`
`EXHIBIT
`EXHIBIT
`1011 – part 1
`1011 — part 1
`
`
`
`
`

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`z=SQo
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`=ol=
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`DATA AND
`COMPUTER
`COMMUNICATIONS
`
`
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`FIFTH EDITION
`
`DATA AND
`COMPUTER
`COMMUNICATIONS
`
`WILLIAM STALLINGS
`
`Prentice-Hall of India Private Limited
`New Delhi - 110 001
`2001
`
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`

`This Fourteenth Indian Reprint—Rs. 250.00
`(Original U.S. Edition—-Rs. 3367.00)
`
`DATA AND COMPUTER COMMUNICATIONS, 5th Ed.
`by William Stallings
`
`© 1997 by Prentice-Hall, Inc., Upper Saddle River, New Jersey 07458, U.S.A. All rights reserved.
`No part of this book may be reproduced in any form, by mimeograph or any other means, without
`permission in writing from the publisher.
`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 shall not be liable in any event for incidental or consequential damages in connection with, or arising
`out of,
`the furnishing, performance, or use of
`these programs.
`
`ISBN-81-203-1240-6
`The export rights of this book are vested solely with the publisher.
`This Eastern Economy Edition is the authorized, complete and unabridged photo-offset reproduction
`of the latest American edition specially published and priced for sale only in Bangladesh, Burma,
`Cambodia, China, Fiji, Hong Kong, India, Indonesia, Laos, Malaysia, Nepal, Pakistan, Philippines,
`Singapore, South Korea, Sri Lanka, Taiwan, Thailand, and Vietnam.
`Reprinted in India by special arrangement with Prentice-Hall,
`Inc., Upper Saddle River, New
`Jersey 07458, U.S.A.
`Fourteenth Printing (Fifth Edition)
`
`tee
`
`we
`
`April, 2001
`
`Published by Asoke K. Ghosh, Prentice-Hall of India Private Limited, M-97, Connaught Circus,
`New Delhi-110001
`and Printed by Mohan Makhijani at Rekha Printers Private Limited,
`New Delhi-110020.
`
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`As always, for Antigone
`and also for her constant
`companion, Geoffroi, Chartreux nonpareil
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`

`PREFACE
`
`Objectives
`
`This book attempts to provide a unified overview of the broad field of data and
`computer communications. The organization of the book reflects an attempt to
`break this massive subject into comprehensibleparts and to build, piece by piece, a
`survey of the state of the art. The book emphasizesbasic principles and topics of
`fundamental importance concerning the technology andarchitectureofthis field, as
`well as providing a detailed discussion of leading-edge topics.
`The following basic themes serve to unify the discussion:
`
`® Principles: Although the scope of this book is broad, there are a number of
`basic principles that appear repeatedly as themes and that unify this field.
`Examples are multiplexing, flow control, and error control. The book high-
`lights these principles and contrasts their application in specific areas of tech-
`nology.
`° Design Approaches: The book examines alternative approaches to meeting
`specific communication requirements. The discussion is bolstered with exam-
`ples from existing implementations.
`¢ Standards: Standards have cometo assume an increasingly important, indeed
`dominant, role in this field. An understandingof the current status and future
`direction of technology requires a comprehensive discussion of the role and
`nature of the related standards.
`
`Plan of the Text
`
`The bookis divided into four parts:
`
`I
`
`I
`
`Data Communications: This part is concerned primarily with the exchange of
`data between twodirectly-connected devices. Within this restricted scope, the
`key aspects of transmission, interfacing, link control, and multiplexing are
`examined,
`Wide-Area Networks: This part examines the internal mechanisms and tech-
`nologies that have been developed to support voice, data, and multimedia
`communications over long-distance networks. Thetraditional technologies of
`packet switching and circuit switching are examined, as well as the more
`recent frame relay and ATM.
`
`Wu
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`viii PREFACE
`
`Ill Local Area Networks: This part explores the quite different technologies and
`architectures that have been developed for networking over shorter distances.
`The transmission media, topologies, and medium accesscontrol protocols that
`are the key ingredients of a LAN design are explored andspecific standard-
`ized LAN systems examined.
`IV Communications Architecture and Protocols: This part explores both the
`architectural principles and the mechanismsrequired for the exchangeof data
`among computers, workstations, servers, and other data among computers.
`workstations, servers, and other data processing devices. Much of the materi-
`al in this part relates to the TCP/IP protocolsuite.
`
`In addition, the book includes an extensive glossary,a list of frequently-used
`acronyms, and a a bibliography. Each chapter includes problems and suggestionsfor
`further reading.
`‘
`The book is intended for both an academic anda professional audience. For
`the professionalinterestedin this field, the book serves as a basic reference volume
`andis suitable for self-study.
`As a textbook, it can be used for a one-semester or two-semester course:It
`covers the material in the Computer Communication Networkscourseofthe joint
`ACM/IEEE Computing Curricula 1991. The chapters and parts of the book are
`sufficiently modular to provide a great dealofflexibility in the design of courses.
`The following are suggestions for course design:
`.
`
`° Fundamentals of Data Communications: Part I, Chapters 8 (circuit switch-
`ing), 9 (packet switching), 12 (protocols and architecture).
`© Communications Networks: If the student has a basic background in data
`communications,
`then this course could cover Parts Il and III, and
`Appendix A.
`° Computer Networks: If the student has a basic background in data commu-
`nications, then this course could cover Chapters 5 (data communication
`interface), 6 (data link control), and PartIV.
`
`In addition, a more streamlined course that covers the entire book is possi-
`ble by eliminating certain chapters that are not essential on a first reading.
`Chaptersthat could be optional are: Chapters 2 (data transmission) and 3 (trans-
`mission media), if the student has a basic understanding of these topics, Chapter
`7 (multiplexing), Chapter 10 (frame relay), Chapter 14 (bridges), andChapter18
`(network security).
`
`INTERNET SERVICES FOR INSTRUCTORS AND STUDENTS
`
`There is a web page for this book that provides support for students andinstruc-
`tors. The page includes links to relevant sites, transparency mastersoffigures in
`the book in PDF (Adobe Acrobat) format, and sign-up information for the book’s
`internet mailinglist. The mailing list has been set up so that instructors using this
`book can exchange information, suggestions, and questions with each other and
`with the author. The web pageis at http:/ /www.shore.net/~ws/DCC5e.html.
`
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`PREFACE ix
`
`As soon as any typos or other errors are discovered, an: errata list for this
`bookwill be available at http://www.shore.net/~ws/welcome.html.
`
`WHAT'S NEWIN THE FIFTH EDITION
`
`This fifth edition is seeing the light of day less than a dozenyearsafter the publi-
`cation of the first edition. Much has happened during those years. Indeed, the
`pace of change, if anything, is increasing. The result is that this revision is more
`comprehensive and thorough than any of the previous ones. As an indication of
`this, about one-half of the figures (233 out of 343) and one-half of the tables (48
`out of 91) in this edition are new. Every chapter has been revised, new chapters
`have been added,and the overall organization of the book has changed.
`To begin this process of revision, the fourth edition of this book was exten-
`sively reviewed by a numberof professors who taught from that edition. The
`result is that, in many places, the narrative has been clarified and tightened and
`illustrations have been improved. Also, a numberof new “field-tested” problems
`have been added.
`Beyond these refinements to improve pedagogy anduser-friendliness, there
`have been major substantive changes throughoutthe book. Highlights include
`
`° ATM:The coverage of ATM has been significantly expanded. There is
`now an entire chapter devoted to ATM and ATM congestion control
`(Chapter 11). New to this edition is the coverage of ATM LANs(Sections
`13.4 and 14.3),
`IPv6 (IPng) and IPv6 Security: IPv6, also knownas IPng (next generation),
`is the key to a greatly expanded use of TCP/IP both on the Internet and
`in other networks. This new topic is thoroughly covered. The protocol
`and its internetworking functions are discussed in Section 16.3, and the
`important material on IPvé6 security is provided in Section 18.4.
`° Wireless and Spread Spectrum: There is greater coverage of wireless tech-
`nology (Section 3.2) and spread spectrum techniques (Section 4.5), New
`to this edition is treatment of the important topic of wireless LANs
`(Sections 12.5 and 13.6).
`e High-speed LANs: Coverageof this important area is significantly expand-
`ed, and includes detailed treatment of leading-edge approaches, includ-
`ing Fast Ethernet (100BASE-T), 100VG-AnyLAN, ATM LANs,and Fibre
`Channel(Sections 13.1 through 13.5).
`e Routing: The coverage of internetwork routing has been updated and
`expanded. There is a longer treatment of OSPF and a discussion of BGP
`has been added.
`e Frame Relay: Frame relay also receives expanded coverage with Chapter
`10 devoted to frame relay and framerelay congestion control.
`Network Security: Coverage of this topic has been expanded to an entire
`chapter (Chapter 18).
`e Network Management: New developments in the specification of SNMPv2
`are covered (Section 19.2).
`
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`x
`
`PREFACE
`
`‘
`
`e SMTP and MIME: Multimedia electronic mail combines the basic func-
`tionality of the Simple Mail Transfer Protocol with the Multi-purpose
`Internet Mail Extension.
`HTTP: (Hypertext Transfer Protocol): HTTPis the foundation of the oper-
`ation of the worldwide web (www). Section 19.3 covers HTTP.
`TCP/IP: TCP/IP is now the focus of the protocol coverage in this book.
`Throughoutthe book, especially in Part IV, there is increased discussion
`of TCP/IP and related protocols and issues.
`
`In addition, throughoutthe book,virtually every topic has been updated to
`reflect the developments in standards and technology that have occurred since
`the publication of the second edition.
`
`ACKNOWLEDGMENTS
`
`This new edition has benefited from review by a numberof people, who gave
`generously of their time and expertise. Kitel Albertson (Trondheim College of
`Engineering), Howard Blum (Pace University), Mike Borella (DePaul University),
`William Clark (University of Alaska, Anchorage), Joe Doupnik (Utah State
`University), Doug Jacobson (Iowa State University), Dave Mallya, Biswath
`Mukherjee (University of California, Davis), and Mark Pullen (George Mason
`University) reviewed all or part of the manuscript.
`Steve Deering of Xerox PARC reviewed the material on IPv6. Ted Doty of
`Network Systems Corporation reviewed IP security. Henrik Nielson reviewed
`HTTP.
`
`_
`
`William Stallings
`
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`BRIEF CONTENTS
`
`CHAPTER 1
`
`INTRODUCTION |
`
`1
`
`PART ONE
`Data Communications
`
`33
`
`CHAPTER 2
`CHAPTER 3.
`CHAPTER 4
`CHAPTER 5
`CHAPTER 6
`CHAPTER 7
`
`33
`DaTATRANSMISSION
`73
`TRANSMISSION MEDIA
`_
`Data ENCODING
`95
`THE DATA COMMUNICATION INTERFACE
`Data LINK CONTROL
`157
`MULTIPLEXING
`197
`
`139
`
`PART TWO
`Wide-Area Networks
`
`229
`
`CHAPTER 8
`CHAPTER 9
`
`CIRCUIT SWITCHING
`PACKET SWITCHING
`
`229
`253
`
`301
`FRAME RELAY
`CHAPTER 10
`CHAPTER 11 ASYNCHRONOUS TRANSFER MODE (ATM)
`
`327
`
`PART THREE
`Local Area Networks
`
`363
`
`CHAPTER 12 LAN TECHNOLOGY.
`CHAPTER 13 LAN SysTEemMs
`401
`CHAPTER 14
`BRIDGES
`465
`
`363
`
`PART FOUR
`Communications Architecture and Protocols
`
`497
`
`497
`
`PROTOCOLS AND ARCHITECTURE
`CHAPTER 15
`INTERNETWORKING
`527
`CHAPTER 16
`585
`TRANSPORT PROTOCOLS
`‘CHAPTER 17
`623
`CHAPTER 18 Nerwork SECURITY
`CHAPTER 19 DISTRIBUTED APPLICATIONS
`
`627
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`xii BRIEF CONTENTS
`
`ISDN AND BROADBAND ISDN
`APPENDIX A
`APPENDIX B RECs Cirep IN THIS BooK =
`GLOSSARY
`773
`REFERENCES
`785
`INDEX
`791
`
`739
`7711
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`CONTENTS
`
`CHAPTER 1
`INTRODUCTION 1
`
`11
`L2
`1.3
`L4
`L5
`1.6
`
`2
`
`A Communications Model
`Data Communications
`5
`Data Communications Networking
`Protocols and Protocol Architecture
`Standards
`21
`Outline of the Book
`
`22
`
`7
`
`11
`
`APPENDIX 1A STANDARDS ORGANIZATIONS
`APPENDIX 1B INTERNET Resources
`31
`
`27
`
`PART ONE
`Data Communications 33
`
`CHAPTER 2
`DATA TRANSMISSION 33
`
`21
`2d
`2.3
`2.4
`2.5
`
`34
`Concepts and Terminology
`Analog and Digital Data Transmission
`Transmission Impairments
`55 .
`Recommended Reading
`Problems
`64
`
`64
`
`45
`
`67
`APPENDIX 2A FOURIER ANALYsiS
`APPENDIX 2B DECIBELS AND SIGNAL STRENGTH 71
`
`CHAPTER 3
`TRANSMISSION MEDIA 73
`
`3,ell
`3.2
`
`Guided Transmission Media
`Wireless Transmission
`85
`
`75
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`xiv CONTENTS
`
`3.3
`3.4
`
`RecommendedReading 93
`Problems
`93
`
`CHAPTER 4
`DATA ENCODING 95
`
`Digital Data, Digital Signals
`4.1
`Digital Data, Analog Signals
`4.2
`43 Analog Data,Digital Signals
`44
`Analog Data, Analog Signals
`4.5
`Spread Spectrum 128
`4.6
`Recommended Reading
`4.7
`Problems
`132
`APPENDIX 4A.1 PROOF OF THE SAMPLING THEOREM 136
`
`97
`107
`115
`121
`
`132
`
`CHAPTER 5
`Tue DATA COMMUNICATION INTERFACE 139
`5.1
`Asynchronous and Synchronous Transmission
`140
`5.2
`Line Configurations
`144
`5.3
`Interfacing
`145
`5.4
`Recommended Reading
`5,8
`Problems
`156
`
`156
`
`CHAPTER 6
`Data LINK CONTRL 157
`
`159
`Flow Control
`6.1
`Error Detection
`164
`6.2
`Error Control
`171
`6.3
`High-Level Data Link Control (HDLC)
`6.4
`Other Data Link Control Protocols
`184
`6.5
`Recommended Reading 186
`6.6
`Problems
`187
`6.7
`APPENDIX 64 PERFORMANCEIssugs
`
`190
`
`176
`
`CHAPTER 7
`MULTIPLEXING 197
`
`7.1.
`7.2.
`7.3
`7.4
`7.5
`
`199
`Frequency-Division Multiplexing
`205
`Synchronous Time-Division Multiplexing
`Statistical Time-Division Multiplexing
`219
`Recommended Reading
`226
`Problems
`226
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`CONTENTS XV
`
`240
`
`PART TWO
`Wide-Area Networks 229
`
`CHAPTER 8
`‘CIRCUIT SWITCHING 229
`
`230
`Switched Networks
`8.1
`8.2: Circuit-Switching Networks
`8.3
`Switching Concepts
`234
`8.4
`Routing in Circuit-Switched Networks
`85
`Control Signaling
`244
`8.6
`Recommended Reading
`8.7
`Problems
`252
`
`252
`
`231
`
`CHAPTER 9
`PACKET SWITCHING 253
`
`253
`
`9.1
`Packet-Switching Principles
`9.2
`Routing
`264
`9.3
`Congestion Control
`9.4
`X25 282
`9.5
`Recommended Reading
`96
`Problems
`291
`APPENDIX 9A Least-Cost ALGORITHMS
`
`278
`
`291
`
`296
`
`304
`
`CHAPTER 190
`FRAME Retay 301
`
`302
`10.1 Background
`10.2
`Frame Relay Protocol Architecture
`10.3.
`Frame Relay Call Control
`307
`10.4 User Data Transfer
`313
`10.5 Network Function
`315
`10.6 Congestion Control
`316
`10.7 Recommended Reading
`10.8
`Problems
`325
`
`325
`
`CHAPTER 11
`ASYNCHRONOUS TRANSFER MOpE (ATM) 327
`11.1
`Protocol Architecture
`328
`11.2 ATM Logical Connections
`11.3 ATMCells
`334
`11.4 Transmission of ATM Cells
`
`329
`
`338
`
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`XVI CONTENTS
`
`iL5
`116
`11.7
`11.8
`
`342
`ATM Adaptation Layer
`Traffic and Congestion Control
`Recommended Reading
`359
`Problems
`360
`
`347
`
`PART THREE
`Local Area Networks 363
`
`CHAPTER 12
`LAN TECHNOLOGY 363
`
`LANArchitecture 364
`Bus/Tree LANs
`337
`Ring LANs
`385
`Star LANs
`389
`393
`Wireless LANs
`Recommended Reading
`Problems
`399
`
`399
`
`402
`
`CHAPTER 13
`LAN Systems 401
`Ethernet and Fast Ethernet (CSMA/CD)
`13.1
`Token Ring and FDDI
`413
`13.2
`100VG-AnyLAN 427
`13.3
`ATMLANs
`431
`13.4
`13.5
`Fibre Channel
`435
`Wireless LANs
`442
`13.6
`Recommended Reading
`13.7
`13.8
`Problems
`448
`Appenpix 13A_ Diarra SIGNAL ENCODING FoR LANs
`APPENDIX 131B
`PERFORMANCE Issues
`458
`
`447
`
`451
`
`CHAPTER 14
`BRIDGES 465
`
`14.1
`142
`14.3
`14.4
`14.5
`
`Bridge Operation 466
`Routing with Bridges
`ATM LAN Emulation
`Recommended Reading
`Problems
`495
`
`470
`487
`495
`,
`
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`

`CONTENTS Xvii
`
`PART FOUR
`Communications Architecture and
`Protocols 497
`
`cHaprer 15
`PROTOCOLS AND ARCHITECTURE 497
`
`498
`
`Protocols
`15.1
`15.2 OSI
`510
`15.3 TCP/IP Protocol Suite
`15.4 Recommended Reading
`15.5
`Problems
`526
`
`520
`526
`
`CHAPTER 16
`INTERNETWORKING 527
`
`529
`Principles of Internetworking
`16.1
`16.2 Connectionless Internetworking
`534
`16.3, The Internet Protocol
`541
`16.4 Routing Protocol
`549
`16.5
`IPv6(IPng)
`559
`16.6
`ICMPv6
`578
`16.7 Recommended Reading
`16.8
`Problems
`582
`
`582
`
`CHAPTER 17
`TRANSPORT PROTOCOLS 585
`
`586
`17.1 Transport Services
`17.2
`Protocol Mechanisms
`591
`17.3 TCP 610
`17.4 UDP 619
`17.5 Recommended Reading
`17.8
`Problems
`620
`
`619
`
`CHAPTER 18
`NETWORK SECURITY 623
`
`624
`Security Requirements and Attacks
`18.1
`Privacy with Conventional Encryption
`627
`18.2
`18.3 Message Authentication and Hash Functions
`18.4
`Public-Key Encryption and Digital Signatures
`
`638
`649
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`XVili_ CONTENTS
`
`659
`IPv4 and IPv6 Security
`18.5
`
`18.6.Recommended Reading 664
`18.8
`Problems
`665
`
`CHAPTER 19
`DISTRIBUTED APPLICATIONS 667
`
`668
`
`19.1 Abstract Syntax Notation One (ASN.1)
`‘ 19.2 Network Management—SNMPV2_
`685
`19.3 Electronic Mail—SMTP and MIME 697
`19.4 Uniform Resource Locators (URL) and Universal Resource Identifiers
`(URD 712
`19.5 Hypertext Transfer Protocol (HTTP)
`19.6 Recommended Reading
`736
`19.7
`Problems
`737
`
`719
`
`APPENDIX A
`ISDN AND BROADBAND ISDN 739
`
`A.l Overview of ISDN 740
`A.2
`ISDN Channels
`747
`A.3 User Access
`750
`752
`A.4
`ISDN Protocols
`A.5 BroadbandISDN 764
`A.6é Recommended Reading
`A.7
`‘Problems
`768
`
`768
`
`APPENDIX 13
`RECs CITED IN THis BOOK 771
`
`GLOSSARY 773
`
`REFERENCES 785
`
`INDEX 791
`
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`

`CHAPTER 1
`
`INTRODUCTION
`
`Transmitter [
`
`Destination |
`
`Receiver
`
`4.14 A Communications Model
`4.2 Data Communications
`1:3 Data Communications Networking
`1.4 Protocols and Protocol Architecture
`1.5 Standards
`1.6 Outline of the Book
`1A Standards Organizations
`
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`

`2 CHAPTER 1 / INTRODUCTION
`
`T=1970sand1980ssawamergerofthefieldsofcomputerscienceanddata
`
`communications that profoundly changed the technology, products, and
`companies of the now-combined computer-communications industry. Al-
`though the consequencesof this revolutionary mergerarestill being worked out,it
`is safe to say that the revolution has occurred, and any investigation of the field of
`data communications must be made within this new context.
`The computer-communications revolution has produced several remarkable
`
`facts:
`
`© Thereis no fundamentaldifference between data processing (computers) and
`data communications (transmission and switching equipment).
`e There are no fundamental differences among data, voice, and video commu-
`nications.
`e The lines between single-processor computer, multi-processor computer,
`local network, metropolitan network, and long-haul network have blurred.
`
`Oneeffect of these trends has been a growing overlap of the computer and
`communications industries, from component fabrication to system integration.
`Anotherresult is the developmentof integrated systems that transmit and process
`all types of data and information. Both the technology andthe technical-standards
`organizations are driving toward a single public system that integrates all commu-
`nications and makesvirtually all data and information sources around the world
`easily and uniformly accessible.
`It is the ambitious purpose of this book to provide a unified view of the broad
`field of data and computer communications. The organization of the bookreflects
`an attempt to break this massive subject into comprehensible parts and to build,
`piece by piece, a surveyofthestate of the art. This introductory chapter begins with
`a general model of communications. Then,a brief discussion introduces each of the
`four major parts of this book. Next, the all-important role of standards is intro-
`duced.Finally, a brief outline of the rest of the bookis provided.
`
`1.1
`
`A COMMUNICATIONS MODEL
`
`Webegin ourstudy with a simple model of communications,illustrated by the block
`diagram in Figure 1.1a.
`The fundamentalpurpose of a communications system is the exchange of data
`between twoparties. Figure 1.1b presents one particular example, which is the com-
`munication between a workstation and a server over a public telephone network.
`Another example is the exchangeof voice signals between two telephonesoverthe
`same network, The key elements of the modelare
`
`Source. This device generates the data to be transmitted; examplesaretele-
`phonesand personal computers.
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`1.1 / A COMMUNICATIONS MODEL 3
`
`Source System
`A
`
`Destination System
`A.
`
`-
`
`Source
`
`Transmitter|;
`
`(a) General block diagram
`
`Destinationeee
` Workstation
`
`-
`Public Telephone Network
`
`Server
`
`FIGURE 1.1 Simplified communications model.
`
`(b) Example
`
`e ‘Transmitter. Usually, the data generated by a source system are not transmit-
`ted directly in the form in which they were generated. Rather, a transmitter
`transforms and encodesthe information in such a way as to produce electro-
`magnetic signals that can be transmitted across somesort of transmission sys-
`tem. For example, a modemtakesa digital bit stream from an attached device
`such as a personal computer and transformsthat bit stream into an analog sig-
`nal that can be handled by the telephone network.
`e Transmission System.This can be a single transmission line or a complex net-
`work connecting source and destination.
`e Receiver. The receiver accepts the signal from the transmission system and
`converts it into a form that can be handled by the destination device. For
`example, a modem will accept an analog signal coming from a network or
`transmission line and convertit into a digital bit stream.
`© Destination. Takes the incoming data from the receiver.
`
`This simple narrative conceals a wealth of technical complexity. To get some
`idea of the scopeof this complexity, Table 1.1 lists some of the key tasks that must
`be performedin a data communications system. The list is somewhatarbitrary: Ele-
`ments could be added; items onthelist could be merged; and some items represent
`several tasks that are performedat different “levels” of the system. However, the
`list as it stands is suggestive of the scope of this book.
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`4 CHAPTER 1 / INTRODUCTION
`
`TABLE 1.1 Communicationstasks.
`
`Addressing
`Transmission system utilization
`Routing
`Interfacing
`Recovery
`Signal generation
`Message formatting
`Synchronization
`Security
`Exchange management
`Network management
`Error detection and correction
`Flow control
`
`Thefirst item, transmission system utilization, refers to the need to makeeffi-
`cientuse of transmission facilities that are typically shared among a numberof com-
`municating devices. Various techniques (referred to as multiplexing) are used to
`allocate the total capacity of a transmission medium among a numberof users.
`Congestion control techniques may be required to assure that the system is not
`overwhelmed by excessive demandfor transmissionservices.
`In order to communicate, a device must interface with the transmission sys-
`tem. All the forms of communication discussed in this book depend, at bottom, on
`the use of electromagnetic signals propagated over a transmission medium. Thus,
`once an interface is established, signal generation is required for communication.
`The properties of the signal,.such as form andintensity, must be such that they are
`(1) capable of being propagated through the transmission system, and (2)inter-
`pretable as data at the receiver.
`Notonly must the signals be generated to conform to the requirements ofthe
`transmission system and receiver, but there must be some form of synchronization
`between transmitter and receiver. The receiver must be able to determine when a
`signal beginsto arrive and whenit ends. It must also know the duration of each sig-
`nal element.
`Beyondthe basic matter of deciding on the nature and timing ofsignals, there
`are a variety of requirements for communication between twoparties that might be
`collected under the term exchange management.If data are to be exchangedin both
`directions over a period of time, the two parties must cooperate. For example, for
`two parties to engagein a telephone conversation, one party must dial the number
`of the other, causing signals to be generatedthat result in the ringing ofthe called
`phone. The called party completes a connectionbylifting the receiver. For data pro-
`cessing devices, more will be needed than simply establishing a connection; certain
`conventions must be decided upon. These conventions may include whether both
`devices may transmit simultaneously or must take turns, the amountof data to be
`sent at onetime, the formatof the data, and whatto doif certain contingencies, such
`as an error,arise.
`The next two items might have been included under exchange management,
`but they are important enoughtolist separately. In all communications systems,
`there is a potential for error; transmittedsignals are distorted to some extent before
`reaching their destination. Error detection and correction are required in circum-
`stances where errors cannotbe tolerated; this is usually the case with data process-
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`1.2 / DATA COMMUNICATIONS §
`
`ing systems. For example, in transferring a file from one computer to another,it is
`simply not acceptable for the contents of the file to be accidentally altered. Flow
`control is required to assure that the source does not overwhelm the destination by
`sending data faster than they can be processed and absorbed.
`Next, we mention the related but distinct concepts of addressing and routing.
`When a transmission facility is shared by more than two devices, a source system
`must somehow indicate the identity of the intended destination. The transmission
`system must assure that the destination system, and only that system, receives the
`data. Further, the transmission system mayitself be a network through which vari-
`ous paths may be taken. A specific route through this network must be chosen,
`Recovery is a concept distinct from that of error correction. Recovery tech-
`niques are neededin situations in which an information exchange, such as a data
`base transactionorfile transfer, is interrupted due to a fault somewhereinthesys-
`tem. The objective is either to be able to resumeactivity at the point of interruption
`or at least to restore the state of the systems involved to the condition prior to the
`beginning of the exchange.
`Message formatting has to do with an agreement between twoparties as to the
`form of the data to be exchanged or transmitted. For example, both sides must-use
`the same binary code for characters.
`Frequently,it isimportant to provide some measure of security in a data com-
`munications system. The sender of data may wish to be assured that only the
`intended party actually receives the data; and the receiver of data may wish to be
`assured that the received data have not been altered in transit and that the data
`have actually come from the purported sender.
`Finally, a data communicationsfacility is a complex system that cannotcreate
`or run itself. Network management capabilities are needed to configure the system,
`monitor its status, react to failures and overloads, and planintelligently for future
`growth.
`Thus we have gone from the simple idea of data communication between
`source and destination to a rather formidable list of data communicationstasks. In
`this book, we further elaborate this list of tasks to describe and encompass the
`entire set of activities that can beclassified under data and computer communi-
`cations.
`"
`
`1.2
`
`DATA COMMUNICATIONS
`
`This book is organized into four parts. The first part deals with the most funda-
`mental aspects of the communicationsfunction, focusing on the transmissionofsig-
`nals in a reliable and efficient manner. For want of a better name, we have given
`Part I the title “Data Communications,” although that term arguably encompasses
`someor even all of the topics of Parts II, I, and IV.
`To get someflavor for the focus of Part I, Figure 1.2 provides a new perspec-
`tive on the. communications model of Figure 1.1a. Let us trace through the details
`of this figure using electronic mail as an example.
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`6 CHAPTER 1 / INTRODUCTION
`
`Digital bit
`stream
`tL
`
`Text
`
`Analog
`signal
`
`Eg Receiver
`
`Analog
`signal
`
`o
`
`Digital bit
`stream
`STL
`
`Text
`
`Destination |
`
`Input
`information
`m
`
`@
`Input data
`g(t)
`
`@
`Transmitted
`signal
`s(t)
`
`Received
`signal
`r(t)
`
`Outputdata
`g(t)!
`
`Output
`information
`m'
`
`FIGURE 1.2 Simplified data communications model.
`
`Consider that the input device and transmitter are components of a personal
`computer. The user of the PC wishes to send a message to another user—for exam-
`ple, “The meeting scheduled for March 25 is canceled” (m). The user activates the
`electronic mail package on the PC andenters the message via the keyboard (input
`device). The character string is briefly buffered in main memory. Wecan view it as
`a sequence of bits (g) in memory. The personal computer is connected to some
`transmission medium,such as a local network or a telephoneline, by an I/O device
`(transmitter), such as a local network transceiver or a modem. The input data are
`transferred to the transmitter as a sequence of voltage shifts [g(4)] representing bits
`on some communications bus or cable. The transmitter is connected directly to the
`medium and converts the incoming stream [g(f)] into a signal [s(t)] suitable for
`transmission. Specific alternatives to this procedure will be described in Chapter4.
`The transmitted signal s(t) presented to the medium is subject to a number of
`impairments, discussed in Chapter 2, before it reaches the receiver. Thus, the
`received signal r(f) may differ to some degree from s(t). The receiver will attempt
`to estimate the original s(t), based on r(t) and its knowledge of the medium,pro-
`ducing a sequenceof bits g'(#). These bits are sent to the output personal computer,
`wherethey are briefly buffered in memoryas a block of bits (g). In many cases, the
`destination system will attempt to.determine if an error has occurred and,if so, will
`cooperate with the source system to eventually obtain a complete, error-free block
`of data. These data are then presented to the user via an output device, such as a
`printeror a screen. The message (m’), as viewedby theuser, will usually be an exact
`copy of the original message (77).
`Nowconsider a telephone conversation. In this case, the input to the tele-
`phoneis a message (m) in the form of sound waves. The sound wavesare converted
`by the telephone into electrical signals of the same frequency. These signals are
`transmitted without modification over the telephone line. Hence, the input signal
`g(t) and the transmitted signal s(¢) are identical. Thesignals(t) will suffer some dis-
`tortion over the medium,so that r(t) will not be identical to s(t). Nevertheless, the
`signal r(t) is converted back into a sound wave withno attempt at correction or
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`1.3 / DATA COMMUNICATIONS NETWORKING 7
`
`improvementof signal quality. Thus m is not an exact replica of m. However, the
`received sound messageis generally comprehensible to the listener.
`The discussion so far does not touch on other key aspects of data communi-
`cations, including data-link control techniques for controlling the flow of data and
`detecting and correcting errors, and multiplexing techniques for transmission effi-
`ciency.