networking the complete reference, third edition - bobbi sandberg.pdf
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Greatthanksandhumbleappreciationtoallofthosewhohelpedwiththisbook.Andtomykidsandtheirkids,andeverandalwaystoSandy.
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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ProtocolAnalysis
CableTesters
Chapter27BackingUp
BackupHardware
BackupCapacityPlanning
HardDiskDrives
RAIDSystems
UsingRAID
Network-AttachedStorage
MagneticTapeDrives
TapeDriveInterfaces
MagneticTapeCapacities
BackupSoftware
SelectingBackupTargets
BackingUpOpenFiles
RecoveringfromaDisaster
JobScheduling
RotatingMedia
BackupAdministration
EventLogging
PerformingRestores
Index
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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!
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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
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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
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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
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PART
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I NetworkBasicsCHAPTER1
WhatIsaNetwork?
CHAPTER2
TheOSIReferenceModel
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CHAPTER
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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
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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
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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.
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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.
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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.
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Figure1-1Commoncabletopographies
Thetopologiesdiscussedherearephysicaltopologies,whichdifferfromlogicaltopologiesthatarediscussedinlaterchapters.Physicaltopologiesrefertotheplacementofcablesandothercomponentsofthenetwork.Logicaltopologiesrefertotheflowofdataonthenetwork.
MediaAccessControlWhenmultiplecomputersareconnectedtothesamebasebandnetworkmedium,theremustbeamediaaccesscontrol(MAC)mechanismthatarbitratesaccesstothenetworktopreventsystemsfromtransmittingdataatthesametime.AMACmechanismisafundamentalpartofalllocalareanetworkingprotocolsthatuseasharednetworkmedium.ThetwomostcommonMACmechanismsareCarrierSenseMultipleAccesswithCollisionDetection(CSMA/CD),whichisusedbyEthernetnetworks,andtokenpassing,whichisusedbyTokenRing,FDDI,andotherprotocols.Thesetwomechanismsarefundamentallydifferent,buttheyaccomplishthesametaskbyprovidingeachsystemonthenetworkwithanequalopportunitytotransmititsdata.(FormoreinformationabouttheseMACmechanisms,seeChapter10forCSMA/CDandChapter12fortokenpassing.)
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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
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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:
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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
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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.
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CHAPTER
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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
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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
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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
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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.
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Figure2-5Eachlayerhaslogicalconnectionswithitscounterpartinothersystems.
Thehorizontalconnectionsbetweenthevariouslayersarelogical;thereisnodirectcommunicationbetweenthem.Theinformationincludedineachprotocolheaderbythetransmittingsystemisamessagethatiscarriedtothesameprotocolinthedestinationsystem.
VerticalCommunicationsTheheadersappliedbythevariousprotocolsimplementthespecificfunctionscarriedoutbythoseprotocols.Inadditiontocommunicatinghorizontallywiththesameprotocolintheothersystem,theheaderinformationenableseachlayertocommunicatewiththelayersaboveandbelowit,asshowninFigure2-6.Forexample,whenasystemreceivesapacketandpassesitupthroughtheprotocolstack,thedatalinklayerprotocolheaderincludesafieldthatidentifieswhichnetworklayerprotocolthesystemshouldusetoprocessthepacket.Thenetworklayerprotocolheaderinturnspecifiesoneofthetransportlayerprotocols,andthetransportlayerprotocolidentifiestheapplicationforwhichthedataisultimatelydestined.Thisverticalcommunicationmakesitpossibleforacomputertosupportmultipleprotocolsateachofthelayerssimultaneously.Aslongasapackethasthecorrectinformationinitsheaders,itcanberoutedontheappropriatepaththroughthestacktotheintendeddestination.
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Figure2-6EachlayerintheOSImodelcommunicateswiththelayeraboveandbelowit.
EncapsulationTerminologyOneofthemostconfusingaspectsofthedataencapsulationprocessistheterminologyusedtodescribethePDUsgeneratedbyeachlayer.Thetermpacketspecificallyreferstothecompleteunittransmittedoverthenetworkmedium,althoughitalsohasbecomea
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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
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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
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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.
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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,
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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.
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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
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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.
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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.
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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.
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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.
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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
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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.
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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
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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.
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Theboundarytothesessionlayerisalsothepoint