121
7.11 InfraRed Communication Technologies 122
7.12 Point-To-Point Laser Communication 122
7.13 Electromagnetic (Radio) Communication 123
7.14 Signal Propagation 124
7.15 Types Of Satellites 125
7.16 GEO Communication Satellites 126
7.17 GEO Coverage Of The Earth 127
7.18 Low Earth Orbit (LEO) Satellites And Clusters 128
7.19 Tradeoffs Among Media Types 128
7.20 Measuring Transmission Media 129
7.21 The Effect Of Noise On Communication 129
7.22 The Significance Of Channel Capacity 130
7.23 Summary 131
Chapter 8 Reliability And Channel Coding 135
8.1 Introduction 135
8.2 The Three Main Sources Of Transmission Errors 135
8.3 Effect Of Transmission Errors On Data 136
8.4 Two Strategies For Handling Channel Errors 137
8.5 Block And Convolutional Error Codes 138
8.6 An Example Block Error Code: Single Parity Checking 139
8.7 The Mathematics Of Block Error Codes And (n,k) Notation 140
Contents xi
8.8 Hamming Distance: A Measure Of A Code’s Strength 140
8.9 The Hamming Distance Among Strings In A Codebook 141
8.10 The Tradeoff Between Error Detection And Overhead 142
8.11 Error Correction With Row And Column (RAC) Parity 142
8.12 The 16-Bit Checksum Used In The Internet 144
8.13 Cyclic Redundancy Codes (CRCs) 145
8.14 An Efficient Hardware Implementation Of CRC 148
8.15 Automatic Repeat reQuest (ARQ) Mechanisms 148
8.16 Summary 149
Chapter 9 Transmission Modes 153
9.1 Introduction 153
9.2 A Taxonomy Of Transmission Modes 153
9.3 Parallel Transmission 154
9.4 Serial Transmission 155
9.5 Transmission Order: Bits And Bytes 156
9.6 Timing Of Serial Transmission 156
9.7 Asynchronous Transmission 157
9.8 RS-232 Asynchronous Character Transmission 157
9.9 Synchronous Transmission 158
9.10 Bytes, Blocks, And Frames 159
9.11 Isochronous Transmission 160
9.12 Simplex, Half-Duplex, and Full-Duplex Transmission 160
9.13 DCE and DTE Equipment 162
9.14 Summary 162
Chapter 10 Modulation And Modems 165
10.1 Introduction 165
10.2 Carriers, Frequency, And Propagation 165
10.3 Analog Modulation Schemes 166
10.4 Amplitude Modulation 166
10.5 Frequency Modulation 167
10.6 Phase Shift Modulation 168
10.7 Amplitude Modulation And Shannon’s Theorem 168
10.8 Modulation, Digital Input, And Shift Keying 169
10.9 Phase Shift Keying 169
10.10 Phase Shift And A Constellation Diagram 171
10.11 Quadrature Amplitude Modulation 173
10.12 Modem Hardware For Modulation And Demodulation 174
10.13 Optical And Radio Frequency Modems 174
xii Contents
10.14 Dialup Modems 175
10.15 QAM Applied To Dialup 175
10.16 V.32 and V.32bis Dialup Modems 176
10.17 Summary 177
Chapter 11 Multiplexing And Demultiplexing (Channelization) 181
11.1 Introduction 181
11.2 The Concept Of Multiplexing 181
11.3 The Basic Types Of Multiplexing 182
11.4 Frequency Division Multiplexing (FDM) 183
11.5 Using A Range Of Frequencies Per Channel 185
11.6 Hierarchical FDM 186
11.7 Wavelength Division Multiplexing (WDM) 187
11.8 Time Division Multiplexing (TDM) 187
11.9 Synchronous TDM 188
11.10 Framing Used In The Telephone System Version Of TDM 189
11.11 Hierarchical TDM 190
11.12 The Problem With Synchronous TDM: Unfilled Slots 190
11.13 Statistical TDM 191
11.14 Inverse Multiplexing 192
11.15 Code Division Multiplexing 193
11.16 Summary 195
Chapter 12 Access And Interconnection Technologies 199
12.1 Introduction 199
12.2 Internet Access Technology: Upstream And Downstream 199
12.3 Narrowband And Broadband Access Technologies 200
12.4 The Local Loop And ISDN 202
12.5 Digital Subscriber Line (DSL) Technologies 202
12.6 Local Loop Characteristics And Adaptation 203
12.7 The Data Rate Of ADSL 204
12.8 ADSL Installation And Splitters 205
12.9 Cable Modem Technologies 205
12.10 The Data Rate Of Cable Modems 206
12.11 Cable Modem Installation 206
12.12 Hybrid Fiber Coax 207
12.13 Access Technologies That Employ Optical Fiber 208
12.14 Head-End And Tail-End Modem Terminology 208
12.15 Wireless Access Technologies 209
12.16 High-Capacity Connections At The Internet Core 209
Contents xiii
12.17 Circuit Termination, DSU / CSU, and NIU 210
12.18 Telephone Standards For Digital Circuits 211
12.19 DS Terminology And Data Rates 212
12.20 Highest Capacity Circuits (STS Standards) 213
12.21 Optical Carrier Standards 213
12.22 The C Suffix 213
12.23 Synchronous Optical NETwork (SONET) 214
12.24 Summary 215
PART III Packet Switching And Network Technologies 219
Chapter 13 Local Area Networks: Packets, Frames, And Topologies 221
13.1 Introduction 221
13.2 Circuit Switching 222
13.3 Packet Switching 223
13.4 Local And Wide Area Packet Networks 224
13.5 Standards For Packet Format And Identification 225
13.6 IEEE 802 Model And Standards 226
13.7 Point-To-Point And Multi-Access Networks 229
13.8 LAN Topologies 229
13.9 Packet Identification, Demultiplexing, MAC Addresses 231
13.10 Unicast, Broadcast, And Multicast Addresses 232
13.11 Broadcast, Multicast, And Efficient Multi-Point Delivery 233
13.12 Frames And Framing 234
13.13 Byte And Bit Stuffing 235
13.14 Summary 237
Chapter 14 The IEEE MAC Sub-Layer 241
14.1 Introduction 241
14.2 A Taxonomy Of Mechanisms For Multi-Access 241
14.3 Static And Dynamic Channel Allocation 242
14.4 Channelization Protocols 243
14.5 Controlled Access Protocols 244
14.6 Random Access Protocols 246
14.7 Summary 252
xiv Contents
Chapter 15 Wired LAN Technology (Ethernet And 802.3) 255
15.1 Introduction 255
15.2 The Venerable Ethernet 255
15.3 Ethernet Frame Format 256
15.4 Ethernet Type Field And Demultiplexing 256
15.5 IEEE’s Version Of Ethernet (802.3) 257
15.6 LAN Connections And Network Interface Cards 258
15.7 Ethernet Evolution And Thicknet Wiring 258
15.8 Thinnet Ethernet Wiring 259
15.9 Twisted Pair Ethernet Wiring And Hubs 260
15.10 Physical And Logical Ethernet Topology 261
15.11 Wiring In An Office Building 261
15.12 Variants Of Twisted Pair Ethernet And Speeds 263
15.13 Twisted Pair Connectors And Cables 263
15.14 Summary 264
Chapter 16 Wireless Networking Technologies 267
16.1 Introduction 267
16.2 A Taxonomy Of Wireless Networks 267
16.3 Personal Area Networks (PANs) 268
16.4 ISM Wireless Bands Used By LANs And PANs 269
16.5 Wireless LAN Technologies And Wi-Fi 269
16.6 Spread Spectrum Techniques 270
16.7 Other Wireless LAN Standards 271
16.8 Wireless LAN Architecture 272
16.9 Overlap, Association, And 802.11 Frame Format 273
16.10 Coordination Among Access Points 274
16.11 Contention And Contention-Free Access 274
16.12 Wireless MAN Technology and WiMax 276
16.13 PAN Technologies And Standards 278
16.14 Other Short-Distance Communication Technologies 279
16.15 Wireless WAN Technologies 280
16.16 Cell Clusters And Frequency Reuse 282
16.17 Generations Of Cellular Technologies 283
16.18 VSAT Satellite Technology 286
16.19 GPS Satellites 287
16.20 Software Radio And The Future Of Wireless 288
16.21 Summary 289