_hBHvnE
`
`Ex.1011
`
`Ex. 1011
`(Part I)
`(Partl)
`
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`
`THE WILLIAM STALLINCS BOOKS
`
`ON COMPUTER AND DATA
`
`COMMUNICATIONS TECHNOLOGY
`
`DATA AND COMPUTER COMMUNICATIONS, THIRD EDITION
`
`A comprehensive survey that has become the standard in the field, covering
`four main areas: (1) data communications, including transmission, media, signal
`encoding, link control, and multiplexing; (2) communication networks, includ-
`ing circuit— and packet-switched, local, packet radio, and satellite; (3) commu-
`nications architecture,
`including the OSI model and related protocols; and
`(4) ISDN and broadband ISDN.
`
`LOCAL AND METROPOLITAN AREA NETWORKS,
`FOURTH EDITION
`
`An in-depth presentation of the technology and architecture of local and met-
`ropolitan area networks. Covers topology, transmission media, medium access
`control, standards, internetworking, and network management. Provides an
`up-to-date coverage of LAN/‘MAN standards.
`
`ISDN AND BROADBAND ISDN: SECOND EDITION
`
`An in-depth presentation of the technology and architecture of integrated ser-
`vices digital networks (ISDN). Covers the integrated digital network (IDN),
`ISDN services, architecture, signaling system no. 7 (S87) and detailed coverage
`of the 1988 and 1990 CCITT standards. This new edition also provides coverage
`of frame relay and broadband ISDN topics including ATM and SONET.
`
`COMPUTER ORGANIZATION AND ARCHITECTURE,
`THIRD EDITION
`
`A unified view of this broad field. Covers fundamentals such as CPU, control
`unit, microprogramming, instruction set, I/O, and memory. Also covers ad-
`vanced topics such as RISC, superscalar, and parallel organization.
`
`BUSINESS DATA COMMUNICATIONS
`
`A comprehensive presentation of data communications and telecommunications
`from a business perspective. Covers voice, data, image, and video communica-
`tions and applications technology and includes a number of case studies.
`
`Page 2 of 569
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`

`
`THE WILLIAM STALLINGS BOOKS
`
`ON COMPUTER AND DATA
`
`COMMUNICATIONS TECHNOLOGY
`
`OPERATING SYSTEMS
`
`A state-of-the art survey of operating system principles. Covers fundamental
`technology as well as contemporary design issues, such as threads, real-time
`systems, multiprocessor scheduling, distributed systems, and security.
`
`HANDBOOK OF COMPUTER-COMMUNICATIONS
`STANDARDS VOLUME 1
`
`THE OPEN SYSTEMS INTERCONNECTION IQSI) MODEL
`AND OSI-RELATED STANDARDS, SECOND EDITION
`
`A description of the master plan for all computer-communications standards:
`the OSI model. The book also provides a detailed presentation of OSI-related
`standards at all 7 layers, including HDLC, X.25, ISO intemet, ISO transport,
`ISO session, ISO presentation, Abstract Syntax ONE (ASN.1), and Common ap-
`plication service elements (CASE).
`
`HANDBOOK OF COMPUTER-COMMUNICATIONS
`STANDARDS VOLUME 2
`
`LOCAL AREA NETWORK STANDARDS, SECOND EDITION
`
`A detailed examination of all current local network standards, including logical
`link control (LLC, IEEE 802.2), CSMA/CD (IEEE 802.3), token bus (IEEE 802.4),
`token ring (IEEE 802.5), and fiber distributed data interface (FDDI, ANS X3T9.5).
`
`HANDBOOK OF COMPUTER-COMMUNICATIONS
`STANDARDS VOLUME 3
`
`THE TCP/IP PROTOCOL SUITE, SECOND EDITION
`
`A description of the protocol standards that are mandated on all DOD computer
`procurements and are becoming increasingly popular on commercial local net-
`work products, including TCP, IP, FTP, SMTP, and TELNET. The network man~
`agement standards, SNMP and CMOT, are also presented.
`
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`

`
` Localand
`Metropolitan
`Area Networks
`
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`
`Localand
`
`Metropolitan
`Area Networks
`
`FOURTH EDITION
`
`William Stallings
`
`MACMILLAN PUBLISHING COMPANY
`New York
`
`MAXWELL MACMILLAN CANADA
`Toronto
`
`MAXWELL MACMILLAN INTERNATIONAL
`
`New York Oxford Singapore Sydney
`
`Page 5 of 569
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`

`
`Editor: Iohn Griffin
`Production Supervisor: John Travis
`Production Manager: Roger Vergnes
`Text Designer: Natasha Sylvester
`Cover Designer: Robert Vega
`
`This book was set in Palatino by Compset, Inc., and printed and bound by
`Book Press. The cover was printed by Lehigh Press.
`
`Copyright © 1993 by Macmillan Publishing Company, a division of Macmillan, Inc.
`
`Printed in the United States of America
`
`All rights reserved. No part of this book may be reproduced or
`transmitted in any form or by any means, electronic or mechanical,
`including photocopying, recording, or any information storage and
`retrieval system, without permission in writing from the publisher.
`
`Earlier editions entitled Local Networks, copyright © 1984, 1987 and 1990 by Macmillan
`Publishing Company.
`
`Macmillan Publishing Company is part
`of the Maxwell Communication Group of Companies.
`Macmillan Publishing Company
`866 Third Avenue, New York, New York 10022
`
`Maxwell Macmillan Canada, Inc.
`1200 Eglinton Avenue East
`Suite 200
`Don Mills, Ontario MSC 3N1
`
`Library of Congress Cataloging-in-Publication Data
`Stallings, William.
`Local and metropolitan area networks I William Stallings. —— 4th
`ed.
`
`cm.
`p.
`Rev. ed. of: Local networks. 3rd ed. © 1990.
`Includes bibliographical references and index.
`ISBN 0-02-415465-2
`
`2. Metropolitan area
`1. Local area "networks (Computer networks)
`networks (Computer networks)
`I. Stallings, William. Local
`networks.
`II. Title.
`TK5105.7.S77
`1993
`004.6-—d.-:20
`
`92-16096
`CIP
`
`Prini1'ng:2345678
`
`Year:345678901
`
`Page 6 of 569
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`

`
`To my wife, Tricia
`
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`

`
`
`
`Preface
`
`The local area network (LAN) has come to play a central role in infor-
`mation distribution and office functioning within businesses and other
`organizations. The major factor driving the widespread introduction of
`the LAN has been the proliferation of small computer systems, espe-
`cially personal computers, but also including workstations and mini-
`computers.
`With the dropping price of LAN hardware and software, LANs have
`become more numerous and larger, and they have taken on more and
`' more functions within the organization. The upshot is that the LAN,
`once installed, quickly becomes almost as essential as the telephone sys-
`tem. At the same time, there is a proliferation of LAN types and options
`and a need to interconnect a number of LANs at the same site and with
`LANs at other sites. This has led to the development of LANs of higher
`and higher data rates and the relatively recent introduction of the met-
`ropolitan area network (MAN).
`
`Objectives
`
`This book focuses on the broad and evolving field of local and metro-
`politan area networks. The aim of the text is to provide a reasoned bal-
`ance among breadth, depth, and timeliness. The book emphasizes
`topics of fundamental importance concerning the technology and archi-
`tecture of these networks. Certain key related areas, such as perfor-
`
`vii
`
`
`
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`
`vi ii
`
`Preface
`
`mance, internetworking, and network management are also treated in
`some detail.
`
`The book explores the key topics in the field in the following general
`, categories:
`
`- Technology and architecture: There is a small collection of ingredients
`that serves to characterize and differentiate local and metropolitan
`area networks, including transmission medium, topology, commu-
`nication protocols, switching technique, and hardware/software in-
`terface.
`
`- Network type: It is convenient to classify the networks covered in this
`book into three types, based partly on technology and partly on
`application: These are local area network (LAN), metropolitan area
`network (MAN), and digital switch/digital private branch exchange
`(PBX).
`- Design approaches: The book examines alternative design choices and
`assesses their relative merits.
`
`Intended Audience
`
`This book is intended for a broad range of readers interested in local
`networks:
`"
`
`- Students and p'rofess1'onals in computer science and data communications.’
`The book is intended as both a textbook for study and a basic ref-
`erence volume for this exciting area within the broader fields of
`computer science and data communications.
`- Local network designers and implementors: The book discusses the crit-
`ical design issues and illustrates alternative approaches to meeting
`user requirements.
`- Local network customers and system managers: The book alerts the
`reader to some of the key issues and tradeoffs, and what to look for
`in the way of network services and performance.
`
`The book is intended for both an academic and a professional audience.
`As a textbook, it is intended as a one-semester course. It covers much
`of the material in the Computer Communication Networks course of
`The oint ACMIIEEE Computing Curricula 1991.
`The book also serves as a basic reference volume and is suitable for
`self-study. For the reader with little or no background in data commu-
`nications, a brief primer is included.
`
`Plan of the Text
`
`The book is organized to clarify both the unifying and the differentiating
`concepts that underlie this field. The organization of the chapters is as
`follows:
`'
`
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`Preface
`
`_
`
`ix
`
`1. Introduction: The chapter defines the term local network and looks
`at some of the applications and benefits.
`2. Topics in Data Communications and Computer Networking: This nec-
`essarily brief survey explains the relevant concepts used through-
`out the book.
`
`3. Overview of LAN/MAN Technology: Introduces the key elements of
`transmission medium and topology. A classification of networks
`into LANs, MANS, and WANS is developed.
`4. Topologies and Transmission Media for LANs and MANS: Examines
`the design issues relating to the implementation of LANs and
`MANs, with emphasis on the topology and transmission medium
`alternatives.
`5. Local Area Network Architecture: Describes the logical link control
`and medium access control architecture of LANs. LAN standards
`are also described.
`
`6. Metropolitan Area Network Architecture: Treats the two most impor-
`tant MANS: FDDI and IEEE 802.6.
`
`7. Circuit-Switched Local Networks: Networks in this category consti-
`tute the major alternative to LANS for meeting general local in-
`terconnection needs. The category includes the data-only digital
`switch and the voice/data digital private branch exchange (PBX).
`8. The Network Interface: The nature of the interface between an at-
`tached device and a LAN or MAN is an important design issue.
`This chapter explores some alternatives.
`9. LAN/MAN Performance: This chapter gives some insight into the
`performance problems and the differences in performance of var-
`ious LANs and MANs.
`
`10.
`
`lnternetworking: In the majority of cases, LANs will be connected
`in some fashion to other networks, either by means of other
`LANS, by way of MANS, or using wide-area networks. The key
`alternatives of bridge and router are explored.
`11. Network Management: Network management tools and systems
`are indispensable for LANs. and MANs. This chapter explores the
`types of systems that are available and examines the standards
`developed for general network management and LAN/MAN
`management.
`
`In addition, the book includes an extensive glossary, a list of fre-
`quently—used acronyms, and a bibliography. Each chapter includes prob-
`lems and suggestions for further reading.
`-
`Throughout, there is a heavy emphasis on standards, including stan-
`dards based on the Open Systems Interconnection (OSI) model and spe-
`cific LAN and MAN standards, such as IEEE 802 and FDDI. This
`
`emphasis reflects the growing importance of such standards in defining
`the available products and future research directions in this field.
`
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`
`
`X Preface
`
`Related Materials
`
`The author has produced other material that may be of interest to stu-
`dents and professionals. Advances in Local and Metropolitan Area Network
`Technology (1993, IEEE Computer Society Press, 10662 Los Vaqueros Cir-
`cle, P.O. Box 3014, Los Alamitos, CA 90720, telephone 714-821-8380) is
`a companion to this text, and follows the same topical organization. It
`contains reprints of many of the key references used herein.
`A set of videotape courses specifically designed for use with this book
`is availabie from The Media Group, Boston University, 565 Common-
`wealth Avenue, Boston, MA 02215; telephone (617) 353-3227.
`Data and Computer Communications, Third Edition (Macmillan, 1991)
`covers fundamental concepts in the areas of data transmission, com-
`munication networks, and computer-communications protocols. ISDN
`and Broadband ISDN, Second Edition (Macmillan, 1992) covers the con-
`cepts and technology of integrated services digital networks (ISDN) and
`broadband ISDN, which are all—dig-ital networks gradually being in-
`troduced to replace existing wide—area networks. Networking Standards
`(Addison-Wesley, 1993) covers the leading~edge standards that are de-
`fining the networks and distributed applications recently introduced or
`currently under development.
`
`The Fourth Edition
`
`I began work on the first edition of this book in 1982. At the time of its
`publication, it was the only book-length technical treatment of LANS
`(and remains the only textbook on the subject). Little did I anticipate
`that it would still be going strong over a decade later. To paraphrase a
`recent Oscar—winner, you like this book! You really like it! Any author is
`bound to feel a sense of pride and satisfaction on being asked to produce
`a fourth edition of a book that covers such a fast-moving field as this
`one. The book has withstood the test of time, and its success confirms
`that the basic organization and emphasis of the book is sound. How-
`ever, because the field is fast-moving, each new edition requires a major
`revision to keep up.
`This edition is no exception. The revision in this case even extends to
`the title, which now includes the phrase Metropolitan Area Networks.
`The inclusion of MANs is dictated by three developments:
`
`1. The fiber distributed data interface (FDDI), which is generally re-
`ferred to as a local area network (LAN), is finding increasing ap-
`plication as a backbone MAN, thanks to the increased demand for
`this service and the maturing of bridge and router technologies.
`2. FDDI—ll has been adopted. This revision of FDDI addresses some
`of the integrated—voice data requirements of a MAN.
`
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`Preface
`
`3. After an almost uncountable number of false starts, the IEEE 802.6
`
`committee has finally settled on a MAN standard, and that stan-
`dard has received broad industry and customer support.
`
`So the inclusion of MANs is a major new addition to the book. An-
`other significant change in this edition is the revision of the chapter on
`internetworking. The material on bridge routing standards has been ex-
`panded. The spanning tree and source routing approaches, introduced
`in the third edition, receive expanded coverage. In addition, a new
`bridge standard, SRT,
`is introduced. In the area of routing, the new
`routing protocol standards, ES-IS and IS-IS are examined.
`A final major change in this edition is the expansion of the coverage
`on network management to an entire chapter. The chapter covers the
`basis technology of network management systems and local network
`management. In addition, the ISO network management standards and
`the IEEE 802 LAN/MAN management standards are covered.
`In addition to these major changes, there have been expansions and
`updates in every chapter. To give some feel for the overall scope of this
`revision, approximately 33% of the tables, 30% of the figures, and 24%
`of the references in this edition are new. All in all, this fourth edition
`
`constitutes a major revision. I have tried in a balanced manner to pro-
`vide a comprehensive survey of the technology and architecture of local
`and metropolitan area networks.
`
`Acknowledgment
`
`My association with Macrnillan's college division now stretches back
`over more than a decade. I have always had the strong and enthusiastic
`support of the division's staff and am grateful for all the support and
`encouragement I have received over the years. Two people in particular
`I would like to thank.
`
`In a changing world, it is remarkable that the production editor for
`every one of my dozen books, going back to the first edition of this
`book, has been the same man: Iohn Travis. Over the years, Iohn has
`caught many errors, both editorial and—more important——technical,
`and he has managed to bring every single one of these books out on
`time. Quite an achievement.
`My current, and I hope permanent, editor is ]ohn Griffin. His feel for
`both the technical and marketing side of the business has helped direct
`my writing into the most fruitful channels.
`Of course, I have dealt with many other people in the College Divi-
`sion over the years. The names have changed from time to time, but the
`supportive atmosphere and the professionalism have not.
`
`W. S.
`
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`
`Contents
`
`Preface
`
`CHAPTER 7
`
`Introduction
`
`1.1 LANs, MANS, and WANS
`1.2 Benefits and Pitfalls
`6
`
`I
`
`7
`1.3 Applications
`1.4
`Information Distribution
`1.5 Outline of the Book
`18
`1.6 Recommended Reading 2?
`1.7 Problems
`21
`
`16
`
`CHAPTER 2
`
`Topics in Data Communications and Computer Networking
`
`23
`
`23
`2.1 Data Communications Concepts
`2.2 Communication Switching Techniques
`2.3 Computer Networking 48
`2.4 Recommended Reading 67
`2.5 Problems
`68
`
`37
`
`Appendix 2A: The Cyclic Redundancy Check 69
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`Contents
`
`CHAPTER 3..M_.M_.M.
`
`Overview of LAN/MAN Technology
`
`73
`
`73
`3.1 Topologies
`3.2 Transmission Media
`
`78
`
`3.3 Relationship Between Medium and Topology
`3.4 Classes of Networks
`95
`
`90
`
`35 Recommended Reading
`3.6 Problems
`I00
`
`700
`
`CHAPTER 4
`
`Topologies and Transmission Media for LANs and MANS
`
`103
`
`703
`
`4.1 Metallic Media: Bus/Tree Topology
`4.2 Metallic Media: Star Topology
`120
`4.3 Metallic Media: Ring Topology
`723
`4.4 Optical Fiber Star
`133
`4.5 Optical Fiber Ring
`738
`4.6. Optical Fiber Bus
`739
`4.7 Recommended Reading
`4.8 Problems
`142
`
`142
`
`Appendix 4A: Characteristic Impedance
`Appendix 4B: Decibels
`146
`Appendix 4C: Scrambling and Descrambling
`
`143
`
`147
`
`CHAPTER 5
`
`Local Area Network Architecture
`
`5.1 LAN Protocols
`
`751
`
`5.2 Link Layer Protocol for LANS 1 T5 7
`53 Medium Access Control—Bus/Tree
`
`170
`
`5.4 Medium Access Control”-Ring
`5.5 Recommended Reading 205
`5.6 Problems 206
`
`193
`
`Appendix 5A: IEEE 802 Standards 207
`Appendix 5B: Service Primitives and Parameters 215
`
`CHAPTER 6M.
`
`Metropolitan Area Network Architecture
`6.1 FDDI 219
`6.2 FDDLII
`241
`6.3
`IEEE802.6 247
`
`6.4 Recommended Reading 272
`6.5 Problems
`273
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`K Contents
`
`CHAPTER 7
`
`Circuit-Switched Local Networks
`
`7.1 Star Topology Networks 275
`7.2 Digital Switching Concepts
`27 7
`291
`7.3 Digital Data Switching Devices
`7.4 The Digital Private Branch Exchange 297
`7.5 Digital PBX Versus LAN 313
`7.6 Recommended Reading 315
`7.7 Problems
`315
`
`CHAPTER 8
`
`The Network Interface
`
`319
`8.1 The Requirement
`8.2 Packet-Switched Interfacing 322
`8.3 The Device/NIU Interface 327
`
`8.4 Terminal Handling for LAN5
`8.5 Circuit—Switched Networks
`
`340
`353
`
`354
`8.6 Analog Devices
`8.7 Recommended Reading 354
`8.8 Problems
`354
`
`CHAPTER 9
`
`LAN/MAN Performance
`
`9.1 LAN/MAN Peformance Considerations
`9.2 LAN Performance 368
`9.3 MAN Performance 395
`9.4 E'nd—to-End Performance
`
`403
`
`358
`
`9.5 Recommended Reading 407
`9.6 Problems 409
`
`
`
`CHAPTER 10 7
`
`lnternetworking
`
`412
`10.1 Bridges
`10.2 Routing with Bridges 417
`10.3 Routers 450
`
`10.4 Routing with Routers 464
`10.5 Recommended Reading 474
`10.6 Problems
`474
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`CHAPTER 11
`
`Network Management
`
`478
`
`11.1 Network Management Requirements
`11.2 Network Management Systems
`483
`11.3 OSI Network Management 485
`11.4 LAN-Specific Network Management 496
`11.5
`IEEE 802 LAN/MAN Management 502
`11.6 FDDI Management
`5 10
`11.7 Recommended Reading 5 74
`11.8 Problems 514
`
`Glossary 5 15
`References 523
`Index 539
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`Metropolitan
`Area Networks
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`CHAPTER 1
`
`Introduction
`
`‘I .1
`
`LANS, MANS, AND WANS
`
`For businesses, government agencies, universities, and other organiza-
`tions, data communications networks have become indispensable. Of
`most importance are networks that interconnect equipment within a sin-
`gle building or a group of buildings. For want of a better term, we will
`refer to such networks as local networks. In fact, this book is concerned
`
`with three types of local networks: local area networks (LANs), metro-
`politan area networks (MANS), and circuit-switching local networks. Be-
`fore defining these terms, we need to understand the trends responsible
`for the importance of local networks.
`Of most importance is the dramatic and continuing decrease in com-
`puter hardware costs, accompanied by an increase in computer hard-
`ware capability. Today's microprocessors have speeds, instruction sets,
`and memory capacities comparable to the most powerful minicornputers
`of a few years ago. This trend has spawned a number of changes in the
`way information is collected, processed, and used in organizations.
`There is increasing use of small, single-function systems, such as word
`processors and small business computers, and of general—purpose mi-
`crocomputers, such as personal computers and Unix—based rnultiuser
`workstations. These small, dispersed systems are more accessible to the
`
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`
`Chapter I
`
`Introduction
`
`user, more responsive, and easier to use than large central time-sharing
`systems.
`All of these factors lead to an increased number of systems at a single
`site: office building, factory, operations center, and so on. At the same
`time there is likely to be a desire to interconnect these systems for a
`variety of reasons, including:
`
`- To share and exchange data between systems
`- To share expensive resources
`
`The ability to exchange data is a compelling reason for interconnec-
`tion. Individual users of computer systems do not work in isolation and
`will want to retain some of the benefits provided by a central system.
`These include the ability to exchange messages with other users, the
`ability to access data from several sources in the preparation of a docu-
`ment or for an analysis, and the opportunity for multiple users to share
`information in a common file.
`
`To appreciate the second reason, consider that although the cost of
`data processing hardware has dropped, the cost of essential electrome-
`chanical equipment, such as bulk storage and line printers, remains
`high. In the past, with a centralized data processing facility, these de-
`vices could be attached directly to the central host computer. With the
`dispersal of computer power, these devices must somehow be shared.
`
`A Definition of Local Networks
`
`We will elaborate on these and other reasons later in this chapter. For
`now, the discussion above should be enough to motivate the following
`definition of a local network:
`
`A local network is a communications network that provides interconnec-
`tion of a variety of data communicating devices within a small area.
`
`There are three elements of significance in this definition. First, a local
`network is a communications network. That is, it is a facility for moving
`bits of data from one attached device to another. The application-level
`software and protocols that are required for attached devices to function
`cooperatively are beyond the scope of this book. As a corollary to this
`definition, note that a collection of devices interconnected by individual
`point-to-point links is not included in the definition or in this book.
`Second, we interpret the phrase data communicating devices broadly, to
`include any device that communicates over a transmission medium. Ex-
`amples:
`
`- Computers
`- Terminals
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`l.ANs, MANS, and WAN5
`
`Peripheral devices
`Sensors (temperature, humidity, security alarm sensors)
`Telephones
`- Television transmitters and receivers
`- Facsimile
`
`Of course, not all types of local networks are capable of handling all
`of these devices.
`
`Third, the geographic scope of a local network is small. The most
`common occurrence is a network that is confined to a single building.
`Networks that span several buildings, such as on a college campus or
`military base, are also common. A borderline case is a network with a
`radius of a few tens of kilometers. With appropriate technology, such a
`system will behave like a local network.
`Another element that could be added to the definition is that a local
`
`network is generally privately owned rather than a public or commer-
`cially available utiiity. Indeed, typically, a single organization will own
`both the network and the attached devices.
`
`Some of the typical characteristics of local networks are:
`
`- High data rates (0.1 to 100 Mbps)
`- Short distances (0.1 to 25 km)
`- Low error rate (1U'3 to 10"‘)
`
`The first two parameters serve to differentiate local networks from
`two cousins: multiprocessor systems and wide-area networks.
`Other distinctions can be drawn between local networks and their
`
`two cousins, and these have a significant impact on design and opera-
`tion. Local networks generally experience significantly fewer data trans-
`mission errors and significantly lower communications costs than those
`of long-haul networks. Cost—perfor1nance trade—offs are thus signifi-
`cantly different. Also, because local networks are generally owned by
`the same organization as the attached devices, it is possible to achieve
`greater integration between the network and the devices; this topic is
`explored in Chapter 8.
`A distinction between local networks and multiprocessor systems is
`the degree of coupling. Multiprocessor systems are tightly coupled, usu-
`ally have some central control, and completely integrate the communica-
`tions function. Local networks tend to exhibit the opposite characteristics.
`
`Types of Local Networks
`
`There are two basic types of local networks: those based on circuit
`switching and those based on a technology referred to as packet broad-
`casting (Figure 1.1). We will define the terms circuit switching and packet
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`Chapter 1
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`Introduction
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`PACKET
`BROADCASTING
`NETWORK
`
`Transmission by
`one is heard by all.
`
`FIGURE 1.1 Transmission Methods for Local
`Networks
`
`broadcasting in more detail in Chapter 2. For now, a brief definition of
`each should suffice:
`
`- Packet broadcasting: Devices share a communications network in
`which a transmission from any one device is heard by all other de-
`vices. Data to be transmitted are broken up into small blocks, called
`packets. Packets include both user data and control information that
`indicate the destination of the data. Each packet is sent onto a net-
`work and may be received by all other devices on the network.
`Circuit switching. The network consists of a central switch to which
`all devices attach. Two devices communicate by setting up a circuit
`through the switch. The circuit consists of a path and dedicated
`resources for transferring data between the two devices through the
`switch.
`
`The key to packet broadcasting is the use of a transmission medium
`shared by a number of devices. An early example of the use of a shared
`transmission medium is the multidrop line. The multidrop line, how-
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`1.1
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`LANS, MANS, and WANS
`
`is used to permit communication between one primary station
`ever,
`(usually a host computer) and a number of secondary stations (usually
`terminals). Communication on the rnultidrop line is controlled by the
`primary, and secondary“-to-secondary exchange is generally not al-
`lowed. For a local network, peer communication among a number of
`cooperating devices is required. This type of local network is generally
`referred to as a local area network (LAN) and has the following key
`characteristics:
`
`- A transmission medium is shared among the attached devices.
`- Transmission is in the form of packets.
`- A transmission from any one station is received by all other stations
`(hence the term packet broadcasting).
`- There is no master station; rather, all of the stations cooperate to
`assure orderly use of the transmission medium.
`
`In recent years, a new type of network, referred to as a metropolitan
`area network (MAN), has been developed. A metropolitan area network
`shares the characteristics listed above with the LAN; the difference is
`that the MAN covers larger distances and, generally, operates at higher
`data rates.
`
`The most familiar example of a circuit—switching local network is the
`private branch exchange (PBX). The PBX was originally developed to
`provide an on—premise telephone exchange system. The voice PBX pro-
`vides a point of interconnection for extension telephones within the of-
`fice and a trunk connection to the nearest central office telephone
`exchange. Calls within the office are made through the PBX; calls out-
`side the office are directed by the PBX to the public telephone network
`or a leased line.
`
`With the advent of digital technology, the digital PBX has appeared
`on the scene and now dominates the PBX market. The digital PBX han-
`dles all signals internally as digital signals but still uses circuit-switching
`technology. The digital PBX is suited to handle both voice and data con-
`nections.
`
`A final example of a local network that employs circuit switching is
`the digital data switch. The digital data switch is designed specifically
`to deal with data rather than voice. The main difference between the
`
`digital data switch and the digital PBX is that the former does not con-
`tain many of the call processing features normally found in the digital
`PBX, such as call forwarding and camp-on.
`All the above networks can be distinguished from wide-area net-
`works (WANS). As the name implies, WANS are networks that cover
`substantial distances. Public telephone networks and packet—switching
`networks are examples of WANs.
`The focus of this book is on LANS and MANS, with a chapter devoted
`to circuit—switching local networks.
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`Chapter 1
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`Introduction
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`1.2
`
`BENEFITS AND PITFALLS
`
`Table 1.1 lists some of the major benefits of a local network. Whether
`these are realized or not, of course, depends on the skill and wisdom of
`those involved in selecting and managing the local network.
`One of the most important potential benefits of a local network relates
`to system evolution. In a nonnetworked installation such as a time—shar-
`ing system, all data processing power is in one or a few systems. In
`order to upgrade hardware, existing applications software must be
`either converted to new hardware or reprogrammed, with the risk of
`error in either case. Even adding new applications on the same hard-
`ware, or enhancing those that exist, involves the risk ‘of introducing er-
`rors and reducing the performance of the entire system. With a local
`network it is possible to gradually replace applications or systems,
`avoiding the ”a1l-or-nothing” approach. Another facet of this capability
`is that old equipment can be left in the system to run a single application
`if the cost of moving that application to a new machine is not justified.
`A local network tends to improve the reliability, availability, and sur-
`vivability of a data processing facility (see Section 12.2.). With multiple
`interconnected systems, the loss of any one system should have mini-
`mal impact. Further, key systems can be made redundant so that other
`systems can quickly take up the load after a failure.
`We have already mentioned resource sharing. This includes not only
`expensive peripheral devices, but data. Data may be housed and con-
`
`TABLE 1.1 Benefits and Pitfalls of Local Networks
`Potential Benefits
`
`System evolution: incremental changes with contained impact
`Reliability/availabiiityfsurvivability: multiple interconnected systems disperse
`functions and provide backup capability
`Resource sharing: expensive