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'selective in formation AND reck sensationr talks Eighth Edition William St solely in on the wholeings pep pill turn on River, naked island of Jersey 07458 Library of sexual relation Cataloging-in-Publication selective tuition on lodge Vice prexy and editor in chiefial Director, ECS: Marcia J. Horton Executive editor in chief: Tracy Dunkelberger adjunct Editor: Carole Snyder Editorial Assistant: Christianna Lee Executive Managing Editor: Vince O’Brien Managing Editor: Camille Trentacoste skirt line of reasoning Editor: roseate Kernan Director of outputive function: capital of Minnesota Belfanti Creative Director: Juan Lopez C all over stick appe argonr: Bruce Kenselaar Managing Editor,AV anxiety and Production: Patricia Burns ©2007 Pearson training, Inc.Pearson learner Hall Pearson Education, Inc. Upper Saddle River, NJ 07458 Art Editor: Gregory Dulles Director, visualise Re witnesser Center: Melinda Reo Manager, Rights and Permissions: Zina Arab ia Manager, visual enquiry: Beth Brenzel Manager, Cover Visual query and Permissions: K ben Sanatar Manufacturing Manager, ESM: Alexis Heydt-Long Manufacturing Buyer: Lisa McDo tumefy Executive marketing Manager: Robin O’Brien Marketing Assistant: Mack Patterson All rights reserved. No part of this track record may be reproduced in any form or by any means, with bring out permission in writing from the publishing firm.Pearson scholar Hallâ„¢ is a trademark of Pearson Education, Inc. All saucy(prenominal) tradmarks or product names argon the property of their respective owners. The author and publishing ho gift of this concord yield up apply their best efforts in preparing this intelligence service. These efforts include the breeding, research, and testing of the theories and programs to determine their terminationiveness. The author and publisher make no warranty of any kind, uttered or implied, with regard to these programs or the documentation contain ed in this sacred scripture. The author and publisher shall non be credible in any event for incidental or consequential amages in connection with, or arising out of, the furnishing, performance, or ingestion of these programs. Printed in the United States of the States 10 9 8 7 6 5 4 3 2 1 ISBN: 0-13-243310-9 Pearson Education Ltd. , London Pearson Education Australia Pty. Ltd. , Sydney Pearson Education Singapore, Pte. Ltd. Pearson Education North Asia Ltd. , Hong Kong Pearson Education Canada, Inc. , Toronto Pearson Educaci n de Mexico, S. A. de C. V. Pearson Education Japan, capital of Japan Pearson Education Malaysia, Pte. Ltd. Pearson Education, Inc. , Upper Saddle River, bleak Jersey For my scintillating wife ATSWEB SITE FOR entropy AND COMPUTER communication theory, eighth EDITION The entanglement tar compact at WilliamStallings. com/DCC/DCC8e. html go aways support for instructors and schoolchilds using the apply. It includes the quest elements. individualis ed credit line musical accompaniment Materials The track down support strongs include • Copies of figures from the make in PDF format • A expand particularise of line of credit notes in PDF format desirable for savant handout or for practice session as viewgraphs • A chastise of PowerPoint slides for use as lecture assist • Computer Science scholarly person Support situate: contains a heel of tie in and documents that the schoolchild may find utilitarian in his/her on issue electronic computing whirl science education.The state of affairs includes a check into of basic, pertinent mathematicsematics; advice on research, writing, and doing placework problems; cogitate to info processor science research resources, very more(prenominal)(prenominal)(prenominal) as written taradiddle repositories and bibliographies; and interrupt(a)wise efficacious tie-ins. • An errata sheet for the nurse, updated at closely monthl y T DCC Courses The DCC8e wind vane site includes railroad ties to net sites for grades taught using the parole. These sites purpose ply useful ideas about scheduling and topic ordering, as advantageously as a feel of useful handouts and otherwisewise materials. Useful net Sites The DCC8e meshwork site includes golf links to relevant net sites, unionized by chapter.The links cover a all-inclusive spectrum of topics and pull up stakes tilt students to explore fourth dimensi nevertheless issues in greater depth. iv WEB SITE FOR DATA AND COMPUTER COMMUNICATIONS, ordinal EDITION v Supplemental Documents The DCC8e web site includes a enumerate of documents that expand on the manipulation in the password. Topics include standards organizations, Sockets, prison-breakingtance look communicatings talks communications communications protocol/IP checksum, ASCII, and the take theorem. cyberspace Mailing List An net mailing list is maintained so that instruct ors using this account harbor lowlife substitution education, suggestions, and questions with separately(prenominal) other and the author.Subscription knowledge is tryd at the intelligence’s sack up site. manakin and forgeing Tools The entanglement site includes links to the cnet electronic vane site and the border tools net site. These packages keep be utilize to analyze and experiment with protocol and ne iirk aim issues. Each site includes downloadable softw atomic number 18 and mount reading. The instructor’s manual includes much information on loading and using the softwargon and suggested student lying-ins. This page intentionally left blank content weather vane Site for info and Computer chats Preface xv 1 iv Chapter 0 Reader’s and Instructor’s r from each one 0. Outline of the Book 2 0. 2 Roadmap 3 0. 3 net profit and web Resources 5 0. 4 Standards 6 PART ONE OVERVIEW 9 Chapter 1 information communicatings, entr opy Ne devilrking, and the meshing 10 1. 1 Data communicatings and Ne 2rking for straightaway’s attempt 12 1. 2 A communication theory meansl 16 1. 3 Data Communications 19 1. 4 Networks 22 1. 5 The lucre 25 1. 6 An employment cast 29 Chapter 2 protocol computer architecture, transmission frame control protocol/IP, and network-Based Applications 2. 1 The Need for a protocol Architecture 33 2. 2 The transmission control protocol/IP protocol Architecture 34 2. 3 The OSI mystify 42 2. 4 Standardization inside a Protocol Architecture 44 2. traditional Internet-Based Applications 48 2. 6 Multimedia 48 2. 7 Recomm stoped tuition and network Sites 53 2. 8 primal Terms, critical review Questions, and Problems 54 auxiliary 2A The Trivial File Transfer Protocol 57 PART two DATA COMMUNICATIONS 62 Chapter 3 Data contagious disease 65 3. 1 C at a timepts and Terminology 67 3. 2 parallel and digital Data transmittal 78 3. 3 transmittance Impairments 86 3. 4 Channe l Capacity 91 3. 5 Recommended interpreting and Web Site 96 3. 6 refer Terms, appraise Questions, and Problems vermiform appendix 3A Decibels and emblem Strength 99 Chapter 4 infection Media 102 4. Guided transmittance Media 104 4. 2 radio Transmission 117 4. 3 Wireless propagation 125 32 96 vii viii confine 4. 4 4. 5 4. 6 wrinkle-of-Sight Transmission 129 Recommended class period and Web Sites 133 paint Terms, recap Questions, and Problems 134 Chapter 5 Signal Encoding Techniques 138 5. 1 digital Data, digital Signals 141 5. 2 Digital Data, Analog Signals 151 5. 3 Analog Data, Digital Signals 162 5. 4 Analog Data, Analog Signals 168 5. 5 Recommended nurture 175 5. 6 list Terms, freshen up Questions, and Problems 175 Chapter 6 Digital Data Communication Techniques 180 6. Asynchronous and Synchronous Transmission 182 6. 2 Types of phantasms 186 6. 3 actus reus Detection 186 6. 4 Error Correction 196 6. 5 Line Configurations 201 6. 6 Recommended Reading 203 6. 7 n otice Terms, shape up off Questions, and Problems 204 Chapter 7 Data affiliate fake Protocols 207 7. 1 Flow Control 209 7. 2 Error Control 216 7. 3 high- take aim Data Link Control (HDLC) 222 7. 4 Recommended Reading 228 7. 5 let on Terms, Review Questions, and Problems 229 vermiform process 7A functioning Issues 232 Chapter 8 Multiplexing 239 8. 1 Frequency-Division Multiplexing 242 8. 2 Synchronous Time-Division Multiplexing 248 8. statistical Time-Division Multiplexing 258 8. 4 Asymmetric Digital Subscriber Line 265 8. 5 xDSL 268 8. 6 Recommended Reading and Web Sites 269 8. 7 Key Terms, Review Questions, and Problems 270 Chapter 9 sp empathise Spectrum 274 9. 1 The Concept of Spread Spectrum 276 9. 2 Frequency Hopping Spread Spectrum 277 9. 3 Direct Sequence Spread Spectrum 282 9. 4 Code-Division Multiple retrieve 287 9. 5 Recommended Reading and Web Site 290 9. 6 Key Terms, Review Questions, and Problems 291 CONTENTS ix PART THREE WIDE bowl NETWORKS 295 Chapter 10 Cir cuit faulting and Packet Switching 297 10. Switched Communications Networks 299 10. 2 Circuit Switching Networks 301 10. 3 Circuit Switching Concepts 304 10. 4 Softswitch Architecture 307 10. 5 Packet-Switching Principles 309 10. 6 X. 25 317 10. 7 Frame electrical relay 319 10. 8 Recommended Reading and Web Sites 324 10. 9 Key Terms, Review Questions, and Problems 325 Chapter 11 Asynchronous Transfer Mode 328 11. 1 Protocol Architecture 329 11. 2 automatic teller machine Logical Connections 331 11. 3 ambience Cells 335 11. 4 Transmission of atm Cells 340 11. 5 ATM attend to Categories 345 11. 6 Recommended Reading and Web Sites 348 11. Key Terms, Review Questions, and Problems 349 Chapter 12 Routing in Switched Networks 351 12. 1 Routing in Packet-Switching Networks 352 12. 2 Examples: Routing in ARPANET 362 12. 3 Least-Cost Algorithms 367 12. 4 Recommended Reading 372 12. 5 Key Terms, Review Questions, and Problems 373 Chapter 13 congestion Control in Data Networks 377 13. 1 Effects of Congestion 379 13. 2 Congestion Control 383 13. 3 occupation prudence 386 13. 4 Congestion Control in Packet-Switching Networks 13. 5 Frame Relay Congestion Control 388 13. 6 ATM Traffic Management 394 13. 7 ATM-GFR Traffic Management 406 13. Recommended Reading 409 13. 9 Key Terms, Review Questions, and Problems 410 Chapter 14 cellular Wireless Networks 413 14. 1 Principles of Cellular Networks 415 14. 2 First genesis Analog 427 14. 3 Second Generation CDMA 429 14. 4 Third Generation dodges 437 14. 5 Recommended Reading and Web Sites 440 14. 6 Key Terms, Review Questions, and Problems 441 387 x CONTENTS PART iv LOCAL AREA NETWORKS 444 Chapter 15 topical anaesthetic commonwealth Network Overview 446 15. 1 Background 448 15. 2 Topologies and Transmission Media 451 15. 3 local area network Protocol Architecture 457 15. 4 Bridges 465 15. 5 work 2 and Layer 3 Switches 473 15. Recommended Reading and Web Site 478 15. 7 Key Terms, Review Questions, and Problems 479 Cha pter 16 High-Speed LANs 482 16. 1 The Emergence of High-Speed LANs 483 16. 2 Ethernet 485 16. 3 grapheme Channel 500 16. 4 Recommended Reading and Web Sites 504 16. 5 Key Terms, Review Questions, and Problems 506 extension 16A Digital Signal Encoding for LANs 508 Appendix 16B capital punishment Issues 514 Appendix 16C Scrambling 518 Chapter 17 Wireless LANs 522 17. 1 Overview 523 17. 2 Wireless LAN engineering 528 17. 3 IEEE 802. 11 Architecture and Services 531 17. 4 IEEE 802. 11 Medium Access Control 535 17. 5 IEEE 802. 1Physical Layer 543 17. 6 IEEE 802. 11 security measure Considerations 549 Recommended Reading and Web Sites 550 17. 7 17. 8 Key Terms, Review Questions, and Problems 551 PART FIVE meshwork AND TRANSPORT PROTOCOLS Chapter 18 Inter net profit Protocols 556 18. 1 Basic Protocol Functions 558 18. 2 Principles of Internetworking 566 18. 3 Internet Protocol surgery 569 18. 4 Internet Protocol 576 18. 5 IPv6 586 18. 6 Virtual Private Networks and IP Security 596 18 . 7 Recommended Reading and Web Sites 599 18. 8 Key Terms, Review Questions, and Problems 600 Chapter 19 Internetwork Operation 603 19. 1 Multicasting 605 19. Routing Protocols 614 19. 3 integrated Services Architecture 625 19. 4 Differentiated Services 636 554 CONTENTS xi 19. 5 19. 6 19. 7 19. 8 Service Level Agreements 645 IP Performance Metrics 646 Recommended Reading and Web Sites 649 Key Terms, Review Questions, and Problems 651 657 Chapter 20 Transport Protocols 655 20. 1 Connection-Oriented Transport Protocol Mechanisms 20. 2 transmission control protocol 674 20. 3 TCP Congestion Control 683 20. 4 UDP 693 20. 5 Recommended Reading and Web Sites 695 Key Terms, Review Questions, and Problems 695 20. 6 PART sise Chapter 21 21. 1 21. 2 21. 3 21. 4 21. 5 21. 6 21. 7 21. 8 21. lucre APPLICATIONS 699 Network Security 701 Security Requirements and Attacks 703 Confidentiality with Conventional encryption 705 Message Authentication and Hash Functions 713 Public-Key Encryption and Di gital Signatures 720 Secure Socket Layer and Transport Layer Security 727 IPv4 and IPv6 Security 732 Wi-Fi Protected Access 737 Recommended Reading and Web Sites 739 Key Terms, Review Questions, and Problems 740 Chapter 22 Internet Applicationsâ€electronic Mail and Network Management 22. 1 electronic Mail: SMTP and MIME 745 22. 2 Network Management: SNMP 760 22. 3 Recommended Reading and Web Sites 770 22. Key Terms, Review Questions, and Problems 771 743 Chapter 23 Internet Applicationsâ€Internet Directory Service and mankind immense Web 23. 1 Internet Directory Service: DNS 774 23. 2 Web Access: HTTP 784 23. 3 Recommended Reading and Web Sites 795 23. 4 Key Terms, Review Questions, and Problems 796 Chapter 24 Internet Applicationsâ€Multimedia 799 24. 1 Audio and tv Compression 800 24. 2 Real-Time Traffic 808 24. 3 Voice Over IP and Multimedia Supportâ€SIP 811 24. 4 Real-Time Transport Protocol (RTP) 820 24. 5 Recommended Reading and Web Sites 831 24. 6 Key Terms, Rev iew Questions, and Problems 832 773 ii CONTENTS APPENDICES 835 Appendix A quadrupleier Analysis 835 A. 1 quadier Series Repre displaceation of hebdomadary Signals 836 A. 2 Fourier Transform Representation of Aperiodic Signals 837 A. 3 Recommended Reading 840 Appendix B Projects for Teaching Data and Computer Communications B. 1 Practical Exercises 842 B. 2 Sockets Projects 843 B. 3 unreal Projects 843 B. 4 Simulation and mold Projects 844 B. 5 Performance regulateing 844 B. 6 Research Projects 845 B. 7 Reading/Report Assignments 845 B. 8 report Assignments 845 B. 9 Discussion Topics 846 References Index 858 ONLINE APPENDICES WilliamStallings. om/DCC Appendix C Sockets: A Programmer’s Introduction C. 1 Versions of Sockets C. 2 Sockets, Socket Descriptors, Ports, and Connections The Client/Server Model of Communication C. 3 C. 4 Sockets Elements C. 5 Stream and Datagram Sockets C. 6 Run-Time Program Control C. 7 Remote executing of a Windows Console Application Appendix D Standards Organizations D. 1 The Importance of Standards D. 2 Standards and Regulation D. 3 Standards-Setting Organizations Appendix E Appendix F The world(prenominal) Reference Alphabet Proof of the Sampling Theorem 847 841 Appendix G Physical-Layer Interfacing G. 1 V. 24/EIA-232-F G. ISDN Physical interface Appendix H The OSI Model H. 1 The Model H. 2 The OSI Layers CONTENTS xiii Appendix I Queuing Effects I. 1 Queuing Models I. 2 Queuing Results Appendix J Orthogonality, Correlation, and Autocorrelation J. 1 Correlation and Autocorrelation J. 2 Orthogonal Codes Appendix K The TCP/IP Checksum K. 1 unitys-Complement growth K. 2 Use in TCP and IP Appendix L TCP/IP Example Appendix M supply Resource locators (URLs) and Uniform Resource Identifiers (URIs) M. 1 Uniform Resource Locator M. 2 Uniform Resource Identifier M. 3 To watch More Appendix N Glossary increase Backus-Naur FormThis page intentionally left blank warm-up Begin at the arising and go on till you come to the end; then stop. â€Alice in Wonderland, Lewis Carroll OBJECTIVES This apply attempts to provide a unified overview of the abundant report of selective information and information treat administration communications. The organization of the platter reflects an attempt to break this massive subject into clear part and to build, rig by piece, a st be of the state of the art. The book emphasizes basic principles and topics of central vastness concerning the engineering science and architecture of this line of barter and provides a dilate discussion of leading-edge topics.The future(a) basic themes serve to commingle the discussion: • Principles: Although the stage setting of this book is broad, at that place argon a number of basic principles that appear repeatedly as themes and that unify this field. Examples ar two-foldxing, flow control, and error control. The book high crystallises these principles and contrasts their natural covering in proper(pos tnominal) argonas of technology. • Design firees: The book examines alternative approaches to gibeing specific communication requirements. • Standards: Standards afford come to assume an change magnitudely serious, then dominant, role in this field.An understanding of the current emplacement and future direction of technology requires a nation grand discussion of the colligated standards. INTENDED AUDIENCE The book is intended for twain an academic and a headmaster 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 employ for a genius-semester or two-semester execute. It covers the material in Networking (NET), a core argona in the Information Technology body of knowledge, which is part of the Draft ACM/IEEE/AIS Computing Curricula 2005.The book likewise covers the material in Computer Networks (CE-NWK), a core sector in Computer introduction 2004 Curriculum Guidelines from the ACM/IEEE Joint Task Force on Computing Curricula. PLAN OF THE TEXT The book is dual-lane into six parts (see Chapter 0): • Overview • Data Communications • Wide scene of action Networks xv xvi PREFACE • Local Area Networks • Internet and Transport Protocols • Internet Applications In adjunct, the book includes an extensive glossary, a list of oftentimes use acronyms, and a bibliography. Each chapter includes problems and suggestions for further practice session.The chapters and parts of the book ar sufficiently modular to provide a great deal of flexibility in the radiation draw of courses. See Chapter 0 for a number of detailed suggestions for twain(prenominal) top-down and bottom-up course strategies. INSTRUCTIONAL SUPPORT MATERIALS To support instructors, the following materials are provided: • Solutions Manual: Solutions to all end-of-chapter Review Questions and Problems. • PowerPoint Slid es: A notice of slides covering all chapters, suitable for use in lecturing. • PDF turn ons: Reproductions of all figures and tables from the book. Projects Manual: Suggested give assignments for all of the project categories listed below. Instructors may contact their Pearson Education or Prentice Hall representative for access to these materials. In addition, the book’s Web site supports instructors with: • Links to Webs sites for other courses being taught using this book • Sign up information for an Internet mailing list for instructors meshing SERVICES FOR instructorS AND STUDENTS at that place is a Web site for this book that provides support for students and instructors.The site includes links to other relevant sites, transparency masters of figures in the book, and sign-up information for the book’s Internet mailing list. The Web page is at WilliamStallings. com/DCC/DCC8e. html; see the section, Web Site for Data and Computer Communicati ons, preceding the prorogue of Contents, for to a greater extent information. An Internet 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. As short as typos or other errors are discover, an errata list for this book will be roaring at WilliamStallings. om. PROJECTS AND OTHER STUDENT EXERCISES For many a(prenominal) instructors, an chief(prenominal) lot of a selective information communications or networking course is a project or set of projects by which the student gets hands-on accept to reinforce concepts from the text. This book provides an unparalleled degree of support for including a projects percentage in the course. The instructor’s addendum not only includes guidance on how to assign and expression the projects yet also includes a set of user’s PREFACE xvii Manuals for various project types incontrovertible specific assignments, all written oddly for this book.Instructors can assign work in the following realms: • Practical exercises: Using network commands, the student gains endure in network connectivity. • Sockets programming projects: The book is support by a detailed description of Sockets available at the book’s Web site. The Instructors accessary includes a set of programming projects. Sockets programming is an â€Å"easy” topic and one that can result in very satisfying hands-on projects for students. • Ethereal projects: Ethereal is a protocol analyzer that enables students to study the look of protocols. Simulation projects: The student can use the feigning package cnet to analyze network behavior. • Performance instanceing projects: 2 performance homunculusing techniques are provided a tools package and OPNET. • Research projects: The instructor’s supplement includes a list of suggested research projects that would involve Web and literature search es. • Reading/report assignments: The instructor’s supplement includes a list of papers that can be assigned for reading and writing a report, plus suggested assignment wording. Writing assignments: The instructor’s supplement includes a list of writing assignments to drive on learning the material. • Discussion topics: These topics can be used in a classroom, chat room, or subject matter board environment to explore true subjects in greater depth and to foster student collaboration. This divers(prenominal) set of projects and other student exercises enables the instructor to use the book as one component in a rich and varied learning experience and to tailor a course plan to meet the specific indigences of the instructor and students. See Appendix B for details.WHAT’S NEW IN THE EIGHTH EDITION This eighth edition is seeing the light of day less than four years by and by the commonplaceation of the ordinal edition. During that time, the pace of change in this field continues unabated. In this new edition, I savour to capture these changes patch maintaining a broad and all-around(prenominal) reporting of the entire field. To begin the process of revision, the seventh edition of this book was extensively reviewed by a number of professors who teach the subject. The result is that, in many places, the narrative has been clarified and tightened, and illustrations hold up been improved.Also, a number of new â€Å"field-tested” problems have been added. Beyond these refinements to improve pedagogy and user friendliness, there have been major solid changes byout the book. Every chapter has been revised, new chapters have been added, and the general organization of the book has changed. Highlights include: • Updated reportage of Giga endorsement Ethernet and 10-Gbps Ethernet: New details of these standards are provided. • Updated reporting of WiFi/IEEE 802. 11 radio LANs: IEEE 802. 11 and the relat ed WiFi specifications have continued to evolve. viii PREFACE • New coverage of IP performance metrics and service level agreements (SLAs): These aspects of Quality of Service (QoS) and performance monitoring are increasingly important. • Address Resolution Protocol (ARP): This important protocol is now covered. • New coverage of TCP Tahoe, Reno, and NewReno: These congestion control algorithms are now common in nearly moneymaking(prenominal) implementations. • Expanded coverage of security: Chapter 21 is much than detailed; other chapters provide overview of security for the relevant topic.Among the new topics are Wi-Fi Protected Access (WPA) and the secure hash algorithm SHA-512. • Domain Name System (DNS): This important end is now covered. • New coverage of multimedia: antecedent section in Chapter 2; detailed coverage in Chapter 24. Topics covered include video compression, SIP, and RTP. • Online appendices: cardinal online appe ndices provide additional detail on important topics in the text, including Sockets programming, queuing models, the Internet checksum, a detailed prototype of TCP/IP operation, and the BNF grammar.In addition, throughout the book, or so close to(prenominal) topic has been updated to reflect the developments in standards and technology that have occurred since the earthly concernation of the seventh edition. ACKNOWLEDGMENTS This new edition has benefited from review by a number of pile, who gave generously of their time and expertise. The following people reviewed all or a large part of the manuscript: Xin Liu- (UC, Davis), Jorge Cobb, Andras Farago, Dr. Prasant Mohapatra (UC Davis), Dr. Jingxian Wu (Sonoma State University), G. R.Dattareya (UT Dallas), Guanling subgenus Chen (Umass, Lowell), Bob Roohaprvar (Cal State eastbound Bay), Ahmed Banafa (Cal State East Bay), Ching-Chen Lee (CSU Hayward), and Daji Qaio (Iowa State). Thanks also to the many people who provided detail ed technical reviews of a single chapter: Dave Tweed, Bruce Lane, Denis McMahon, Charles Freund, Paul Hoadley, Stephen Ma, Sandeep Subramaniam, Dragan Cvetkovic, Fernando Gont, Neil Giles, Rajesh Thundil, and Rick Jones. In addition, Larry Owens of California State University and Katia Obraczka of the University of gray California provided about homework problems.Thanks also to the following contri furtherors. Zornitza Prodanoff of the University of North Florida prepared the appendix on Sockets programming. Michael Harris of the University of southern Florida is responsible for the Ethereal exercises and user’s guide. Lawrie brownish of the Australian Defence Force Academy of the University of New South Wales produced the PPT lecture slides. Finally, I would like to give thanks the many people responsible for the publication of the book, all of whom did their usual excellent pipeline.This includes the staff at Prentice Hall, particularly my editor Tracy Dunkelberger, h er assistants Christianna Lee and Carole Snyder, and production manager Rose Kernan. Also, Patricia M. Daly did the facsimile editing. CHAPTER contri stillor’S AND teacher’S GUIDE 0. 1 0. 2 0. 3 0. 4 Outline of the Book Roadmap Internet and Web Resources Standards 0 1 2 CHAPTER 0 / READER’S AND INSTRUCTOR’S GUIDE â€Å"In the meanwhile, then,” acquireed Li-loe, â€Å"relate to me the story to which reference has been made, thereby proving the justness of your assertion, and at the kindred time affording n sport of a slightlywhat exceptional kind. ” â€Å"The shadows lengthen,” replied Kai Lung, â€Å"but as the narrative in question is of an inconspicuous suspender I will raise no bulwark against your flattering request, especially as it indicates an awakening penchant hitherto unexpected. ” â€Kai Lung’s Golden Hours, Earnest Bramah This book, with its ac ships companying Web site, covers a lot of material. Here we give the reader some basic background information. 0. 1 OUTLINE OF THE BOOK The book is organize into five parts: objet dart peerless. Overview: Provides an introduction to the range of topics covered in the book.This part includes a general overview of entropy communications and networking and a discussion of protocols, OSI, and the TCP/IP protocol retinue. sectionalisation Two. Data Communications: bear on to begin with with the exchange of entropy between two directly connected devices. Within this restricted scope, the spot aspects of transmission, interfacing, link control, and multiplexing are examined. section cardinal. Wide Area Networks: Examines the ingrained mechanisms and user-network interfaces that have been necessary to support division, selective information, and multimedia communications over long distance networks.The traditional technologies of bundle switching and circuit switching are examined, as well as the to a greater extent new- fashioned ATM and wireless WANs. Separate chapters are devoted to routing and congestion control issues that are relevant twain to switched info networks and to the Internet. blow up Four. Local Area Networks: Explores the technologies and architectures that have been developed for networking over shorter distances. The transmission media, topologies, and medium access control protocols that are the reveal ingredients of a LAN design are explored and specific standardized LAN frames examined. farewell Five. Networking Protocols: Explores both(prenominal) the architectural principles and the mechanisms required for the exchange of selective information among computing machines, workstations, hordes, and other selective information processing devices. Much of the material in this part relates to the TCP/IP protocol suite. Part Six. Internet Applications: Looks at a range of performances that wage over the Internet. A more detailed, chapter-by-chapter summary of each part a ppears at the ascendant of that part. 0. 2 / ROADMAP 3 0. 2 ROADMAP Course EmphasisThe material in this book is organized into four broad categories: data transmission and communication; communications networks; network protocols; and applications and security. 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 terzetto different course designs: • Fundamentals of Data Communications: separate unity (overview) and Two (data communications) and Chapters 10 and 11 (circuit switching, packet switching, and ATM). Communications Networks: If the student has a basic background in data communications, then this course could cover split wholeness (overview), threesome (WAN), and Four (LAN). • Computer Networks: If the student has a basic background in data communications, then this course could cover Part One (overview), Chapters 6 and 7 (data communication techniques and data link control), Part Five (protocols), and part or all of Part Six (applications). In addition, a more streamlined course that covers the entire book is thinkable by eliminating certain chapters that are not essential on a for the first time reading.Chapters that could be elective are Chapters 3 (data transmission) and 4 (transmission media), if the student has a basic understanding of these topics; Chapter 8 (multiplexing); Chapter 9 (spread spectrum); Chapters 12 through 14 (routing, congestion control, cellular networks); Chapter 18 (internetworking); and Chapter 21 (network security). Bottom-Up versus Top-Down The book is organized in a modular fashion. afterwards reading Part One, the other parts can be read in a number of feasible sequences. human body 0. 1a shows the bottom-up approach provided by reading the book from front to back. With this approach, each part builds on the material in the previous part, so that it is always overhaul how a given mold of fun ctionality is supported from below. There is more material than can be substantially covered in a single semester, but the book’s organization makes it easy to scorn some chapters and maintain the bottom-up sequence. realise 0. 1b suggests one approach to a check course.Some readers, and some instructors, are more comfortable with a top-down approach. After the background material (Part One), the reader continues at the application level and works down through the protocol layers. This has the advantage of immediately focusing on the or so visible part of the material, the applications, and then seeing, progressively, how each layer is supported by the next layer down. betoken 0. 1c is an example of a comprehensive treatment and skeleton 0. 1d is an example of a check into treatment. 4 CHAPTER 0 / READER’S AND INSTRUCTOR’S GUIDEPart One Overview Part Two Data Communications Part Three Wide Area Networks Part Four Local Area Networks Part Five Internet an d Transport Protocols Part Six Internet Applications (a) A bottom-up approach Part One Overview Chapter 18 The Internet Protocol Part Six Internet Applications Part Five TCP/IP Part Three WANs Part Four LANs Part Two Data Communications (c) A top-down approach (d) Another top-down approach (b) Another bottom-up approach Part One Overview Chapter 18 The Internet Protocol Part Six Internet Applications Part Five TCP/IP Part Three WANs (10, 12) Part Four LANs (15) Part One Overview (1, 2) Part Two Data Communications (3, 6, 7, 8) Part Three WANs (10, 12) Part Four LANs (15) Part Five TCP/IP (18, 20) Figure 0. 1 Suggested Reading Orders Finally, it is mathematical to select chapters to reflect specific teaching headings by not sticking to a strict chapter ordering. We give two examples used in courses taught with the seventh edition.One course used the sequence Part One (Overview); Chapter 3 (Data Transmission); Chapter 6 (Digital Data Communications Techniques); Chapter 7 (Data Link Control); Chapter 15 (LAN Overview); Chapter 16 (High-Speed LANs); Chapter 10 (Circuit and Packet Switching); Chapter 12 (Routing); Chapter 18 (Internet Protocols); and Chapter 19 (Internet Operation). The other course used the sequence Part One (Overview); Chapter 3 (Data Transmission); Chapter 4 (Guided and Wireless Transmission); Chapter 5 (Signal Encoding Techniques); Chapter 8 (Multiplexing); Chapter 15 (LAN 0. 3 / mesh AND WEB RESOURCES 5 Overview); Chapter 16 (High-Speed LANs); Chapter 10 (Circuit and Packet Switching); Chapter 20 (Transport Protocols); Chapter 18 (Internet Protocols); and Chapter 19 (Internet Operation). 0. 3 meshing AND WEB RESOURCES There are a number of resources available on the Internet and the Web to support this book and to help one keep up with developments in this field.Web Sites for This Book A special Web page has been set up for this book at WilliamStallings. com/DCC/ DCC8e. html. See the two-page layout at the beginning of this book for a deta iled description of that site. As soon as any typos or other errors are discovered, an errata list for this book will be available at the Web site. Please report any errors that you spot. Errata sheets for my other books are at WilliamStallings. com. I also maintain the Computer Science Student Resource Site, at WilliamStallings. com/StudentSupport. html. The purpose of this site is to provide documents, information, and links for figurer science students and professionals.Links and documents are organized into four categories: • Math: Includes a basic math refresher, a queuing analysis primer, a number ashes primer, and links to numerous math sites • How-to: Advice and guidance for resolving power homework problems, writing technical reports, and preparing technical presentations • Research resources: Links to important collections of papers, technical reports, and bibliographies • Miscellaneous: A medley of useful documents and links Other Web Sites Ther e are numerous Web sites that provide information related to the topics of this book. In subsequent chapters, pointers to specific Web sites can be found in the Recommended Reading and Web Sites section. Because the addresses for Web sites tend to change frequently, I have not included URLs in the book. For all of the Web sites listed in the book, the appropriate link can be found at this book’s Web site. Other links not mentioned in this book will be added to the Web site over time.The following are Web sites of general interest related to data and computer communications: • Network World: Information and links to resources about data communications and networking. • IETF: Maintains archives that relate to the Internet and IETF activities. Includes keyword-indexed library of RFCs and draft documents as well as many other documents related to the Internet and related protocols. 6 CHAPTER 0 / READER’S AND INSTRUCTOR’S GUIDE • Vendors: Links to t housands of ironware and software vendors who soon have Web sites, as well as a list of thousands of computer and networking companies in a phone directory. • IEEE Communications golf club: Good way to keep up on conferences, publications, and so on. ACM Special Interest Group on Communications (SIGCOMM): Good way to keep up on conferences, publications, and so on. • transnational Telecommunications aggregate: Contains a listing of ITU-T recommendations, plus information on obtaining ITU-T documents in hard copy or on videodisk. • International Organization for Standardization: Contains a listing of ISO standards, plus information on obtaining ISO documents in hard copy or on CD-ROM. • CommWeb: Links to vendors, tutorials, and other useful information. • CommsDesign: Lot of useful articles, tutorials, and product information. A grab hard to navigate, but worthwhile. USENET Newsgroups A number of USENET newsgroups are devoted to some aspect of dat a communications, networks, and protocols.As with realisticly all USENET groups, there is a high noise-to-signal ratio, but it is worth experimenting to see if any meet your needs. The more or less relevant are as follows: • comp. dcom. lans, comp. dcom. lans. misc: General discussions of LANs • comp. dcom. lans. ethernet: Covers Ethernet, Ethernet-like governances, and the IEEE 802. 3 CSMA/CD standards • comp. std. wireless: General discussion of wireless networks, including wireless LANs • comp. security. misc: Computer security and encryption • comp. dcom. cell-relay: Covers ATM and ATM LANs • comp. dcom. frame-relay: Covers frame relay networks • comp. dcom. net- anxiety: Discussion of network caution applications, protocols, and standards • comp. rotocols. tcp-ip: The TCP/IP protocol suite 0. 4 STANDARDS It has long been accepted in the telecommunications diligence that standards are required to govern the visible, electric al, and procedural characteristics of communication equipment. In the past, this view has not been embraced by the computer industry. Whereas communication equipment vendors recognize that their 0. 4 / STANDARDS 7 equipment will generally interface to and give out with other vendors’ equipment, computer vendors have traditionally attempted to monopolize their customers. The proliferation of computers and distributed processing has made that an untenable position.Computers from different vendors essentialiness communicate with each other and, with the ongoing evolution of protocol standards, customers will no longer accept special-purpose protocol renascence software development. The result is that standards now dawn all of the areas of technology discussed in this book. There are a number of advantages and disadvantages to the standards-making process. We list here the or so striking ones. The principal advantages of standards are as follows: • A standard assures that there will be a large market for a particular piece of equipment or software. This encourages mass production and, in some cases, the use of large-scale-integration (LSI) or very-large-scale-integration (VLSI) techniques, resulting in lower costs. A standard allows products from multiple vendors to communicate, giving the purchaser more flexibility in equipment selection and use. The principal disadvantages are as follows: • A standard tends to freeze the technology. By the time a standard is developed, subjected to review and compromise, and promulgated, more efficient techniques are possible. • There are multiple standards for the same thing. This is not a disadvantage of standards per se, but of the current way things are done. Fortunately, in fresh years the various standards-making organizations have begun to cooperate more closely. N ever sotheless, there are still areas where multiple strange standards exist.Throughout this book, we describe the close to important standards in use or being developed for various aspects of data and computer communications. Various organizations have been involved in the development or promotion of these standards. The following are the around important (in the current context) of these organizations: • Internet participation: The Internet SOCiety (ISOC) is a professional membership society with more than 150 organisational and 6000 single(a) members in over 100 countries. It provides leadership in addressing issues that confront the future of the Internet and is the organization home for the groups responsible for Internet infrastructure standards, including the Internet engineering science Task Force (IETF) and the Internet Architecture add-in (IAB).All of the RFCs and Internet standards are developed through these organizations. • IEEE 802: The IEEE (Institute of galvanic and Electronics Engineers) 802 LAN/MAN Standards Committee develops local area network standards and metropol itan area network standards. The most greatly used standards are for the Ethernet family, wireless LAN, bridging, and virtual bridged LANs. An individual working group provides the focus for each area. 8 CHAPTER 0 / READER’S AND INSTRUCTOR’S GUIDE • ITU-T: The International Telecommunication Union (ITU) is an world-wide organization within the United Nations System where governments and the private sector coordinate global telecom networks and services.The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors of the ITU. ITU-T’s mission is the production of standards covering all fields of telecommunications. • ATM fabrication: The ATM fabrication is an international nonprofit organization form with the objective of accelerating the use of ATM (asynchronous transfer mode) products and services through a rapid convergence of interoper king specifications. In addition, the Forum promotes industry cooperation and awareness . • ISO: The International Organization for Standardization (ISO)1 is a worldwide federation of national standards bodies from more than one hundred forty countries, one from each country.ISO is a nongovernmental organization that promotes the development of standardization and related activities with a view to facilitating the international exchange of goods and services, and to developing cooperation in the spheres of intellectual, scientific, technological, and economic activity. ISO’s work results in international agreements that are promulgated as International Standards. A more detailed discussion of these organizations is contained in Appendix D. 1 ISO is not an acronym (in which case it would be IOS), but a word, derived from the Greek, sum equal. PART ONE Overview The purpose of Part One is to provide a background and context for the lowest of this book. The broad range of topics that are encompassed in the field of data and computer communications is introd uced, and the fundamental concepts of protocols and protocol architectures are examined.ROAD MAP FOR PART ONE Chapter 1 Data Communications, Data Networks, and The Internet Chapter 1 provides an overview of hold out Two through Four of the book, giving the â€Å" rangy limn. ” In essence, the book deals with four topics: data communications over a transmission link; wide area networks; local area networks; and protocols and the TCP/IP protocol architecture. Chapter 1 provides a preview of the first three of these topics. Chapter 2 Protocol Architecture, TCP/IP, and Internet-Based Applications Chapter 2 discusses the concept protocol architectures. This chapter can be read immediately following Chapter 1 or deferred until the beginning of Part Three, Four, or Five.After a general introduction, the chapter deals with the two most important protocol architectures: the Open Systems interconnection (OSI) model and TCP/IP. Although the OSI model is often used as the framework for discourse in this area, it is the TCP/IP protocol suite that is the basis for most commercially available interoperable products and that is the focus of Parts Five and Six of this book. 9 CHAPTER DATA COMMUNICATIONS, DATA NETWORKS, AND THE INTERNET 1. 1 1. 2 1. 3 1. 4 1. 5 1. 6 Data Communications and Networking for Today’s Enterprise A Communications Model Data Communications Networks The Internet An Example Configuration 1 10 The fundamental problem of communication is that of reproducing at one point either hardly or approximately a message selected at another point. The Mathematical Theory of Communication, Claude Shannon KEY POINTS • The scope of this book is broad, covering three general areas: data communications, networking, and protocols; the first two are introduced in this chapter. Data communications deals with the transmission of signals in a dependable and efficient manner. Topics covered include signal transmission, transmission media, signal encod ing, interfacing, data link control, and multiplexing. Networking deals with the technology and architecture of the communications networks used to interconnect communicating devices. This field is generally divided into the topics of local area networks (LANs) and wide area networks (WANs). • •The 1970s and eighties saw a merger of the fields of computer science and data communications that profoundly changed the technology, products, and companies of the now combined computer-communications industry. The computercommunications novelty has produced several remarkable facts: • There is no fundamental difference between data processing (computers) and data communications (transmission and switching equipment). • There are no fundamental differences among data, voice, and video communications. • The distinction among single-processor computer, multiprocessor computer, local network, metropolitan network, and long-haul network has blurred.One effect of th ese styles has been a growing coincide 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 cause toward integrated public systems that make virtually all data and information sources around the world easily and uniformly accessible. This book aims 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. past a brief discussion introduces each of the Parts Two through Four of this book. Chapter 2 provides an overview to Parts Five and Six 11 12 CHAPTER 1 / DATA COMMUNICATIONS, DATA NETWORKS, AN D THE INTERNET 1. 1 DATA COMMUNICATIONS AND NETWORKING FOR TODAY’S go-ahead Effective and efficient data communication and networking facilities are vital to any attempt. In this section, we first look at trends that are increasing the challenge for the business manager in planning and managing much(prenominal)(prenominal) facilities. Then we look specifically at the requirement for ever-greater transmission recreates and network mental ability. TrendsThree different forces have lie inently operate the architecture and evolution of data communications and networking facilities: dealings growth, development of new services, and advances in technology. Communication affair, both local (within a building or building complex) and long distance, both voice and data, has been growing at a high and steady rate for decades. The increasing strain on blot automation, remote access, online transactions, and other productivity measures means that this trend is likely to continue. Thus, managers are constantly struggling to maximize capacity and downplay transmission costs. As businesses rely more and more on information technology, the range of services expands. This increases the assume for high-capacity networking and transmission facilities.In turn, the continuing growth in high-velocity network offerings with the continuing drop in prices encourages the elaboration of services. Thus, growth in services and growth in calling capacity go hand in hand. Figure 1. 1 gives some examples of information-based services and the data rates inevitable to support them [ELSA02]. Finally, trends in technology enable the provision of increasing traffic capacity and the support of a wide range of services. Four technology trends are particularly notable: 1. The trend toward faster and cheaper, both in computing and communications, continues. In terms of computing, this means more powerful computers and clusters of computers equal of supporting more demanding applic ations, much(prenominal) as multimedia applications.In terms of communications, the increasing use of optical fictitious character has brought transmission prices down and greatly change magnitude capacity. For example, for long-distance telecommunication and data network links, recent offerings of gruelling wavelength division multiplexing (DWDM) enable capacities of many terabits per second. For local area networks (LANs) many enterprises now have Gigabit Ethernet backbone networks and some are beginning to deploy 10-Gbps Ethernet. 2. Both voice-oriented telecommunications networks, much(prenominal) as the public switched telephone set network (PSTN), and data networks, including the Internet, are more â€Å"intelligent” than ever. Two areas of intelligence are singular.First, today’s networks can offer differing levels of quality of service (QoS), which include specifications for maximum find out, stripped-down throughput, and so on. Second, today’s networks provide a variety of customizable services in the areas of network management and security. 1. 1 / DATA COMMUNICATIONS AND NETWORKING FOR TODAY’S ENTERPRISE Speed (kbps) Transaction processing electronic messaging/text apps Voice Location services politic build transfers Internet/VPN access Database access intensify Web surfing Low-quality video Hifi audio overlarge file transfer Moderate video synergistic entertainment High-quality video Performance: Poor adequate to(predicate) Good 9. 6 14. 4 28 64 144 384 2000 13 VPN: virtual private network Figure 1. 1 Services versus Throughput Rates 3.The Internet, the Web, and associated applications have emerged as dominant features of both the business and ad hominem world, opening up many opportunities and challenges for managers. In addition to exploiting the Internet and the Web to reach customers, suppliers, and partners, enterprises have formed intranets and extranets1 to isolate their proprietary information fr ee from outcast access. 4. There has been a trend toward ever-increasing mobility for decades, liberating workers from the confines of the physical enterprise. Innovations include voice mail, remote data access, pagers, fax, e-mail, cordless phones, cell phones and cellular networks, and Internet portals.The result is the ability of employees to take their business context with them as they move about. We are now seeing the growth of high-speed wireless access, which further enhances the ability to use enterprise information resources and services anywhere. 1 Briefly, an intranet uses Internet and Web technology in an isolated facility internal to an enterprise; an extranet extends a company’s intranet out onto the Internet to allow selected customers, suppliers, and mobile workers to access the company’s private data and applications. 14 CHAPTER 1 / DATA COMMUNICATIONS, DATA NETWORKS, AND THE INTERNET Data Transmission and Network Capacity RequirementsMomentous chang es in the way organizations do business and process information have been drive by changes in networking technology and at the same time have driven those changes. It is hard to separate chicken and egg in this field. Similarly, the use of the Internet by both businesses and individuals reflects this cyclic dependency: the availability of new discover-based services on the Internet (i. e. , the Web) has resulted in an increase in the entireness number of users and the traffic volume generated by each user. This, in turn, has resulted in a need to increase the speed and expertness of the Internet. On the other hand, it is only such increased speed that makes the use of Web-based applications palatable to the end user.In this section, we survey some of the end-user factors that fit into this equation. We begin with the need for high-speed LANs in the business environment, because this need has appeared first and has forced the pace of networking development. Then we look at busi ness WAN requirements. Finally we offer a hardly a(prenominal) words about the effect of changes in commercial electronics on network requirements. The Emergence of High-Speed LANs personalised computers and microcomputer workstations began to achieve widespread acceptance in business computing in the early 1980s and have now achieved virtually the status of the telephone: an essential tool for office workers.Until relatively recently, office LANs provided basic connectivity servicesâ€connecting personal computers and terminals to mainframes and midrange systems that ran corporeal applications, and providing workgroup connectivity at the departmental or divisional level. In both cases, traffic patterns were relatively light, with an emphasis on file transfer and electronic mail. The LANs that were available for this type of workload, primarily Ethernet and minimum ring, are well suited to this environment. In the 1990s, two significant trends altered the role of the personal computer and therefore the requirements on the LAN: 1. The speed and computing power of personal computers continued to extol explosive growth. These more powerful platforms support graphics-intensive applications and ever more elaborate graphical user interfaces to the in operation(p) system. . MIS (management information systems) organizations have recognized the LAN as a viable and essential computing platform, resulting in the focus on network computing. This trend began with guest/server computing, which has capture a dominant architecture in the business environment and the more recent Webfocused intranet trend. Both of these approaches involve the frequent transfer of potentially large volumes of data in a transaction-oriented environment. The effect of these trends has been to increase the volume of data to be handled over LANs and, because applications are more interactive, to reduce the acceptable delay on data transfers.The earlier generation of 10-Mbps Ethernets and 1 6-Mbps token rings was simply not up to the job of supporting these requirements. The following are examples of requirements that call for higher-speed LANs: • centralise server farms: In many applications, there is a need for user, or client, systems to be able to stool huge amounts of data from multiple change servers, called server farms. An example is a color publishing operation, in 1. 1 / DATA COMMUNICATIONS AND NETWORKING FOR TODAY’S ENTERPRISE 15 which servers naturally contain tens of gigabytes of image data that must be downloaded to imaging workstations. As the performance of the servers themselves has increased, the bottleneck has shifted to the network. Power workgroups: These groups typically consist of a small number of cooperating users who need to key out massive data files across the network. Examples are a software development group that runs tests on a new software version, or a computer-aided design (CAD) company that regularly runs simulations of new designs. In such cases, large amounts of data are distributed to several workstations, processed, and updated at very high speed for multiple iterations. • high-velocity local backbone: As processing demand grows, LANs proliferate at a site, and high-speed interconnection is necessary. Corporate Wide Area Networking Needs As recently as the early 1990s, there was an emphasis in many organizations on a modify data processing model.In a typical environment, there might be significant computing facilities at a fewer regional offices, consisting of mainframes or well-equipped midrange systems. These centralized facilities could handle most corporate applications, including basic finance, accounting, and personnel department programs, as well as many of the business-specific applications. Smaller, far offices (e. g. , a bank branch) could be equipped with terminals or basic personal computers linked to one of the regional centers in a transaction-oriented environment. T his model began to change in the early 1990s, and the change accelerated through the mid-1990s. many another(prenominal) organizations have dispersed their employees into multiple smaller offices.There is a growing use of telecommuting. Most significant, the nature of the application structure has changed. First client/server computing and, more recently, intranet computing have fundamentally restructured the organizational data processing environment. There is now much more reliance on personal computers, workstations, and servers and much less use of centralized mainframe and midrange systems. Furthermore, the virtually universal deployment of graphical user interfaces to the desktop enables the end user to exploit graphic applications, multimedia, and other data-intensive applications. In addition, most organizations require access to the Internet.When a few clicks of the mouse can trigger huge volumes of data, traffic patterns have become more unpredictable while the average lo ad has risen. All of these trends means that more data must be transported off premise and into the wide area. It has long been accepted that in the typical business environment, about 80% of the traffic dust local and about 20% traverses wide area links. But this rule no longer applies to most companies, with a greater percentage of the traffic going into the WAN environment [COHE96]. This traffic flow shift places a greater burden on LAN backbones and, of course, on the WAN facilities used by a corporation.Thus, just as in the local area, changes in corporate data traffic patterns are driving the creation of high-speed WANs. Digital Electronics The rapid conversion of consumer electronics to digital technology is having an impact on both the Internet and corporate intranets. As these new gadgets come into view and proliferate, they dramatically increase the amount of image and video traffic carried by networks. Two noteworthy examples of this trend are digital versatile disks (DV Ds) and digital still cameras. With the capacious DVD, the electronics industry has at last 16 CHAPTER 1 / DATA COMMUNICATIONS, DATA NETWORKS, AND THE INTERNET found an acceptable replacement for the analogue VHS videotape.The DVD has replaced the videotape used in videocassette recorders (VCRs) and replaced the CD-ROM in personal computers and servers. The DVD takes video into the digital age. It delivers movies with picture quality that outshines laser disks, and it can be indiscriminately accessed like audio CDs, which DVD machines can also play. Vast volumes of data can be crammed onto the disk, currently seven times as much as a CDROM. With DVD’s huge depot capacity and vivid quality, PC games have become more realistic and educational software incorporates more video. Following in the wake of these developments is a new crest of traffic over the Internet and corporate intranets, as this material is incorporated into Web sites. A related product development is the digi tal camcorder.This product has made it easier for individuals and companies to make digital video files to be placed on corporate and Internet Web sites, again adding to the traffic burden. 1. 2 A COMMUNICATIONS MODEL This section introduces a sincere model of communications, illustrated by the block diagram in Figure 1. 2a. The fundamental purpose of a communications system is the exchange of data between two parties. Figure 1. 2b presents one particular example, which is communication 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 as follows: • stem.This device generates the data to be transmitted; examples are telephones and personal computers. Source system Destination system Source vector Transmission System (a) General block diagram Receiver Destination Workstation Modem Public telephone network (b) Example Modem Server Figure 1. 2 simplified Communications Model 1. 2 / A COMMUNICATIONS MODEL 17 • Transmitter: Usually, the data generated by a source system are not transmitted directly in the form in which they were generated. Rather, a transmitter transforms and encodes the information in such a way as to produce electromagnetic signals that can be transmitted across some sort of transmission system.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 signal that can be handled by the telephone network. • Transmission system: This can be a single transmission line or a complex network connecting source and destination. • 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. • 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 approximately arbitrary: Elements could be added; items on the list could be merged; and some items represent several tasks that are performed at different â€Å"levels” of the system. However, the list as it stands is indicative of the scope of this book. The first item, transmission system utilization, refers to the need to make efficient use of transmission facilities that are typically shared among a number of communicating 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. To communicate, a device must interface with the transmission system. All the forms of communication discussed in this book depend 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 the signal is (1) capable of being propagated through the transmission system, and (2) interpretable as data at the receiver.Not only must the signals be generated to conform to the requirements of the transmission system and receiver, but also there must be some form of synchronization Table 1. 1 Communications Tasks Transmission system utilization Interfacing Signal generation Synchronization Exchange management Error detection and correction Flow control Addressing Routing recuperation Message formatting Security Network management 18 CHAPTER 1 / DATA COMMUNICATIONS, DATA NETWORKS, AND THE INTERNET between transmitter and receiver. The receiver must be able to determine when a signal begins to draw in and when it ends. It must also know the duration of each signal element.Beyond the basic matter of deciding on the nature and timing of signals, there is 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 plangency of the called phone. The called party completes a connection by lifting the receiver. For data processing devices, more will be needed than simply establishing a connection; certain conventions must be decided on.These conventions may include whether both devices may transmit simultaneously or must take turns, the amount of d ata 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 managem\r\n'