INTERNATIONAL EDITION
`
`TH
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`I R D EDITION
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`COMPUTER NETWORKS
`
`ANDREW S. TANENBAUM
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`Broadcasting
`
`WELCOME
`TO THE
`FORMATION
`SUPER
`HIGHWAY
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`FORD EX. 1014
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`

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`Computer Networks
`Third Edition
`
`Andrew S. Tanenbaum
`
`Vrije Universiteit
`Amsterdam, The Netherlands
`
`For book and bookstore information
`
`http://www.prenhall.com
`
`Prentice-Hall International, Inc.
`
`FORD EX. 1014
`
`

`

`ISBN 0-13-394248-1
`
`This edition may be sold only in those countries to which it is consigned
`by Prentice-Hall International. It is not to be re-exported , and it is not for
`sale in the U.S .A., Mexico, or Canada.
`
`ii A Simon & Schuster Company
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`_
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`==
`
`© 1996 by Prentice-Hall, Inc.
`
`Upper Saddle River , New Jersey 07458
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`All rights reserved. No part of this book may be reproduced, in any form
`or by any means, without permission in writing from the publisher .
`
`All product names mentioned herein are the trademarks of their respective
`owners.
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`Printed in the United States of America
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`ISBN 0-13-394248-1
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`Prentice-Hall, Inc ., Upper Saddle River; New Jersey
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`FORD EX. 1014
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`

`

`1
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`INTRODUCTION
`
`Each of the past three centuries has been dominated by a single technology.
`The 18th Century was the time of the great mechanical systems accompanying the
`Industrial Revolution. The 19th Century was the age of the steam engine. During
`the 20th Century, the key technology has been information gathering, processing,
`and distribution. Among other developments, we have seen the installation of
`worldwide telephone networks, the invention of radio and television, the birth and
`unprecedented growth of the computer industry, and the launching of communica(cid:173)
`tion satellites.
`Due to rapid technological progress, these areas are rapidly converging, and
`the differences between collecting, transporting, storing, and processing informa(cid:173)
`tion are quickly disappearing . Organizations with hundreds of offices spread over
`a wide geographical area routinely expect to be able to examine the current status
`of even their most remote outpost at the push of a button. As our ability to gather,
`process, and distribute information grows, the demand for even more sophisti(cid:173)
`cated information processing grows even faster.
`Although the computer industry is young compared to other industries (e.g.,
`automobiles and air transportation), computers have made spectacular progress in
`a short time. During the first two decades of their existence, computer systems
`were highly centralized, usually within a single large room. Not infrequently, this
`room had glass walls, through which visitors could gawk at the great electronic
`wonder inside. A medium-size company or university might have had one or two
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`1
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`FORD EX. 1014
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`1.1.1.
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`-
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`2
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`INTRODUCTION
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`CHAP . 1
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`computers, while large institutions had at most a few dozen. The idea that within
`20 years equally powerful computers smaller than postage stamps would be mass
`produced by the millions was pure science fiction.
`The merging of computers and communications has had a profound influence
`on the way computer systems are organized. The concept of the "computer
`center" as a room with a large computer to which users bring their work for pro(cid:173)
`cessing is now totally obsolete. The old model of a single computer serving all of
`the organization's computational needs has been replaced by one in which a large
`number of separate but interconnected computers do the job. These systems are
`called computer networks. The design and organization of these networks are
`the subjects of this book.
`Throughout the book we will use the term "computer network" to mean an
`interconnect ed collection of autonomous computers. Two computers are said to
`be interconnected if they are able to exchange information . The connection need
`not be via a copper wire; fiber optics, microwaves, and communication satellites
`can also be used. By requiring the computers to be autonomous, we wish to
`exclude from our definition systems in which there is a clear master/slave rela(cid:173)
`tion. If one computer can forcibly start, stop, or control another one, the comput(cid:173)
`ers are not autonomous . A system with one control unit and many slaves is not a
`network; nor is a large computer with remote printers and terminals.
`There is considerable confusion in the literature between a computer network
`and a distributed system. The key distinction is that in a distributed system, the
`existence of multiple autonomous computers is transparent (i.e., not visible) to the
`user. He t can type a command to run a program, and it runs. It is up to the
`operating system to select the best processor, find and transport all the input files
`to that processor, and put the results in the appropriate place.
`In other words, the user of a distributed system is not aware that there are
`multiple processors; it looks like a virtual uniprocessor. Allocation of jobs to pro(cid:173)
`cessors and files to disks, movement of files between where they are stored and
`where they are needed, and all other system functions must be automatic.
`With a network, users must explicitl y log onto one machine, explicitly submit
`jobs remotely, explicitly move files around and generally handle all the network
`management personally. With a distributed system, nothing has to be done expli(cid:173)
`citly ; it is all automatically done by the system without the users' knowledge .
`In effect, a distributed system is a software system built on top of a network.
`The software gives it a high degree of cohesiveness and transparency. Thus the
`distinction between a network and a distributed system lies with the software
`(especially the operating system), rather than with the hardware.
`Nevertheless, there is considerable overlap between the two subjects. For
`example, both distributed systems and computer networks need to move files
`around . The difference lies in who invokes the movement, the system or the user.
`t "He" should be read as "he or she" throughout this book.
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`FORD EX. 1014
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`

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`SEC. 1.1
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`USES OF COMPUTER NETWORKS
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`3
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`Although this book primarily focuses on networks, many of the topics are also
`important in distributed systems . For more information about distributed systems,
`see (Coulouris et al., 1994; Mullender, 1993; and Tanenbaum, 1995).
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`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.
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`1.1.1. Networks for Companies
`
`Many organization s have a substantial number of computers in operation,
`often located far apart. For example , a company with many factories may have a
`computer at each location to keep track of inventories, monitor productivity, and
`do the local payroll. Initially, each of these computers may have worked in isola(cid:173)
`tion 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. In other words, the mere fact that a user happens to be 1000 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."
`A second goal is to provide high reliability by having alternative sources of
`supply. For example, all files could be replicated on two or three machines , so if
`one of them is unavailable (due to a hardware failure), the other copies could be
`used. In addition, the presence of multiple CPUs means that if one goes down, the
`others may be able to take over its work, although at reduced performance . For
`military , banking, air traffic control, nuclear reactor safety, and many other appli(cid:173)
`cations, the ability to continue operating in the face of hardware problems is of
`utmost importance.
`Another goal is saving money. Small computers have a much better
`price/performance ratio than large ones. Mainframes (room-size computers) are
`roughly a factor of ten faster than personal computers, but they cost a thousand
`times more. This imbalance has caused many systems designers to build systems
`consisting of personal computers, one per user , with data kept on one or more
`hared file server machines.
`In this model, the users are called clients, and the
`whole arrangement is called the client-server model. It is illustrated in Fig. 1-1.
`In the client-server model, communication generally takes the form of a
`request message from the client to the server asking for some work to be done.
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`FORD EX. 1014
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`SEC. 1.4
`REFERENCE MODELS
`35
`headers are stripped off one by one as the message propagates up the layers until
`it finally arrives at the receiving process.
`Thekey idea throughoutis that althoughactual data transmissionis vertical in
`Fig. 1-17, each layer is programmedas thoughit were horizontal. Whenthe send-
`ing transport layer, for example, gets a message from thesessionlayer, it attaches
`a transport header and sendsit to the receiving transport layer. From its point of
`view, the fact that it must actually hand the message to the network layer onits
`own machine is an unimportant technicality. As an analogy, when a Tagalog-
`speaking diplomat
`is addressing the United Nations, he thinks of himself as
`addressing the other assembled diplomats. That, in fact, he is really only speaking
`to his translator is seen as a technical detail.
`
`1.4.2. The TCP/IP Reference Model
`
`Let us now turn from the OSI reference model to the reference model used in
`the grandparentof all computer networks, the ARPANET,andits successor, the
`worldwide Internet. Although wewill give a brief history of the ARPANETlater,
`it
`is useful
`to mention a few key aspects of it now. The ARPANET was a
`research network sponsored by the DoD (U.S. Departmentof Defense).
`It eventu-
`ally connected hundredsof universities and governmentinstallations using leased
`telephone lines. Whensatellite and radio networks were added later, the existing
`protocols had trouble interworking with them, so a new reference architecture was
`needed. Thusthe ability to connect multiple networks together in a seamless way
`was one of the major design goals from the very beginning. This architecture
`later became knownas the TCP/IP Reference Model, after its two primary pro-
`tocols.
`It wasfirst defined in (Cerf and Kahn, 1974). A later perspective is given
`in (Leiner et al., 1985). The design philosophy behind the model is discussed in
`(Clark, 1988).
`Given the DoD’s worry that some ofits precious hosts, routers, and internet-
`work gateways might get blown to pieces at a moment’s notice, another major
`goal wasthatthe networkbeable to survive loss of subnet hardware, with existing
`conversations not being broken off.
`In other words, DoD wanted connections to
`remain intact as long as the source and destination machines were functioning,
`even if someof the machines or transmission lines in between were suddenly put
`out of operation. Furthermore, a flexible architecture was needed, since applica-
`tions with divergent requirements were envisioned, ranging from transferring files
`to real-time speech transmission.
`
`The Internet Layer
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`All these requirements led to the choice of a packet-switching network based
`on a connectionless internetwork layer. This layer, called the internet layer,is
`the linchpin that holds the whole architecture together.
`Its job is to permit hosts to
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`FORD EX. 1014
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`The layer above the internet layer in the TCP/IP model is now usually called
`the transport layer.
`It is designed to allow peer entities on the source and desti-
`nation hosts to carry on a conversation, the same as in the OSI transport layer.
`Two end-to-end protocols have been defined here. The first one, TCP
`(Transmission Control Protocol) is a reliable connection-oriented protocol that
`allows a byte stream originating on one machine to be delivered without error on
`
`
`
`INTRODUCTION
`
`CHAP.1
`
`inject packets into any network and have them travel independently to the destina-
`tion (potentially on a different network). They may even arrive in a different
`order than they were sent, in which caseit is the job of higher layers to rearrange
`them, if in-order delivery is desired. Note that “internet’’ is used here in a generic
`sense, even thoughthis layer is present in the Internet.
`The analogy here is with the (snail) mail system. A person can drop a
`sequence of international letters into a mail box in one country, and with a little
`luck, most of them will be delivered to the correct address in the destination coun-
`try. Probably the letters will travel through one or more international mail gate-
`ways along the way, but this is transparent to the users. Furthermore, that each
`country (i.e., each network) has its own stamps, preferred envelope sizes, and
`delivery rules is hidden from the users.
`The internet layer defines an official packet format and protocol called IP
`(Internet Protocol). The job of the internet layer is to deliver IP packets where
`they are supposed to go. Packet routing is clearly the major issue here, as is
`avoiding congestion. For these reasons, it is reasonable to say that the TCP/IP
`internet layer is very similar in functionality to the OSI network layer. Figure
`1-18 shows this correspondence.
`
`np©f-&om&@«I
`
`Osl
`
`
`
`ret
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`TCP/IP
`
`ms
`
`Not present
`
`in the model
`
`
`
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`Host-to-network
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`Fig. 1-18. The TCP/IP reference model.
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`The Transport Layer
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` 36
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`FORD EX. 1014
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`

`

`SEC. 1.4
`
`REFERENCE MODELS
`
`37
`
`It fragments the incoming byte stream into
`any other machine in the internet.
`discrete messages and passes each oneontotheinternet layer. At the destination,
`the receiving TCP process reassembles the received messages into the output
`steam. TCPalso handles flow control to make sure a fast sender cannot swamp a
`slow receiver with more messagesthan it can handle.
`is an
`The second protocol
`in this layer, UDP (User Datagram Protocol),
`unreliable, connectionless protocol
`for applications that do not want TCP’s
`sequencing or flow control and wish to provide their own.
`It is also widely used
`for one-shot, client-server type request-reply queries and applications in which
`prompt delivery is more important than accurate delivery, such as transmitting
`speech or video. The relation of IP, TCP, and UDPis shownin Fig. 1-19. Since
`the model was developed, IP has been implemented on many other networks.
`
`
`
`
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`
`
`UDP
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`TCP
`
`Layer (OSI names)
`
`Application
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`Transport
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`Neto
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`
`
`[om
`[Fagg]
`[ samen]
`data link
`
`Physical +
`
`Protocols
`
`Now|
`
`Fig. 1-19. Protocols and networks in the TCP/IP modelinitially.
`
`The Application Layer
`
`The TCP/IP model does not havesession or presentation layers. No need for
`them was perceived, so they were not included. Experience with the OSI model
`has proven this view correct: they are oflittle use to most applications.
`the
`On top of the transport layer is the application layer.
`It contains all
`higher-level protocols. The early ones included virtual terminal (TELNET),file
`transfer (FTP), and electronic mail (SMTP), as shown in Fig. 1-19. The virtual
`terminal protocol allows a user on one machineto log into a distant machine and
`work there. The file transfer protocol provides a way to move data efficiently
`from one machine to another. Electronic mail wasoriginally just a kind offile
`transfer, but later a specialized protocol was developed for it. Many other proto-
`cols have been added to these over the years, such as the Domain Name Service
`(DNS) for mapping host namesonto their network addresses, NNTP,the protocol
`used for moving newsarticles around, and HTTP,the protocol used for fetching
`pages on the World Wide Wide, and manyothers.
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`FORD EX. 1014
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`FORD EX. 1014
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`SEC. 2.4
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`town, and
`that either
`me ntor he
`100 years.
`Strowger ge~
`boar d ope
`·~
`What is the F
`The m
`of the ' ·cop
`links onto
`-
`is valid: on~e
`will contin e
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`-
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`0
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`
`
`~ L -
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`. ~
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`130
`
`THE PHYSICAL LA YER
`
`CHAP . 2
`
`repeaters, which just amplify and regenerate the bits, but do not change or process
`them in any way.
`The line sublayer is concerned with multiplexing multiple tributaries onto a
`single line and demultiplexing
`them at the other end. To the line sublayer, the
`repeaters are transparent. When a multiplexer puts out bits on a fiber, it expects
`them to arrive at the next multiplexer unchanged, no matter how many repeaters
`are used in between. The protocol in the line sublayer is thus between two multi(cid:173)
`plexers and deals with issues such as how many inputs are being multiplexed
`together and how . In contrast, the path sublayer and protocol deal with end-to(cid:173)
`end issues.
`
`2.4.5. Switching
`
`From the point of view of the average telephone engineer, the phone system is
`divided into two parts: outside plant (the local loops and trunks, since they are
`outside the switching offices), and inside plant (the switches). We have just
`looked at outside plant. Now it is time to examine inside plant.
`Two different switching techniques are used inside the telephone system: cir(cid:173)
`cuit switching and packet switching. We will give a brief introduction to each of
`them below. Then we will go into circuit switching in detail, because that is how
`the current telephone system works . Later in the chapter we will go into packet
`switching in detail in the context of the next generation telephone system, broad(cid:173)
`band ISDN.
`
`Circuit Switching
`
`When you or your computer places a telephone call, the switching equipment
`within the telephone system seeks out a physical "copper"
`(including fiber and
`radio) path all the way from your telephone to the receiver's
`telephone. This tech(cid:173)
`nique is called circuit switching and is shown schematically
`in Fig. 2-34(a).
`Each of the six rectangles represents a carrier switching office (end office, toll
`office, etc.). In this example, each office has three incoming lines and three out(cid:173)
`going lines. When a call passes through a switching office, a physical connection
`is (conceptually) established between the line on which the call came in and one
`of the output lines, as shown by the dotted lines .
`In the early days of the telephone, the connection was made by having the
`operator plug a jumper cable into the input and output sockets. In fact, there is a
`surprising little story associated with the invention of automatic circuit switching
`equipment.
`It was invented by a 19th Century undertaker named Almon B.
`Strowger. Shortly after the telephone was invented, when someone died, one of
`the survivors would call the town operator and say: "Please connect me to an
`undertaker." Unfortunately for Mr. Strowger, there were two undertakers
`in his
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`SEC. 2.4
`
`THE TELEPHONE SYSTEM
`
`131
`
`town, and the other one's wife was the town telephone operator. He quickly saw
`that either he was going to have to invent automatic telephone switching equip(cid:173)
`ment or he was going to go out of business. He chose the first option. For nearly
`100 years, the circuit switching equipment used worldwide was known as
`Strowger gear. (History does not record whether the now-unemployed switch(cid:173)
`board operator got a job as an information operator, answering questions such as:
`What is the phone number of an undertaker?
`The model shown in Fig. 2-34(a) is highly simplified of course, because parts
`of the "copper" path between the two telephones may, in fact, be microwave
`links onto which thousands of calls are multiplexed. Nevertheless, the basic idea
`is valid: once a call has been set up, a dedicated path between both ends exists and
`will continue to exist until the call is finished.
`
`Physical copper
`connection set up
`when call is made
`
`\
`I
`
`Switching office
`
`Packets queued up
`for subsequent
`transmission
`
`Computer
`
`(a)
`
`(b)
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`Fig. 2-34. (a) Circuit switching. (b) Packet switching.
`
`An important property of circuit switching is the need to set up an end-to-end
`path before any data can be sent. The elapsed time between the end of dialing and
`the start of ringing can easily be 10 sec, more on long-distance or international
`calls. During this time interval, the telephone system is hunting for a copper path,
`as shown in Fig. 2-35(a). Note that before data transmission can even begin, the
`call request signal must propagate all the way to the destination, and be
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`132
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`THE PHYSICAL LA YER
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`CHAP. 2
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`acknowledged. For many computer applications (e.g., point-of-sale credit verifi(cid:173)
`cation), long setup times are undesirable.
`
`Call request signal
`+
`
`r---
`r---
`
`(l)
`E
`i=
`
`j
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`1{
`
`Time
`spent
`hunting
`for an
`outgoin~
`trunk
`
`Data
`
`Pkt 1
`
`Propagation
`delay Msg
`
`i -t ----
`
`Msg
`
`Queuing
`delay
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`r------J./
`
`Msg
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`Pkt 3
`
`Call
`accept
`signal
`
`AB
`trunk
`
`BC
`trunk
`
`CD
`trunk
`
`------------------
`
`A
`
`B
`
`C
`
`D
`
`A
`
`B
`
`C
`
`D
`
`A
`
`B
`
`C
`
`D
`
`(a)
`
`(b)
`
`(c)
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`Fig. 2-35. Timing of events in (a) circuit switching, (b) message switching,
`(c) packet switching.
`
`As a consequence of the copper path between the calling parties, once the
`setup has been completed, the only delay for data is the propagation time for the
`electromagnetic signal, about 5 msec per 1000 km. Also as a consequence of the
`established path, there is no danger of congestion-that
`is, once the call has been
`put through, you never get busy signals, although you might get one before the
`connection has been established due to lack of switching or trunk capacity.
`An alternative switching strategy is message switching, shown in Fig. 2-
`35(b ). When this form of switching is used, no physical copper path is established
`in advance between sender and receiver. Instead, when the sender has a block of
`data to be sent, it is stored in the first switching office (i.e., router) and then for(cid:173)
`warded later, one hop at a time. Each block is received in its entirety, inspected
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`SEC. 2.4
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`THE TELEPHONE SYSTEM
`
`133
`
`for errors, and then retransmitted. A network using this technique is called a
`store-and-forward network, as mentioned in Chap. 1.
`The first electromechanical telecommunication systems used message switch(cid:173)
`ing, namely for telegrams. The message was punched on paper tape off-line at the
`sending office, and then read in and transmitted over a communication line to the
`next office along the way, where it was punched out on paper tape. An operator
`there tore the tape off and read it in on one of the many tape readers, one per out(cid:173)
`going trunk. Such a switching office was called a torn tape office.
`With message switching, there is no limit on block size, which means that
`routers (in a modem system) must have disks to buffer long blocks. It also means
`that a single block may tie up a router-router line for minutes, rendering message
`switching useless for interactive traffic. To get around these problems, packet
`switching was invented. Packet-switching networks place a tight upper limit on
`~lock size, allowing packets to be buffered in router main memory instead of on
`disk. By making sure that no user can monopolize any transmission line very long
`(milliseconds), packet-switching networks are well suited to handling interactive
`traffic. A further advantage of packet switching over message switching is shown
`in Fig. 2-35(b) and (c): the first packet of a multipacket message can be forwarded
`before the second one has fully arrived, reducing delay and improving throughput.
`For these reasons, computer networks are usually packet switched, occasionally
`circuit switched, but never message switched .
`Circuit switching and packet switching differ in many respects. The key
`difference is that circuit switching statically reserves the required bandwidth in
`advance, whereas p~cket switching acquires and releases it as it is needed. With
`circuit switching, any unused bandwidth on an allocated circuit is just wasted.
`With packet switching it may be utilized by other packets from unrelated sources
`going to unrelated destinations, because circuits are never dedicated. However,
`just because no circuits are dedicated, a sudden surge of input traffic may
`overwhelm a router, exceeding its storage capacity and causing it to lose packets.
`In contrast, with circuit switching, when packet switching is used, it is
`straightforward for the routers to provide speed and code conversion. Als~, they
`can provide error correction to some extent. In some packet-switched networks,
`however, packets may be delivered in the wrong order to the destination. Reor(cid:173)
`dering of packets can never happen with circuit switching.
`Another difference is that circuit switching is completely transparent. The
`sender and receiver can use qny bit rate, format, or framing method they want to.
`The carrier does not know or care. With packet switching, the carrier determines
`the basic parameters. A rough analogy is a road versus a railroad. In the former,
`the user determines the size, speed, and nature of the vehicle; in the latter, the car(cid:173)
`rier does. It is this transparency that allows voice, data, and fax to coexist within
`the phone system.
`A final difference between circuit and packet switching is the charging algo(cid:173)
`rithm. Packet carriers usually base their charge on both the number of bytes ( or
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`134
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`THE PHYSICAL LA YER
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`CHAP . 2
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`packets) carried and the connect time. Furthermore, transmission distance usually
`does not matter, except perhaps internationally. With circuit switching, the
`charge is based on the distance and time only, not the traffic. The differences are
`summarized in Fig. 2-36.
`
`Item
`
`Circuit-switched
`
`Packet-switched
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`Dedicated "copper" path
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`Bandwidth available
`
`Potentially wasted bandwidth
`
`Store-and-forward transmission
`
`Each packet follows the same route
`
`Yes
`
`Fixed
`
`Yes
`
`No
`
`Yes
`
`No
`
`Dynamic
`
`No
`
`Yes
`
`No
`
`Call setup
`
`Required
`
`Not needed
`
`When can congestion occur
`
`At setup time
`
`On every packet
`
`Charging
`
`Per minute
`
`Per packet
`
`Fig. 2-36. A comparison of circuit-switched and packet-switched networks.
`
`Both circuit switching and packet switching are so important, we will come
`back to them shortly and describe the various technologies used in detail.
`
`The Switch Hierarchy
`
`It is worth saying a few words about how the routing between switches is
`done within the current circuit-switched telephone system. We will describe the
`AT&T system here, but other companies and countries use the same general prin(cid:173)
`ciples. The telephone system has five classes of switching offices, as illustrated
`in Fig. 2-37. There are 10 regional switching offices, and these are fully intercon(cid:173)
`nected by 45 high-bandwidth fiber optic trunks. Below the regional offices are 67
`sectional offices, 230 primary offices, 1300 toll offices, and 19,000 end offices.
`The lower four levels were originally connected as a tree.
`Calls are generally connected at the lowest possible level. Thus if a sub(cid:173)
`scriber connected to end office 1 calls another subscriber connected to end office
`1, the call will be completed in that office. However , a call from a customer
`attached to end office 1 in Fig. 2-37 to a customer attached to end office 2 will
`have to go toll office 1. However , a call from end office 1 to end office 4 will
`hav'e to go up to primary office 1, and so on. With a pure tree, there is only one
`minimal route, and that would normally be taken.
`During years of operation, the telephone companies noticed that some routes
`were busier than others. For example, there were many calls from New York to
`Los Angeles. Rather than go all the way up the hierarchy, they simply installed
`direct trunks for the busy routes. A few of these are shown in Fig. 2-37 as
`
`FORD EX. 1014
`
`

`

`\ .
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`These are unabridged paperback reprint¥
`establis
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`these lowetc~i&d
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`This edition may be sold only in those coun~s
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`I 90000
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`ISBN 0-13-394248-1
`
`Prentice Hall International Editions
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`9 780133 942484
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`FORD EX. 1014
`
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