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

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`DATA AND
`
`COMPUTER
`
`COMMUNICATIONS
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`

`FIFTH EDITI0§
`
`DATA AND
`
`COMPUTER
`COMMUNICATIONS
`
`WILLIAM STALLINGS
`
`PrenticeoHa|| of India E3L?fivi}@ flfimmfifi@ccfl
`New Delhi - 110 001
`
`2001
`
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`This Fourteenth Indian Fteprint—Rs. 250.00
`(Original U.S. 'Edition——Rs. 3367.00)
`
`DATA AND COMPUTER COMMUNICATIONS, 5th Ed.
`by William Stallings
`
`© 1997 by Prentice-Hall, lnc., 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 oi
`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 lndia by special arrangement with Prentice-Hall,
`Jersey 07458, U.S.A.
`
`lnc., Upper Saddle River, New
`
`Fourteenth Printing (Fifth Edition‘)
`
`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 Fiekha 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 comprehensible parts and to build, piece by piece, a
`survey of the state of the art. The book emphasizes basic principles and topics of
`fundamental importance concerning the technology and architecture of this 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 isbolstered with exam-
`ples from existing implementations.
`
`° Standards: Standards have come to assume an increasingly important, indeed
`dominant, role in this field. An understanding of 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 book is divided into four parts:
`
`I Data Communications: This part is concerned primarily with the exchange of
`data between two directly-connected devices. Within this restricted scope, the
`key aspects of transmission, interfacing, link control, and multiplexing are
`examined.
`
`II 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. The traditional technologies of
`packet switching and circuit switching are examined, as well as the more
`recent frame relay and ATM.
`
`V11
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`PREFACE
`
`III 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 access control protocols that
`are the key ingredients of a LAN design are explored and specific standard-
`ized LAN systems examined.
`IV Communications Architecture and Protocols: This part explores both the
`architectural principles and the mechanisms required for the exchange of 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 protocol suite.
`
`In addition, the book includes an extensive glossary, a list of frequently-used
`acronyms, and a a bibliography. Each chapter includes problems and suggestions for
`further reading.
`'
`The book is intended for both an academic and a professional audience. For
`the professional interested in this field, the book serves as a basic reference volume
`and is 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 Networks course of the joint
`ACM/IEEE Computing Curricula 1991. The chapters and parts of the book are
`sufficiently modular to provide a great deal of flexibility in the design of courses.
`The following are suggestions for course design:
`A
`
`0 Fundamentals of Data Communications: Part 1, Chapters 8 (circuit switch-
`ing), 9 (packet switching), 12 (protocols and architecture).
`9 Communications Networks: If the student has a basic background in data
`communications,
`then this course could cover Parts 11 and III, and
`Appendix A.
`9 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 Part IV.
`
`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.
`Chapters that 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), and-Chapter 18
`(network security).
`
`INTERNET SERVICES FOR INSTRUCTORS AND STUDENTS
`
`There is a web page for this book that provides support for students and instruc-
`tors. The page includes links to relevant sites, transparency masters of figures in
`the book in PDF (Adobe Acrobat) format, and sign-up information for the book's
`intemet mailing list. 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 page is at http:/ /www.shore.net/ ~ws/DCC5e.htrnl.
`
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`PREFACE ix
`
`As soon as any typos or other errors are discovered, an errata list for this
`book will be available at http:/ /www.shore.net/ ~ws/welcome.htrnl.
`
`WHAT'S NEW IN THE FIFTH EDITION
`
`This fifth edition is seeing the light of day less than a dozen years after 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 number of 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 number of new ”field-tested” problems
`have been added.
`
`Beyond these refinements to improve pedagogy and user-friendliness, there
`have been major substantive changes throughout the book. Highlights include
`
`9 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).
`
`0
`
`IPv6 (IPng) and IPv6 Security: IPv6, also known as 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 IPV6 security is provided in Section 18.4.
`0 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).
`
`0 High—speed LANS: Coverage of 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
`Charmel (Sections 13.1 through 13.5).
`0 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.
`
`0 Frame Relay: Frame relay also receives expanded coverage with Chapter
`10 devoted to frame relay and frame relay congestion control.
`0 Network Security: Coverage of this topic has been expanded to an entire
`chapter (Chapter 18).
`0 Network Management: New developments in the specification of SNMPVZ
`are covered (Section 19.2).
`
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`X
`
`PREFACE
`
`0 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): HTTP is 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.
`Throughout the book, especially in Part IV, there is increased discussion
`of TCP/IP and related protocols and issues.
`
`In addition, throughout the 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 number of 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 A
`University), Doug Iacobson (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
`
`-DATATRANSMISSION
`TRANSMISSION MEDIA
`
`33
`73
`
`CHAPTER 4
`
`DATA ENCODING
`
`95
`
`1
`
`CHAPTER 5
`
`THE DATA COMMUNICATION INTERFACE
`
`139
`
`CHAPTER 6
`
`DATA LINK CONTROL
`
`157
`
`CHAPTER 7
`
`MULTIPLEXING
`
`197
`
`PART TWO
`
`Wide-Area Networks
`
`229
`
`CHAPTER 8
`
`CIRCUIT SWITCHING
`
`229
`
`CHAPTER 9
`
`PACKET SWITCHING
`
`253
`
`CHAPTER 10
`
`FRAME RELAY
`
`301
`
`CHAPTER 1 1
`
`ASYNCHRONOUS TRANSFER MODE (ATM)
`
`327
`
`PA RT THREE
`
`Local Area Networks
`
`363
`
`CHAPTER 12 LAN TECHNOLOGY
`CHAPTER 13 LAN SYSTEMS
`401
`
`363
`
`CHAPTER 14
`
`BRIDGES
`
`465
`
`PART FOUR
`
`Communications Architecture and Protocols
`
`497
`
`CHAPTER 15
`
`PROTOCOLS AND ARCHITECTURE
`
`497
`
`CHAPTER 16
`
`INTERNETWORKING
`
`527
`
`CHAPTER 17
`
`TRANSPORT PROTOCOLS
`
`585
`
`CHAPTER 18 NETWORK SECURITY
`
`623
`
`CHAPTER 19 DISTRIBUTED APPLICATIONS
`
`627
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`

`xii BRIEF CONTENTS
`
`ISDN AND BROADBAND ISDN
`APPENDIX A
`APPENDIX B RFCs CITED IN THIS BOOK‘
`GLOSSARY
`773
`
`739
`771
`
`REFERENCES
`
`785
`
`INDEX
`
`791
`
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`CONTENTS
`
`CHAPTER 1
`
`INTRODUCTION 1
`
`1.1
`1.2
`
`L3
`1.4
`L5
`1.6
`
`A Communications Model
`Data Communications
`5
`
`2
`
`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
`
`PA RT ONE
`
`Data Communications 33
`
`CHAPTER 2
`
`DATA TRANSMISSION 33
`
`2.1
`2.2
`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
`
`Guided Transmission Media
`3.1
`3.2 Wireless Transmission
`85
`
`75
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`xiv CONTENTS
`
`3.3
`3.4
`
`Recommended‘Reading
`Problems
`93
`
`93
`
`CHAPTER 4
`
`DATA ENCODING 95
`
`4.1
`4.2
`p 4.3
`4.4
`4.5
`4.6
`4.7
`
`Digital Data, Digital Signals
`Digital Data, Analog Signals
`Analog Data, Digital Signals
`Analog Data, Analog Signals
`Spread Spectrum 128
`Recommended Reading
`Problems
`132
`
`132
`
`97
`107
`115
`121
`
`APPENDIX 4A1 PROOF OF THE SAMPLING THEOREM 136
`
`CHAPTER 5
`THE DATA COMMUNICATION INTERFACE 139
`
`5.1
`5.2
`5.3
`5.4
`5.5
`
`Asynchronous and Synchronous Transmission
`Line Configurations
`144
`Interfacing
`145
`Recommended Reading
`Problems
`156
`
`156
`
`140
`
`CHAPTER 6
`
`DATA LINK. CONTRL 157
`
`6.1
`6.2
`6.3
`6.4
`6.5
`6.6
`6.7
`
`159
`Flow Control
`Error Detection
`164
`Error Control
`171
`High-Level Data Link Control (HDLC)
`Other Data Link Control Protocols
`184
`Recommended Reading _186
`Problems
`187
`
`176
`
`APPENDIX 6A PERFORMANCE ISSUES
`
`190
`
`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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`PA RT TWO
`
`Wide-Area Networks 229
`
`CHAPTER 8
`
`CIRCUIT SWITCHING 229
`
`8.1
`
`Switched Networks
`
`230
`
`231
`
`8.2~ Circuit-Switching Networks
`8.3
`Switching Concepts
`234
`8.4
`Routing in Circuit-Switched Networks
`8.5
`Control Signaling
`244
`8.6
`Recommended Reading
`8.7
`Problems
`252
`
`252
`
`CONTENTS XV
`
`240
`
`CHAPTER 9
`
`PACKET SWITCHING 253
`
`9.1
`9.2
`9.3
`9.4
`
`9.5
`9.6
`
`Packet—Switching Principles
`Routing
`264
`Congestion Control
`X.25
`282
`
`278
`
`253
`
`Recommended Reading
`Problems
`291
`
`291
`
`APPENDIX 9A LEAST=-COST ALGORITHMS
`
`296
`
`CHAPTER 1 9
`
`FRAME RELAY 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
`
`304
`
`316
`10.6 Congestion Control
`10.7 Recommended Reading
`10.8
`Problems
`.325
`
`325
`
`CHAPTER 1 1
`
`ASYNCHRONOUS TRANSFER MODE (ATM) 327
`11.1
`Protocol Architecture
`328
`
`11.2 ATM Logical Connections
`11.3 ATM Cells
`334
`11.4 Transmission of ATM Cells
`
`329
`
`338
`
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`xvi CONTENTS
`
`11.5
`11.6
`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 1 2
`
`LAN TECHNOLOGY 363
`
`364
`12.1 LAN Architecture
`12.2 Bus/Tree LANs
`337
`
`385
`12.3 Ring LANs
`389
`12.4
`Star LANs
`12.5 Wireless LANs
`
`393
`
`12.6 Recommended Reading
`12.7
`Problems
`399
`
`399
`
`CHAPTER 13
`
`LAN SYSTEMS 401
`
`13.1 Ethernet and Fast Ethernet (CSMA/CD)
`13.2 Token Ring and FDDI
`413
`13.3
`100VG-AnyLAN 427
`13.4 ATM LANs
`431
`13.5
`Fibre Channel
`435
`13.6 Wireless LANs
`442
`
`402
`
`13.7 Recommended Reading
`1.3.8
`Problems
`448
`
`447
`
`APPENDIX 13A DIGITAL SIGNAL ENCODING FOR LANS
`
`451
`
`APPENDIX 13B PERFORMANCE Issuns
`
`458
`
`CHAPTER 14
`
`BRIDGES 465
`
`14.1 Bridge Operation 466
`14.2 Routing with Bridges
`14.3 ATM LAN Emulation
`
`470
`487
`
`14.4 Recommended Reading
`14.5
`Problems
`495
`
`495
`’
`
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`CONTENTS xvii
`
`PA RT F 0 UR
`Communications Architecture and
`Protocols 497
`
`CHAPTER 1 5
`PROTOCOLS AND ARCHITECTURE 497
`
`15.1
`15.2
`15.3
`15.4
`15.5
`
`498
`
`Protocols
`OSI
`510
`VTCP/IP Protocol Suite
`
`Recommended Reading
`Problems
`526
`
`520
`
`526
`
`CHAPTER 1 6
`
`INTERNETWORKING 527
`
`16.1
`16.2
`16.3 7
`16.4
`16.5
`16.6
`16.7
`16.8
`
`529
`Principles of Internetworking
`Connectionless Intemetworking
`534
`The Internet Protocol
`541
`
`Routing Protocol
`IPv6 (IPng)
`559
`ICMPV6
`578
`
`549
`
`Recommended Reading
`Problems
`582
`
`582
`
`CHAPTER 1 7
`
`TRANSPORT PROTOCOLS 585
`
`17.1
`17.2
`17.3
`17.4
`17.5
`
`17.3
`
`586
`Transport Services
`Protocol Mechanisms
`591
`TCP 610
`UDP 619
`
`Recommended Reading
`Problems
`620
`
`619
`
`CHAPTER 1 8
`
`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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`CONTENTS
`
`IPv4 and IPv6 Security
`18.5
`18.6 Recommended Reading
`18.8
`Problems
`665
`
`659
`664
`
`CHAPTER 19
`
`DISTRIBUTED APPLICATIONS 667
`
`19.1 Abstract Syntax Notation One (ASN.1)
`~ 19.2 Network Management—SNMPV2
`685
`19.3 Electronic Mail—SMTP and MIME 697
`
`668
`
`19.4 Uniform Resource Locators (URL) and Universal Resource Identifiers
`(URI)
`712
`19.5 Hypertext Transfer Protocol (HTTP)
`19.6 Recommended Reading
`736
`19.7
`Problems
`737
`
`719
`
`APPENDIX A
`
`ISDN AND BROADBAND ISDN 739
`
`A1 Overview of ISDN 740
`A.2
`ISDN Channels
`747
`A3 User Access
`750
`752
`AA ISDN Protocols
`A.5 Broadband ISDN 764
`
`A.6 Recommended Reading
`A.’7‘ Problems
`768
`
`768
`
`APPENDIX B
`
`RFCS CITED IN THIS BOOK 771
`
`GLOSSARY 773
`
`REFERENCES 785
`
`INDEX 791
`
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`

`

`CHAPTER 1
`
`INTRODUCTION
`
`
`
`A Communications Model
`Data Communications
`
`Data Communications Networking
`Protocols and Protocol Architecture
`Standards
`Outline of the Book
`
`11
`12
`13
`1.4
`15
`0
`16
`1
`
`A Standards Organizations
`
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`2 CHAPTER 1 / INTRODUCTION
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`The 1970s and 1980ssawamergerofthefieldsofcomputerscience and data
`
`communications that profoundly changed the technology, products, and
`companies of the now-combined computer—communications industry. Al-
`though the consequences of this revolutionary merger are still 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:
`
`- There is no fundamental difference between data processing (computers) and
`data communications (transmission and switching equipment).
`0 There are no fundamental differences among data, voice, and video commu-
`nications.
`
`o The lines between single-processor computer, multi-processor computer,
`local network, metropolitan network, and long-haul network have blurred.
`
`One effect of these trends has been a growing overlap of the computer and
`communications industries, from component fabrication to system integration.
`Another result is the development of integrated systems that transmit and process
`all types of data and information. Both the technology and the technical-standards
`organizations are driving toward a single public system that integrates all commu-
`nications and makes virtually 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 book reflects
`an attempt to break this massive subject into comprehensible parts and to build,
`piece by piece, a survey of the state 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 book is provided.
`
`1.1
`
`A COMMUNICATIONS MODEL
`
`We begin our study with a simple model of communications, illustrated by the block
`diagram in Figure 1.1a.
`The fundamental purpose of a communications system is the exchange of data
`between two parties. 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 exchange of voice signals between two telephones over the
`same network. The key elements of the model are
`
`Source. This device generates the data to be transmitted; examples are tele-
`phones and personal computers.
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`1.1 / A COMMUNICATIONS MODEL 3
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`Source System
`A
`
`r
`
`K
`
`Destination System
`J\
`
`\
`/ .
`
`
`fl
`
`
`
`
`
`Transmission
`
`system
`
`
`
`
`
`Transmitter
`
`Source
`
`(a) General block diagram
`
`Workstation
`
`_
`Public Telephone Network
`
`(b) Example
`
`FIGURE 1.1 Simplified communications model.
`
`Server
`
`0 -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 encodes the information in such a way as to produce electro-
`magnetic signals that can be transmitted across some sort of transmission sys-
`tem. For example, a modem takes a digital bit stream from an attached device
`such as a personal computer and transforms that bit stream into an analog sig-
`nal that can be handled by the telephone network.
`- Transmission System. This can be a single transmission line or a complex net-
`work connecting source and destination.
`0 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 convert it into a digital bit stream.
`0 Destination. Takes the incoming data from the receiver.
`
`This simple narrative conceals a wealth of technical complexity. To get some
`idea of the scope of this complexity, Table 1.1 lists some of the key tasks that must
`be performed in a data communications system. The list is somewhat arbitrary: Ele-
`ments could be added; items on the list could be merged; and some items represent
`several tasks that are performed at 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
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`/I INTRODUCTION
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`TABLE 1.1 Communications tasks.
`
`Transmission system utilization
`Interfacing
`Signal generation
`Synchronization
`Exchange management
`Error detection and correction
`Flow control
`
`Addressing
`Routing
`Recovery
`Message formatting
`Security
`Network management
`
`The first item, transmission system utilization, refers to the need to make effi-
`cient use of transmission facilities that are typically shared among a number of com-
`municating devices. Various techniques (referred to as multiplexing) are used to
`allocate the total capacity of a transmission medium among a number of users.
`Congestion control techniques may be required to assure that the system is not
`overwhelmed by excessive.demand for transmission services.
`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 and intensity, must be such that they are
`(1) capable of being propagated through the transmission system, and (2) inter-
`pretable as data at the receiver.
`Not only must the signals be generated to conform to the requirements of the
`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 begins to arrive and when it ends. It must also know the duration of each sig-
`nal element.
`
`Beyond the basic matter of deciding on the nature and timing of signals, there
`are a variety of requirements for communication between two parties that might be
`collected under the term exchange management. If data are to be exchanged in both
`directions over a period of time, the two parties must cooperate. For example, for
`two parties to engage in a telephone conversation, one party must dial the number
`of the other, causing signals to be generated that result in the ringing of the called
`phone. The called party completes a connection by lifting 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 amount of data to be
`sent at one time, the format of the data, and what to do if certain contingencies, such
`as an error, arise.
`The next two items might have been included under exchange management,
`but they are important enough to list 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 cannot be tolerated; this is usually the case with data process-
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`1.2 / DATA COMMUNICATIONS 5
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`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 may itself 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 needed in situations in which an information exchange, such as a data
`base transaction or file transfer, is interrupted due to a fault somewherein the sys-
`tem. The objective is either to be able to resume activity 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 two parties 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 is important 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 communications facility is a complex system that cannot create
`or run itself. Network management capabilities areneeded to configure the system,
`monitor its status, react to failures and overloads,iand plan intelligently 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 communications tasks. In
`
`this book, we further elaborate this list of tasks to describe and encompass the
`entire set of activities that can be classified under data and computer communi-
`cations.
`“
`
`1.2
`
`DATA COMMUNICATIQNS
`
`This book is organized into four parts. The first part deals with the most funda-
`mental aspects of the communications’ function, focusing on the transmission of sig
`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
`some or even all /of the topics of Parts II, III, and IV.
`To get some flavor for the focus of Part I, Figure 1.2 provides a new perspec-
`tive on tihecommunications 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
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`Digital bit
`stream
`
`Text
`
`Text
`
`Destination
`
`
`
`<9
`
`Input
`information
`In
`
`@‘
`
`Input data
`g(t)
`
`Transmitted
`signal
`'s(t)
`
`Received
`signal
`r(t)
`
`I
`
`<9
`
`Output data
`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 and enters the message via the keyboard (input
`device). The character string is briefly buffered in main memory. We can 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 telephone line, 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(t)] representing bits
`on some communications bus or cable. The transmitter is connected directly to the
`medium and converts the incoming stream [g(t)] into a signal [s(t)] suitable for
`transmission. Specific alternatives to this procedure will be described in Chapter 4.
`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(t) 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 sequence of bits g’(t). These bits are sent to the output personal computer,
`where they are briefly buffered in memory as a block of bits (g). In many cases, the
`destination system will attempt todetermine 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
`printer or a screen. The message (m’), as viewed by the user, will usually be an exact
`copy of the original message (m).
`Now consider a telephone conversation. In this case, the input to the tele-
`phone is a message (m) in the form of sound waves. The sound waves are 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(t) are identical. The signal s(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 with no attempt at correction or
`
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`_1.3\ / DATA COMMUNICATIONS NETWORKING 7
`
`improvement of signal quality. Thus m is not an exact replica of m. However, the
`received sound messa