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Greatthanksandhumbleappreciationtoallofthosewhohelpedwiththisbook.Andtomykidsandtheirkids,andeverandalwaystoSandy.

AbouttheAuthorBobbiSandbergisasmallbusinessconsultantandretiredCPAwhohasbeenatrainer,instructor,andteacherofallthingscomputerinthePacificNorthwestformorethan40years.Shehasplayedwithcomputerssincetheyoccupiedentireroomsandrequiredperforatedpapertapeandpunchcards.Today,sheteacheshardwareandsoftwareclasses,solveshardwareandsoftwareissuesforanumberofclients,andkeepsnetworksfunctionalonaregularbasis.Bobbiistheauthororcoauthorofseveralcomputerbooks,includingQuickBooks2015:TheSmallBusinessGuide,Quicken2015:TheOfficialGuide,Quicken2014:TheOfficialGuide,MicrosoftOffice2013QuickSteps,andComputingforSeniorsQuickSteps.

AbouttheTechnicalEditorsRandalNollanhasbeenworkingwithtechnologysincethelate1970swhenhewrotehisfirstprogramonpinkpunchcards.RandaljoinedtheU.S.Navyin1980asanAviationOrdnancemanandretiredin2001.Duringthattime,hemaintainedthedBaseIIIvaccinationdatabaseforthesquadroncorpsmanandwasalwaysinthethickofmaintainingthetokenringnetwork,computers,andterminalstheyhadatthetime.HegraduatedfromSkagitValleyCollegeCIS(networking)andMIT(programming)in2003.HeworkedinInternettechsupportfrom2003to2005andhassincebeenworkingincomputerrepairforalocaltelephonecompanyonWhidbeyIsland,Washington.Inhissparetime,heenjoystheoutdoorsbyfishing,crabbing,bicycling,camping,andhunting.Indoorfunincludesplayingwithanythingtechrelated,remodelinghishome,andmakingwinefromanyfruitthatlandsonhisdoorstep;sometimehemayevenstopworkinganddrinkit.

DwightSpiveyistheauthorofmorethan20booksoncomputersandtechnologyandhashappilylenthisexpertiseasatechnicaleditortoseveralmoretitles.DwightishappilymarriedtoCindy,andtheyresideontheGulfCoastofAlabamaalongwiththeirfourchildren.Hestudiestheology,drawscomicstrips,androotsfortheAuburnTigersinhisever-decreasingsparetime.

VanAguirreisaninformationtechnologyspecialistwhohasbroadexperienceinthefield.Sincethelate1990s,hehasdevelopedandtaughtcoursesinnetworkingandmultimediatechnology,computingsecurity,computercrimeforensics,ITriskmanagement,ITbusinesscontinuity,anddisasterrecoveryplanning.WorkingwithotherITprofessionals,hehasplannedandmanagedtheimplementationofevolvingtechnologies,includingvirtualization,mobile,andcloudcomputingtosupportinstitutionalbusinessandstrategicinitiatives.Asaprojectmanagerineducationaltechnology,VanhasestablishedandpromotedsuccessfulapprenticeshipprogramsinITdeskservicemanagementforcollegestudents,integratingLEANprinciplesandITILprocessestosupplementtechnicalskills.

Contents

Acknowledgments

Introduction

PartINetworkBasicsChapter1WhatIsaNetwork?

LocalAreaNetwork

Basebandvs.Broadband

PacketSwitchingvs.CircuitSwitching

CablesandTopologies

MediaAccessControl

Addressing

Repeaters,Bridges,Switches,andRouters

WideAreaNetworks

ProtocolsandStandards

ClientsandServers

OperatingSystemsandApplications

Chapter2TheOSIReferenceModel

CommunicationsBetweentheLayers

DataEncapsulation

HorizontalCommunications

VerticalCommunications

EncapsulationTerminology

ThePhysicalLayer

PhysicalLayerSpecifications

PhysicalLayerSignaling

TheDataLinkLayer

Addressing

MediaAccessControl

ProtocolIndicator

ErrorDetection

TheNetworkLayer

Routing

Fragmenting

Connection-OrientedandConnectionlessProtocols

TheTransportLayer

ProtocolServiceCombinations

TransportLayerProtocolFunctions

SegmentationandReassembly

FlowControl

ErrorDetectionandRecovery

TheSessionLayer

DialogControl

DialogSeparation

ThePresentationLayer

TheApplicationLayer

PartIINetworkHardwareChapter3NetworkInterfaceAdapters

NICFunctions

NICFeatures

FullDuplex

BusMastering

ParallelTasking

Wake-on-LANorWake-on-Wireless-LAN

SelectingaNIC

Protocol

TransmissionSpeed

NetworkInterface

BusInterface

Bottlenecks

ISAorPCI?

IntegratedAdapters

Fiber-OpticNICs

PortableSystems

HardwareResourceRequirements

PowerRequirements

Servervs.WorkstationNICs

Chapter4NetworkInterfaceAdaptersandConnectionDevices

Repeaters

Hubs

PassiveHubs

Repeating,Active,andIntelligentHubs

TokenRingMAUs

HubConfigurations

TheUplinkPort

StackableHubs

ModularHubs

Bridges

TransparentBridging

BridgeLoops

SourceRouteBridging

BridgingEthernetandTokenRingNetworks

Routers

RouterApplications

RouterFunctions

RoutingTables

WindowsRoutingTables

RoutingTableParsing

StaticandDynamicRouting

SelectingtheMostEfficientRoute

DiscardingPackets

PacketFragmentation

RoutingandICMP

RoutingProtocols

Switches

SwitchTypes

Routingvs.Switching

VirtualLANs

Layer3Switching

Multiple-LayerSwitching

Chapter5CablingaNetwork

CableProperties

CablingStandards

DataLinkLayerProtocolStandards

CoaxialCable

ThickEthernet

ThinEthernet

CableTelevision

Twisted-PairCable

UnshieldedTwisted-Pair

Category5e

Cat6and6a

Cat7

ConnectorPinouts

ShieldedTwisted-Pair

Fiber-OpticCable

Fiber-OpticCableConstruction

Fiber-OpticConnectors

Chapter6WirelessLANs

WirelessNetworks

AdvantagesandDisadvantagesofWirelessNetworks

TypesofWirelessNetworks

WirelessApplications

TheIEEE802.11Standards

ThePhysicalLayer

PhysicalLayerFrames

TheDataLinkLayer

DataLinkLayerFrames

MediaAccessControl

Chapter7WideAreaNetworks

IntroductiontoTelecommunications

WANUtilization

SelectingaWANTechnology

PSTN(POTS)Connections

LeasedLines

Leased-LineTypes

Leased-LineHardware

Leased-LineApplications

ISDN

ISDNServices

ISDNCommunications

ISDNHardware

DSL

SwitchingServices

Packet-SwitchingServices

Circuit-SwitchingServices

FrameRelay

Frame-RelayHardware

VirtualCircuits

Frame-RelayMessaging

ATM

ThePhysicalLayer

TheATMLayer

TheATMAdaptationLayer

ATMSupport

SONET

Chapter8ServerTechnologies

PurchasingaServer

UsingMultipleProcessors

ParallelProcessing

ServerClustering

UsingHierarchicalStorageManagement

FibreChannelNetworking

NetworkStorageSubsystems

Chapter9DesigningaNetwork

ReasoningtheNeed

SeekingApproval

DesigningaHomeorSmall-OfficeNetwork

SelectingComputers

SelectingaNetworkingProtocol

ChoosingaNetworkMedium

ChoosingaNetworkSpeed

DesigninganInternetwork

SegmentsandBackbones

DistributedandCollapsedBackbones

BackboneFaultTolerance

SelectingaBackboneLANProtocol

ConnectingtoRemoteNetworks

SelectingaWANTopology

PlanningInternetAccess

LocatingEquipment

WiringClosets

DataCenters

FinalizingtheDesign

PartIIINetworkProtocolsChapter10EthernetBasics

EthernetDefined

EthernetStandards

EthernetII

IEEE802.3

DIXEthernetandIEEE802.3Differences

IEEEShorthandIdentifiers

CSMA/CD

Collisions

LateCollisions

PhysicalLayerGuidelines

10Base-5(ThickEthernet)

10Base-2(ThinEthernet)

10Base-Tor100Base-T(Twisted-PairEthernet)

Fiber-OpticEthernet

CablingGuidelines

ExceedingEthernetCablingSpecifications

TheEthernetFrame

TheIEEE802.3Frame

TheEthernetIIFrame

TheLogicalLinkControlSublayer

TheSNAPHeader

Full-DuplexEthernet

Full-DuplexRequirements

Full-DuplexFlowControl

Full-DuplexApplications

Chapter11100BaseEthernetandGigabitEthernet

100BaseEthernet

PhysicalLayerOptions

CableLengthRestrictions

Autonegotiation

GigabitEthernet

GigabitEthernetArchitecture

MediaAccessControl

TheGigabitMedia-IndependentInterface

ThePhysicalLayer

EthernetTroubleshooting

EthernetErrors

IsolatingtheProblem

100VG-AnyLAN

TheLogicalLinkControlSublayer

TheMACandRMACSublayers

ThePhysicalMediumIndependentSublayer

TheMedium-IndependentInterfaceSublayer

ThePhysicalMediumDependentSublayer

TheMedium-DependentInterface

Workingwith100VG-AnyLAN

Chapter12NetworkingProtocols

TokenRing

TheTokenRingPhysicalLayer

TokenPassing

TokenRingFrames

TokenRingErrors

FDDI

FDDITopology

PartIVNetworkSystemsChapter13TCP/IP

TCP/IPAttributes

TCP/IPArchitecture

TheTCP/IPProtocolStack

IPVersions

IPv4Addressing

SubnetMasking

IPAddressRegistration

SpecialIPAddresses

Subnetting

PortsandSockets

TCP/IPNaming

TCP/IPProtocols

SLIPandPPP

ARP

IP

Chapter14OtherTCP/IPProtocols

IPv6

IPv6Addresses

IPv6AddressStructure

OtherProtocols

ICMP

UDP

TCP

Chapter15TheDomainNameSystem

HostTables

HostTableProblems

DNSObjectives

DomainNaming

Top-LevelDomains

Second-LevelDomains

Subdomains

DNSFunctions

ResourceRecords

DNSNameResolution

ReverseNameResolution

DNSNameRegistration

ZoneTransfers

DNSMessaging

TheDNSHeaderSection

TheDNSQuestionSection

DNSResourceRecordSections

DNSMessageNotation

NameResolutionMessages

RootNameServerDiscovery

ZoneTransferMessages

Chapter16InternetServices

WebServers

SelectingaWebServer

HTML

HTTP

FTPServers

FTPCommands

FTPReplyCodes

FTPMessaging

E-mail

E-mailAddressing

E-mailClientsandServers

SimpleMailTransferProtocol

PostOfficeProtocol

InternetMessageAccessProtocol

PartVNetworkOperatingServicesChapter17Windows

TheRoleofWindows

Versions

ServicePacks

MicrosoftTechnicalSupport

OperatingSystemOverview

KernelModeComponents

UserModeComponents

Services

TheWindowsNetworkingArchitecture

TheNDISInterface

TheTransportDriverInterface

TheWorkstationService

TheServerService

APIs

FileSystems

FAT16

FAT32

NTFS

ResilientFileSystem

TheWindowsRegistry

OptionalWindowsNetworkingServices

ActiveDirectory

MicrosoftDHCPServer

MicrosoftDNSServer

WindowsInternetNamingService

Chapter18ActiveDirectory

ActiveDirectoryArchitecture

ObjectTypes

ObjectNaming

Domains,Trees,andForests

DNSandActiveDirectory

GlobalCatalogServer

DeployingActiveDirectory

CreatingDomainControllers

DirectoryReplication

Sites

MicrosoftManagementConsole

DesigninganActiveDirectory

PlanningDomains,Trees,andForests

Chapter19Linux

UnderstandingLinux

LinuxDistributions

AdvantagesandDisadvantagesofLinux

FileSystems

LinuxInstallationQuestions

DirectoryStructure

QuickCommandsinLinux

WorkingwithLinuxFiles

Journaling

Editing

LackofFragmentation

Chapter20Unix

UnixPrinciples

UnixArchitecture

UnixVersions

UnixSystemV

BSDUnix

UnixNetworking

UsingRemoteCommands

BerkeleyRemoteCommands

DARPACommands

NetworkFileSystem

Client-ServerNetworking

Chapter21OtherNetworkOperatingSystemsandNetworkingintheCloud

HistoricalSystems

FreeBSD

NetBSD

OpenBSD

OracleSolaris

OperatingintheCloud

HistoryoftheCloud

BenefitsoftheCloud

DisadvantagesintheCloud

HowtheCloudWorks

CloudTypes

CloudServiceModels

InfrastructureasaService

PlatformasaService

SoftwareasaService

NetworkasaService

PartVINetworkServicesChapter22NetworkClients

WindowsNetworkClients

WindowsNetworkingArchitecture

NetWareClients

MacintoshClients

ConnectingMacintoshSystemstoWindowsNetworks

UnixClients

Applications

UnixAccess

Windows7Interface

Windows8Interface

Chapter23NetworkSecurityBasics

SecuringtheFileSystem

TheWindowsSecurityModel

WindowsFileSystemPermissions

UnixFileSystemPermissions

VerifyingIdentities

FTPUserAuthentication

Kerberos

PublicKeyInfrastructure

DigitalCertificates

Token-BasedandBiometricAuthentication

SecuringNetworkCommunications

IPsec

SSL

Firewalls

PacketFilters

NetworkAddressTranslation

ProxyServers

Circuit-LevelGateways

CombiningFirewallTechnologies

Chapter24WirelessSecurity

WirelessFunctionality

WirelessNetworkComponents

WirelessRouterTypes

WirelessTransmission

WirelessAccessPoints

CreatingaSecureWirelessNetwork

SecuringaWirelessHomeNetwork

SecuringaBusinessNetwork

SecuringaWirelessRouter

SecuringMobileDevices

WhatAretheRisks?

UnsecuredHomeNetworks

WirelessInvasionTools

UnderstandingEncryption

Chapter25OverviewofNetworkAdministration

LocatingApplicationsandDatainWindowsSystems

Server-BasedOperatingSystems

Server-BasedApplications

StoringDataFiles

ControllingtheWorkstationEnvironment

DriveMappingsinWindows

UserProfiles

ControllingtheWorkstationRegistry

UsingSystemPolicies

Chapter26NetworkManagementandTroubleshootingTools

OperatingSystemUtilities

WindowsUtilities

TCP/IPUtilities

NetworkAnalyzers

FilteringData

TrafficAnalysis

ProtocolAnalysis

CableTesters

Chapter27BackingUp

BackupHardware

BackupCapacityPlanning

HardDiskDrives

RAIDSystems

UsingRAID

Network-AttachedStorage

MagneticTapeDrives

TapeDriveInterfaces

MagneticTapeCapacities

BackupSoftware

SelectingBackupTargets

BackingUpOpenFiles

RecoveringfromaDisaster

JobScheduling

RotatingMedia

BackupAdministration

EventLogging

PerformingRestores

Index

TAcknowledgments

hisbook,likemostothers,istheendproductofalotofhardworkbymanypeople.Allofthepeopleinvolveddeservegreatthanks.Aspecialthank-youtothefollowing:

RogerStewart,acquisitionseditoratMcGraw-HillEducation,forhissupport,understanding,andalwaysavailableear.Heandhisteamareunbeatable.

Twoothermembersoftheteam,PattyMonandAmandaRussell.Pattyisthefinesteditorialsupervisoraround.Sheisbeyondhelpful,alwaysconsiderateandthoughtful,andjustthereforanyquestions.Sheisagem.Thegenerous,organized,andalwaysontopofanyconcernorissue,editorialcoordinatorAmandaRussell.Amandaeitherhastheanswerathandorfindsoutquicklyandreliably.Thesefewdescriptivewordsareonlythetipoftheicebergwhendiscussingtheirtalent,professionalism,andalwaysgenerousspirits.

Thetechnicaleditors,RandyNollanandDwightSpivey,forthesupport,suggestions,andideas.Theseskilledandproficientgentlemenmadetheprocessfun.Andaspecialthank-youtoVanAguirreforhishardworkatthebeginningoftheproject.

AsheeshRatraandhisteamatMPSLimited,whodeservegreatthanksandappreciationfortheirhardworkandexpertise.Itwasapleasureandhonorworkingwiththem!

TIntroduction

hisbookisdesignedasathorough,practicalplanningguideandunderpinningofknowledgeforITnetworkingprofessionalsaroundtheworld,includingstudentsofIT

networkingcourses,beginningnetworkadministrators,andthoseseekingworkintheITnetworkingfield.

BenefittoYou,theReaderAfterreadingthisbook,youwillbeabletosetupaneffectivenetwork.Thebookteacheseverything,includingmethodology,analysis,caseexamples,tips,andallthetechnicalsupportingdetailsneededtosuitanITaudiencesrequirements,soitwillbenefiteveryonefrombeginnerstothosewhoareintermediate-levelpractitioners.

WhatThisBookCoversThisbookcoversthedetailsaswellasthebigpicturefornetworking,includingbothphysicalandvirtualnetworks.Itdiscusseshowtoevaluatethevariousnetworkingoptionsandexplainshowtomanagenetworksecurityandtroubleshooting.

OrganizationThisbookislogicallyorganizedintosixparts.Withineachpart,thechaptersstartwithbasicconceptsandprocedures,mostofwhichinvolvespecificnetworkingtasks,andthenworktheirwayuptomoreadvancedtopics.

Itisnotnecessarytoreadthisbookfrombeginningtoend.Skiparoundasdesired.Thefollowingsectionssummarizethebooksorganizationandcontents.

PartI:NetworkBasicsThispartofthebookintroducesnetworkingconceptsandexplainsboththeOSIandTCP/IPmodels.

Chapter1:WhatIsaNetwork?

Chapter2:TheOSIReferenceModel

PartII:NetworkHardwareThispartofthebookdiscussesthevarioushardwareitemsusedinacomputernetwork.Italsoexplainssomebasicswhendesigninganetwork.

Chapter3:NetworkInterfaceAdapters

Chapter4:NetworkInterfaceAdaptersandConnectionDevices

Chapter5:CablingaNetwork

Chapter6:WirelessLANs

Chapter7:WideAreaNetworks

Chapter8:ServerTechnologies

Chapter9:DesigningaNetwork

PartIII:NetworkProtocolsThispartofthebookexplainsthevariousrulesandprotocolsfornetworks.

Chapter10:EthernetBasics

Chapter11:100BaseEthernetandGigabitEthernet

Chapter12:NetworkingProtocols

PartIV:NetworkSystemsThispartofthebookdiscussesthevariousnetworkoperatingsystems.

Chapter13:TCP/IP

Chapter14:OtherTCP/IPProtocols

Chapter15:TheDomainNameSystem

Chapter16:InternetServices

PartV:NetworkOperatingServicesInthispartofthebook,youwilllearnabitmoreaboutthebasicsofsomeoftheotherservicesavailable,includingcloudnetworking.InChapter23,youwilllearnsomeofthebasicsneededtosecureyournetwork.

Chapter17:Windows

Chapter18:ActiveDirectory

Chapter19:Linux

Chapter20:Unix

Chapter21:OtherNetworkOperatingSystemsandNetworkingintheCloud

PartVI:NetworkServicesFromclientstosecuritytotheall-importantbackup,thissectioncoverssomeoftheday-to-dayoperationsinnetworking.

Chapter22:NetworkClients

Chapter23:NetworkSecurityBasics

Chapter24:WirelessSecurity

Chapter25:OverviewofNetworkAdministration

Chapter26:NetworkManagementandTroubleshootingTools

Chapter27:BackingUp

ConventionsAllhow-tobooksespeciallycomputerbookshavecertainconventionsforcommunicatinginformation.Heresabriefsummaryoftheconventionsusedthroughoutthisbook.

MenuCommandsWindowsandmostotheroperatingsystemsmakecommandsaccessibleonthemenubaratthetopoftheapplicationwindow.Throughoutthisbook,youaretoldwhichmenucommandstochoosetoopenawindowordialogortocompleteatask.Thefollowingformatisusedtoindicatemenucommands:Menu|Submenu(ifapplicable)|Command.

KeystrokesKeystrokesarethekeysyoumustpresstocompleteatask.Therearetwokindsofkeystrokes:

KeyboardshortcutsCombinationsofkeysyoupresstocompleteataskmorequickly.Forexample,theshortcutforclickingaCancelbuttonmaybetopresstheEsckey.Whenyouaretopressakey,youwillseethenameofthekeyinsmallcaps,likethis:ESC.Ifyoumustpresstwoormorekeyssimultaneously,theyareseparatedwithahyphen,likethis:CTRL-P.

LiteraltextTextyoumusttypeinexactlyasitappearsinthebook.Althoughthisbookdoesntcontainmanyinstancesofliteraltext,thereareafew.Literaltexttobetypedisinboldfacetype,likethis:Typehelpattheprompt.

MonospacefontTextthatyouseeatthecommandline.Itlookslikethis:Nslookupnameserver

PART

I NetworkBasicsCHAPTER1

WhatIsaNetwork?

CHAPTER2

TheOSIReferenceModel

CHAPTER

1 WhatIsaNetwork?Atitscore,anetworkissimplytwo(ormore)connectedcomputers.Computerscanbeconnectedwithcablesortelephonelines,ortheycanconnectwirelesslywithradiowaves,fiber-opticlines,oreveninfraredsignals.Whencomputersareabletocommunicate,theycanworktogetherinavarietyofways:bysharingtheirresourceswitheachother,bydistributingtheworkloadofaparticulartask,orbyexchangingmessages.Today,themostwidelyusednetworkistheInternet.Thisbookexaminesindetailhowcomputersonanetworkcommunicate;whatfunctionstheyperform;andhowtogoaboutbuilding,operating,andmaintainingthem.

Theoriginalmodelforcollaborativecomputingwastohaveasinglelargecomputerconnectedtoaseriesofterminals,eachofwhichwouldserviceadifferentuser.Thiswascalledtimesharingbecausethecomputerdivideditsprocessorclockcyclesamongtheterminals.Usingthisarrangement,theterminalsweresimplycommunicationsdevices;theyacceptedinputfromusersthroughakeyboardandsentittothecomputer.Whenthecomputerreturnedaresult,theterminaldisplayeditonascreenorprinteditonpaper.Theseterminalsweresometimescalleddumbterminalsbecausetheydidntperformanycalculationsontheirown.Theterminalscommunicatedwiththemaincomputer,neverwitheachother.

Astimepassedandtechnologyprogressed,engineersbegantoconnectcomputerssothattheycouldcommunicate.Atthesametime,computerswerebecomingsmallerandlessexpensive,givingrisetomini-andmicrocomputers.Thefirstcomputernetworksusedindividuallinks,suchastelephoneconnections,toconnecttwosystems.Thereareanumberofcomputernetworkingtypesandseveralmethodsofcreatingthesetypes,whichwillbecoveredinthischapter.

LocalAreaNetworkSoonafterthefirstIBMPCshitthemarketinthe1980sandrapidlybecameacceptedasabusinesstool,theadvantagesofconnectingthesesmallcomputersbecameobvious.Ratherthansupplyingeverycomputerwithitsownprinter,anetworkofcomputerscouldshareasingleprinter.Whenoneuserneededtogiveafiletoanotheruser,anetworkeliminatedtheneedtoswapfloppydisks.Theproblem,however,wasthatconnectingadozencomputersinanofficewithindividualpoint-to-pointlinksbetweenallofthemwasnotpractical.Theeventualsolutiontothisproblemwasthelocalareanetwork(LAN).

ALANisagroupofcomputersconnectedbyasharedmedium,usuallyacable.Bysharingasinglecable,eachcomputerrequiresonlyoneconnectionandcanconceivablycommunicatewithanyothercomputeronthenetwork.ALANislimitedtoalocalareabytheelectricalpropertiesofthecablesusedtoconstructthemandbytherelativelysmallnumberofcomputersthatcanshareasinglenetworkmedium.LANsaregenerallyrestrictedtooperationwithinasinglebuildingor,atmost,acampusofadjacentbuildings.

Sometechnologies,suchasfiberoptics,haveextendedtherangeofLANstoseveral

kilometers,butitisntpossibletouseaLANtoconnectcomputersindistantcities,forexample.Thatistheprovinceofthewideareanetwork(WAN),asdiscussedlaterinthischapter.

Inmostcases,aLANisabaseband,packet-switchingnetwork.Anunderstandingofthetermsbasebandandpacketswitching,whichareexaminedinthefollowingsections,isnecessarytounderstandhowdatanetworksoperatebecausethesetermsdefinehowcomputerstransmitdataoverthenetworkmedium.

Basebandvs.BroadbandAbasebandnetworkisoneinwhichthecableorothernetworkmediumcancarryonlyasinglesignalatanyonetime.Abroadbandnetwork,ontheotherhand,cancarrymultiplesignalssimultaneously,usingadiscretepartofthecablesbandwidthforeachsignal.Asanexampleofabroadbandnetwork,considerthecabletelevisionserviceyouprobablyhaveinyourhome.AlthoughonlyonecablerunstoyourTV,itsuppliesyouwithdozensofchannelsofprogrammingatthesametime.Ifyouhavemorethanonetelevisionconnectedtothecableservice,theinstallerprobablyusedasplitter(acoaxialfittingwithoneconnectorfortheincomingsignalsandtwoconnectorsforoutgoingsignals)torunthesinglecableenteringyourhousetotwodifferentrooms.ThefactthattheTVscanbetunedtodifferentprogramsatthesametimewhileconnectedtothesamecableprovesthatthecableisprovidingaseparatesignalforeachchannelatalltimes.Abasebandnetworkusespulsesapplieddirectlytothenetworkmediumtocreateasinglesignalthatcarriesbinarydatainencodedform.Comparedtobroadbandtechnologies,basebandnetworksspanrelativelyshortdistancesbecausetheyaresubjecttodegradationcausedbyelectricalinterferenceandotherfactors.Theeffectivemaximumlengthofabasebandnetworkcablesegmentdiminishesasitstransmissionrateincreases.ThisiswhylocalareanetworkingprotocolssuchasEthernethavestrictguidelinesforcableinstallations.

NOTEAcablesegmentisanunbrokennetworkcablethatconnectstwonodes.

PacketSwitchingvs.CircuitSwitchingLANsarecalledpacket-switchingnetworksbecausetheircomputersdividetheirdataintosmall,discreteunitscalledpacketsbeforetransmittingit.Thereisalsoasimilartechniquecalledcellswitching,whichdiffersfrompacketswitchingonlyinthatcellsarealwaysaconsistent,uniformsize,whereasthesizeofpacketsisvariable.MostLANtechnologies,suchasEthernet,TokenRing,andFiberDistributedDataInterface(FDDI),usepacketswitching.AsynchronousTransferMode(ATM)isthecell-switchingLANprotocolthatismostcommonlyused.

UnderstandingPacketsE-mailmaybetheeasiestwaytounderstandpackets.Eachmessageisdividedbythesendingserviceintoaspecificnumberofbytes,oftenbetween1,000and1,500.Theneachpacketissentusingthemostefficientroute.Forexample,ifyouaresendingan

e-mailtoyourcompanyshomeofficefromyourvacationcabin,eachpacketwillprobablytravelalongadifferentroute.Thisismoreefficient,andifanyonepieceofequipmentisnotworkingproperlyinthenetworkwhileamessageisbeingtransferred,thepacketthatwouldusethatpieceofequipmentcanberoutedaroundtheproblemareaandsentonanotherroute.Whenthemessagereachesitsdestination,thepacketsarereassembledfordeliveryoftheentiremessage.

SegmentingthedatainthiswayisnecessarybecausethecomputersonaLANshareasinglecable,andacomputertransmittingasingleunbrokenstreamofdatawouldmonopolizethenetworkfortoolong.Ifyouweretoexaminethedatabeingtransmittedoverapacket-switchingnetwork,youwouldseethepacketsgeneratedbyseveraldifferentsystemsintermixedonthecable.Thereceivingsystem,therefore,musthaveamechanismforreassemblingthepacketsintothecorrectorderandrecognizingtheabsenceofpacketsthatmayhavebeenlostordamagedintransit.

Theoppositeofpacketswitchingiscircuitswitching,inwhichonesystemestablishesadedicatedcommunicationchanneltoanothersystembeforeanydataistransmitted.Inthedatanetworkingindustry,circuitswitchingisusedforcertaintypesofwideareanetworkingtechnologies,suchasIntegratedServicesDigitalNetwork(ISDN)andframerelay.Theclassicexampleofacircuit-switchingnetworkisthepublictelephonesystem.Whenyouplaceacalltoanotherperson,aphysicalcircuitisestablishedbetweenyourtelephoneandtheirs.Thiscircuitremainsactivefortheentiredurationofthecall,andnooneelsecanuseit,evenwhenitisnotcarryinganydata(thatis,whennooneistalking).

Intheearlydaysofthetelephonesystem,everyphonewasconnectedtoacentralofficewithadedicatedcable,andoperatorsusingswitchboardsmanuallyconnectedacircuitbetweenthetwophonesforeverycall.Whiletodaytheprocessisautomatedandthetelephonesystemtransmitsmanysignalsoverasinglecable,theunderlyingprincipleisthesame.

LANswereoriginallydesignedtoconnectasmallnumberofcomputersintowhatlatercametobecalledaworkgroup.Ratherthaninvestingahugeamountofmoneyintoalarge,mainframecomputerandthesupportsystemneededtorunit,businessownerscametorealizethattheycouldpurchaseafewcomputers,cablethemtogether,andperformmostofthecomputingtaskstheyneeded.Asthecapabilitiesofpersonalcomputersandapplicationsgrew,sodidthenetworks,andthetechnologyusedtobuildthemprogressedaswell.

CablesandTopologiesMostLANsarebuiltaroundcoppercablesthatusestandardelectricalcurrentstorelaytheirsignals.Originally,mostLANsconsistedofcomputersconnectedwithcoaxialcables,buteventually,thetwisted-paircablingusedfortelephonesystemsbecamemorepopular.Anotheralternativeisfiber-opticcable,whichdoesntuseelectricalsignalsatallbutinsteadusespulsesoflighttoencodebinarydata.Othertypesofnetworkinfrastructureseliminatecablesentirelyandtransmitsignalsusingwhatisknownasunboundedmedia,suchasradiowaves,infrared,andmicrowaves.

NOTEFormoreinformationaboutthevarioustypesofcablesusedindatanetworking,seeChapter5.

LANsconnectcomputersusingvarioustypesofcablingpatternscalledtopologies(seeFigure1-1),whichdependonthetypeofcableusedandtheprotocolsrunningonthecomputers.Themostcommontopologiesareasfollows:

BusAbustopologytakestheformofacablethatrunsfromonecomputertothenextoneinadaisy-chainfashion,muchlikeastringofChristmastreelights.Allofthesignalstransmittedbythecomputersonthenetworktravelalongthebusinbothdirectionstoalloftheothercomputers.Thetwoendsofthebusmustbeterminatedwithelectricalresistorsthatnullifythevoltagesreachingthemsothatthesignalsdonotreflectintheotherdirection.Theprimarydrawbackofthebustopologyisthat,likethestringofChristmaslightsitresembles,afaultinthecableanywherealongitslengthsplitsthenetworkintwoandpreventssystemsonoppositesidesofthebreakfromcommunicating.Inaddition,thelackofterminationateitherhalfcanpreventcomputersthatarestillconnectedfromcommunicatingproperly.AswithChristmaslights,findingasinglefaultyconnectioninalargebusnetworkcanbetroublesomeandtimeconsuming.Mostcoaxialcablenetworks,suchastheoriginalEthernetLANs,useabustopology.

Star(hubandspoke)Astartopologyusesaseparatecableforeachcomputerthatrunstoacentralcablingnexuscalledahuborconcentrator.Thehubpropagatesthesignalsenteringthroughanyoneofitsportsoutthroughalloftheotherportssothatthesignalstransmittedbyeachcomputerreachalltheothercomputers.Hubsalsoamplifythesignalsastheyprocessthem,enablingthemtotravellongerdistanceswithoutdegrading.Astarnetworkismorefaulttolerantthanabusbecauseabreakinacableaffectsonlythedevicetowhichthatcableisconnected,nottheentirenetwork.Mostofthenetworkingprotocolsthatcallfortwisted-paircable,suchas10Base-Tand100Base-TEthernet,usethestartopology.

StarbusAstarbustopologyisonemethodforexpandingthesizeofaLANbeyondasinglestar.Inthistopology,anumberofstarnetworksarejoinedtogetherusingaseparatebuscablesegmenttoconnecttheirhubs.Eachcomputercanstillcommunicatewithanyothercomputeronthenetworkbecauseeachofthehubstransmitsitsincomingtrafficoutthroughthebusportaswellastheotherstarports.Designedtoexpand10Base-TEthernetnetworks,thestarbusisrarelyseentodaybecauseofthespeedlimitationsofcoaxialbusnetworks,whichcanfunctionasabottleneckthatdegradestheperformanceoffasterstarnetworktechnologiessuchasFastEthernet.

RingThistopologyissimilartoabustopology,exceptthesetopologiestransmitinonedirectiononlyfromstationtostation.Aringtopologyoftenusesseparatephysicalportsandwirestosendandreceivedata.Aringtopologyisfunctionallyequivalenttoabustopologywiththetwoendsconnectedsothatsignalstravelfromonecomputertothenextinanendlesscircularfashion.However,thecommunicationsringisonlyalogicalconstruct,notaphysicalone.

Thephysicalnetworkisactuallycabledusingastartopology,andaspecialhubcalledamultistationaccessunit(MSAU)implementsthelogicalringbytakingeachincomingsignalandtransmittingitoutthroughthenextdownstreamportonly(insteadofthroughalloftheotherports,likeastarhub).Eachcomputer,uponreceivinganincomingsignal,processesit(ifnecessary)andsendsitrightbacktothehubfortransmissiontothenextstationonthering.Becauseofthisarrangement,systemsthattransmitsignalsontothenetworkmustalsoremovethesignalsaftertheyhavetraversedtheentirering.Networksconfiguredinaringtopologycanuseseveraldifferenttypesofcable.TokenRingnetworks,forexample,usetwisted-paircables,whileFDDInetworksusetheringtopologywithfiber-opticcable.

DaisychainsThesetopologiesarethesimplestformasonedeviceisconnectedtoanotherthroughserialports.Thinkofacomputerhookedtoaprinterandtheprinter,inturn,beinghookedtoalaptop.

HierarchicalstarThehierarchicalstartopologyisthemostcommonmethodforexpandingastarnetworkbeyondthecapacityofitsoriginalhub.Whenahubsportsareallfilledandyouhavemorecomputerstoconnecttothenetwork,youcanconnecttheoriginalhubtoasecondhubusingacablepluggedintoaspecialportdesignatedforthispurpose.Trafficarrivingateitherhubisthenpropagatedtotheotherhubaswellastotheconnectedcomputers.ThenumberofhubsthatasingleLANcansupportisdependentontheprotocolituses.

Figure1-1Commoncabletopographies

Thetopologiesdiscussedherearephysicaltopologies,whichdifferfromlogicaltopologiesthatarediscussedinlaterchapters.Physicaltopologiesrefertotheplacementofcablesandothercomponentsofthenetwork.Logicaltopologiesrefertotheflowofdataonthenetwork.

MediaAccessControlWhenmultiplecomputersareconnectedtothesamebasebandnetworkmedium,theremustbeamediaaccesscontrol(MAC)mechanismthatarbitratesaccesstothenetworktopreventsystemsfromtransmittingdataatthesametime.AMACmechanismisafundamentalpartofalllocalareanetworkingprotocolsthatuseasharednetworkmedium.ThetwomostcommonMACmechanismsareCarrierSenseMultipleAccesswithCollisionDetection(CSMA/CD),whichisusedbyEthernetnetworks,andtokenpassing,whichisusedbyTokenRing,FDDI,andotherprotocols.Thesetwomechanismsarefundamentallydifferent,buttheyaccomplishthesametaskbyprovidingeachsystemonthenetworkwithanequalopportunitytotransmititsdata.(FormoreinformationabouttheseMACmechanisms,seeChapter10forCSMA/CDandChapter12fortokenpassing.)

AddressingForsystemsonasharednetworkmediumtocommunicateeffectively,theymusthavesomemeansofidentifyingeachother,usuallysomeformofnumericaladdress.Inmostcases,thenetworkinterfacecard(NIC)installedintoeachcomputerhasanaddresshard-codedintoitatthefactory,calleditsMACaddressorhardwareaddress,whichuniquelyidentifiesthatcardamongallothers.Everypacketthateachcomputertransmitsoverthenetworkcontainstheaddressofthesendingcomputerandtheaddressofthesystemforwhichthepacketisintended.

InadditiontotheMACaddress,systemsmayhaveotheraddressesoperatingatotherlayers.Forexample,TransmissionControlProtocol/InternetProtocol(TCP/IP)requiresthateachsystembeassignedauniqueIPaddressinadditiontotheMACaddressitalreadypossesses.Systemsusethevariousaddressesfordifferenttypesofcommunications.(SeeChapter3formoreinformationonMACaddressingandChapter13formoreinformationonIPaddressing.)

Repeaters,Bridges,Switches,andRoutersLANswereoriginallydesignedtosupportonlyarelativelysmallnumberofcomputers30forthinEthernetnetworksand100forthickEthernetbuttheneedsofbusinessesquicklyoutgrewtheselimitations.Tosupportlargerinstallations,engineersdevelopedproductsthatenabledadministratorstoconnecttwoormoreLANsintowhatisknownasaninternetwork,whichisessentiallyanetworkofnetworksthatenablesthecomputersononenetworktocommunicatewiththoseonanother.DontconfusethegenericterminternetworkwiththeInternet.TheInternetisanexampleofanextremelylargeinternetwork,butanyinstallationthatconsistsoftwoormoreLANsconnectedisalsoaninternetwork.Thisterminologyisconfusingbecauseitissooftenmisused.Sometimeswhatusersmeanwhentheyrefertoanetworkisactuallyaninternetwork,andatothertimes,whatmayseemtobeaninternetworkisactuallyasingleLAN.Strictlyspeaking,aLANoranetworksegmentisagroupofcomputersthatshareanetworkcablesothatabroadcastmessagetransmittedbyonesystemreachesalloftheothersystems,evenifthatsegmentisactuallycomposedofmanypiecesofcable.Forexample,onatypical10Base-TEthernetLAN,allofthecomputersareconnectedtoahubusingindividuallengthsofcable.Regardlessofthatfact,thisarrangementisstillanexampleofanetworksegmentorLAN.IndividualLANscanbeconnectedusingseveraldifferenttypesofdevices,someofwhichsimplyextendtheLANwhileanothercreatesaninternetwork.Thesedevicesareasfollows:

RepeatersArepeaterisapurelyelectricaldevicethatextendsthemaximumdistanceaLANcablecanspanbyamplifyingthesignalspassingthroughit.Thehubsusedonstarnetworksaresometimescalledmultiportrepeatersbecausetheyhavesignalamplificationcapabilitiesintegratedintotheunit.Stand-alonerepeatersarealsoavailableforuseoncoaxialnetworkstoextendthemoverlongerdistances.UsingarepeatertoexpandanetworksegmentdoesnotdivideitintotwoLANsorcreateaninternetwork.

BridgesAbridgeprovidestheamplificationfunctionofarepeater,along

withtheabilitytoselectivelyfilterpacketsbasedontheiraddresses.Packetsthatoriginateononesideofthebridgearepropagatedtotheothersideonlyiftheyareaddressedtoasystemthatexiststhere.Becausebridgesdonotpreventbroadcastmessagesfrombeingpropagatedacrosstheconnectedcablesegments,they,too,donotcreatemultipleLANsortransformanetworkintoaninternetwork.

SwitchesSwitchesarerevolutionarydevicesthatinmanycaseseliminatethesharednetworkmediumentirely.Aswitchisessentiallyamultiportrepeater,likeahub,exceptthatinsteadofoperatingatapurelyelectricallevel,theswitchreadsthedestinationaddressineachincomingpacketandtransmitsitoutonlythroughtheporttowhichthedestinationsystemisconnected.

RoutersArouterisadevicethatconnectstwoLANstoformaninternetwork.Likeabridge,arouterforwardsonlythetrafficthatisdestinedfortheconnectedsegment,butunlikerepeatersandbridges,routersdonotforwardbroadcastmessages.Routerscanalsoconnectdifferenttypesofnetworks(suchasEthernetandTokenRing),whereasbridgesandrepeaterscanconnectonlysegmentsofthesametype.

WideAreaNetworksInternetworkingenablesanorganizationtobuildanetworkinfrastructureofalmostunlimitedsize.InadditiontoconnectingmultipleLANsinthesamebuildingorcampus,aninternetworkcanconnectLANsatdistantlocationsthroughtheuseofwideareanetworklinks.AWANisacollectionofLANs,someorallofwhichareconnectedusingpoint-to-pointlinksthatspanrelativelylongdistances.AtypicalWANconnectionconsistsoftworouters,oneateachLANsite,connectedusingalong-distancelinksuchasaleasedtelephoneline.AnycomputerononeoftheLANscancommunicatewiththeotherLANbydirectingitstraffictothelocalrouter,whichrelaysitovertheWANlinktotheothersite.

WANlinksdifferfromLANsinthattheydonotuseasharednetworkmediumandtheycanspanmuchlongerdistances.Becausethelinkconnectsonlytwosystems,thereisnoneedformediaaccesscontrolorasharednetworkmedium.Anorganizationwithofficeslocatedthroughouttheworldcanbuildaninternetworkthatprovidesuserswithinstantaneousaccesstonetworkresourcesatanylocation.TheWANlinksthemselvescanusetechnologiesrangingfromtelephonelinestopublicdatanetworkstosatellitesystems.UnlikeaLAN,whichisnearlyalwaysprivatelyownedandoperated,anoutsideserviceprovider(suchasatelephonecompany)isnearlyalwaysinvolvedinaWANconnectionbecauseprivateorganizationsdontusuallyownthetechnologiesneededtocarrysignalsoversuchlongdistances.Generallyspeaking,WANconnectionscanbeslowerandmoreexpensivethanLANs,andsometimesmuchmoreso.Asaresult,oneofthegoalsofthenetworkadministratoristomaximizetheefficiencyofWANtrafficbyeliminatingunnecessarycommunicationsandchoosingthebesttypeoflinkfortheapplication.SeeChapter7formoreinformationonWANtechnologies.

TherearealsowirelessLAN/WANnetworksandmetropolitanareanetworks(MANs).AMANhasthreefeaturesthatdifferentiateitfrombothaLANandaWAN:

AMANssizeisusuallybetweenthatofaLANandaWAN.Typically,itcoversbetween3and30miles(5to50km).AMANcanencompassseveralbuildings,acompanycampus,orasmalltown.

AswithWANs,MANsarenormallyownedbyagrouporanetworkprovider.

MANsareoftenusedasawaytoprovidesharedaccesstooneormoreWANs.

ProtocolsandStandardsCommunicationsbetweencomputersonanetworkaredefinedbyprotocols,standardizedmethodsthatthesoftwareprogramsonthecomputershaveincommon.Theseprotocolsdefineeverypartofthecommunicationsprocess,fromthesignalstransmittedovernetworkcablestothequerylanguagesthatenableapplicationsondifferentmachinestoexchangemessages.Networkedcomputersrunaseriesofprotocols,calledaprotocolstack,thatspansfromtheapplicationuserinterfaceatthetoptothephysicalnetworkinterfaceatthebottom.Thestackistraditionallysplitintosevenlayers.TheOpenSystemsInterconnection(OSI)referencemodeldefinesthefunctionsofeachlayerandhowthelayersworktogethertoprovidenetworkcommunications.Chapter2coverstheOSIreferencemodelindetail.

Earlynetworkingproductstendedtobeproprietarysolutionscreatedbyasinglemanufacturer,butastimepassed,interoperabilitybecameagreaterpriority,andorganizationswereformedtodevelopandratifynetworkingprotocolstandards.Mostofthesebodiesareresponsibleforlargenumbersoftechnicalandmanufacturingstandardsinmanydifferentdisciplines.Today,mostoftheprotocolsincommonusearestandardizedbythesebodies,someofwhichareasfollows:

InstituteofElectricalandElectronicEngineers(IEEE)AU.S.-basedsocietyresponsibleforthepublicationoftheIEEE802workinggroup,whichincludesthestandardsthatdefinetheprotocolscommonlyknownasEthernetandTokenRing,aswellasmanyothers.

InternationalOrganizationforStandardization(ISO)Aworldwidefederationofstandardsbodiesfrommorethan100countries,responsibleforthepublicationoftheOSIreferencemodeldocument.

InternetEngineeringTaskForce(IETF)AnadhocgroupofcontributorsandconsultantswhocollaboratetodevelopandpublishstandardsforInternettechnologies,includingtheTCP/IPprotocols.

ClientsandServersLocalareanetworkingisbasedontheclient-serverprinciple,inwhichtheprocessesneededtoaccomplishaparticulartaskaredividedbetweencomputersfunctioningasclientsandservers.Thisisindirectcontrasttothemainframemodel,inwhichthecentralcomputerdidalloftheprocessingandsimplytransmittedtheresultstoauserataremoteterminal.Aserverisacomputerrunningaprocessthatprovidesaservicetoother

computerswhentheyrequestit.Aclientisthecomputerrunningaprogramthatrequeststheservicefromaserver.

Forexample,aLAN-baseddatabaseapplicationstoresitsdataonaserver,whichstandsby,waitingforclientstorequestinformationfromit.Usersatworkstationcomputersrunadatabaseclientprograminwhichtheygeneratequeriesthatrequestspecificinformationinthedatabaseandtransmitthosequeriestotheserver.Theserverrespondstothequerieswiththerequestedinformationandtransmitsittotheworkstations,whichformatitfordisplaytotheusers.Inthiscase,theworkstationsareresponsibleforprovidingauserinterfaceandtranslatingtheuserinputintoaquerylanguageunderstoodbytheserver.Theyarealsoresponsiblefortakingtherawdatafromtheserveranddisplayingitinacomprehensibleformtotheuser.Theservermayhavetoservicedozensorhundredsofclients,soitisstillapowerfulcomputer.Byoffloadingsomeoftheapplicationsfunctionstotheworkstations,however,itsprocessingburdenisnowherenearwhatitwouldbeonamainframesystem.

OperatingSystemsandApplicationsClientsandserversareactuallysoftwarecomponents,althoughsomepeopleassociatethemwithspecifichardwareelements.Thisconfusionisbecausesomenetworkoperatingsystemsrequirethatacomputerbededicatedtotheroleofserverandthatothercomputersfunctionsolelyasclients.Thisisaclient-serveroperatingsystem,asopposedtoapeer-to-peeroperatingsystem,inwhicheverycomputercanfunctionasbothaclientandaserver.Themostbasicclient-serverfunctionalityprovidedbyanetworkoperatingsystem(NOS)istheabilitytosharefilesystemdrivesandprinters,andthisiswhatusuallydefinestheclientandserverroles.Atitscore,aNOSmakesservicesavailabletoitsnetworkclients.Thesystemcanprovidethefollowing:

Printerservices,includingmanagingdevices,printjobs,whoisusingwhatasset,andwhatassetsarenotavailabletothenetwork

Managinguseraccesstofilesandotherresources,suchastheInternet

Systemmonitoring,includingprovidingnetworksecurity

Makingnetworkadministrationutilitiesavailabletonetworkadministrators

Apartfromtheinternalfunctionsofnetworkoperatingsystems,manyLANapplicationsandnetworkservicesalsooperateusingtheclient-serverparadigm.Internetapplications,suchastheWorldWideWeb,consistofserversandclients,asdoadministrativeservicessuchastheDomainNameSystem(DNS).

MostoftodaysdesktopoperatingsystemsarecapableofprovidingsomeoftheservicestraditionallyascribedtoNOSssincemanysmall-office/home-office(SOHO)LANimplementationstakeadvantageofthefact.UnderstandingthismayhelpclarifythedistinctionbetweenLANsthataretrulyclient-server,relyingonnetworkoperatingsystems,andthosenetworkconfigurationsthatleveragepowerfulcomputerswithtodaysoperatingsystems.Theseoperatingsystemsarenotlimitedtocomputers,butcanincludecellphones,tablets,andotherproductsthatarenotconsideredtobecomputers.

CHAPTER

2 TheOSIReferenceModelNetworkcommunicationstakeplaceonmanylevelsandcanbedifficulttounderstand,evenfortheknowledgeablenetworkadministrator.TheOpenSystemsInterconnection(OSI)referencemodelisatheoreticalconstructionthatseparatesnetworkcommunicationsintosevendistinctlayers,asshowninFigure2-1.Eachcomputeronthenetworkusesaseriesofprotocolstoperformthefunctionsassignedtoeachlayer.Thelayerscollectivelyformwhatisknownastheprotocolstackornetworkingstack.Atthetopofthestackistheapplicationthatmakesarequestforaresourcelocatedelsewhereonthenetwork,andatthebottomisthephysicalmediumthatactuallyconnectsthecomputersandformsthenetwork,suchasacable.

Figure2-1TheOSIreferencemodelwithitssevenlayers

TheOSIreferencemodelwasdevelopedintwoseparateprojectsbytheInternationalOrganizationforStandardization(ISO)andtheComitConsultatifInternationalTlphoniqueetTlgraphique(ConsultativeCommitteeforInternationalTelephoneandTelegraphy,orCCITT),whichisnowknownastheTelecommunicationsStandardizationSectoroftheInternationalTelecommunicationsUnion(ITU-T).Eachofthesetwobodiesdevelopeditsownseven-layermodel,butthetwoprojectswerecombinedin1983,resultinginadocumentcalledTheBasicReferenceModelforOpenSystemsInterconnectionthatwaspublishedbytheISOasISO7498andbytheITU-TasX.200.

TheOSIstackwasoriginallyconceivedasthemodelforthecreationofaprotocolsuitethatwouldconformexactlytothesevenlayers.Thissuitenevermaterializedinacommercialform,however,andthemodelhassincebeenusedasateaching,reference,andcommunicationstool.Networkingprofessionals,educators,andauthorsfrequentlyrefertoprotocols,devices,orapplicationsasoperatingataparticularlayeroftheOSImodelbecauseusingthismodelbreaksacomplexprocessintomanageableunitsthatprovideacommonframeofreference.Manyofthechaptersinthisbookusethelayersofthemodeltohelpdefinenetworkingconcepts.However,itisimportanttounderstandthatnoneoftheprotocolstacksincommonusetodayconformsexactlytothelayersoftheOSImodel.Inmanycases,protocolshavefunctionsthatoverlaptwoormorelayers,suchasEthernet,whichisconsideredadatalinklayerprotocolbutwhichalsodefineselementsof

thephysicallayer.

TheprimaryreasonwhyrealprotocolstacksdifferfromtheOSImodelisthatmanyoftheprotocolsusedtoday(includingEthernet)wereconceivedbeforetheOSImodeldocumentswerepublished.Infact,theTCP/IPprotocolshavetheirownlayeredmodel,whichissimilartotheOSImodelinseveralwaysbutusesonlyfourlayers(seeFigure2-2).Inaddition,developersareusuallymoreconcernedwithpracticalfunctionalitythanwithconformingtoapreexistingmodel.Theseven-layermodelwasdesignedtoseparatethefunctionsoftheprotocolstackinsuchawayastomakeitpossibleforseparatedevelopmentteamstoworkontheindividuallayers,thusstreamliningthedevelopmentprocess.However,ifasingleprotocolcaneasilyprovidethefunctionsthataredefinedasbelonginginseparatelayersofthemodel,whydivideitintotwoseparateprotocolsjustforthesakeofconformity?

Figure2-2TheOSIreferencemodelandtheTCP/IPprotocolstack

CommunicationsBetweentheLayersNetworkingistheprocessofsendingmessagesfromoneplacetoanother,andtheprotocolstackillustratedintheOSImodeldefinesthebasiccomponentsneededtotransmitmessagestotheirdestinations.Thecommunicationprocessiscomplexbecausetheapplicationsthatgeneratethemessageshavevaryingrequirements.Somemessageexchangesconsistofbriefrequestsandrepliesthathavetobeexchangedasquicklyaspossibleandwithaminimumamountofoverhead.Othernetworktransactions,suchasprogramfiletransfers,involvethetransmissionoflargeramountsofdatathatmustreachthedestinationinperfectcondition,withoutalterationofasinglebit.Stillothertransmissions,suchasstreamingaudioorvideo,consistofhugeamountsofdatathatcansurvivethelossofanoccasionalbit,byte,orpacket,butthatmustreachthedestinationinatimelymanner.

Thenetworkingprocessalsoincludesanumberofconversionsthatultimatelytaketheapplicationprogramminginterface(API)callsgeneratedbyapplicationsandtransformthemintoelectricalcharges,pulsesoflight,orothertypesofsignalsthatcanbetransmittedacrossthenetworkmedium.Finally,thenetworkingprotocolsmustseetoitthatthetransmissionsreachtheappropriatedestinationsinatimelymanner.Justasyou

packagealetterbyplacingitinanenvelopeandwritinganaddressonit,thenetworkingprotocolspackagethedatageneratedbyanapplicationandaddressittoanothercomputeronthenetwork.

DataEncapsulationTosatisfyalloftherequirementsjustdescribed,theprotocolsoperatingatthevariouslayersworktogethertosupplyaunifiedqualityofservice.Eachlayerprovidesaservicetothelayersdirectlyaboveandbelowit.Outgoingtraffictravelsdownthroughthestacktothenetworkphysicalmedium,acquiringthecontrolinformationneededtomakethetriptothedestinationsystemasitgoes.Thiscontrolinformationtakestheformofheaders(andinonecaseafooter)thatsurroundthedatareceivedfromthelayerabove,inaprocesscalleddataencapsulation.Theheadersandfooterarecomposedofindividualfieldsthatcontaincontrolinformation(necessary/requiredbythesystemtodeliver)usedtogetthepackettoitsdestination.Inasense,theheadersandfooterformtheenvelopethatcarriesthemessagereceivedfromthelayerabove.

Inatypicaltransaction,showninFigure2-3,anapplicationlayerprotocol(whichalsoincludespresentationandsessionlayerfunctions)generatesamessagethatispasseddowntoatransportlayerprotocol.Theprotocolatthetransportlayerhasitsownpacketstructure,calledaprotocoldataunit(PDU),whichincludesspecializedheaderfieldsandadatafieldthatcarriesthepayload.Inthiscase,thepayloadisthedatareceivedfromtheapplicationlayerprotocol.BypackagingthedatainitsownPDU,thetransportlayerencapsulatestheapplicationlayerdataandthenpassesitdowntothenextlayer.

Figure2-3Theapplicationlayerdataisencapsulatedfortransmissionbytheprotocolsatthelowerlayersinthestack.

ThenetworklayerprotocolthenreceivesthePDUfromthetransportlayerand

encapsulatesitwithinitsownPDUbyaddingaheaderandusingtheentiretransportlayerPDU(includingtheapplicationlayerdata)asitspayload.ThesameprocessoccursagainwhenthenetworklayerpassesitsPDUtothedatalinklayerprotocol,whichaddsaheaderandfooter.Toadatalinklayerprotocol,thedatawithintheframeistreatedaspayloadonly,justaspostalemployeeshavenoideawhatisinsidetheenvelopestheyprocess.Theonlysystemthatreadstheinformationinthepayloadisthecomputerpossessingthedestinationaddress.Thatcomputertheneitherpassesthenetworklayerprotocoldatacontainedinthepayloadupthroughitsprotocolstackorusesthatdatatodeterminewhatthenextdestinationofthepacketshouldbe.Inthesameway,theprotocolsoperatingattheotherlayersareconsciousoftheirownheaderinformationbutareunawareofwhatdataisbeingcarriedinthepayload.

Onceitisencapsulatedbythedatalinklayerprotocol,thecompletedpacket(nowcalledaframe)isthenreadytobeconvertedtotheappropriatetypeofsignalusedbythenetworkmedium.Thus,thefinalpacket,astransmittedoverthenetwork,consistsoftheoriginalapplicationlayerdataplusseveralheadersappliedbytheprotocolsatthesucceedinglayers,asshowninFigure2-4.

Figure2-4Anencapsulatedframe,readyfortransmission

NOTEEachlayermusttranslatedataintoitsspecificformatbeforesendingiton.Therefore,eachlayercreatesitsownPDUtotransmittothenextlayer.Aseachlayerreceivesdata,thePDUofthepreviouslayerisread,andanewPDUiscreatedusingthatlayersprotocol.Remember,aPDUisacompletemessage(orpacket)thatincludestheprotocolofthesendinglayer.Atthephysicallayer,youendupwithamessagethatconsistsofallthedatathathasbeenencapsulatedwiththeheadersand/orfootersfromeachofthepreviouslayers.

HorizontalCommunicationsFortwocomputerstocommunicateoveranetwork,theprotocolsusedateachlayeroftheOSImodelinthetransmittingsystemmustbeduplicatedatthereceivingsystem.Whenthepacketarrivesatitsdestination,theprocessbywhichtheheadersareappliedatthesourceisrepeatedinreverse.Thepackettravelsupthroughtheprotocolstack,andeachsuccessiveheaderisstrippedoffbytheappropriateprotocolandprocessed.Inessence,theprotocolsoperatingatthevariouslayerscommunicatehorizontallywiththeircounterpartsintheothersystem,asshowninFigure2-5.

Figure2-5Eachlayerhaslogicalconnectionswithitscounterpartinothersystems.

Thehorizontalconnectionsbetweenthevariouslayersarelogical;thereisnodirectcommunicationbetweenthem.Theinformationincludedineachprotocolheaderbythetransmittingsystemisamessagethatiscarriedtothesameprotocolinthedestinationsystem.

VerticalCommunicationsTheheadersappliedbythevariousprotocolsimplementthespecificfunctionscarriedoutbythoseprotocols.Inadditiontocommunicatinghorizontallywiththesameprotocolintheothersystem,theheaderinformationenableseachlayertocommunicatewiththelayersaboveandbelowit,asshowninFigure2-6.Forexample,whenasystemreceivesapacketandpassesitupthroughtheprotocolstack,thedatalinklayerprotocolheaderincludesafieldthatidentifieswhichnetworklayerprotocolthesystemshouldusetoprocessthepacket.Thenetworklayerprotocolheaderinturnspecifiesoneofthetransportlayerprotocols,andthetransportlayerprotocolidentifiestheapplicationforwhichthedataisultimatelydestined.Thisverticalcommunicationmakesitpossibleforacomputertosupportmultipleprotocolsateachofthelayerssimultaneously.Aslongasapackethasthecorrectinformationinitsheaders,itcanberoutedontheappropriatepaththroughthestacktotheintendeddestination.

Figure2-6EachlayerintheOSImodelcommunicateswiththelayeraboveandbelowit.

EncapsulationTerminologyOneofthemostconfusingaspectsofthedataencapsulationprocessistheterminologyusedtodescribethePDUsgeneratedbyeachlayer.Thetermpacketspecificallyreferstothecompleteunittransmittedoverthenetworkmedium,althoughitalsohasbecomea

generictermforthedataunitatanystageintheprocess.Mostdatalinklayerprotocolsaresaidtoworkwithframesbecausetheyincludebothaheaderandafooterthatsurroundthedatafromthenetworklayerprotocol.ThetermframereferstoaPDUofvariablesize,dependingontheamountofdataenclosed.AdatalinklayerprotocolthatusesPDUsofauniformsize,suchasAsynchronousTransferMode(ATM),issaidtodealincells.

Whentransportlayerdataisencapsulatedbyanetworklayerprotocol,suchastheInternetProtocol(IP)orInternetworkPacketExchange(IPX),theresultingPDUiscalledadatagram.Duringthecourseofitstransmission,adatagrammightbesplitintofragments,eachofwhichissometimesincorrectlycalledadatagram.Theterminologyatthetransportlayerismoreprotocol-specificthanatthelowerlayers.TCP/IP,forexample,hastwotransportlayerprotocols.Thefirst,calledtheUserDatagramProtocol(UDP),alsoreferstothePDUsitcreatesasdatagrams,althoughthesearenotsynonymouswiththedatagramsproducedatthenetworklayer.

WhentheUDPprotocolatthetransportlayerisencapsulatedbytheIPprotocolatthenetworklayer,theresultisadatagrampackagedwithinanotherdatagram.ThedifferencebetweenUDPandtheTransmissionControlProtocol(TCP),whichalsooperatesatthetransportlayer,isthatUDPdatagramsareself-containedunitsthatweredesignedtocontaintheentiretyofthedatageneratedbytheapplicationlayerprotocol.Therefore,UDPistraditionallyusedtotransmitsmallamountsofdata,whileTCP,ontheotherhand,isusedtotransmitlargeramountsofapplicationlayerdatathatusuallydonotfitintoasinglepacket.Asaresult,eachofthePDUsproducedbytheTCPprotocoliscalledasegment,andthecollectionofsegmentsthatcarrytheentiretyoftheapplicationlayerprotocoldataiscalledasequence.ThePDUproducedbyanapplicationlayerprotocolistypicallycalledamessage.Thesessionandpresentationlayersareusuallynotassociatedwithindividualprotocols.Theirfunctionsareincorporatedintootherelementsoftheprotocolstack,andtheydonothavetheirownheadersorPDUs.Allofthesetermsarefrequentlyconfused,anditisnotsurprisingtoseeevenauthoritativedocumentsusethemincorrectly.

NOTEWhileTCPisoftenusedtotransmitdatapacketstoday,thereareinstanceswhereUDPissuitable.Forexample,UDPisusedwhennewerdatawillreplacepreviousdata,suchasinvideostreamingorgaming.Asanotherexampleoftheneedfornewerdata,considerweatherinformationthatmustbeupdatedquicklyduringinclementweather.Also,sinceTCPisaconnection-oriented,streamingprotocol,UDPisthepreferredwaytomulticast(senddataacrossanetworktoseveralusersatthesametime).

ThefollowingsectionsexamineeachofthesevenlayersoftheOSIreferencemodelinturn,thefunctionsthatareassociatedwitheach,andtheprotocolsthataremostcommonlyusedatthoselayers.Asyouproceedthroughthisbook,youwilllearnmoreabouteachoftheindividualprotocolsandtheirrelationshipstotheotherelementsoftheprotocolstack.

ThePhysicalLayer

ThephysicallayeroftheOSImodeldefinestheactualmediumthatcarriesdatafromonecomputertoanother.Thetwomostcommontypesofphysicallayerusedindatanetworkingarecopper-basedelectricalcableandfiber-opticcable.Anumberofwirelessphysicallayerimplementationsuseradiowaves,infraredorlaserlight,microwaves,andothertechnologies.Thephysicallayerincludesthetypeoftechnologyusedtocarrythedata,thetypeofequipmentusedtoimplementthattechnology,thespecificationsofhowtheequipmentshouldbeinstalled,andthenatureofthesignalsusedtoencodethedatafortransmission.

Forexample,formanyyears,themostpopularphysicallayerstandardsusedforlocalareanetworkingwas10Base-TEthernet.Ethernetisprimarilythoughtofasadatalinklayerprotocol.However,aswithmostprotocolsfunctioningatthedatalinklayer,Ethernetincludesspecificphysicallayerimplementations,andthestandardsfortheprotocoldefinetheelementsofthephysicallayeraswell.10Base-TreferredtothetypeofcableusedtoformaparticulartypeofEthernetnetwork.TheEthernetstandarddefined10Base-Tasanunshieldedtwisted-paircable(UTP)containingfourpairsofcopperwiresenclosedinasinglesheath.Today,Ethernetisfoundatmuchfasterspeedssuchas100Base-Trunningat100megabitspersecond,or1000Base-T,whichrunsat1gigabitpersecond.

NOTEThephysicallayerusesthebinarydatasuppliedbythedatalinklayerprotocoltoencodethedataintopulsesoflight,electricalvoltages,orotherimpulsessuitablefortransmissionoverthenetworkmedium.

However,theconstructionofthecableitselfisnottheonlyphysicallayerelementinvolved.ThestandardsusedtobuildanEthernetnetworkalsodefinehowtoinstallthecable,includingmaximumsegmentlengthsanddistancesfrompowersources.Thestandardsspecifywhatkindofconnectorsyouusetojointhecable,thetypeofnetworkinterfacecard(NIC)toinstallinthecomputer,andthetypeofhubyouusetojointhecomputersintoanetworktopology.Finally,thestandardspecifieshowtheNICshouldencodethedatageneratedbythecomputerintoelectricalimpulsesthatcanbetransmittedoverthecable.

Thus,youcanseethatthephysicallayerencompassesmuchmorethanatypeofcable.However,yougenerallydonthavetoknowthedetailsabouteveryelementofthephysicallayerstandard.WhenyoubuyEthernetNICs,cables,andhubs,theyarealreadyconstructedtotheEthernetspecificationsanddesignedtousethepropersignalingscheme.Installingtheequipment,however,canbemorecomplicated.

PhysicalLayerSpecificationsWhileitisrelativelyeasytolearnenoughaboutaLANtechnologytopurchasetheappropriateequipment,installingthecable(orothermedium)ismuchmoredifficultbecauseyoumustbeawareofallthespecificationsthataffecttheprocess.Forexample,theEthernetstandardspublishedbytheIEEE802.3workinggroupspecifythebasicwiringconfigurationguidelinesthatpertaintotheprotocolsmediaaccesscontrol(MAC)andcollisiondetectionmechanisms.Theserulesspecifyelementssuchasthemaximumlengthofacablesegment,thedistancebetweenworkstations,andthenumberofrepeaters

permittedonanetwork.TheseguidelinesarecommonknowledgetoEthernetnetworkadministrators,buttheserulesalonearenotsufficienttoperformalargecableinstallation.Inaddition,therearelocalbuildingcodestoconsider,whichmighthaveagreateffectonacableinstallation.Forthesereasons,largephysicallayerinstallationsshould,inmostcases,beperformedbyprofessionalswhoarefamiliarwithallofthestandardsthatapplytotheparticulartechnologyinvolved.SeeChapter4formoreinformationonnetworkcablingandcableinstallation.

NOTEThelatestrevisiontotheIEEE802.3StandardforEthernetwaspublishedinSeptember2012.Itwasamendedtoaddressnewmarkets,bandwidthspeeds,andmediatypesaccordingtotheIEEEwebsiteathttp://standards.ieee.org.

NOTECollisiondetectioniswhenonedevice(ornode)onanetworkdeterminesthatdatahascollided.Thisissimilartotwopeoplecomingthrougharevolvingdooratthesametime,butinthatcase,onepersoncanseetheotherpersonandstops.Ifonenodehearsadistortedversionofitsowntransmission,thatnodeunderstandsthatacollisionhasoccurredand,justlikethepersonwhostopstoallowtheothertogothroughtherevolvingdoor,thatnodewillstopthetransmissionandwaitforsilenceonthenetworktosenditsdata.

PhysicalLayerSignalingTheprimaryoperativecomponentofaphysicallayerinstallationisthetransceiverfoundinNICs,repeatinghubs,andotherdevices.Thetransceiver,asthenameimplies,isresponsiblefortransmittingandreceivingsignalsoverthenetworkmedium.Onnetworksusingcoppercable,thetransceiverisanelectricaldevicethattakesthebinarydataitreceivesfromthedatalinklayerprotocolandconvertsitintosignalsofvariousvoltages.Unlikealloftheotherlayersintheprotocolstack,thephysicallayerisnotconcernedinanywaywiththemeaningofthedatabeingtransmitted.Thetransceiversimplyconvertszerosandonesintovoltages,pulsesoflight,radiowaves,orsomeothertypeofsignal,butitiscompletelyoblivioustopackets,frames,addresses,andeventhesystemreceivingthesignal.

Thesignalsgeneratedbyatransceivercanbeeitheranalogordigital.Mostdatanetworksusedigitalsignals,butsomeofthewirelesstechnologiesuseanalogradiotransmissionstocarrydata.Analogsignalstransitionbetweentwovaluesgradually,formingthesinewavepatternshowninFigure2-7,whiledigitalvaluetransitionsareimmediateandabsolute.Thevaluesofananalogsignalcanbedeterminedbyvariationsinamplitude,frequency,phase,oracombinationoftheseelements,asinamplitudemodulated(AM)orfrequencymodulated(FM)radiosignalsorinanalogphaselooplock(PLL)circuits.

Figure2-7Analogsignalsformwavepatterns.

Theuseofdigitalsignalsismuchmorecommonindatanetworking,however.Allofthestandardcopperandfiber-opticmediausevariousformsofdigitalsignaling.Thesignalingschemeisdeterminedbythedatalinklayerprotocolbeingused.AllEthernetnetworks,forexample,usetheManchesterencodingscheme,whethertheyarerunningovertwisted-pair,coaxial,orfiber-opticcable.Digitalsignalstransitionbetweenvaluesalmostinstantaneously,producingthesquarewaveshowninFigure2-8.Dependingonthenetworkmedium,thevaluescanrepresentelectricalvoltages,thepresenceorabsenceofabeamoflight,oranyotherappropriateattributeofthemedium.Inmostcases,thesignalisproducedwithtransitionsbetweenapositivevoltageandanegativevoltage,althoughsomeuseazerovalueaswell.Givenastablevoltagewithincircuitspecifications,thetransitionscreatethesignal.

Figure2-8Polarencoding

NOTEDigitalsignalsaresusceptibletovoltagedegradation;adigitalcircuitdesignedfora5-voltapplicationwillmostlikelybehaveerroneouslyifvoltageattenuationresultsinsignalsof3volts,meaningthecircuitwillnownotbeabletodistinguishwhethertherewasatransitioneventsincethesignalisbelowthedesignthreshold.

Figure2-8illustratesasimplesignalingschemecalledpolarsignaling.Inthisscheme,

thesignalisbrokenupintounitsoftimecalledcells,andthevoltageofeachcelldenotesitsbinaryvalue.Apositivevoltageisazero,andanegativevoltageisaone.Thissignalingcodewouldseemtobeasimpleandlogicalmethodfortransmittingbinaryinformation,butithasonecrucialflaw,andthatistiming.Whenthebinarycodeconsistsoftwoormoreconsecutivezerosorones,thereisnovoltagetransitionforthedurationoftwoormorecells.Unlessthetwocommunicatingsystemshaveclocksthatarepreciselysynchronized,itisimpossibletotellforcertainwhetheravoltagethatremainscontinuousforaperiodoftimerepresentstwo,three,ormorecellswiththesamevalue.Rememberthatthesecommunicationsoccuratincrediblyhighratesofspeed,sothetimingintervalsinvolvedareextremelysmall.

Somesystemscanusethistypeofsignalbecausetheyhaveanexternaltimingsignalthatkeepsthecommunicatingsystemssynchronized.However,manydatanetworksrunoverabasebandmediumthatpermitsthetransmissionofonlyonesignalatatime.Asaresult,thesenetworksuseadifferenttypeofsignalingscheme,onethatisself-timing.Inotherwords,thedatasignalitselfcontainsatimingsignalthatenablesthereceivingsystemtocorrectlyinterpretthevaluesandconvertthemintobinarydata.

TheManchesterencodingschemeusedonEthernetnetworksisaself-timingsignalbyvirtueofthefactthateverycellhasavaluetransitionatitsmidpoint.Thisdelineatestheboundariesofthecellstothereceivingsystem.Thebinaryvaluesarespecifiedbythedirectionofthevaluetransition;apositive-to-negativetransitionindicatesavalueofzero,andanegative-to-positivetransitionindicatesavalueofone(seeFigure2-9).Thevaluetransitionsatthebeginningsofthecellshavenofunctionotherthantosetthevoltagetotheappropriatevalueforthemidcelltransition.

Figure2-9TheManchesterencodingscheme

TokenRingnetworksuseadifferentencodingschemecalledDifferentialManchester,whichalsohasavaluetransitionatthemidpointofeachcell.However,inthisscheme,thedirectionofthetransitionisirrelevant;itexistsonlytoprovideatimingsignal.Thevalueofeachcellisdeterminedbythepresenceorabsenceofatransitionatthebeginningofthecell.Ifthetransitionexists,thevalueofthecelliszero;ifthereisnotransition,thevalueofthecellisone(seeFigure2-10).Aswiththemidpointtransition,thedirectionofthetransitionisirrelevant.

Figure2-10TheDifferentialManchesterencodingscheme

TheDataLinkLayerThedatalinklayerprotocolprovidestheinterfacebetweenthephysicalnetworkandtheprotocolstackonthecomputer.Adatalinklayerprotocoltypicallyconsistsofthreeelements:

Theformatfortheframethatencapsulatesthenetworklayerprotocoldata

Themechanismthatregulatesaccesstothesharednetworkmedium

Theguidelinesusedtoconstructthenetworksphysicallayer

Theheaderandfooterappliedtothenetworklayerprotocoldatabythedatalinklayerprotocolaretheoutermostonthepacketasitistransmittedacrossthenetwork.Thisframeis,inessence,theenvelopethatcarriesthepackettoitsnextdestinationand,therefore,providesthebasicaddressinginformationneededtogetitthere.Inaddition,datalinklayerprotocolsusuallyincludeanerror-detectionfacilityandanindicatorthatspecifiesthenetworklayerprotocolthatthereceivingsystemshouldusetoprocessthedataincludedinthepacket.

OnmostLANs,multiplesystemsaccessasinglesharedbasebandnetworkmedium.Thismeansthatonlyonecomputercantransmitdataatanyonetime.Iftwoormoresystemstransmitsimultaneously,acollisionoccurs,andthedataislost.Thedatalinklayerprotocolisresponsibleforcontrollingaccesstothesharedmediumandpreventinganexcessofcollisions.

Whenspeakingofthedatalinklayer,thetermsprotocolandtopologyareoftenconfused,buttheyarenotsynonymous.Ethernetissometimescalledatopologywhenthetopologyactuallyreferstothewayinwhichthecomputersonthenetworkarecabledtogether.SomeformsofEthernetuseabustopology,inwhicheachofthecomputersiscabledtothenextoneinadaisy-chainfashion,whilethestartopology,inwhicheachcomputeriscabledtoacentralhub,ismoreprevalenttoday.Aringtopologyisabuswiththeendsjoinedtogether,andameshtopologyisoneinwhicheachcomputerhasacableconnectiontoeveryothercomputeronthenetwork.Theselasttwotypesaremainlytheoretical;LANstodaydonotusethem.TokenRingnetworksusealogicalring,butthe

computersareactuallycabledusingastartopology.Thisconfusionisunderstandablesincemostdatalinklayerprotocolsincludeelementsofthephysicallayerintheirspecifications.Itisnecessaryforthedatalinklayerprotocoltobeintimatelyrelatedtothephysicallayerbecausemediaaccesscontrolmechanismsarehighlydependentonthesizeoftheframesbeingtransmittedandthelengthsofthecablesegments.

AddressingThedatalinklayerprotocolheadercontainstheaddressofthecomputersendingthepacketandthecomputerthatistoreceiveit.Theaddressesusedatthislayerarethehardware(orMAC)addressesthatinmostcasesarehard-codedintothenetworkinterfaceofeachcomputerandrouterbythemanufacturer.OnEthernetandTokenRingnetworks,theaddressesare6byteslong,thefirst3bytesofwhichareassignedtothemanufacturerbytheInstituteofElectricalandElectronicEngineers(IEEE),andthesecond3bytesofwhichareassignedbythemanufacturer.Someolderprotocolsusedaddressesassignedbythenetworkadministrator,butthefactory-assignedaddressesaremoreefficient,insofarastheyensurethatnoduplicationcanoccur.

Thedatalinklayerprotocoldoesthefollowing:

Providespacketaddressingservices

Packagesthenetworklayerdatafortransmission

Arbitratesnetworkaccess

Checkstransmittedpacketsforerrors

Datalinklayerprotocolsarenotconcernedwiththedeliveryofthepackettoitsultimatedestination,unlessthatdestinationisonthesameLANasthesource.Whenapacketpassesthroughseveralnetworksonthewaytoitsdestination,thedatalinklayerprotocolisresponsibleonlyforgettingthepackettotherouteronthelocalnetworkthatprovidesaccesstothenextnetworkonitsjourney.Thus,thedestinationaddressinadatalinklayerprotocolheaderalwaysreferencesadeviceonthelocalnetwork,eveniftheultimatedestinationofthemessageisacomputeronanetworkmilesaway.

ThedatalinklayerprotocolsusedonLANsrelyonasharednetworkmedium.Everypacketistransmittedtoallofthecomputersonthenetworksegment,andonlythesystemwiththeaddressspecifiedasthedestinationreadsthepacketintoitsmemorybuffersandprocessesit.Theothersystemssimplydiscardthepacketwithouttakinganyfurtheraction.

MediaAccessControlMediaaccesscontrolistheprocessbywhichthedatalinklayerprotocolarbitratesaccesstothenetworkmedium.Inorderforthenetworktofunctionefficiently,eachoftheworkstationssharingthecableorothermediummusthaveanopportunitytotransmititsdataonaregularbasis.Thisiswhythedatatobetransmittedissplitintopacketsinthefirstplace.Ifcomputerstransmittedalloftheirdatainacontinuousstream,theycouldconceivablymonopolizethenetworkforextendedperiodsoftime.

TwobasicformsofmediaaccesscontrolareusedonmostoftodaysLANs.Thetokenpassingmethod,usedbyTokenRingandFDDIsystems,usesaspecialframecalledatokenthatispassedfromoneworkstationtoanother.Onlythesysteminpossessionofthetokenisallowedtotransmititsdata.Aworkstation,onreceivingthetoken,transmitsitsdataandthenreleasesthetokentothenextworkstation.Sincethereisonlyonetokenonthenetworkatanytime(assumingthatthenetworkisfunctioningproperly),itisntpossiblefortwosystemstotransmitatthesametime.

Theothermethod,usedonEthernetnetworks,iscalledCarrierSenseMultipleAccesswithCollisionDetection(CSMA/CD).Inthismethod,whenaworkstationhasdatatosend,itlistenstothenetworkcableandtransmitsifthenetworkisnotinuse.OnCSMA/CDnetworks,itispossible(andevenexpected)forworkstationstotransmitatthesametime,resultinginpacketcollisions.Tocompensateforthis,eachsystemhasamechanismthatenablesittodetectcollisionswhentheyoccurandretransmitthedatathatwaslost.

BothoftheseMACmechanismsrelyonthephysicallayerspecificationsforthenetworktofunctionproperly.Forexample,anEthernetsystemcandetectcollisionsonlyiftheyoccurwhiletheworkstationisstilltransmittingapacket.Ifanetworksegmentistoolong,acollisionmayoccurafterthelastbitofdatahasleftthetransmittingsystemandthusmaygoundetected.Thedatainthatpacketisthenlost,anditsabsencecanbedetectedonlybytheupperlayerprotocolsinthesystemthataretheultimatedestinationsofthemessage.Thisprocesstakesarelativelylongtimeandsignificantlyreducestheefficiencyofthenetwork.Thus,whiletheOSIreferencemodelmightcreateaneatdivisionbetweenthephysicalanddatalinklayers,intherealworld,thefunctionalityofthetwoismorecloselyintertwined.

ProtocolIndicatorMostdatalinklayerprotocolimplementationsaredesignedtosupporttheuseofmultiplenetworklayerprotocolsatthesametime.Thismeansthereareseveralpossiblepathsthroughtheprotocolstackoneachcomputer.Tousemultipleprotocolsatthenetworklayer,thedatalinklayerprotocolheadermustincludeacodethatspecifiesthenetworklayerprotocolthatwasusedtogeneratethepayloadinthepacket.Thisrequirementissothatthereceivingsystemcanpassthedataenclosedintheframeuptotheappropriatenetworklayerprocess.

ErrorDetectionMostdatalinklayerprotocolsareunlikealloftheupperlayerprotocolsinthattheyincludeafooterthatfollowsthepayloadfieldinadditiontotheheaderthatprecedesit.Thisfootercontainsaframechecksequence(FCS)fieldthatthereceivingsystemusestodetectanyerrorsthathaveoccurredduringthetransmission.Todothis,thesystemtransmittingthepacketcomputesacyclicalredundancycheck(CRC)valueontheentireframeandincludesitintheFCSfield.Whenthepacketreachesitsnextdestination,thereceivingsystemperformsthesamecomputationandcomparesitsresultswiththevalueintheFCSfield.Ifthevaluesdonotmatch,thepacketisassumedtohavebeendamagedintransitandissilentlydiscarded.

Thereceivingsystemtakesnoactiontohavediscardedpacketsretransmitted;thisisleftuptotheprotocolsoperatingattheupperlayersoftheOSImodel.Thiserror-detectionprocessoccursateachhopinthepacketsjourneytoitsdestination.Someupper-layerprotocolshavetheirownmechanismsforend-to-enderrordetection.

TheNetworkLayerThenetworklayerprotocolistheprimaryend-to-endcarrierformessagesgeneratedbytheapplicationlayer.Thismeansthat,unlikethedatalinklayerprotocol,whichisconcernedonlywithgettingthepackettoitsnextdestinationonthelocalnetwork,thenetworklayerprotocolisresponsibleforthepacketsentirejourneyfromthesourcesystemtoitsultimatedestination.Anetworklayerprotocolacceptsdatafromthetransportlayerandpackagesitintoadatagrambyaddingitsownheader.Likeadatalinklayerprotocolheader,theheaderatthenetworklayercontainstheaddressofthedestinationsystem,butthisaddressidentifiesthepacketsfinaldestination.Thus,thedestinationaddressesinthedatalinklayerandnetworklayerprotocolheadersmayactuallyrefertotwodifferentcomputers.Thenetworklayerprotocoldatagramisessentiallyanenvelopewithinthedatalinklayerenvelope,andwhilethedatalinklayerenvelopeisopenedbyeverysystemthatprocessesthepacket,thenetworklayerenveloperemainssealeduntilthepacketreachesitsfinaldestination.

Thenetworklayerprotocolprovides

End-to-endaddressing

Internetroutingservices

Packetfragmentationandreassembly

Errorchecking

RoutingNetworklayerprotocolsusedifferenttypesofaddressingsystemstoidentifytheultimatedestinationofapacket.Themostpopularnetworklayerprotocol,theInternetProtocol(IP),providesitsown32-bitaddressspacethatidentifiesboththenetworkonwhichthedestinationsystemresidesandthesystemitself.

Anaddressbywhichindividualnetworkscanbeuniquelyidentifiedisvitaltotheperformanceofthenetworklayerprotocolsprimaryfunction,whichisrouting.WhenapackettravelsthroughalargecorporateinternetworkortheInternet,itispassedfromroutertorouteruntilitreachesthenetworkonwhichthedestinationsystemislocated.Properlydesignednetworkshavemorethanonepossibleroutetoaparticulardestination,forfault-tolerancereasons,andtheInternethasmillionsofpossibleroutes.Eachrouterisresponsiblefordeterminingthenextrouterthatthepacketshouldusetotakethemostefficientpathtoitsdestination.Becausedatalinklayerprotocolsarecompletelyignorantofconditionsoutsideofthelocalnetwork,itisleftuptothenetworklayerprotocoltochooseanappropriateroutewithaneyeontheend-to-endjourneyofthepacket,notjustthenextinterimhop.

Thenetworklayerdefinestwotypesofcomputersthatcanbeinvolvedinapackettransmission:endsystemsandintermediatesystems.Anendsystemiseitherthecomputergeneratingandtransmittingthepacketorthecomputerthatistheultimaterecipientofthepacket.Anintermediatesystemisarouterorswitchthatconnectstwoormorenetworksandforwardspacketsonthewaytotheirdestinations.Onendsystems,allsevenlayersoftheprotocolstackareinvolvedineitherthecreationorthereceptionofthepacket.Onintermediatesystems,packetsarriveandtravelupthroughthestackonlyashighasthenetworklayer.Thenetworklayerprotocolchoosesarouteforthepacketandsendsitbackdowntoadatalinklayerprotocolforpackagingandtransmissionatthephysicallayer.

NOTEOnintermediatesystems,packetstravelnohigherthanthenetworklayer.

Whenanintermediatesystemreceivesapacket,thedatalinklayerprotocolchecksitforerrorsandforthecorrecthardwareaddressandthenstripsoffthedatalinkheaderandfooterandpassesituptothenetworklayerprotocolidentifiedbytheEthernet-typefieldoritsequivalent.Atthispoint,thepacketconsistsofadatagramthatis,anetworklayerprotocolheaderandapayloadthatwasgeneratedbythetransportlayerprotocolonthesourcesystem.Thenetworklayerprotocolthenreadsthedestinationaddressintheheaderanddetermineswhatthepacketsnextdestinationshouldbe.Ifthedestinationisaworkstationonalocalnetwork,theintermediatesystemtransmitsthepacketdirectlytothatworkstation.Ifthedestinationisonadistantnetwork,theintermediatesystemconsultsitsroutingtabletoselecttherouterthatprovidesthemostefficientpathtothatdestination.

Thecompilationandstorageofroutinginformationinareferencetableisaseparatenetworklayerprocessthatisperformedeithermanuallybyanadministratororautomaticallybyspecializednetworklayerprotocolsthatroutersusetoexchangeinformationaboutthenetworkstowhichtheyareconnected.Onceithasdeterminedthenextdestinationforthepacket,thenetworklayerprotocolpassestheinformationdowntothedatalinklayerprotocolwiththedatagramsothatitcanbepackagedinanewframeandtransmitted.WhentheIPprotocolisrunningatthenetworklayer,anadditionalprocessisrequiredinwhichtheIPaddressofthenextdestinationisconvertedintoahardwareaddressthatthedatalinklayerprotocolcanuse.

FragmentingBecauserouterscanconnectnetworksthatusedifferentdatalinklayerprotocols,itissometimesnecessaryforintermediatesystemstosplitdatagramsintofragmentstotransmitthem.If,forexample,aworkstationonaTokenRingnetworkgeneratesapacketcontaining4,500bytesofdata,anintermediatesystemthatjoinstheTokenRingnetworktoanEthernetnetworkmustsplitthedataintofragmentsbetween64and1,518bytesbecause1,518bytesisthelargestamountofdatathatanEthernetframecancarry.

Dependingonthedatalinklayerprotocolsusedbythevariousintermediatenetworks,thefragmentsofadatagrammaybefragmentedthemselves.Datagramsorfragmentsthatarefragmentedbyintermediatesystemsarenotreassembleduntiltheyreachtheirfinaldestinations.

Connection-OrientedandConnectionlessProtocolsTherearetwotypesofend-to-endprotocolsthatoperateatthenetworkandtransportlayers:connection-orientedandconnectionless.Thetypeofprotocolusedhelpstodeterminewhatotherfunctionsareperformedateachlayer.Aconnection-orientedprotocolisoneinwhichalogicalconnectionbetweenthesourceandthedestinationsystemisestablishedbeforeanyupper-layerdataistransmitted.Oncetheconnectionisestablished,thesourcesystemtransmitsthedata,andthedestinationsystemacknowledgesitsreceipt.Afailuretoreceivetheappropriateacknowledgmentsservesasasignaltothesenderthatpacketshavetoberetransmitted.Whenthedatatransmissioniscompletedsuccessfully,thesystemsterminatetheconnection.Byusingthistypeofprotocol,thesendingsystemiscertainthatthedatahasarrivedatthedestinationsuccessfully.Thecostofthisguaranteedserviceistheadditionalnetworktrafficgeneratedbytheconnectionestablishment,acknowledgment,andterminationmessages,aswellasasubstantiallylargerprotocolheaderoneachdatapacket.

Aconnectionlessprotocolsimplypackagesdataandtransmitsittothedestinationaddresswithoutcheckingtoseewhetherthedestinationsystemisavailableandwithoutexpectingpacketacknowledgments.Inmostcases,connectionlessprotocolsareusedwhenaprotocolhigherupinthenetworkingstackprovidesconnection-orientedservices,suchasguaranteeddelivery.Theseadditionalservicescanalsoincludeflowcontrol(amechanismforregulatingthespeedatwhichdataistransmittedoverthenetwork),errordetection,anderrorcorrection.

MostoftheLANprotocolsoperatingatthenetworklayer,suchasIPandIPX,areconnectionless.Inbothcases,variousprotocolsareavailableatthetransportlayertoprovidebothconnectionlessandconnection-orientedservices.Ifyouarerunningaconnection-orientedprotocolatonelayer,thereisusuallynoreasontouseoneatanotherlayer.Theobjectoftheprotocolstackistoprovideonlytheservicesthatanapplicationneeds,andnomore.

TheTransportLayerOnceyoureachthetransportlayer,theprocessofgettingpacketsfromtheirsourcetotheirdestinationisnolongeraconcern.Thetransportlayerprotocolsandallthelayersabovethemrelycompletelyonthenetworkanddatalinklayersforaddressingandtransmissionservices.Asdiscussedearlier,packetsbeingprocessedbyintermediatesystemstravelonlyashighasthenetworklayer,sothetransport-layerprotocolsoperateononlythetwoendsystems.ThetransportlayerPDUconsistsofaheaderandthedataithasreceivedfromtheapplicationlayerabove,whichisencapsulatedintoadatagrambythenetworklayerbelow.

Thetransportlayerprovidesdifferentlevelsofservicedependingontheneedsoftheapplication:

Packetacknowledgment

Guaranteeddelivery

Flowcontrol

End-to-enderrorchecking

Oneofthemainfunctionsofthetransportlayerprotocolistoidentifytheupper-layerprocessesthatgeneratedthemessageatthesourcesystemandthatwillreceivethemessageatthedestinationsystem.ThetransportlayerprotocolsintheTCP/IPsuite,forexample,useportnumbersintheirheaderstoidentifyupper-layerservices.

ProtocolServiceCombinationsDatalinkandnetworklayerprotocolsoperatetogetherinterchangeably;youcanusealmostanydatalinklayerprotocolwithanynetworklayerprotocol.However,transportlayerprotocolsarecloselyrelatedtoaparticularnetworklayerprotocolandcannotbeinterchanged.Thecombinationofanetworklayerprotocolandatransportlayerprotocolprovidesacomplementarysetofservicessuitableforaspecificapplication.Asatthenetworklayer,transportlayerprotocolscanbeconnectionoriented(CO)orconnectionless(CL).TheOSImodeldocumentdefinesfourpossiblecombinationsofCOandCLprotocolsatthesetwolayers,dependingontheservicesrequired,asshowninFigure2-11.Theprocessofselectingacombinationofprotocolsforaparticulartaskiscalledmappingatransportlayerserviceontoanetworklayerservice.

Figure2-11Anyconfigurationofconnection-orientedandconnectionlessprotocolscanbeused.

Theselectionofaprotocolatthetransportlayerisbasedontheneedsoftheapplicationgeneratingthemessageandtheservicesalreadyprovidedbytheprotocolsatthelowerlayers.TheOSIdocumentdefinesfivetheoreticalclassesoftransportlayerprotocol,asshownhere:

TP0Thisclassdoesnotprovideanyadditionalfunctionalitybeyondfragmentingandreassemblyfunctions.ThisclassdeterminesthesizeofthesmallestPDUrequiredbyanyoftheunderlyingnetworksandsegmentsasneeded.

TP1ThisclassperformsthefunctionsofTP0plusprovidingthecapabilitytocorrecterrorsthathavebeendetectedbytheprotocolsoperatingatthelowerlayers.

TP2Thisclassprovidesfragmentationandreassemblyfunctions,multiplexing,anddemultiplexingandincludescodesthatidentifytheprocessthatgeneratedthepacketandthatwillprocessitatthedestination,thusenablingthetrafficfrommultipleapplicationstobecarriedoverasinglenetworkmedium.

TP3Thisclassofferserrorrecovery,segmentation,reassembly,multiplexing,anddemultiplexing.ItcombinestheservicesprovidedbyTP1andTP2.

TP4Thisclassprovidescompleteconnection-orientedservice,includingerrordetectionandcorrection,flowcontrol,andotherservices.Itassumestheuseofaconnectionlessprotocolatthelowerlayersthatprovidesnoneoftheseservices.

ThisclassificationoftransportlayerservicesisanotherplacewherethetheoreticalconstructsoftheOSImodeldiffersubstantiallyfromreality.Noprotocolsuiteincommonusehasfivedifferenttransportlayerprotocolsconformingtotheseclasses.Mostofthesuites,likeTCP/IP,havetwoprotocolsthatbasicallyconformtotheTP0andTP4classes,providingconnectionlessandconnection-orientedservices,respectively.

TransportLayerProtocolFunctionsTheUDPprotocolisaconnectionlessservicethat,togetherwithIPatthenetworklayer,providesminimalservicesforbrieftransactionsthatdonotneedtheservicesofaconnection-orientedprotocol.DomainNameSystem(DNS)transactions,forexample,generallyconsistofshortmessagesthatcanfitintoasinglepacket,sonoflowcontrolisneeded.Atypicaltransactionconsistsofarequestandareply,withthereplyfunctioningasanacknowledgment,sonootherguaranteeddeliverymechanismisneeded.UDPdoeshaveanoptionalerror-detectionmechanismintheformofachecksumcomputationperformedonboththesourceanddestinationsystems.BecausetheUDPprotocolprovidesaminimumofadditionalservices,itsheaderisonly8byteslong,providinglittleadditionalcontroloverheadtothepacket.

TCP,ontheotherhand,isaconnection-orientedprotocolthatprovidesafullrangeofservicesbutatthecostofmuchhigheroverhead.TheTCPheaderis20byteslong,andtheprotocolalsogeneratesalargenumberofadditionalpacketssolelyforcontrolprocedures,suchasconnectionestablishment,termination,andpacketacknowledgment.

SegmentationandReassemblyConnection-orientedtransportlayerprotocolsaredesignedtocarrylargeamountsofdata,butthedatamustbesplitintosegmentstofitintoindividualpackets.Thesegmentationofthedataandthenumberingofthesegmentsarecriticalelementsinthetransmissionprocessandalsomakefunctionssuchaserrorrecoverypossible.Theroutingprocessperformedatthenetworklayerisdynamic;inthecourseofatransmission,itispossibleforthesegmentstotakedifferentroutestothedestinationandarriveinadifferentorderfromthatinwhichtheyweresent.Itisthenumberingofthesegmentsthatmakesitpossibleforthereceivingsystemtoreassemblethemintotheiroriginalorder.Thisnumberingalsomakesitpossibleforthereceivingsystemtonotifythesenderthatspecific

packetshavebeenlostorcorrupted.Asaresult,thesendercanretransmitonlythemissingsegmentsandnothavetorepeattheentiretransmission.

FlowControlOneofthefunctionscommonlyprovidedbyconnection-orientedtransportlayerprotocolsisflowcontrol,whichisamechanismbywhichthesystemreceivingthedatacannotifythesenderthatitmustdecreaseitstransmissionrateorriskoverwhelmingthereceiverandlosingdata.TheTCPheader,forexample,includesaWindowfieldinwhichthereceiverspecifiesthenumberofbytesitcanreceivefromthesender.Ifthisvaluedecreasesinsucceedingpackets,thesenderknowsthatithastoslowdownitstransmissionrate.Whenthevaluebeginstoriseagain,thesendercanincreaseitsspeed.

ErrorDetectionandRecoveryTheOSImodeldocumentdefinestwoformsoferrorrecoverythatcanbeperformedbyconnection-orientedtransportlayerprotocols.Oneisaresponsetosignalederrorsdetectedbyotherprotocolsinthestack.Inthismechanism,thetransportlayerprotocoldoesnothavetodetectthetransmissionerrorsthemselves.Instead,itreceivesnotificationfromaprotocolatthenetworkordatalinklayerthatanerrorhasoccurredandthatspecificpacketshavebeenlostorcorrupted.Thetransportlayerprotocolonlyhastosendamessagebacktothesourcesystemlistingthepacketsandrequestingtheirretransmission.

Themorecommonlyimplementedformoferrorrecoveryatthetransportlayerisacompleteprocessoferrordetectionandcorrectionthatisusedtocopewithunsignalederrors,whichareerrorsthathavenotyetbeendetectedbyothermeans.Eventhoughmostdatalinklayerprotocolshavetheirownerror-detectionandcorrectionmechanisms,theyfunctiononlyovertheindividualhopsbetweentwosystems.Atransportlayererror-detectionmechanismprovideserrorcheckingbetweenthetwoendsystemsandincludesthecapabilitytorecoverfromtheerrorsbyinformingthesenderwhichpacketshavetoberesent.Todothis,thechecksumincludedinthetransportlayerprotocolheaderiscomputedonlyonthefieldsthatarenotmodifiedduringthejourneytothedestination.Fieldsthatroutinelychangeareomittedfromthecalculation.

TheSessionLayerWhenyoureachthesessionlayer,theboundariesbetweenthelayersandtheirfunctionsstarttobecomemoreobscure.Therearenodiscreteprotocolsthatoperateexclusivelyatthesessionlayer.Rather,thesessionlayerfunctionalityisincorporatedintootherprotocols,withfunctionsthatfallintotheprovincesofthepresentationandapplicationlayersaswell.NetworkBasicInput/OutputSystem(NetBIOS)andNetBIOSExtendedUserInterface(NetBEUI)aretwoofthebestexamplesoftheseprotocols.Thesessionlayerprovidesmechanismsbywhichthemessagedialogbetweencomputersisestablished,maintained,andterminated.Forspecificexamplesthatmayfurtherclarify,seetheISO8327standardthatdefinessessionlayerprotocolsandisassumedtobeusedbyvariousIOS8823standardprotocolsinthepresentationlayer.

Theboundarytothesessionlayerisalsothepoint