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`
`“Computer Networks=
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`Major Data UABv.Bright Data Ltd.
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`Computer Networks
`Fourth Edition
`
`ISBN
`
`BN
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`QO-13
`O-13-Obb102-4
`
`|
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`9"7901 30"661028
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`|
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`

`

`Computer Networks
`Fourth Edition
`
`Andrew S. Tanenbaum
`
`Vrije Universiteit
`Amsterdam, The Netherlands
`
`el
`PTR—
`
`Prentice Hall PTR
`UpperSaddle River, NJ 07458
`www.phptr.com
`
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`
`Library of Congress Cataloging-in-Publication Data
`
`Tanenbaum, Andrew S.
`Computer networks / Andrew S. Tanenbaum.--4th ed.
`p.cm.
`Includes bibliographical references.
`ISBN 0-13-066102-3
`1. Computer networks.
`TK5105.5 .T36 2002
`004.6--dce21
`
`I. Title.
`
`2002029263
`
`Editorial/production supervision: Patti Guerrieri
`Coverdesign director: Jerry Votta
`Coverdesigner: Anthony Gemmellaro
`Coverdesign: Andrew S. Tanenbaum
`Art director: Gail Cocker-Bogusz
`Interior Design: Andrew S. Tanenbaum
`Interior graphics: Hadel Studio
`Typesetting: Andrew S. Tanenbaum
`Manufacturing buyer: Maura Zaldivar
`Executive editor: Mary Franz
`Editorial assistant: Noreen Regina
`Marketing manager: Dan DePasquale
`
`© 2003, 1996 Pearson Education, Inc.
`atl
`Publishing as Prentice Hall PTR
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`SEC. 1.1
`
`USES OF COMPUTER NETWORKS
`
`a
`
`Although this book primarily focuses on networks, many of the topics are also
`important in distributed systems. For more information aboutdistributed systems,
`see (Tanenbaum and Van Steen, 2002).
`
`1.1 USES OF COMPUTER NETWORKS
`
`Before we start to examine the technical issues in detail, it is worth devoting
`some time to pointing out why people are interested in computer networks and
`what they can be used for. After all, if nobody were interested in computer net-
`works, few of them would be built. We will start with traditional uses at com-
`panies and for individuals and then move on to recent developments regarding
`mobile users and home networking.
`
`1.1.1 Business Applications
`
`Many companies have a substantial number of computers. For example, a
`company may have separate computers to monitor production, keep track of
`inventories, and do the payroll.
`Initially, each of these computers may have
`worked in isolation from the others, but at some point, management may have
`decided to connect them to be able to extract and correlate information about the
`entire company.
`Put in slightly more general form, the issue here is resource sharing, and the
`goal is to make all programs, equipment, and especially data available to anyone
`on the network without regard to the physical location of the resource and the
`user. An obvious and widespread example is having a group of office workers
`share a common printer. None of the individuals really needs a private printer,
`and a high-volume networkedprinter is often cheaper, faster, and easier to main-
`tain than a large collection of individual printers.
`However, probably even more important than sharing physical resources such
`as printers, scanners, and CD burners,
`is sharing information. Every large and
`medium-sized company and many small companiesare vitally dependent on com-
`puterized information. Most companies have customer
`records,
`inventories,
`accounts receivable, financial statements,
`tax information, and much more on-
`line. If all of its computers went down, a bank could not last more than five min-
`utes. A modern manufacturing plant, with a computer-controlled assembly line,
`would not last even that long. Even a small travel agency or three-person law
`firm is now highly dependent on computer networks for allowing employees to
`access relevant information and documents instantly.
`For smaller companies, all the computers are likely to be in a single office or
`perhaps a single building, but for larger ones, the computers and employees may
`be scattered over dozens of offices and plants in many countries. Nevertheless, a
`sales person in New York might sometimes need access to a product inventory
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`wa
`
`4
`
`INTRODUCTION
`
`CHAP.
`
`1
`
`In other words, the mere fact that a user happens to be
`database in Singapore.
`15,000 km away from his data should not prevent him from using the data as
`though they were local. This goal may be summarized by saying that it is an
`attempt to end the “tyranny of geography.”
`In the simplest of terms, one can imagine a company’s information system as
`consisting of one or more databases and some number of employees who need to
`access them remotely.
`In this model, the data are stored on powerful computers
`called servers. Often these are centrally housed and maintained by a system
`administrator.
`In contrast, the employees have simpler machines, called clients,
`on their desks, with which they access remote data, for example,
`to include in
`spreadsheets they are constructing.
`(Sometimes we will refer to the human user
`of the client machine as the “client,” but it should be clear from the context
`whether we mean the computerorits user.) The client and server machines are
`connected by a network, asillustrated in Fig. 1-1. Note that we have shown the
`network as a simple oval, without any detail. We will use this form when we
`mean a network in the abstract sense. When more detail is required, it will be
`provided.
`
`
`
`
`
`
` Hing
`
`Figure 1-1. A network with twoclients and one server.
`
`It is widely used
`This whole arrangementis called the client-server model.
`and forms the basis of much network usage.
`It is applicable when the client and
`server are both in the same building (e.g., belong to the same company), but also
`whenthey are far apart. For example, when a person at home accesses a page on
`the World Wide Web, the same model is employed, with the remote Webserver
`being the server and the user’s personal computerbeing the client. Under most
`conditions, one server can handle a large numberofclients.
`If we look at the client-server model in detail, we see that two processes are
`involved, one on the client machine and one on the server machine. Communica-
`tion takes the formofthe client process sending a message overthe network to the
`server process. The client process then waits for a reply message. Whenthe serv-
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`SEC. 1.1
`
`USES OF COMPUTER NETWORKS
`
`5
`
`er process gets the request, it performs the requested work or looks up the re-
`quested data and sends back a reply. These messages are shown in Fig. 1-2.
`Client machine
`Server machine
`
`Request
`
`__———~—_
`
`
`
`
`+—€ NetworkSS
`
`
`Client process
`
`Server process
`
`Reply
`
`Figure 1-2. The client-server model involves requests and replies.
`A second goalof setting up a computer network has to do with people rather
`than information or even computers. A computer network can provide a powerful
`communication medium among employees. Virtually every company that has
`two or more computers now hase-mail (electronic mail), which employees gen-
`erally use for a great deal of daily communication.
`In fact, a common gripe
`around the water cooler is how much e-mail everyone has to deal with, muchofit
`meaningless because bosses have discovered that they can send the same (often
`content-free) messageto all their subordinates at the push of a button.
`But e-mail is not the only form of improved communication made possible by
`computer networks. With a network, it is easy for two or more people who work
`far apart to write a report together. When one worker makes a change to an on-
`line document,
`the others can see the change immediately,
`instead of waiting
`several days for a letter. Such a speedup makes cooperation among far-flung
`groups of people easy whereit previously had been impossible.
`Yet another form of computer-assisted communication is videoconferencing.
`Using this technology, employees at distant locations can hold a meeting, seeing
`and hearing each other and even writing on a shared virtual blackboard. Video-
`conferencing is a powerful
`tool for eliminating the cost and time previously
`devoted to travel.
`It is sometimes said that communication and transportation are
`having a race, and whicheverwins will makethe otherobsolete.
`A third goal for increasingly many companiesis doing business electronically
`with other companies, especially suppliers and customers. For example, manufac-
`turers of automobiles, aircraft, and computers, among others, buy subsystems
`from a variety of suppliers and then assemble the parts. Using computer net-
`works, manufacturers can place orders electronically as needed. Being able to
`place orders in real time (i.e., as needed) reduces the need for large inventories
`and enhancesefficiency.
`A fourth goal that is starting to become more importantis doing business with
`consumers over the Internet. Airlines, bookstores, and music vendors have
`discovered that many customers like the convenience of shopping from home.
`Consequently, many companies provide catalogs of their goods and services on-
`line and take orders on-line. This sector is expected to grow quickly in the future.
`It is called e-commerce(electronic commerce).
`
`—————SSS
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`Wii alli.AAU
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`alle
`
`FOURTH EDITION
`
`
`Computer Networks
`
`ANDREW S. TANENBAUM
`
`The world’s leading introduction to networking—fully updated for tomorrow’s key technologies
`Computer Networks, fourth edition,is the ideal introduction to today’s networks—and tomorrow’s. This
`classic bestseller has been thoroughly updatedto reflect the newest and most important networking technologies
`with a special emphasis on wireless networking, including 802.11, Bluetooth™, broadband wireless, ad hoc
`networks,i-mode, and WAP. But fixed networks havenot been ignoredeither, with coverage of ADSL, Internet
`over cable, gigabit Ethernet, peer-to-peer networks, NAT, and MPLS. Andthereis lots of new material on
`applications, including over 60 pages on the Web,plus Internet radio, Voice over IP, and video on demand.
`Finally, the coverage of network security has been revised and expandedtofill an entire chapter.
`Author, educator, and researcher Andrew S. Tanenbaum,winner of the ACM KarlV. Karlstrom Outstanding
`Educator Award, carefully explains how networks work on the inside, from underlying hardwareat the physical
`layer up through the top-level application layer. Tanenbaum coversall this and more:
`
`® Physical layer (e.g., copper,fiber, wireless, satellites, and Internet over cable)
`& Datalink layer (e.g., protocol principles, protocolverification, HDLC, and PPP)
`= MAC sublayer(e.g., gigabit Ethernet, 802.11, broadbandwireless, and switching)
`= Network layer (e.g., routing algorithms, congestion control, QoS, IPv4, and IPv6)
`= Transportlayer (e.g., socket programming, UDP, TCP, RTP, and network performance)
`® Application layer (e.g., e-mail, the Web, PHP, wireless Web, MP3, and streaming audio)
`
`= Network security (e.g., AES, RSA, quantum cryptography, IPsec, and Websecurity)
`
`The book gives detailed descriptions of the principles associatedwith each layer and presents many
`examples drawn from the Internet and wireless networks.
`
`and Scientific Director of ASCI, a ——
`Netherlands. Heis also a Fellow ¢ SSS COMPUTER NETWORKS
`
`About the Author
`von
`ANDREWS. TANENBAUMis Profe aes TONENMEGUM
`co-authoredinclude Structured(
`4TH 3
`PH
`
`Implementation, secondedition;
`SSS
`;
`;
`5
`Principles andParadigms (Pren
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`See al
`
`973-B-135-H66 182-9
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`PRENTICE HALL
`Upper Saddle River, NJ 07458
`
`www.phptr.com
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