Ethernet: wireless networks.

Stallings Pearson International Edition tenth edition This is a special edition of an established title widely used by colleges and universities throughout the world. Pearson published this exclusive edition for the benefit of students outside the United States and Canada. If you purchased this book within the United States or Canada you should be aware that it has been imported without the approval of the Publisher or Author. Data and Computer Communications The editorial team at Pearson has worked closely with educators around the globe to inform students of the ever-changing world in a broad variety of disciplines. Pearson Education offers this product to the international market, which may or may not include alterations from the United States version. international edition international edition international edition Data and Computer Communications tenth edition William Stallings Data and Computer Communications Tenth Edition William Stallings International Edition contributions by Moumita Mitra Manna Bangabasi College, Kolkata Boston Columbus Indianapolis New York San Francisco Upper Saddle River Amsterdam Cape Town Dubai London Madrid Milan Munich Paris Montréal Toronto Delhi Mexico City São Paulo Sydney Hong Kong Seoul Singapore Taipei Tokyo A01_STAL4388_10_PIE_FM.indd 1 10/24/13 2:03 PM Editorial Director, ECS: Marcia Horton Executive Editor: Tracy Johnson (Dunkelberger) Editorial Assistant: Jenah Blitz-Stoehr Director of Marketing: Christy Lesko Marketing Manager: Yez Alayan Marketing Assistant: Jon Bryant Director of Program Management: Erin Gregg Program Management-Team Lead: Scott Disanno Program Manager: Carole Snyder Project Management-Team Lead: Laura Burgess Project Manager: Robert Engelhardt Publishing Operations Director, International Edition: Angshuman Chakraborty Manager, Publishing Operations, International Edition: Shokhi Shah Khandelwal Associate Print and Media Editor, International Edition: Anuprova Dey Chowdhuri Acquisitions Editor, International Edition: Sandhya Ghoshal Publishing Administrator, International Edition: Hema Mehta Project Editor, International Edition: Daniel Luiz Editorial Assistant, International Edition: Sinjita Basu Procurement Specialist: Linda Sager Senior Manufacturing Controller, Production, International Edition: Trudy Kimber Art Director: Jayne Conte Cover Designer: Karen Noferi Cover Photo Credit: Fotolia/Female photographer Cover Printer: Courier Westford Pearson Education Limited Edinburgh Gate Harlow Essex CM20 2JE England and Associated Companies throughout the world Visit us on the World Wide Web at: www.pearsoninternationaleditions.com © Pearson Education Limited 2014 The rights of William Stallings to be identified as the author of this work have been asserted by him in accordance with the Copyright, Designs and Patents Act 1988. Authorized adaptation from the United States edition, entitled Data and Computer Communications, 10th edition, ISBN 978-0-133-50648-8, by William Stallings, published by Pearson Education © 2014. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, ­electronic, mechanical, photocopying, recording or otherwise, without either the prior written permission of the publisher or a license permitting restricted copying in the United Kingdom issued by the Copyright Licensing Agency Ltd, Saffron House, 6–10 Kirby Street, London EC1N 8TS. All trademarks used herein are the property of their respective owners.The use of any trademark in this text does not vest in the author or publisher any trademark ownership rights in such trademarks, nor does the use of such trademarks imply any affiliation with or endorsement of this book by such owners. 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Printed and bound by Courier Westford in The United States of America A01_STAL4388_10_PIE_FM.indd 2 10/24/13 2:03 PM For Tricia A01_STAL4388_10_PIE_FM.indd 3 10/24/13 2:03 PM A01_STAL4388_10_PIE_FM.indd 4 10/24/13 2:03 PM Contents Preface 13 Acknowledgments 21 About the Author 23 Chapter 0 0.1 0.2 0.3 0.4 Guide for Readers and Instructors 25 Outline of the Book 26 A Roadmap for Readers and Instructors 27 Internet and Web Resources 28 Standards 29 Unit One Fundamentals 31 Part One Overview 32 Chapter 1 Data Communications, Data Networks, and the Internet 32 1.1 Data Communications and Networking for Today’s Enterprise 33 1.2 A Communications Model 39 1.3 Data Communications 42 1.4 Networks 45 1.5 The Internet 48 1.6 An Example Configuration 53 Chapter 2 Protocol Architecture, TCP/IP, and Internet-Based Applications 55 2.1 The Need for a Protocol Architecture 56 2.2 A Simple Protocol Architecture 57 2.3 The TCP/IP Protocol Architecture 61 2.4 Standardization within a Protocol Architecture 69 2.5 Traditional Internet-Based Applications 72 2.6 Multimedia 72 2.7 Sockets Programming 76 2.8 Recommended Reading and Animation 85 2.9 Key Terms, Review Questions, and Problems 87 2.10 Sockets Programming Assignments 90 Appendix 2A The Trivial File Transfer Protocol 90 Part Two Data Communications 95 Chapter 3 Data Transmission 95 3.1 Concepts and Terminology 96 3.2 Analog and Digital Data Transmission 108 3.3 Transmission Impairments 116 3.4 Channel Capacity 122 3.5 Recommended Reading 128 3.6 Key Terms, Review Questions, and Problems Appendix 3A Decibels and Signal Strength 131 A01_STAL4388_10_PIE_FM.indd 5 128 5 10/24/13 2:03 PM 6  Contents Chapter 4 4.1 4.2 4.3 4.4 4.5 4.6 Chapter 5 5.1 5.2 5.3 5.4 5.5 Chapter 6 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 Chapter 7 7.1 7.2 7.3 7.4 7.5 Chapter 8 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 Transmission Media 134 Guided Transmission Media 136 Wireless Transmission 151 Wireless Propagation 159 Line-of-Sight Transmission 164 Recommended Reading 168 Key Terms, Review Questions, and Problems 169 Signal Encoding Techniques 173 Digital Data, Digital Signals 175 Digital Data, Analog Signals 186 Analog Data, Digital Signals 197 Recommended Reading and Animations 204 Key Terms, Review Questions, and Problems 205 Error Detection and Correction 210 Types of Errors 212 Error Detection 213 Parity Check 214 The Internet Checksum 216 Cyclic Redundancy Check (CRC) 218 Forward Error Correction 225 Recommended Reading and Animations 231 Key Terms, Review Questions, and Problems 232 Data Link Control Protocols 235 Flow Control 237 Error Control 244 High-Level Data Link Control (HDLC) 250 Recommended Reading and Animations 257 Key Terms, Review Questions, and Problems 257 Multiplexing 260 Frequency-Division Multiplexing 262 Synchronous Time-Division Multiplexing 268 Cable Modem 278 Asymmetric Digital Subscriber Line 279 xDSL 284 Multiple Channel Access 285 Recommended Reading and Animations 289 Key Terms, Review Questions, and Problems 290 Part Three Wide Area Networks 293 Chapter 9 WAN Technology and Protocols 293 9.1 Switched Communications Networks 295 9.2 Circuit-Switching Networks 296 9.3 Circuit-Switching Concepts 299 9.4 Softswitch Architecture 305 A01_STAL4388_10_PIE_FM.indd 6 10/24/13 2:03 PM Contents  7 9.5 Packet-Switching Principles 307 9.6 Asynchronous Transfer Mode 316 9.7 Recommended Reading 321 9.8 Key Terms, Review Questions, and Problems Chapter 10 Cellular Wireless Networks 326 10.1 Principles of Cellular Networks 327 10.2 Cellular Network Generations 340 10.3 LTE-Advanced 344 10.4 Recommended Reading 352 10.5 Key Terms, Review Questions, and Problems 322 353 Part Four Local Area Networks 355 Chapter 11 Local Area Network Overview 355 11.1 Bus and Star Topologies 356 11.2 LAN Protocol Architecture 358 11.3 Bridges 366 11.4 Hubs and Switches 374 11.5 Virtual LANs 377 11.6 Recommended Reading and Animations 382 11.7 Key Terms, Review Questions, and Problems 383 Chapter 12 Ethernet 385 12.1 Traditional Ethernet 387 12.2 High-Speed Ethernet 395 12.3 IEEE 802.1Q VLAN Standard 405 12.4 Recommended Reading and Animations 407 12.5 Key Terms, Review Questions, and Problems 407 Appendix 12A Digital Signal Encoding for LANs 409 Appendix 12B Scrambling 416 Chapter 13 Wireless LANs 419 13.1 Overview 420 13.2 IEEE 802.11 Architecture and Services 424 13.3 IEEE 802.11 Medium Access Control 428 13.4 IEEE 802.11 Physical Layer 436 13.5 Gigabit Wi-Fi 443 13.6 IEEE 802.11 Security Considerations 446 13.7 Recommended Reading 447 13.8 Key Terms, Review Questions, and Problems 448 Part Five Internet and Transport Protocols Chapter 14 The Internet Protocol 451 14.1 Principles of Internetworking 452 14.2 Internet Protocol Operation 457 14.3 Internet Protocol 464 14.4 IPv6 474 14.5 Virtual Private Networks and IP Security 484 A01_STAL4388_10_PIE_FM.indd 7 451 10/24/13 2:03 PM 8  Contents 14.6 Recommended Reading and Animations 487 14.7 Key Terms, Review Questions, and Problems 488 Chapter 15 Transport Protocols 491 15.1 Connection-Oriented Transport Protocol Mechanisms 15.2 TCP 511 15.3 UDP 518 15.4 Recommended Reading and Animations 519 15.5 Key Terms, Review Questions, and Problems 520 492 Unit Two Advanced Topics in Data Communications and Networking 523 Part Six Data Communications and Wireless Networks Chapter 16 Advanced Data Communications Topics 524 16.1 Analog Data, Analog Signals 525 16.2 Forward Error-Correcting Codes 532 16.3 ARQ Performance Issues 547 16.4 Recommended Reading and Animations 554 16.5 Key Terms, Review Questions, and Problems 556 Chapter 17 Wireless Transmission Techniques 558 17.1 MIMO Antennas 559 17.2 OFDM, OFDMA, and SC-FDMA 562 17.3 Spread Spectrum 568 17.4 Direct Sequence Spread Spectrum 569 17.5 Code Division Multiple Access 574 17.6 Recommended Reading 577 17.7 Key Terms, Review Questions, and Problems 578 Chapter 18 Wireless Networks 582 18.1 Fixed Broadband Wireless Access 583 18.2 WiMAX/IEEE 802.16 585 18.3 Bluetooth Overview 597 18.4 Bluetooth Radio Specification 601 18.5 Bluetooth Baseband Specification 601 18.6 Bluetooth Logical Link Control and Adaptation Protocol 610 18.7 Recommended Reading 612 18.8 Key Terms, Review Questions, and Problems 612 524 Part Seven Internetworking 614 Chapter 19 Routing 614 19.1 Routing in Packet-Switching Networks 615 19.2 Examples: Routing in ARPANET 625 19.3 Internet Routing Protocols 631 19.4 Least-Cost Algorithms 642 19.5 Recommended Reading and Animations 648 19.6 Key Terms, Review Questions, and Problems 649 A01_STAL4388_10_PIE_FM.indd 8 10/24/13 2:03 PM Contents  9 Chapter 20 Congestion Control 653 20.1 Effects of Congestion 655 20.2 Congestion Control 660 20.3 Traffic Management 662 20.4 Congestion Control in Packet-Switching Networks 667 20.5 TCP Congestion Control 667 20.6 Datagram Congestion Control Protocol 679 20.7 Recommended Reading and Animations 684 20.8 Key Terms, Review Questions, and Problems 685 Chapter 21 Internetwork Operation 690 21.1 Multicasting 691 21.2 Software-Defined Networks 703 21.3 OpenFlow 707 21.4 Mobile IP 714 21.5 Dynamic Host Configuration Protocol 725 21.6 Recommended Reading and Animations 727 21.7 Key Terms, Review Questions, and Problems 728 Chapter 22 Internetwork Quality of Service 732 22.1 QOS Architectural Framework 734 22.2 Integrated Services Architecture 737 22.3 Resource Reservation Protocol 744 22.4 Differentiated Services 755 22.5 Service Level Agreements 763 22.6 IP Performance Metrics 765 22.7 Recommended Reading and Web Sites 768 22.8 Key Terms, Review Questions, and Problems 770 Chapter 23 Multiprotocol Label Switching 773 23.1 The Role of MPLS 775 23.2 Background 777 23.3 MPLS Operation 779 23.4 Labels 784 23.5 FECs, LSPs, and Labels 787 23.6 Label Distribution 789 23.7 Traffic Engineering 794 23.8 Virtual Private Networks 798 23.9 Recommended Reading 801 23.10 Key Terms, Review Questions, and Problems 801 Part Eight Internet Applications 803 Chapter 24 Electronic Mail, DNS, and HTTP 803 24.1 Electronic Mail—SMTP and MIME 804 24.2 Internet Directory Service: DNS 817 24.3 Web Access and HTTP 826 24.4 Recommended Reading and Animations 837 24.5 Key Terms, Review Questions, and Problems 838 A01_STAL4388_10_PIE_FM.indd 9 10/24/13 2:03 PM 10  Contents Chapter 25 Internet Multimedia Support 841 25.1 Real-Time Traffic 842 25.2 Voice Over IP 845 25.3 Session Initiation Protocol 848 25.4 Real-Time Transport Protocol (RTP) 852 25.5 Recommended Reading 862 25.6 Key Terms, Review Questions, and Problems APPENDICES  Appendix A Fourier Analysis 863 864 A.1 Fourier Series Representation of Periodic Signals 864 A.2 Fourier Transform Representation of Aperiodic Signals 865 A.3 Recommended Reading 868 Appendix B Projects and Other Student Exercises for Teaching Data and Computer Communications 869 B.1 Animations and Animation Assignments 870 B.2 Practical Exercises 870 B.3 Sockets Projects 870 B.4 Wireshark Projects 871 B.5 Simulation and Modeling Projects 871 B.6 Performance Modeling 872 B.7 Research Projects 872 B.8 Reading/Report Assignments 873 B.9 Writing Assignments 873 B.10 Discussion Topics 873 References 874 Index 887 Online Chapters And Appendices1 Part Nine Network Security Chapter 26 Computer and Network Security Threats 26.1 Computer Security Concepts 26.2 Threats, Attacks, and Assets 26.3 Intruders 26.4 Malicious Software Overview 26.5 Viruses, Worms, and Bots 26.6 Recommended Reading 26.7 Key Terms, Review Questions, and Problems Chapter 27 Computer and Network Security Techniques 27.1 Virtual Private Networks and IPsec 27.2 SSL and TLS 1 Online chapters and appendices are Premium Content, available via the access card at the front of this book. A01_STAL4388_10_PIE_FM.indd 10 10/24/13 2:03 PM Contents  11 27.3 Wi-Fi Protected Access 27.4 Intrusion Detection 27.5 Firewalls 27.6 Malware Defense 27.7 Recommended Reading 27.8 Key Terms, Review Questions, and Problems Appendix C Standards Organizations Appendix D Asynchronous and Synchronous Transmission Appendix E The OSI Model Appendix F The International Reference Alphabet Appendix G Proof of the Sampling Theorem Appendix H Ones Complement Representation and Addition Appendix I Statistical TDM Appendix J The Spanning Tree Algorithm Appendix K LAN Performance Issues Appendix L Matrix Multiplication and Determinants Appendix M Queuing Effects Appendix N Orthogonality, Correlation, and Autocorrelation Appendix O TCP/IP Example Appendix P Queue Management and Queueing Discipline Appendix Q Cryptographic Algorithms Appendix R Uniform Resource Locators (URLs) and Uniform Resource Identifiers (URIs) Appendix S Augmented Backus-Naur Form Appendix T Derivations of Equations and Examples Glossary A01_STAL4388_10_PIE_FM.indd 11 10/24/13 2:03 PM A01_STAL4388_10_PIE_FM.indd 12 10/24/13 2:03 PM Preface What’s New in The Tenth Edition Since the ninth edition of this book went to press, the pace of change in this field continues unabated. In this new edition, I try to capture these changes while maintaining a broad and comprehensive coverage of the entire field. To begin the process of revision, the ninth edition of this book was extensively reviewed by a number of professors who teach the subject and by professionals working in the field. The result is that, in many places, the narrative has been clarified and tightened, and illustrations have been improved. Beyond these refinements to improve pedagogy and user friendliness, there have been major substantive changes throughout the book. The chapter organization has been changed somewhat so that now the material is organized into two Units, with Unit Two containing more advanced material and an expansion of the material related to the Internet. Beyond this organizational revision, the most noteworthy changes include the following: • Sockets programming: A new section introduces sockets programming. Plus a number of sockets programming assignments, with sample solutions, are available for instructors. • Software-defined networks: A new section covers this widely used technology. • Wireless transmission technology: The book provides a unified treatment of important transmission technologies for wireless networks, including FDD, TDD, FDMA, TDMA, CDMA, OFDM, OFDMA, SC-FDMA, and MIMO. • 4G cellular networks: A new section covers 4G networks and the LTEAdvanced specification. • Gigabit Wi-Fi: A new section covers the two new Wi-Fi standards, IEEE 802.11ac and 802.11ad, which provide Wi-Fi in the Gbps range. • Fixed broadband wireless access: New sections cover fixed broadband wireless access to the Internet and the related WiMAX standard. • Forward error correction: Forward error correction techniques are essential in wireless networks. This new edition contains substantially expanded coverage of this important topic. • Personal area networks: New sections cover personal area networks and the Bluetooth standard. • Dynamic Host Configuration Protocol (DHCP): DHCP is a widely used protocol that enables dynamic IP address assignment. A new section covers this protocol. • Datagram Congestion Control Protocol: DCCP is a new protocol that meets the needs of multimedia applications for a congestion control transport protocol without the overhead of TCP. A new section covers DCCP. 13 A01_STAL4388_10_PIE_FM.indd 13 10/24/13 2:03 PM 14  Preface • Protocol Independent Multicast (PIM): PIM, the most important Internet multicast routing algorithm, is covered in a new section. • Quality of service (QoS) architectural framework: A new section covers ITU-T Recommendation Y.1291, which provides an overall framework for provision of Internet QoS facilities. • Electronic mail: The section on e-mail in Chapter 24 has been expanded to include a discussion of the standard Internet mail architecture. • Animations: As a powerful aid to understanding the material, over 150 online animations are provided covering a wide range of topics from the book. An icon at the beginning of many chapters indicates that supporting animations are available to enhance the student’s understanding. • Learning objectives: Each chapter now begins with a list of learning objectives. • Sample syllabus: The text contains more material than can be conveniently covered in one semester. Accordingly, instructors are provided with several sample syllabi that guide the use of the text within limited time (e.g., 16 weeks or 12 weeks). These samples are based on real-world experience by professors with the ninth edition. In addition, the material that carries over from the ninth edition has been revised, with new figures and revised and updated content. 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 and provides 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 highlights these principles and contrasts their application in specific areas of technology. • Design approaches: The book examines alternative approaches to meeting specific communication requirements. • 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 related standards. A01_STAL4388_10_PIE_FM.indd 14 10/24/13 2:03 PM Preface  15 Support of ACM/IEEE Computer Science Curricula 2013 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 selfstudy. As a textbook, it can be used for a one-semester or two-semester course. This edition is designed to support the recommendations of the current (February 2013) draft version of the ACM/IEEE Computer Science Curricula 2013 (CS2013). The CS2013 curriculum recommendation includes Networking and Communication (NC) as one of the Knowledge Areas in the Computer Science Body of Knowledge. CS2013 divides all course work into three categories: Core-Tier 1 (all topics should be included in the curriculum), Core-Tier-2 (all or almost all topics should be included), and elective (desirable to provide breadth and depth). In the NC area, CS2013 includes two Tier 1 topics and five Tier 2 topics, each of which has a number of subtopics. This text covers all of the topics and subtopics listed by CS2013 in these two tiers. Table P.1 shows the support for the NC Knowledge Area provided in this textbook. Table P.1 Coverage of CS2013 Networking and Communication (NC) Knowledge Area Topic Chapter Coverage Introduction (Tier 1) 1-Data Communications —Organization of the Internet (Internet Service Providers, Content Providers, etc.) 2-Protocol Architecture 9-WAN Technology —Switching techniques (Circuit, packet, etc.) —Physical pieces of a network (hosts, routers, switches, ISPs, wireless, LAN, access point, firewalls, etc.) —Layering principles (encapsulation, multiplexing) —Roles of the different layers (application, transport, network, datalink, physical) Networked Applications (Tier 1) 24-Electronic mail, DNS, HTTP —Naming and address schemes (DNS, IP addresses, Uniform Resource Identifiers, etc.) 2-Protocol Architecture —Distributed applications (client/server, peer-to-peer, cloud, etc.) —HTTP as an application layer protocol —Multiplexing with TCP and UDP —Socket APIs Reliable Data Delivery (Tier 2) 6-Error Detection and Correction —Error control (retransmission techniques, timers) 7-Data Link Control —Flow control (acknowledgments, sliding window) 15-Transport Protocols —Performance issues (pipelining) —TCP A01_STAL4388_10_PIE_FM.indd 15 10/24/13 2:03 PM 16  Preface Table P.1 Continued Topic Chapter Coverage Routing And Forwarding (Tier 2) 19-Routing —Routing versus forwarding 14-The Internet Protocol —Static routing —Internet Protocol (IP) —Scalability issues (hierarchical addressing) Local Area Networks (Tier 2) —Multiple Access Problem —Common approaches to multiple access (exponential-backoff, time division multiplexing, etc.) 11-Local Area Network Overview 12-Ethernet —Local Area Networks —Ethernet —Switching Resource Allocation (Tier 2) 8-Multiplexing —Need for resource allocation 20-Congestion Control —Fixed allocation (TDM, FDM, WDM) versus dynamic allocation 21-Internetwork QoS —End-to-end versus network-assisted approaches —Fairness —Principles of congestion control —Approaches to Congestion (Content Distribution Networks, etc.) Mobility (Tier 2) 10-Cellular Wireless Networks —Principles of cellular networks 13-Wireless LANs —802.11 networks —Issues in supporting mobile nodes (home agents) Plan of the Text The book is divided into two units, comprising nine parts, which are described in Chapter 0: • Unit One: Fundamentals of Data Communications and Networking —Overview —Data Communications —Wide Area Networks —Local Area Networks —Internet and Transport Layers • Unit Two: Advanced Topics in Data Communications and Networking —Data Communications and Wireless Networks —Internetworking —Internet Applications —Network Security A01_STAL4388_10_PIE_FM.indd 16 10/24/13 2:03 PM Preface  17 The book includes a number of pedagogic features, including the use of animations and numerous figures and tables to clarify the discussions. Each chapter includes a list of key words, review questions, homework problems, and suggestions for further reading. The book also includes an extensive online glossary, a list of frequently used acronyms, and a reference list. In addition, a test bank is available to instructors. The chapters and parts of the book are sufficiently modular to provide a great deal of flexibility in the design of courses. See Chapter 0 for a number of detailed suggestions for both top-down and bottom-up course strategies. Instructor Support Materials The major goal of this text is to make it as effective a teaching tool for this exciting and fastmoving subject as possible. This goal is reflected both in the structure of the book and in the supporting material. The text is accompanied by the following supplementary material to aid the instructor: • Solutions manual: Solutions to all end-of-chapter Review Questions and Problems. • Projects manual: Suggested project assignments for all of the project categories in the next section. • PowerPoint slides: A set of slides covering all chapters, suitable for use in lecturing. • PDF files: Reproductions of all figures and tables from the book. • Test bank: A chapter-by-chapter set of questions with a separate file of answers. • Sample syllabuses: The text contains more material than can be conveniently covered in one semester. Accordingly, instructors are provided with several sample syllabuses that guide the use of the text within limited time. These samples are based on real-world experience by professors with the ninth edition. All of these support materials are available at the Instructor Resource Center (IRC) for this textbook, which can be reached through the publisher’s Web site www. pearsoninternationaleditions.com/stallings or by clicking on the link labeled Pearson Resources for Instructors at this book’s Companion Web site at WilliamStallings.com/ DataComm. To gain ­access to the IRC, please contact your local Pearson sales representative. The Companion Web site, at WilliamStallings.com/DataComm (click on Instructor Resources link), includes the following: • Links to Web sites for other courses being taught using this book. • Sign-up information for an Internet mailing list for instructors using this book to exchange information, suggestions, and questions with each other and with the author. A01_STAL4388_10_PIE_FM.indd 17 10/24/13 2:03 PM 18  Preface Projects and Other Student Exercises For many instructors, an important component of a data communications or networking course is a project or set of projects by which the student gets hands-on experience to reinforce concepts from the text. This book provides an unparalleled degree of support for ­including a projects component in the course. The IRC not only provides guidance on how to assign and structure the projects but also includes a set of User’s Manuals for various project types plus specific assignments, all written especially for this book. Instructors can assign work in the following areas: • Animation assignments: Described in the following section. • Practical exercises: Using network commands, the student gains experience in network connectivity. • Sockets programming projects: Described subsequently in this Preface. • Wireshark projects: Wireshark is a protocol analyzer that enables students to study the behavior of protocols. A video tutorial is provided to get students started, in addition to a set of Wireshark assignments. • Simulation projects: The student can use the simulation package cnet to analyze network behavior. The IRC includes a number of student assignments. • Performance modeling projects: Two performance modeling techniques are introduced: a tools package and OPNET. The IRC includes a number of ­student assignments. • Research projects: The IRC includes a list of suggested research projects that would involve Web and literature searches. • Reading/report assignments: The IRC includes a list of papers that can be ­assigned for reading and writing a report, plus suggested assignment wording. • Writing assignments: The IRC includes a list of writing assignments to facilitate learning the material. • Discussion topics: These topics can be used in a classroom, chat room, or message board environment to explore certain areas in greater depth and to foster student collaboration. This diverse 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 needs of the instructor and students. See Appendix B for details. Animations Animations provide a powerful tool for understanding the complex mechanisms discussed in this book, including forward error correction, signal encoding, and protocols. Over 150 Web-based animations are used to illustrate many of the data communications and protocol concepts in this book. These animations are available online at the Premium Web site. For those chapters for which animations are available, this icon appears at the beginning of the chapter:   . A n im a ti o n A01_STAL4388_10_PIE_FM.indd 18 10/24/13 2:03 PM Preface  19 Twelve of the animations have been designed to allow for two types of assignments. First, the student can be given a specific set of steps to invoke and watch the animation, and then be asked to analyze and comment on the results. Second, the student can be given a specific end point and is required to devise a sequence of steps that achieve the desired result. The IRC includes a set of assignments for each of these animations, plus suggested solutions so that instructors can assess the student’s work. Sockets Programming Sockets are the fundamental element behind any kind of network communication using the TCP/IP protocol suite. Sockets programming is a relatively straightforward topic that can result in very satisfying and effective hands-on projects for students. This book provides considerable support to enable students to learn and use Sockets programming to enhance their understanding of networking, including: 1. Chapter 2 provides a basic introduction to Sockets programming and includes a detailed analysis of a TCP server and a TCP client program. 2. Chapter 2 also includes some end-of-chapter programming assignments using Sockets. Sample solutions are available at the IRC for this book. 3. Additional Sockets programming assignments, plus sample solutions, are available for instructors at the IRC. These include a number of moderate-size ­assignments and a more substantial project that, step by step, implements a simplified instant messaging client and server. 4. A different, additional set of Sockets assignments, plus sample solutions, are included in the supplemental homework problems available to students at the Premium Web site. Taken together, these resources provide students with a solid understanding of Sockets programming and experience in developing networking applications. Online Documents for Students For this new edition, a substantial amount of original supporting material for students has been made available online, at two Web locations. The Companion Web site, at WilliamStallings.com/DataComm (click on Student Resources link), includes a list of relevant links organized by chapter and an errata sheet for the book. Purchasing this textbook new also grants the reader six months of access to the Premium Content site, which includes the following materials: • Online chapters: To limit the size and cost of the book, two chapters of the book, covering security, are provided in PDF format. The chapters are listed in this book’s table of contents. • Online appendices: There are numerous interesting topics that support material found in the text but whose inclusion is not warranted in the printed text. A01_STAL4388_10_PIE_FM.indd 19 10/24/13 2:03 PM 20  Preface A total of 18 online appendices cover these topics for the interested student. The appendices are listed in this book’s table of contents. • Homework problems and solutions: To aid the student in understanding the material, a separate set of homework problems with solutions is available. To access the Premium Content site, click on the Premium Content link at the Companion Web site or at www.pearsoninternationaleditions.com/stallings and enter the student access code found on the card in the front of the book. A01_STAL4388_10_PIE_FM.indd 20 10/24/13 2:03 PM Acknowledgments Through its multiple editions this book has benefited from review by hundreds of instructors and professionals, who gave generously of their time and expertise. Here I acknowledge those whose help contributed to this latest edition. The following instructors reviewed all or a large part of the manuscript: Tibor Gyires (Illinois State University), Hossein Hosseini (University of Wisconsin-Milwaukee), Naeem Shareef (Ohio State University), Adrian Lauf (University of Louisville), and Michael Fang (University of Florida). Thanks also to the many people who provided detailed technical reviews of a single chapter: Naji A. Albakay, Prof. (Dr). C. Annamalai, Rakesh Kumar Bachchan, Alan ­ Cantrell, Colin Conrad, Vineet Chadha, George Chetcuti, Rajiv Dasmohapatra, Ajinkya Deshpande, Michel Garcia, Thomas Johnson, Adri Jovin, Joseph Kellegher, Robert Knox, Bo Lin, Yadi Ma, Luis Arturo Frigolet Mayo, Sushil Menon, Hien Nguyen, Kevin SanchezCherry, Mahesh S. Sankpal, Gaurav Santhalia, Stephanie Sullivan, Doug Tiedt, Thriveni Venkatesh, and Pete Zeno. Thanks also to the following contributors. Yadi Ma contributed homework problems on Sockets programming. Yunzhao Li developed some of the animation applets. Larry Tan of the University of Stirling in Scotland developed the animation assignments. Michael Harris of Indiana University initially developed the Wireshark exercises and user’s guide. Dave Bremer, a principal lecturer at Otago Polytechnic in New Zealand, updated the material for the most recent Wireshark release; he also developed an online video tutorial for using Wireshark. Kim McLaughlin produced the PPT lecture slides. Finally, I thank the many people responsible for the publication of this book, all of whom did their usual excellent job. This includes the staff at Pearson, particularly my editor Tracy Johnson, her assistant Jenah Blitz-Stoehr, program manager Carole Snyder, and permissions supervisor Bob Engelhardt. I also thank Shiny Rajesh and the production staff at Integra for another excellent and rapid job. Thanks also to the marketing and sales staffs at Pearson, without whose efforts this book would not be in front of you. The publishers wish to thank Somitra Kumar Sanadhya, of the Indraprastha Institute of Information Technology, Delhi, for reviewing the content of the International Edition. 21 A01_STAL4388_10_PIE_FM.indd 21 10/24/13 2:03 PM A01_STAL4388_10_PIE_FM.indd 22 10/24/13 2:03 PM About the Author Dr. William Stallings has authored 17 titles, and counting revised editions, over 40 books on computer security, computer networking, and computer architecture. His writings have ­appeared in numerous publications, including the Proceedings of the IEEE, ACM Computing Reviews and Cryptologia. He has 12 times received the award for the best Computer Science textbook of the year from the Text and Academic Authors Association. In over 30 years in the field, he has been a technical contributor, technical manager, and an executive with several high-technology firms. He has designed and implemented both TCP/ IP-based and OSI-based protocol suites on a variety of computers and operating systems, ranging from microcomputers to mainframes. As a consultant, he has advised government agencies, computer and software vendors, and major users on the design, selection, and use of networking software and products. He created and maintains the Computer Science Student Resource Site at ComputerScienceStudent.com. This site provides documents and links on a variety of subjects of general interest to computer science students (and professionals). He is a member of the editorial board of Cryptologia, a scholarly journal devoted to all aspects of cryptology. Dr. Stallings holds a PhD from MIT in Computer Science and a BS from Notre Dame in electrical engineering. 23 A01_STAL4388_10_PIE_FM.indd 23 10/24/13 2:03 PM A01_STAL4388_10_PIE_FM.indd 24 10/24/13 2:03 PM Chapter Guide for Readers and Instructors 0.1 Outline of the Book 0.2 A Roadmap for Readers and Instructors 0 Course Emphasis Bottom–Up versus Top–Down 0.3 Internet and Web Resources Web Sites for This Book Computer Science Student Resource Site Other Web Sites 0.4 Standards 25 M01_STAL4388_10_PIE_INTR.indd 25 02/10/13 11:43 AM 26   Chapter 0 / Guide for Readers and Instructors This book, with its accompanying Web support, covers a lot of material. Here, we give the reader some basic background information. 0.1 Outline of the Book The book is organized into two units. Unit One provides a survey of the fundamentals of data communications, networks, and Internet protocols. Unit Two ­covers more advanced or difficult topics in data communications and networks, and ­provides a more comprehensive discussion of Internet protocols and operation. Unit One is organized into five parts: Part One. Overview: Provides an introduction to the range of topics covered in the book. This part includes a general overview of data communications and networking, and a discussion of protocols and the TCP/IP protocol suite. Part Two. Data Communications: Presents material concerned primarily with the exchange of data between two directly connected devices. Within this restricted scope, the key aspects of transmission, transmission media, error detection, link control, and multiplexing are examined. Part Three. Wide Area Networks: Examines the technologies and protocols that have been developed to support voice, data, and multimedia communications over long-distance networks. The traditional t­echnologies of packet switching and circuit switching, as well as the more contemporary ATM and cellular networks, are examined. Part 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 key ingredients of a LAN design are explored. This is followed by a detailed discussion of Ethernet and Wi-Fi networks. Part Five. Internet and Transport Protocols: Discusses protocols at the Internet and Transport layers. Unit Two consists of three parts: Part Six. Data Communications and Wireless Networks: Treats important topics in these areas not covered in Unit One. Part Seven. Internetworking: Examines a range of protocols and standards related to the operation of the Internet, including routing, congestion control, and quality of service. Part Eight. Internet Applications: Looks at a range of applications that operate over the Internet. In addition, there is an online Part Nine. Security: It covers security threats and techniques for countering these threats. A number of online appendices cover additional topics relevant to the book. M01_STAL4388_10_PIE_INTR.indd 26 02/10/13 11:43 AM 0.2 / A Roadmap for Readers and Instructors   27 0.2 A Roadmap for Readers and Instructors The text contains more material than can be conveniently covered in one semester. Accordingly, the Instructor Resource Center (IRC) for this book includes several sample syllabi that guide the use of the text within limited time (e.g., 16 weeks or 12 weeks). Each alternative syllabus suggests a selection of chapters and a weekly schedule. These samples are based on real-world experience by professors with the previous edition. The organization of the book into two units is intended to divide the material, roughly, into introductory and fundamental topics (Unit One) and advanced topics (Unit Two). Thus, a one-semester course could be limited to all or most of the material in Unit One. In this section, we provide some other suggestions for organizing the material for a course. Course Emphasis The 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 three different course designs: • Fundamentals of Data Communications: Parts One (overview), Two (data communications), and Three (wired WANs and cellular networks). • Communications Networks: If the student has a basic background in data communications, then this course could cover Parts One (overview), Three (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 (error detection and correction, and data link control), Part Five (internet and transport protocols), and part or all of Parts Seven (internetworking) and Eight (applications). In addition, a more streamlined course that covers the entire book is possible by eliminating certain chapters that are not essential on a first reading. The sample syllabi document at the IRC provides guidance on chapter selection. Bottom–Up versus Top–Down The book is organized in a modular fashion. After reading Part One, the other parts can be read in a number of possible sequences. Table 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 clear how M01_STAL4388_10_PIE_INTR.indd 27 02/10/13 11:43 AM 28   Chapter 0 / Guide for Readers and Instructors Table 0.1 Suggested Reading Orders (a) A bottom–up approach (b) A shorter bottom–up approach Part One: Overview Part One: Overview Part Two: Data Communications Part Two: Data Communications (Chapters 3, 6, 7, 8) Part Three: Wide Area Networks Part Three: Wide Area Networks Part Four: Local Area Networks Part Four: Local Area Networks Part Five: Internet and Transport Layers Part Five: Internet and Transport Layers Part Seven: Internetworking Part Eight: Internet Applications (c) A top–down approach (d) A shorter top–down approach Part One: Overview Part One: Overview Chapter 14: The Internet Protocol Chapter 14: The Internet Protocol Part Eight: Internet Applications Part Eight: Internet Applications Chapter 15: Transport Protocols Chapter 15: Transport Protocols Part Seven: Internetworking Part Seven: Internetworking (Chapters 19, 20, 21) Part Three: Wide Area Networks Part Three: Wide Area Networks Part Four: Local Area Networks Part Four: Local Area Networks (Chapter 11) Part Two: Data Communications a given layer of functionality is supported from below. There is more material than can be comfortably covered in a single semester, but the book’s organization makes it easy to eliminate some chapters and maintain the bottom–up sequence. Table 0.1b suggests one approach to a survey 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 most visible part of the material, the applications, and then seeing, progressively, how each layer is supported by the next layer down. Table 0.1c is an example of a comprehensive treatment, and Table 0.1d is an example of a survey treatment. 0.3 Internet and Web Resources There are a number of resources available on the Internet and the Web that support this book and help readers keep up with developments in this field. Web Sites for This Book Three Web sites provide additional resources for students and instructors. There is a Companion Website for this book at http://williamstallings.com/ DataComm. For students, this Web site includes a list of relevant links, organized by chapter, and an errata list for the book. For instructors, this Web site provides M01_STAL4388_10_PIE_INTR.indd 28 02/10/13 11:43 AM 0.4 / Standards   29 links to course pages by professors teaching from this book and provides a number of other useful documents and links. There is also an access-controlled Premium Content Website, which provides a wealth of supporting material, including additional online chapters, additional online appendices, and a set of homework problems with solutions. See the card at the front of this book for access information. Finally, additional material for instructors, including a solutions manual and a projects manual, is available at the Instructor Resource Center (IRC) for this book. See Preface for details and access information. Computer Science Student Resource Site I also maintain the Computer Science Student Resource Site, at ComputerScience Student.com. The purpose of this site is to provide documents, information, and links for computer science students and professionals. Links and documents are organized into seven categories: • Math: Includes a basic math refresher, a queuing analysis primer, a number system primer, and links to numerous math sites. • How-to: Advice and guidance for solving homework problems, writing technical reports, and preparing technical presentations. • Research resources: Links to important collections of papers, technical reports, and bibliographies. • Other useful: A variety of other useful documents and links. • Computer science careers: Useful links and documents for those considering a career in computer science. • Writing help: Help in becoming a clearer, more effective writer. • Miscellaneous topics and humor: You have to take your mind off your work once in a while. Other Web Sites Numerous Web sites provide information related to the topics of this book. The Companion Website provides links to these sites, organized by chapter. 0.4 Standards Standards have come to play a dominant role in the information communications marketplace. Virtually all vendors of products and services are committed to ­supporting international standards. Throughout this book, we describe the most important standards in use or being developed for various aspects of data communications and networking. Various organizations have been involved in the development or promotion of these standards. The most important (in the current context) of these organizations are as follows: • Internet Society: The Internet SOCiety (ISOC) is a professional membership society with worldwide organizational and individual membership. It provides M01_STAL4388_10_PIE_INTR.indd 29 02/10/13 11:43 AM 30   Chapter 0 / Guide for Readers and Instructors 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 Task Force (IETF) and the Internet Architecture Board (IAB). These organizations develop Internet standards and related specifications, all of which are published as Requests for Comments (RFCs). • IEEE 802: The IEEE (Institute of Electrical and Electronics Engineers) 802 LAN/MAN Standards Committee develops local area network standards and metropolitan area network standards. The most widely used standards are for the Ethernet family, wireless LAN, bridging, and virtual bridged LANs. An individual working group provides the focus for each area. • ITU-T: The International Telecommunication Union (ITU) is a United Nations agency in which 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. ITU-T standards are referred to as Recommendations. • ISO: The International Organization for Standardization (ISO)1 is a worldwide federation of national standards bodies from more than 140 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 published as International Standards. A more detailed discussion of these organizations is contained in Appendix C. 1 ISO is not an acronym (in which case it would be IOS), but a word, derived from the Greek, meaning equal. M01_STAL4388_10_PIE_INTR.indd 30 02/10/13 11:43 AM UNIT ONE Fundamentals PART ONE OVERVIEW Chapter 1 Data Communications, Data Networks, and the Internet Chapter 2 Protocol Architecture, TCP/IP, and Internet-Based Applications PART TWO Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 DATA COMMUNICATIONS Data Transmission Transmission Media Signal Encoding Techniques Error Detection and Correction Data Link Control Protocols Multiplexing PART THREE WIDE AREA NETWORKS Chapter 9 WAN Technology and Protocols Chapter 10 Cellular Wireless Networks PART FOUR Chapter 11 Chapter 12 Chapter 13 LOCAL AREA NETWORKS Local Area Network Overview Ethernet Wireless LANs PART FIVE INTERNET AND TRANSPORT PROTOCOLS Chapter 14 The Internet Protocol Chapter 15 Transport Protocols M02_STAL4388_10_PIE_C01.indd 31 02/10/13 11:51 AM Part One: Overview Chapter 1 Data Communications, Data Networks, and the Internet 1.1 Data Communications and Networking for Today’s Enterprise Trends Data Transmission and Network Capacity Requirements Convergence 1.2 A Communications Model 1.3 Data Communications A Data Communications Model The Transmission of Information 1.4 Networks Wide Area Networks Local Area Networks Wireless Networks 1.5 The Internet Origins of the Internet Key Elements Internet Architecture 1.6 An Example Configuration 32 M02_STAL4388_10_PIE_C01.indd 32 02/10/13 11:51 AM 1.1 / Data Communications and Networking for Today’s Enterprise   33 Learning Objectives After studying this chapter, you should be able to: ◆ Present an overview of data communications traffic volume trends. ◆ Understand the key elements of a data communications system. ◆ Summarize the types of data communications networks. ◆ Present an overview of the overall architecture of the Internet. 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. Then a brief discussion introduces each of the Parts Two through Four and Six of this book. Chapter 2 provides an overview to Parts Five, Eight, and Nine. 1.1 Data Communications and Networking for Today’s Enterprise Effective and efficient data communication and networking facilities are vital to any enterprise. In this section, we first look at trends that are increasing the challenge for the business manager in planning and managing such facilities. Then we look specifically at the requirement for ever-greater transmission speeds and network capacity. Trends Three different forces have consistently driven the architecture and evolution of data communications and networking facilities: traffic growth, development of new services, and advances in technology. Communication traffic, both local (within a building or business campus) and long distance, has been growing at a high and steady rate for decades. Network traffic is no longer limited to voice and data and increasingly includes image and video. Increasing business emphasis on web services, remote access, online transactions, and social networking means that this trend is likely to continue. Thus, ­business managers are constantly pressured to increase communication capacity in costeffective ways. As businesses rely more and more on information technology, the range of services that business users desire to consume is expanding. For example, mobile M02_STAL4388_10_PIE_C01.indd 33 02/10/13 11:51 AM 34   Chapter 1 / Data Communications, Data Networks, and the Internet broadband traffic growth is exploding as is the amount of data being pushed over mobile networks by business users’ smart phones and tablets. In addition, over time, mobile users are increasingly demanding high-quality services to support their high-resolution camera phones, favorite video streams, and high-end audio. Similar demand growth is seen in landline access to the Internet and private networks. To keep up with mushrooming traffic generated by both consumers and business users, mobile service providers have to keep investing in high-capacity networking and transmission facilities. In turn, the growth in high-speed network offerings at competitive price points encourages the expansion of mobile applications and services. Thus, growth in services and in traffic capacity go hand in hand. As an example, Figure 1.1 [IEEE12] shows the mix of traffic and the growth trend for cable Internet subscribers. 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, in both computing and communications, continues. In terms of computing, this means more powerful computers and clusters of computers capable of supporting more demanding applications, such as multimedia applications. In terms of communications, the increasing use of optical fiber and high-speed wireless has brought transmission prices down and greatly increased capacity. For example, for long-distance telecommunication and data network links, dense wavelength division multiplexing (DWDM) 160 Average data rate per subscriber (kbps) 140 120 Other protocols Web browsing 100 Peer-to-peer 80 Streaming media 60 40 20 January 2010 Figure 1.1 M02_STAL4388_10_PIE_C01.indd 34 January 2011 Average Downstream Traffic per Internet Subscriber 02/10/13 11:51 AM 1.1 / Data Communications and Networking for Today’s Enterprise   35 1 Tbps Ethernet data rate standard Core Networking/ Service provider/WAN Doubling ≈ 18 months 100 Gbps 10 Gbps Enterprise/ server/LAN Doubling ≈ 24 months 1 Gbps 100 Mbps 1995 2000 2005 2010 2015 2020 Figure 1.2 Past and Projected Growth in Ethernet Data Rate Demand Compared to Existing Ethernet Data Rates enables communication traffic to be carried by fiber optic cables at rates of multiple terabits per second. For local area networks (LANs), many enterprises now have 40-Gbps Ethernet or 100-Gbps Ethernet backbone networks.1 Figure 1.2 [IEEE12] indicates the Ethernet demand trend. As shown, usage statistics indicate that Internet backbone data rate demand in the ­network core doubles approximately every 18 months, while demand in enterprise/LAN ­applications doubles approximately every 24 months. 2. Today’s networks are more “intelligent” than ever. Two areas of intelligence are noteworthy. First, today’s networks can offer differing levels of quality of service (QoS), which include specifications for maximum delay, minimum throughput, and so on to ensure high-quality support for applications and services. Second, today’s networks provide a variety of customizable services in the areas of network management and security. 3. The Internet, the Web, and associated applications have emerged as dominant features for both business and personal network landscapes. The migration to “everything over IP” continues and has created many opportunities and challenges for information and communications technology (ICT) managers. In addition to exploiting the Internet and the Web to reach customers, suppliers, and partners enterprises have formed intranets and extranets2 to isolate proprietary information to keep it free from unwanted access. 1 An explanation of numerical prefixes, such as tera and giga, is provided in the document Prefix.pdf, available at box.com/dcc10e. 2 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. M02_STAL4388_10_PIE_C01.indd 35 02/10/13 11:51 AM 36   Chapter 1 / Data Communications, Data Networks, and the Internet 4. Mobility is newest frontier for ICT managers, and popular consumer devices such as the iPhone, Droid, and iPad have become drivers of the evolution of business networks and their use. While there has been a trend toward mobility for decades, the mobility explosion has occurred and has liberated workers from the confines of the physical enterprise. Enterprise applications traditionally supported on terminals and office desktop computers are now routinely delivered on mobile devices. Cloud computing is being embraced by all major business software vendors including SAP, Oracle, and Microsoft, and this ensures that further mobility innovations will be forthcoming. Industry experts predict that mobile devices will become the dominant business computing platform by 2015 and that enhanced ability to use enterprise information resources and services anywhere-anytime will be a dominant trend for the remainder of the decade. Data Transmission and Network Capacity Requirements Momentous changes in the way organizations do business and process information have been driven 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 image-based services on the Internet (i.e., the Web) has resulted in an increase in the total number of users and the traffic volume generated by each user. This, in turn, has resulted in a need to increase the speed and efficiency 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 business WAN requirements. Finally, we offer a few words about the effect of changes in commercial electronics on network requirements. The Emergence of High-Speed LANs Personal 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 services—connecting personal computers and terminals to mainframes and midrange systems that ran corporate 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 token ring, are well suited to this environment. In the last 20 years, 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 enjoy explosive growth. These more powerful platforms support graphics-intensive applications and ever more elaborate graphical user interfaces to the operating system. M02_STAL4388_10_PIE_C01.indd 36 02/10/13 11:51 AM 1.1 / Data Communications and Networking for Today’s Enterprise   37 2. 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 client/server computing, which has become a dominant architecture in the business environment and the more recent Web-focused 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 16-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: • Centralized server farms: In many applications, there is a need for user, or client, systems to be able to draw huge amounts of data from multiple centralized servers, called server farms. An example is a color publishing operation, in which servers typically 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 draw 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-speed 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 centralized data processing model. In a typical environment, there might be significant computing facilities at a few regional offices, consisting of mainframes or well-equipped midrange systems. These centralized facilities could handle most corporate applications, including basic finance, accounting, and personnel programs, as well as many of the business-specific applications. Smaller, outlying 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. This model began to change in the early 1990s, and the change accelerated since then. Many 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, M02_STAL4388_10_PIE_C01.indd 37 02/10/13 11:51 AM 38   Chapter 1 / Data Communications, Data Networks, and the Internet 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 load has risen. All of these trends mean that more data must be transported off premises and into the wide area. It has long been accepted that in the typical business environment, about 80% of the traffic remains 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. 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 (DVDs) and digital still cameras. With the capacious DVD, the electronics industry at last found an acceptable replacement for the analog video home system (VHS) tapes. The DVD has replaced the videotape used in videocassette recorders (VCRs) and 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 randomly accessed like audio CDs, which DVD machines can also play. Vast volumes of data can be crammed onto the disk. With DVD’s huge storage 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 digital 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. Convergence Convergence refers to the merger of previously distinct telephony and information technologies and markets. We can think of this convergence in terms of a threelayer model of enterprise communications: • Applications: These are seen by the end users of a business. Convergence integrates communications applications, such as voice calling (telephone), voice mail, e-mail, and instant messaging, with business applications, such as workgroup collaboration, customer relationship management, and other back-office functions. With convergence, applications provide features that incorporate voice, data, and video in a seamless, organized, and value-added manner. One example is multimedia messaging, which enables a user to employ a single interface to access messages from a variety of sources (e.g., office voice mail, office e-mail, beeper, and fax). M02_STAL4388_10_PIE_C01.indd 38 02/10/13 11:51 AM 1.2 / A Communications Model   39 • Enterprise services: At this level, the manager deals with the information network in terms of the services it provides to support applications. The network manager needs design, maintenance, and support services related to the deployment of convergence-based facilities. Also at this level, network managers deal with the enterprise network as a function-providing system. Such management services may include setting up authentication schemes; capacity management for various users, groups, and applications; and QoS provision. • Infrastructure: The network and communications infrastructure consists of the communication links, LANs, WANs, and Internet connections available to the enterprise. Increasingly, enterprise network infrastructure also includes private and/or public cloud connections to data centers which host high-volume data storage and Web services. A key aspect of convergence at this level is the ability to carry voice, image, and video over networks that were originally designed to carry data traffic. Infrastructure convergence has also occurred for networks that were designed for voice traffic. For example, video, image, text, and data are routinely delivered to smart phone users over cell phone networks. In simple terms, convergence involves moving voice into a data infrastructure, integrating all the voice and data networks inside a user organization into a single data network infrastructure, and then extending that into the wireless arena. The foundation of this convergence is packet-based transmission using the Internet Protocol (IP). Convergence increases the function and scope of both the infrastructure and the application base. 1.2 A Communications Model This section introduces a simple model of communications, illustrated by the block diagram in Figure 1.3a. The fundamental purpose of a communications system is the exchange of data between two parties. Figure 1.3b 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 following are key elements of the model: • Source: This device generates the data to be transmitted; examples are telephones and personal computers. • Transmitter: Usually, the data generated by a source system are not t­ ransmitted 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 a­ nalog 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. M02_STAL4388_10_PIE_C01.indd 39 02/10/13 11:51 AM 40   Chapter 1 / Data Communications, Data Networks, and the Internet Source system Destination system Transmission system Transmitter Source Destination Receiver (a) General block diagram Workstation Modem Public telephone network Modem Server (b) Example Figure 1.3 Simplified Communications Model • 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 somewhat arbitrary: Elements 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. 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. Table 1.1 Communications Tasks Transmission system utilization Addressing Interfacing Routing Signal generation Recovery Synchronization Message formatting Exchange management Security Error detection and correction Network management Flow control M02_STAL4388_10_PIE_C01.indd 40 02/10/13 11:51 AM 1.2 / A Communications Model   41 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 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 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 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 ringing 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 might 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 seem important enough to list separately. In all communications systems, there is a potential for error; transmitted signals are distorted to some extent before reaching their destination. Error detection and correction are required in circumstances where errors cannot be tolerated. This is usually the case with data processing 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 are the related but distinct concepts of addressing and routing. When more than two devices share a transmission facility, a source system must 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 various paths may be taken. A specific route through this network must be chosen. Recovery is a concept distinct from that of error correction. Recovery techniques are needed in situations in which an information exchange, such as a d ­ atabase transaction or file transfer, is interrupted due to a fault somewhere in the system. 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. M02_STAL4388_10_PIE_C01.indd 41 02/10/13 11:51 AM 42   Chapter 1 / Data Communications, Data Networks, and the Internet Message formatting has to do with an agreement between two parties as to the form of the data to be exchanged or transmitted, such as the binary code for characters. Frequently, it is important to provide some measure of security in a data communications system. The sender of data may wish to be assured that only the intended receiver 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 actually come from the purported sender. Finally, a data communications facility is a complex system that cannot create or run itself. Network management capabilities are needed to configure the system, monitor its status, react to failures and overloads, and 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 elaborate this list of tasks to describe and encompass the entire set of activities that can be classified under data and computer communications. 1.3 Data Communications A Data Communications Model To get some flavor for the focus of Part Two, Figure 1.4 provides a new perspective on the communications model of Figure 1.3a. We trace the details of this figure using electronic mail as an example. Suppose that the input device and transmitter are components of a personal computer. The user of the PC wishes to send a message m to another user. 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 area network, a digital subscriber line, or a wireless connection, by an I/O device (transmitter), such as a local network transceiver or a DSL modem. The input data are transferred to the transmitter as a Digital bit stream Analog signal Analog signal Digital bit stream Text Text Transmitter Source 1 Input information m Figure 1.4 M02_STAL4388_10_PIE_C01.indd 42 2 Input data g(t) Transmission system 3 Transmitted signal s(t) Receiver 4 Received signal r(t) Destination 5 Output data g'(t) 6 Output information m’ Simplified Data Communications Model 02/10/13 11:51 AM 1.3 / Data Communications   43 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 will be described in Chapter 5. The transmitted signal s(t) presented to the medium is subject to a number of impairments, discussed in Chapter 3, before it reaches the receiver. Thus, the received signal r(t) may differ from s(t). The receiver will attempt to estimate the original s(t), based on r(t) and its knowledge of the medium, producing 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 to determine if an error has occurred and, if so, 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 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 telephone 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 distortion 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 improvement of signal quality. Thus, m′ is not an exact replica of m. However, the received sound message is generally comprehensible to the listener. The discussion so far does not touch on other key aspects of data communications, including data link control techniques for controlling the flow of data and detecting and correcting errors, and multiplexing techniques for transmission efficiency. The Transmission of Information The basic building block of any enterprise network infrastructure is the transmission line. Much of the technical detail of how information is encoded and ­transmitted across a line is of no real interest to the business manager. The manager is concerned with whether the particular facility provides the required capacity, with acceptable reliability, at minimum cost. However, there are certain aspects of transmission technology that a manager must understand to ask the right questions and make informed decisions. One of the basic choices facing a business user is the transmission medium. For use within the business premises, this choice is generally completely up to the business. For long-distance communications, the choice is generally but not always made by the long-distance carrier. In either case, changes in technology are rapidly changing the mix of media used. Of particular note are fiber optic transmission and wireless transmission (e.g., satellite and cellular communications). These two media are now driving the evolution of data communications transmission. The ever-increasing availability of fiber optic communication circuits is ­making channel capacity a virtually free resource. Since the early 1980s, the growth of the market for optical fiber transmission systems is without precedent. During M02_STAL4388_10_PIE_C01.indd 43 02/10/13 11:51 AM 44   Chapter 1 / Data Communications, Data Networks, and the Internet the past 10 years, the cost of fiber optic transmission has dropped by more than an order of magnitude, and the capacity of such systems has grown at almost as rapid a rate. Almost all of the long-distance telephone communications trunks within the United States and the highest speed links on the Internet consist of fiber optic cable. Because of its high capacity and its security characteristics (fiber is difficult to tap), it is becoming increasingly used within office buildings and local area networks to carry the growing load of business information. The spreading use of fiber optic cable is also spurring advancements in communication switching technologies and network management architectures. The increasing use of the second medium, wireless transmission, is a result of the trend toward universal personal telecommunications and universal access to communications. The first concept refers to the ability of a person to a single account to use any communication system anytime-anywhere, ideally globally. The second refers to the ability to use one’s preferred computing device in a wide variety of environments to connect to information services (e.g., to have a laptop, smartphone, or tablet that will work equally well in the office, on the street, and on an ­airplane, bus, or train). Today, both concepts are subsumed under the business push to support mobility. Wireless LANs have become common components of enterprise networks as well as small office/home office networks, and smartphones and tablets with wireless capabilities are rapidly becoming mainstream business user communications devices. Mobility has the potential to unleash higher performance at all business levels: personal, workgroup, and enterprise-wide. This provides compelling rationale for further business investment in wireless technologies. Despite the growth in the capacity and the drop in cost of transmission facilities, transmission services remain the most costly component of a communications budget for most businesses. Thus, the manager needs to be aware of techniques that increase the efficiency of the use of these facilities. The two major approaches to greater efficiency are multiplexing and compression. Multiplexing refers to the ability of a number of devices to share a transmission facility. If each device needs the facility only a fraction of the time, then a sharing arrangement allows the cost of the facility to be spread over many users. Compression, as the name indicates, involves squeezing the data down so that a lower-capacity, cheaper transmission facility can be used to meet a given demand. These two techniques show up separately and in combination in a number of types of communications equipment. The manager needs to understand these technologies to assess the appropriateness and cost-effectiveness of the various products on the market. Transmission and Transmission Media Information can be communicated by converting it into an electromagnetic signal and transmitting that signal over some medium, such as a twisted-pair telephone line. The most commonly used transmission media are twisted-pair lines, coaxial cable, optical fiber cable, and terrestrial and satellite microwave. The data rates that can be achieved and the rate at which errors can occur depend on the nature of the signal and the type of medium. Chapters 3 and 4 examine the significant properties of electromagnetic signals and compare the various transmission media in terms of cost, performance, and applications. Communication Techniques The transmission of information across a transmission medium involves more than simply inserting a signal on the medium. The M02_STAL4388_10_PIE_C01.indd 44 02/10/13 11:51 AM 1.4 / Networks   45 technique used to encode the information into an electromagnetic signal must be determined. There are various ways in which the encoding can be done, and the choice affects performance and reliability. Furthermore, the successful transmission of information involves a high degree of cooperation between the various components. The interface between a device and the transmission medium must be agreed on. Some means of controlling the flow of information and recovering from its loss or corruption must be used. These latter functions are performed by a data link control protocol. All these issues are examined in Chapters 5 through 7. Transmission Efficiency A major cost in any computer/communications facility is transmission cost. Because of this, it is important to maximize the amount of information that can be carried over a given resource or, alternatively, to m ­ inimize the transmission capacity needed to satisfy a given information communications requirement. Two ways of achieving this objective are multiplexing and compression. The two techniques can be used separately or in combination. Chapter 8 ­examines the three most common multiplexing techniques: frequency division, synchronous time division, and statistical time division, as well as the important c­ompression techniques. 1.4 Networks Globally, the number of Internet users is forecast to increase from approximately 2 billion in 2011 to 3 billion users in 2016. This figure is in fact misleading, as a single end user may have multiple types of devices. The estimate for 2016 is that there will be over 20 billion fixed and mobile networked devices and machineto-machine connections, up from about 7 billion devices in 2011. The increase in the number of user devices, especially broadband devices, affects traffic volume in a number of ways. It enables a user to be continuously consuming network capacity, as well as to be consuming capacity on multiple devices simultaneously. Also, different broadband devices enable different applications, which may have greater traffic generation capability. The result is that the total annual traffic generated over the Internet and other IP-based networks is forecast to rise from 372 exabytes (372 × 260 bytes) to 1.3 zettabytes (1.3 × 270 bytes) in 2016 [CISC12a]. This traffic demand imposes stiff performance requirements on communications protocols, which is previewed in Chapter 2, and on communications and computer networks. One type of network that has become commonplace is the local area network. Indeed, LANs are to be found in virtually all medium- and large-size office buildings. LANs, especially Wi-Fi LANs, are also increasingly used for small office and home networks. As the number and power of computing devices have grown, so have the number and capacity of LANs found in business networks. The development of internationally recognized standards for LANs has contributed to their proliferation in enterprises. Although Ethernet has emerged as the dominant LAN architecture, business managers still have choices to make about transmission rates (ranging from 100 Mbps to 100 Gbps) and the degree to which both wired and wireless LANs will be combined within an enterprise network. Interconnecting and managing a M02_STAL4388_10_PIE_C01.indd 45 02/10/13 11:51 AM 46   Chapter 1 / Data Communications, Data Networks, and the Internet diverse collection of local area networks and computing devices within today’s business networks presents ongoing challenges for networking professionals. A business need for a robust network to support voice, data, image, and video traffic is not confined to a single office building or LAN; today, it is an enterprisewide communication requirement. Advances in LAN switches and other data communication technologies have led to greatly increased local area network transmission capacities and the concept of integration. Integration means that the communication equipment and networks can deal simultaneously with voice, data, image, and even video. Thus, a memo or report can be accompanied by voice ­commentary, presentation graphics, and perhaps even a short video introduction, demonstration, or ­summary. Image and video services that perform adequately within LANs often impose large demands on wide area network transmission and can be costly. Moreover, as LANs become ubiquitous and as their transmission rates increase, the need for enterprise networks to support interconnections among geographically dispersed areas has increased. This, in turn, has forced businesses increase wide area network transmission and switching capacity. Fortunately, the enormous and ever-increasing capacity of fiber optic and wireless transmission services provides ample resources to meet these business data communication needs. However, the development of switching systems that are capable of responding to the increasing capacities of transmission links and business communication traffic requirements is an ongoing challenge not yet conquered. The opportunities for a business to use its enterprise network as an aggressive competitive tool and as a means of enhancing productivity and slashing costs are great. When business managers understand these technologies, they can deal effectively with data communication equipment vendors and service providers to enhance the company’s competitive position. In the remainder of this section, we provide a brief overview of various networks. Parts Three and Four cover these topics in depth. Wide Area Networks Wide area networks generally cover a large geographical area. They often require the crossing of public right-of-ways, and typically rely at least in part on circuits provided by one or more common carriers—companies that offer communication services to the general public. Typically, a WAN consists of a number of interconnected switching nodes. A trans-mission from any one device is routed through these internal nodes to the specified destination device. These nodes (including the boundary nodes) are not concerned with the content of the data; rather, their purpose is to provide a switching facility that will move the data from node to node until they reach their destination. Traditionally, WANs have been implemented using one of two technologies: circuit switching and packet switching. Subsequently, frame relay and ATM networks assumed major roles. While ATM and, to some extent frame relay, are still widely used, their use is gradually being supplanted by services based on gigabit Ethernet and Internet Protocol technologies. Circuit Switching In a circuit-switching network, a dedicated communications path is established between two stations through the nodes of the network. That M02_STAL4388_10_PIE_C01.indd 46 02/10/13 11:51 AM 1.4 / Networks   47 path is a connected sequence of physical links between nodes. On each link, a logical channel is dedicated to the connection. Data generated by the source station are transmitted along the dedicated path as rapidly as possible. At each node, incoming data are routed or switched to the appropriate outgoing channel without delay. The most common example of circuit switching is the telephone network. Packet Switching In a packet-switching network, it is not necessary to dedicate transmission capacity along a path through the network. Rather, data are sent out in a sequence of small chunks, called packets. Each packet is passed through the network from node to node along some path leading from source to destination. At each node, the entire packet is received, stored briefly, and then transmitted to the next node. Packet-switching networks are commonly used for terminal-to-computer and computer-to-computer communications. Frame Relay Packet switching was developed at a time when digital long-distance transmission facilities exhibited a relatively high error rate compared to today’s facilities. As a result, there is a considerable amount of overhead built into packetswitching schemes to compensate for errors. The overhead includes additional bits added to each packet to introduce redundancy and additional processing at the end stations and the intermediate switching nodes to detect and recover from errors. With modern high-speed telecommunications systems, this overhead is unnecessary and counterproductive. It is unnecessary because the rate of errors has been dramatically lowered and any remaining errors can easily be caught in the end ­systems by logic that operates above the level of the packet-switching logic. It is counterproductive because the overhead involved soaks up a significant fraction of the high capacity provided by the network. Frame relay was developed to take advantage of these high data rates and low error rates. Whereas the original packet-switching networks were designed with a data rate to the end user of about 64 kbps, frame relay networks are designed to operate efficiently at user data rates of up to 2 Mbps. The key to achieving these high data rates is to strip out most of the overhead involved with error control. ATM Asynchronous transfer mode, sometimes referred to as cell relay, is a culmination of developments in circuit switching and packet switching. ATM can be viewed as an evolution from frame relay. The most obvious difference between frame relay and ATM is that frame relay uses variable-length packets, called frames, and ATM uses fixed-length packets, called cells. As with frame relay, ATM provides little overhead for error control, depending on the inherent reliability of the transmission system and on higher layers of logic in the end systems to catch and correct errors. By using a fixed packet length, the processing overhead is reduced even further for ATM compared to frame relay. The result is that ATM is designed to work in the range of 10s and 100s of Mbps, and in the Gbps range. ATM can also be viewed as an evolution from circuit switching. With circuit switching, only fixed-data-rate circuits are available to the end system. ATM allows the definition of multiple virtual channels with data rates that are dynamically defined at the time the virtual channel is created. By using small, fixed-size cells, ATM is so efficient that it can offer a constant-data-rate channel even though it is M02_STAL4388_10_PIE_C01.indd 47 02/10/13 11:51 AM 48   Chapter 1 / Data Communications, Data Networks, and the Internet using a packet-switching technique. Thus, ATM extends circuit switching to allow multiple channels with the data rate on each channel dynamically set on demand. Local Area Networks As with WANs, a LAN is a communications network that interconnects a variety of devices and provides a means for information exchange among those devices. There are several key distinctions between LANs and WANs: 1. The scope of the LAN is small, typically a single building or a cluster of buildings. This difference in geographic scope leads to different technical solutions, as we shall see. 2. It is usually the case that the LAN is owned by the same organization that owns the attached devices. For WANs, this is less often the case, or at least a significant fraction of the network assets is not owned. This has two implications. First, care must be taken in the choice of LAN, because there may be a substantial capital investment (compared to dial-up or leased charges for WANs) for both purchase and maintenance. Second, the network management responsibility for a LAN falls solely on the user. 3. The internal data rates of LANs are typically much greater than those of WANs. LANs come in a number of different configurations. The most common are switched LANs and wireless LANs. The most common switched LAN is a switched Ethernet LAN, which may consist of a single switch with a number of attached devices, or a number of interconnected switches. The most common type of wireless LANs are Wi-Fi LANs. Wireless Networks As was just mentioned, wireless LANs are widely used in business environments. Wireless technology is also common for both wide area voice and data networks. Wireless networks provide advantages in the areas of mobility and ease of installation and configuration. 1.5 The Internet Origins of the Internet The Internet evolved from the ARPANET, which was developed in 1969 by the Advanced Research Projects Agency (ARPA) of the U.S. Department of Defense. It was the first operational packet-switching network. ARPANET began operations in four locations. Today the number of hosts is in the hundreds of millions, the number of users in the billions, and the number of countries participating nearing 200. The number of connections to the Internet continues to grow exponentially. The network was so successful that ARPA applied the same packet-switching technology to tactical radio communication (packet radio)…

Ethernet: wireless networks.

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