Cisco SYSTEMS
`
`
`
`Computer Networking
`Essentials
`
`An essential guide to understanding networking
`theory, implementation, and interoperability
`
`001
`
`AppleInc.
`APL1114
`U.S. Patent No. 8,724,622
`
`Apple Inc.
`APL1114
`U.S. Patent No. 8,724,622
`
`001
`
`

`

`~ Computer Networking Essentials
`
`DebraLittlejohn Shinder
`
`CiscoPress
`Cisco Press ss”
`201 West 103rd Street
`Indianapolis, IN 46290 USA
`
`002
`
`002
`
`

`

`
`
`Computer Networking Essentials
`Debra Littlejohn Shinder
`Copyright © 2002 Cisco Systems, Inc.
`Published by:
`Cisco Press
`201 West 103rd Street
`Indianapolis, IN 46290 USA
`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 by any information storage and retrieval system, without
`written permission from the publisher, except for the inclusion of brief quotations in a review.
`Printed in the United States of America 34567890
`Third Printing
`January 2002
`Library of Congress Cataloging-in-Publication Number: 2001090429
`ISBN: 1-58713-038-6
`
`Trademark Acknowledgments
`All terms mentionedin this book that are known to be trademarksor service marks have been appropriately capital-
`ized. Cisco Press or Cisco Systems, Inc., cannotattest to the accuracy of this information. Use of a term in this book
`should not be regardedas affecting the validity of any trademark or service mark.
`
`Warning and Disclaimer
`This bookis designed to provide information about basic networking and operating system technologies. Every
`effort has been madeto make this book as complete and as accurate as possible, but no warranty orfitness is
`implied.
`The information is provided on an “as is” basis. The author, Cisco Press, and Cisco Systems,Inc., shall have neither
`liability nor responsibility to any person or entity with respect to any loss or damagesarising from the information
`contained in this book or from the use ofthe discs or programs that may accompany it.
`The opinions expressed in this book belongto the authorand are not necessarily those of Cisco Systems,Inc.
`
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`003
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`
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`Publisher
`Editor-in-Chief
`Executive Editor
`Cisco Systems Management
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`Reviewers
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`Carl Lindholm
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`Kitty Wilson Jarrett
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`Dr. Thomas W.Shinder
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`Cisco SYSTEMS
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`Workgroup Director, and Workgroup Stack are trademarks of Cisco Systems, Inc.; Changing the Way We Work, Live, Play, and Learn, Empowering the Internet Generation, are
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`LightSwitch, MICA, NetRanger, Post-Routing, Pre-Routing, Registrar, StrataView Plus, Stratm, SwitchProbe, TeleRouter, are registered trademarks of Cisco Systems, Inc. or its
`affiliates in the U.S. and certain other countries.
`All otherbrands, names,or trademarks mentioned in this document or Web site are the property oftheir respective owners. The use of the word partner does not imply a partnership
`relationship between Cisco and any other company. (0010R)
`
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`xxii
`
`
`Introduction
`Computer Networking Essentials helps you understand the fundamentals of computer networking concepts and
`implementation and introduces youto the client and server operating systems that run on networked PCs.
`Concepts covered in this book include the history of networking, networking terminology, networking theory and
`established standards, and implementation of local-area and wide-area networks. Special emphasis is placed on
`understanding network protocols and how they operateatall layers of the networking model. Emphasisalso is
`placed on the interoperability of networks that run on multiple protocols, platforms, and operating systems.
`Specialty areas such as security, remote access, virtual private networking, thin client networking, monitoring, man-
`agement, and troubleshooting are covered thoroughly. Emerging technologies that are expected to impact the future
`of networking are also introduced.
`
`WhoShould Read This Book
`This book’s primary audienceis professionals who are beginning training in the networking industry and those who
`need a review of basic concepts.
`The secondary audience includes corporate training faculties and staff and members of the business world who
`work with information technology personnel and require a broad overview ofthe concepts involved in networking
`from the small business to the enterprise-level corporation.
`A third target audienceis the general user who wants to know more about how computers communicate over net-
`works. The book’s approachis designedto be user-friendly and accessible to the non-technical reader whois
`overwhelmed by the jargon found in vendor documentation andtechnical manuals.
`
`This Book’s Organization
`This bookis organized into four parts and includes 19 chapters, an appendix,and a glossary. The following sections
`describe the contents of each part of the book.
`
`PartI: Introduction to Networking Concepts
`Chapter 1, “Introduction to PC Networking,” introduces you to the basic concepts of PC networking by providing a
`brief history of electronic communications and networking and a summary of where PC networkingis today.
`Chapter2, “Categorizing Networks,” discusses the categorization of networks according to physical scope, admin-
`istrative model, network operating system, protocols in use, topology, and architecture.
`Chapter 3, “Networking Concepts, Models, and Standards,” provides an overview of binary communications and
`introduces two popular networking models: the Department of Defense (DoD) model on which the TCP/IP proto-
`cols are based and the Open Systems Interconnection (OSI) model, which was developed by the International Orga-
`nization for Standardization (ISO). Specifications set forth by the Institute of Electrical and Electronics Engineers
`(IEEE) and vendor-specific models are also covered.
`Chapter4, “Networking Communications Methods,”discusses signaling methods and provides an understanding of
`analog, digital, broadband, baseband, asynchronous, synchronous, simplex, duplex, and multiplexed signaling.
`Media access methods are described, including CSMA/CD, CSMA/CA,token passing, and demandpriority.
`Chapter 5, “LANLinks,” discusses popular LAN types, including Ethernet, Token Ring, FDDI, AppleTalk, and
`ARCnet.
`Chapter 6, “WAN Links,” provides an overview of WANconnections such as PSTN, ISDN,t-carriers, Frame Relay,
`X.25, and CATV network, as well as high-speed connectivity solutions such as ATM, SONET,and SMDS. This
`
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`xxiii
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`
`chapter also covers LAN-to-WANconnection solutions, including Internet Connection Sharing (ICS), Network
`Address Translation (NAT), proxy servers, and routed connections.
`
`Part II: Networking Hardware and Software
`Chapter 7, “Physical Components of the Network,” introduces students to the many types of networking media,
`including coax, twisted-pair cable, and fiber-optic cable, as well as to wireless technologies such aslaser, infrared,
`radio, and satellite/microwave communications. Connectivity devices such as repeaters, hubs, bridges, routers, and
`switchesare also discussed.
`
`Chapter8, “Networking Protocols and Services,” describes common LAN protocols—TCP/IP, NetBEUI, IPX/
`SPX—and discusses the OSI protocol suite. PPP and SLIP, which are WANlink protocols, and PPTP and L2TP,
`which are common tunneling protocols, are also presented.
`Chapter 9, “The Widest Area Network: The GlobalInternet,” discusses the evolution of the Internet, the protocols
`used for Internet communications—HTTP, FTP, NNTP, SMTP, and POP—and the TCP/IP protocolsuite.
`Chapter 10, “Network Operating Systems,” discusses general network administration practices and then looksat the
`specifics of commonserver operating systems, including Windows NT, Windows 2000, NetWare, UNIX, and Linux.
`Chapter 11, “Directory Services,” describes the Directory Services Protocol (DAP) and the Lightweight Directory
`Access Protocol (LDAP), as well as the X.500 standards developed by the ISO to promote directory services com-
`patibility and interoperability. Novell’s NDS, Microsoft’s Active Directory, and Banyan VINES’ StreetTalk direc-
`tory services are covered in some depth.
`Chapter 12, “Desktop Operating Systems,” looksat the client side of the client/server network and discusses the
`advantages and disadvantages of common desktopclients, such as DOS, Windows, Linux, Macintosh, and OS/2,
`and how each can be integrated into popular NOS environments.
`Chapter 13, “Hybrid Networks,” provides information about interoperability solutions and protocol gateways that
`allow PCs running different operating systems, protocols, and platforms to communicate with one another. This
`chapteralso looks at PC-to-mainframe communications using Systems Network Architecture (SNA) solutions.
`
`Part III: Network Specialty Areas
`Chapter 14, “Protecting the Network,” addresses security issues and provides an overview of basic cryptography
`concepts, public and private key encryption, certificate services, firewalls and proxies, and internal security mea-
`sures such as “smart cards” and advanced authentication technologies.It also provides guidance for developing
`security policies for your network. The secondhalf of the chapter discusses disaster recovery plans, including
`implementation of disk fault tolerance (or RAID), regular scheduled backups, and server
`clustering.
`Chapter 15, “Remote Access,” discusses methods of connecting to a server from a remotelocation using remote
`connectivity devices such as modems, ISDN terminal adapters, and customer premises equipment (CPE) for dedi-
`cated lines. Dial-in server configuration and special security considerations are also covered.
`Chapter 16, “Virtual Private Networking,” provides an overview of VPN concepts and discussesthe tunneling pro-
`tocols used to provide VPN security.
`Chapter 17, “Thin Client Networking,” discusses Network Computers, Net PCs, and Windows-based terminals.
`Windowsterminal services, Citrix Metaframe, web-based computing, the X Window system and Javavirtual
`machines—andthe role each plays in thin client networking—arealso discussed.
`Chapter 18, “Monitoring, Management, and Troubleshooting Tools,” presents an introduction to the TCP/IP utilities
`and other tools built into the various operating systems. This chapter also examines commercial products such as
`Sniffer Pro, LANanalzyer, Microsoft’s Systems Management Server, Novell’s ManageWise, and IBM’sTivoli.
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`
`Part IV: The Future of Networking
`Chapter 19, “Tomorrow’s Technologies,” takes a look into the future of PC networking. It discusses ways of over-
`comingthe currentlimits of IP, including the new version of IP—IPv6. The goal of universal connectivity is
`addressed, and more exotic possibilities such asartificial intelligence, quantum computing, and cyberneticlife
`formsare presented as possible components of tomorrow’s networks.
`
`This Book’s Features
`This book contains several elements that help you learn about operating systems and networking:
`Figures,listings, and tables—This book containsfigures,listings, and tables that help to
`explain concepts, commands,and procedural sequences. Diagramsillustrate network layouts
`and processes, and screenshots assist students in visualization configuration procedures. In addi-
`tion,listings and tables provide summaries and comparisonsof features and characteristics.
`Author’s notes,tips, sidebars, and cautions—These elements are included to provide you
`with extra information on a subject. You will probably find these asides to be very beneficial in
`real-world implementations.
`Chapter summaries—Atthe end of each chapteris a summary of the concepts covered in the
`chapter, which provides a synopsis of the chapter and canserve as a study aid.
`Further Reading—Eachchapter includesa list of resources for additional information about
`the topics covered in the chapter, including website URLs and booksandarticles that cover the
`topic in more detail.
`Review questions—After the Further Reading section in each chapter are 10 review questions
`that serve as an end-of-chapter assessment. The questions are designed to reinforce the con-
`cepts introducedin the chapter andto help students evaluate their understanding before moving
`onto the next chapter.
`The conventionsused to present command syntax in this book are the same conventions used in the Cisco IOS
`Command Reference, as follows:
`Boldface indicates commands and keywordsthatare enteredliterally as shown. In examples
`(not syntax), boldface indicates user input (for example, a show command).
`Italics indicates arguments for which you supply values.
`Square brackets [ ] indicate optional elements.
`Vertical bars(I) separate alternative, mutually exclusive elements.
`Braces and vertical bars within square brackets—for example,[x {y | z}|—indicate a required
`choice within an optional element. You do notneedto enter what is in the brackets, but if you
`do, you have some required choices in the braces.
`
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`[inna
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`
`
`WANLinks
`
`
`
`The technologies, media, and equipmentthat work well for the short distances spanned by
`a LAN or MANare generally not suitable for long-distance wide-area networks (WANs).
`In today’s very mobile world, high-performance, cost-effective WAN technologies are a
`necessity for many reasons:
`
`® Executives and other employees need access to their corporate networks while on the
`road or at home.
`
`® Companies with branch offices in widely dispersed geographic locations need
`network connectivity between locations.
`
`® Organizations want to share information with other organizations physically
`separated by long distances.
`
`® Commercial, governmental, and educational bodies and individuals need access to the
`resources available on the global Internet.
`
`It is obviously impossible to string Ethernet cable from the homeoffice in Denverto the
`branch office in Houston. Even if cabling distance limitations did not apply, this would not
`be a viable solution for connecting internationalsites.
`
`WANsrequire a whole newset of technologies andrules of implementation.In this chapter,
`wediscuss the concept of networking over long distances and the technologies commonly
`used to connect computers that are located in different states, countries, or even different
`continents. These range from the Public Switched Telephone Network (PSTN)already in
`place in most of the world to modern high-tech solutions such assatellite communications
`technologies that enable us to “talk” to computers in space.
`
`Wide-area networking presents many challenges not encountered in implementing a
`network that is confined to one geographic area. A WANis not just a really big LAN.
`Rather, it is a collection of many separate LANs, connected by linksthatare different in
`many ways from LANlinks. WANsthat span international boundaries require
`consideration of even more factors, including time zones and language differences.
`
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`

`160 Chapter 6: WAN Links
`
`Designing a WANis a complex task. Choosing the appropriate technology involves
`analyzing the purpose(s) the WAN will serve, the numberof users, the bandwidth
`requirements, and the patterns of use. We can categorize these considerationsas follows:
`© WANhardware
`
`® WANtopologies
`
`© Network switching types
`
`® New and emerging WANtechnologies
`
`® LAN/WANconnectivity
`
`Welook at each issue in the sections that follow.
`
`WAN Hardware
`The hardware necessary to implement a WANlink can be as simple and inexpensive as a
`telephoneline and a modem ateach end. Ontheotherhand, it can be complex andcostly.
`In general, equipment cost and complexity increases with increased speedandreliability.
`In the following sections, we discuss common WANdevices,including modems, ISDN and
`digital subscriber line (DSL)terminal adapters, and customer premises equipment (CPE)
`used with dedicated links such as T-carrier connections and X.25.
`
`Modems
`
`To establish a network connection (to an Internet service provider orto a dial-up server on
`a private network) over public telephonelines, you use a device called a modem.
`
`
`
`TIP
`
`- The word “modem”is derived from the actions it performs; a modem modulates and
`demodulates a signal. In other words, it converts the sending computer’s digital signal to
`analog for transmission overthe analog line and then converts it back to digital for
`processing by the receiving computer.
`
`Modemscome in two physical types: internal and external. Each has advantages and
`disadvantages, and configuration is slightly different depending on the type. Either way,
`modemsare serial devices, which meansbits are sent one at a time. This can be contrasted
`with parallel devices, such as printers, to which multiple bits can be sent simultaneously.
`A serial transmission is analogous to a group of people marching in a straight line, and a
`parallel transmissionis like having the same group marching in rowsofthree across.
`
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`161
`WAN Hardware
`
`
`internal Modems
`One advantage of the internal modem is compactness.It is a circuit board card thatfits in
`an ISA (Industry Standard Architecture) or PCI (Peripheral Component Interconnect) slot
`inside the computer, as shownin Figure 6-1. This means that you don’t haveto find room
`for an extra device on your desk. In addition, you are not required to buy a serial cable,
`which you might be forced to do if you use an external model that doesn’t include onein
`the box.
`
`Figure 6-1
`
`Aninternal modemis a circuit board thatfits inside the computer.
`
`Jumperedpins
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`|
`
`
`
`
`
`
`cc
`
`
`
`
`
`RJ-1 4 jack
`for attaching
`phoneline
`
`eintannatnennamnnctmtereenctntainiinimnmencncicncai|
`Typical
`internal modem card
`
`Internal Modem Configuration Parameters
`Internal modemsare traditionally more difficult to configure than external modems. You
`mustset the IRQ,the input/output addresses, and the virtual com ports to ensure that they
`don’t conflict with the settings of some other device in your computer. Let’s look more
`closely at each setting and howit is used:
`®
`Interrupt Request (RQ)—Thisis an assigned location that designates where the
`system expects the device to interrupt it when the device sendsa signal. Signals from
`different devices that go to the processor on the same interrupt line would interfere
`with each other, so a separate IRQ must be assigned to each device.
`Input/Output(I/O) address—Thisis the location where data sent from the device is
`stored beforeit is processed by the CPU.Aswith the IRQ,if multiple devices attempt
`to use the same I/O address, one or both devices might not work properly.
`® Virtual com port—This is a logical port number, by whichthe operating system
`identifies a serial port. You mustset each serial device to use a different com port.
`
`©
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`

`162
`Chapter 6: WAN Links
`
`All popular operating systems provide a means by which you can view how resources are
`being used, and which onesare not in use, so that you can choose free resources to assign
`to your new device.
`
`Changing the Internal Modem Seitings
`Internal modemsgenerally provide a way to change the configuration settings. Depending
`on the manufacturer and model, you can change IRQ,I/O, and com port settings with the
`following:
`
`®
`
`® Dip switches—These are small switches on the circuit board that can be moved to a
`different position. The position of the switch designates whichsetting is to be used.
`Jumpers—Pairs of metal pinsbuilt into the circuit board, these represent anelectrical
`contact point. Jumpers are configured by placing a small plug onthe pins to complete
`the circuit. The instructions that come with your internal modem tell you how the
`jumpers should beset to use a specific IRQ, I/O address, or com port.
`® Software—Some modems do not have physical switches or jumpers, but do come
`with a software program that is run to change the configuration.
`
`Plug and Play
`Many modern modemssupport Plug and Play (PnP) technology, which enables the
`operating system to detect the device, install the necessary software drivers, detect what
`resourcesare free on the computer, and assign those resourcesto the device automatically.
`Little or no intervention is required from the user.
`
`PnPis great—when it works and when you are aware of a few caveats. If you buy a modem
`or other device that is advertised as Plug and Play, it is automatically configured only if the
`followingis true:
`
`®
`
`‘Your computer’s BIOS (Basic Input/Output System) supports PnP.
`
`® You are running a PnP operating system.
`
`Both criteria must be met. Computer motherboards produced after 1995 usually support
`PnP. Operating systems that support PnP include Windows 95, 98, ME, and 2000.
`
`
`
`NOTE
`
`WindowsNTis not a PnP operating system. However,it does have limited PnP
`functionality and detects some modem types.
`
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`

`WAN Hardware
`
`
`163
`
`External Modems
`External modemshave a couple advantages over the internal variety:
`
`® Most external modemsprovide status lights, which indicate when the modem is
`powered on, connected, or transferring data. See Figure 6-2.
`® External modemsare generally easier to install and configure. There are no switches
`or jumpers to set, and you don’t have to open the computercase.
`
`Figure 6-2.
`
`External modemsprovide status indicatorlights.
`
`RJ-11 jacks to connect
`phoneline
`
`Power cord
`connector
`
`:
`Typical external modem
`
`MOTOROLA
`
`RD
`
`
` Serial cable
`
`Status indicater
`lights
`
`connector
`
`External modemsrequire power cordsto plug into an electrical outlet, but internal modems
`run off the computer’s power. A serial cable connects the modem to oneoftheserial ports
`on the back of the computer.
`
`Serial Port Considerations
`To use an external modem, you need a free serial port. Most computers have two built-in
`serial ports, labeled COM 1 and COM 2, with connectors on the back of the computer.
`Manydevices, such as scanners, digital cameras, and serial pointing devices, also use serial
`ports. If your computer does not havea free serial port, you have a few options:
`
`® Use an internal modem.
`
`®
`
`©
`
`Install an expansion card in your computer, which enables youto add serial port
`connections.
`
`If your computer’s motherboard supports universalserial bus (USB), you can chain
`multiple serial devices, such as modems,off a single serial port. You might haveto
`add a card to provide a USB connector, and you need a USB modem.
`
`UART Chips
`Serial ports use a chip called a UART (Universal Asynchronous Receiver/Transmitter) to
`handle serial communications. This chip comes in different types, and the type used
`determines how fast data can be transferred over that serial port.
`
`012
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`

`

`164 Chapter 6: WAN Links
`
`
`Thefirst PCs had 8250 UARTchips. The top speed for this chip is 9600 bps, which means
`that even if you attach a high speed (56 kbps) modem to oneofthese ports, your speed
`would be limited by the UART.
`
`Modern computers have UARTchips in the 16450 or 16550 series. These serial ports can
`support transfer speeds of up to 115,200 bps.
`
`16650 and 16750 UARTchipsare also available as add-on “enhancedserial port”cards.
`Internal modems have their own UARTchipsbuilt into the card, so the speed ofthe
`computer’s com port is irrelevant.
`
`
`
`NOTE
`
`If you have a high-speed modem and a modern computer, butare able to connectonly at
`low speeds, check the com port configuration settings. Some operating systemsset the com
`ports to 9600 bps by default; you need to changethis setting to realize the port’s full
`capacity.
`
`Modem Drivers
`
`Drivers are software programsthat act as a liaison between the hardware device and the
`operating system. Driver software is usually supplied by the modem manufacturer with the
`device, or it can be downloaded from the manufacturer’s Website.
`
`You mustinstall the correct driver software for your device because if operating system
`code included supportfor all hardware devices that could possibly be used with it, the
`operating system would require significantly more disk space—muchofit wasted on driver
`software that would never be used.
`
`Modem Configuration
`In additionto installing the driver software that enables the operating system to recognize
`the modem,and setting the IRQ, I/O address, and com port that the modem will use, you
`have to configure the modem to dial and maintain a connection. Modern operating systems
`have built-in support for dialup networking. You might have to install the remote access
`services if the modem was not present when the operating system wasinstalled.
`
`Modem Banks
`
`A computer can be configured as a dialup server(also called a remote access server) to
`enable other computers to dial into it and connect to it over the phone lines. Computers
`running powerful server software can support many incoming remote access connections
`simultaneously; for instance, Windows NT Server supports up to 256 connections.
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`How can you connect 256 modemsto a remote server? When you have many simultaneous
`dial-in connections (for example, when the server belongs to a company with many
`telecommuters who need to connect to the corporate network from home), you can use a
`modembank. Modem banksare also called modemnests or modem pools.
`
`A modem bankenables you to use a group of modems(usually mounted together in a rack)
`with a single server, and host multiple remote connections. The rack of modem cardsis
`controlled by an interface that connects to the server, to a router, or directly to the local
`network. Of course, you need a phoneline for each separate connection.
`
`ISDN and DSL Adapters
`The device used to connect a computer to an Integrated Services Digital Network (ISDN)
`or DSL telephoneline is often referred to as a modem.It is more accurately called a
`terminal adapter because it does not modulate and demodulate signals because ISDNlines
`are digital, unlike the analog PSTN lines.
`
`ISDN Adapters
`ISDNadapters, such as modems, comein both internal and external varieties. They are
`configured similarly to modems,but the typical 128-kbps ISDNservice consists of two data
`channels that each run at 64 kbps. The two channels are commonly used in a multilink
`configuration to provide the 128-kbps bandwidth. We discuss ISDN technologylater in this
`chapterin the section, “ISDN.”
`
`The two data channels have separate telephone numbers in most cases. ISDN adapters are
`configured with information about the service profile identifier (SPID) for each channel,
`which consists of the telephone number, a two-digit sharing terminal identifier, and a two-
`digit terminal identifier (TID). Some modern ISDN adapters support automated SPID
`selection and do not require you to enterthis information.
`
`DSL Adapters
`Both ends of a DSL connection require a device called an endpoint (and often referred to
`as a DSL modem), which connects to an Ethernet NIC installed in the computer. In some
`cases, the endpoint/modemis external. In others, the endpoint and the NIC are placed
`together on the same card.
`
`Customer Premises Equipment
`Customer premises equipment (CPE)is a general term that encompassesseveral different
`devices. The customer’s site requires this hardware to process incoming transmissions from
`WANlinks such as T-carrier lines, X.25 connections, and Frame Relay links.
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`166 Chapter 6: WANLinks
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`Commontypes of CPE include the following:
`
`® Achannelservice unit/digital service unit (CSU/DSU), used with circuit-switched
`connections such as a T-1 line. The CSU receives and transmits signals to and from
`the WANline. The DSU managesline control, timing errors, and signal regeneration.
`® A packet assembler/disassembler (PAD), used with packet-switched connections such
`as X.25. The PADis an asynchronousdevice that enables multiple terminals to share
`a network line. Users dial into PADs through modems.
`
`WANTopologies
`We discussed LANtopologies in Chapter 2, “Categorizing Networks,” and some of those
`same concepts apply to WANs.In the context of WANs, however, the topology describes
`the arrangementofthe transmission facilities.
`
`The simplest WAN topologyis a simple point-to-point connection. The WAN,like the
`LAN,also can use traditional networking topologies such as a ring orstar.
`
`The Point-to-Point WAN
`
`A point-to-point WANis similar to the LAN topology referredto as a linear bus. A remote
`access link, which can be anything from a 56-kbps dial-up modem connection to a
`dedicated T-1 line, connects each point on the WANto the next. See Figure 6-3 for an
`illustration ofthis.
`
`Figure 6-3 A point-to-point WANdirectly connects two endpoints.
`
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`San Francisco
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`WAN Topologies
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`This is a relatively inexpensive way to connect a small number of WANsites. However, it
`is not fault tolerant. For example, in Figure 6-3, if the equipmentat the Dallas office fails,
`San Francisco and Boston cannot communicate with one another. Limited scalability (the
`capability to “grow gracefully,” that is, to continue to function efficiently as the network
`grows larger) is another disadvantage. If you add anotherpoint to the WANat Nashville,
`between Dallas and Boston, you increase the number of hops required for Boston to
`communicate with Dallas or San Francisco.
`
`NOTE
`
`
`
`In wide-area networking, a hop is defined as the trip from one routerto the next. The hop
`countis the numberof