《数据与计算机通信》PPT课件

1,WilliamStallingsDataandComputerCommunications,第15单元局域网概述,Chapter15LocalAreaNetworkOverview,2,15.1LANApplications(1),PersonalcomputerLANsLowcostLimiteddatarateBackendnetworksInterconnectinglargesystems(mainframesandlargestoragedevices)HighdatarateHighspeedinterfaceDistributedaccessLimiteddistanceLimitednumberofdevices,3,LANApplications(2),StorageAreaNetworksSeparatenetworkhandlingstorageneedsDetachesstoragetasksfromspecificserversSharedstoragefacilityacrosshigh-speednetworkHarddisks,tapelibraries,CDarraysImprovedclient-serverstorageaccessDirectstoragetostoragecommunicationforbackupHighspeedofficenetworksDesktopimageprocessingHighcapacitylocalstorageBackboneLANsInterconnectlowspeedlocalLANsReliabilityCapacityCost,4,StorageAreaNetworks,5,个人计算机局域网客户/服务器计算模式,后端网络和存储区域网大型系统互联：大型机、海量存储特征：高速高可靠性、有限数量和距离,高速办公室网络大信息量数据处理,骨干局域网高带宽、高可靠性实现速率相对较低的LAN的互联,局域网应用,6,后端网络与存储区域网(1),大系统的互联大型机（Mainframes）巨型机（Supercomputers）海量存储设备（Massstoragedevices）主要需求小范围内有限数量的设备之间的批量数据传送典型特征高数据率高速接口分布式接入有限距离有限设备数量,7,后端网络与存储区域网(2),SAN网络存储设备硬盘磁带库CD阵列,8,15.2TopologiesandTransmissionmedium,KeyelementsofaLANareTopologiesTransmissionmediumLayoutMediumaccesscontrol,9,15.2.1Topologies,TreeBusSpecialcaseoftreeOnetrunk,nobranchesRingStar,10,LANTopologies,11,BusandTreeTopology,MultipointmediumTransmissionpropagatesthroughoutmediumHeardbyallstationsNeedtoidentifytargetstationEachstationhasuniqueaddressFullduplexconnectionbetweenstationandtapAllowsfortransmissionandreceptionNeedtoregulatetransmission需要规定传输Toavoidcollisions避免信号冲突Toavoidhogging(扰乱)避免信号混淆Datainsmallblocks-framesTerminatorabsorbsframesatendofmedium,12,FrameTransmissiononBusLAN,13,RingTopology,RepeatersjoinedbypointtopointlinksinclosedloopReceivedataononelinkandretransmitonanotherLinksunidirectionalStationsattachtorepeatersDatainframesCirculatepastallstationsDestinationrecognizesaddressandcopiesframeFramecirculatesbacktosourcewhereitisremovedMediaaccesscontroldetermineswhenstationcaninsertframe,14,FrameTransmissionRingLAN,15,StarTopology,EachstationconnecteddirectlytocentralnodeUsuallyviatwopointtopointlinksCentralnodecanbroadcastPhysicalstar,logicalbusOnlyonestationcantransmitatatimeCentralnodecanactasframeswitch,16,15.2.2ChoiceofTopology,ReliabilityExpandabilityPerformanceNeedsconsideringincontextof:MediumWiringlayoutAccesscontrol,17,BusLANTransmissionMedia(1),TwistedpairEarlyLANsusedvoicegradecableDidntscaleforfastLANsNotusedinbusLANsnowBasebandcoaxialcableUsesdigitalsignallingOriginalEthernet,18,BusLANTransmissionMedia(2),BroadbandcoaxialcableAsincableTVsystemsAnalogsignalsatradiofrequenciesExpensive,hardtoinstallandmaintainNolongerusedinLANsOpticalfiberExpensivetapsBetteralternativesavailableNotusedinbusLANsAllhardtoworkwithcomparedwithstartopologytwistedpairCoaxialbasebandstillusedbutnotofteninnewinstallations,19,RingandStarUsage,RingVeryhighspeedlinksoverlongdistancesSinglelinkorrepeaterfailuredisablesnetworkStarUsesnaturallayoutofwiringinbuildingBestforshortdistancesHighdataratesforsmallnumberofdevices,20,15.2.3ChoiceofMedium,ConstrainedbyLANtopologyCapacityReliabilityTypesofdatasupportedEnvironmentalscope,21,MediaAvailable(1),Voicegradeunshieldedtwistedpair(UTP)Cat3CheapWellunderstoodUseexistingtelephonewiringinofficebuildingLowdataratesShieldedtwistedpairandbasebandcoaxialMoreexpensivethanUTPbuthigherdataratesBroadbandcableStillmoreexpensiveandhigherdatarate,22,MediaAvailable(2),HighperformanceUTPCat5andaboveHighdatarateforsmallnumberofdevicesSwitchedstartopologyforlargeinstallationsOpticalfiberElectromagneticisolationHighcapacitySmallsizeHighcostofcomponentsHighskillneededtoinstallandmaintainPricesarecomingdownasdemandandproductrangeincreases,23,15.3ProtocolArchitecture,LowerlayersofOSImodelIEEE802referencemodelPhysicalLogicallinkcontrol(LLC)Mediaaccesscontrol(MAC),24,协议体系结构,LAN的标准化协议体系结构物理层逻辑链路控制(LLC)层媒体接入控制(MAC)层标准IEEE802ISO8802OSI参考模型的低层IEEE802比之ISO/OSI高层协议与网络体系结构无关主要关心OSI模型的低层将数据链路层划分为MAC和LLC两级子层,25,IEEE802vs.OSI,26,802LayersPhysical物理层的功能,Encoding/decoding信号的编码/解码Preamblegeneration/removal前导码的产生/去除(用于同步)Bittransmission/reception比特传输/接收Transmissionmedium传输媒体,27,802Layers-LogicalLinkControlLLC子层的功能,Interfacetohigherlevels与高层进行接口Flowanderrorcontrol流量控制与差错控制为什么要将传统的DL层拆分为LLC与MAC两个子层？,28,15.3.2LogicalLinkControl,TransmissionoflinklevelPDUsbetweentwostationsMustsupportmultiaccess,sharedmediumRelievedofsomelinkaccessdetailsbyMAClayerAddressinginvolvesspecifyingsourceanddestinationLLCusersReferredtoasserviceaccesspoints(SAP)Typicallyhigherlevelprotocol,29,逻辑链路控制,LLC的特性两站之间传输链路级不需中间交换节点必须支持多点接入的共享媒体由MAC承担了一些链路接入的细节寻址包括指定源LLC用户和目的LLC用户称为LLC服务访问点(LSAP)典型的用户是高层协议,30,LLCServices,BasedonHDLCUnacknowledgedconnectionlessserviceConnectionmodeserviceAcknowledgedconnectionlessservice提供三种服务不确认的无连接服务（Unacknowledgedconnectionlessservice）连接方式服务（Connection-modeservice）带确认的无连接服务（Acknowledgedconnectionlessservice）,31,LLCProtocol,ModeledafterHDLCAsynchronousbalancedmodetosupportconnectionmodeLLCservice(type2operation)UnnumberedinformationPDUstosupportunAcknowledgedconnectionlessservice(type1)TwonewunnumberedinformationPDUstosupportAcknowledgedconnectionlessservice(type3)MultiplexingusingLSAPs,32,LLC协议,以HDLC为蓝本第1类操作无编号信息PDU支持不确认的无连接服务第2类操作异步平衡方式支持连接方式LLC服务第3类操作两个新的无编号PDU支持带确认的无连接服务利用LSAP实现多路复用,33,15.3.3802Layers-MediaAccessControl,AssemblyofdataintoframewithaddressanderrordetectionfieldsDisassemblyofframeAddressrecognitionErrordetectionGovernaccesstotransmissionmediumNotfoundintraditionallayer2datalinkcontrolForthesameLLC,severalMACoptionsmaybeavailable,34,MAC子层的功能,在发送方，将数据配以地址与差错检测字段组装成帧在接收方,拆解帧地址辨识差错检测监管传输媒体的接入在传统的第二层数据链路控制中无此功能,35,分裂数据链路层的原因,在传统的数据链路(DL)层中不具有管理共享访问媒体的接入所需的逻辑对于相同的LLC,可以提供几种MAC选项,36,LANProtocolsinContext,37,MediaAccessControl,WhereCentralGreatercontrolSimpleaccesslogicatstationAvoidsproblemsofco-ordinationSinglepointoffailurePotentialbottleneckDistributedHowSynchronousSpecificcapacitydedicatedtoconnectionAsynchronousInresponsetodemand,38,媒体接入控制,重要参数Where集中式（Central）更多的控制简单的站点接入逻辑避免协商问题单点故障可能形成瓶颈分布式（Distributed）集体完成MAC功能动态确定哪个站发送优缺点是集中式方案的镜像How同步为每个连接指定固有带宽异步按需响应,39,AsynchronousSystems,RoundrobinGoodifmanystationshavedatatotransmitoverextendedperiodReservationGoodforstreamtrafficContentionGoodforburstytrafficAllstationscontendfortimeDistributedSimpletoimplementEfficientundermoderateloadTendtocollapseunderheavyload,40,异步系统,循环（Roundrobin）若许多站都需持续一段时间发送数据则很有效轮询：集中式技术令牌环：分布式控制预约（Reservation）适用于流式通信量streamtraffic争用（Contention）适用于突发通信量burstytraffic所有站争用占用媒体时间分布式(e.g.CSMA/CD)实现简单在负荷适中时有效在重负荷时可能会崩溃,41,MACFrameFormat,MAClayerreceivesdatafromLLClayerMACcontrolDestinationMACaddressSourceMACaddressLLCCRCMAClayerdetectserrorsanddiscardsframesLLCoptionallyretransmitsunsuccessfulframes,42,MAC帧格式,MAC层从LLC层接收数据MAC帧的字段MAC控制目的站MAC地址源站MAC地址LLCCRC(FCS)MAC层检测错误并丢弃帧LLC可选设成重传不成功接收的帧,43,TypicalFrameFormat,44,小结,局域网的应用拓扑结构和传输媒体局域网的体系结构IEEE802参考模型媒体接入控制（Mediaaccesscontrol）逻辑链路控制（LogicalLinkControl）,45,15.4Bridges网桥,AbilitytoexpandbeyondsingleLANProvideinterconnectiontootherLANs/WANsUseBridgeorrouterBridgeissimplerConnectssimilarLANsIdenticalprotocolsforphysicalandlinklayersMinimalprocessingRoutermoregeneralpurposeInterconnectvariousLANsandWANsseelater,46,WhyBridge?,ReliabilityPerformanceSecurityGeography,47,15.4.1FunctionsofaBridge,ReadallframestransmittedononeLANandacceptthoseaddresstoanystationontheotherLANUsingMACprotocolforsecondLAN,retransmiteachframeDothesametheotherwayround,48,BridgeOperation,49,BridgeDesignAspects,NomodificationtocontentorformatofframeNoencapsulationExactbitwisecopyofframeMinimalbufferingtomeetpeakdemandContainsroutingandaddressintelligenceMustbeabletotellwhichframestopassMaybemorethanonebridgetocrossMayconnectmorethantwoLANsBridgingistransparenttostationsAppearstoallstationsonmultipleLANsasiftheyareononesingleLAN,50,15.4.2BridgeProtocolArchitecture,IEEE802.1DMAClevelStationaddressisatthislevelBridgedoesnotneedLLClayerItisrelayingMACframesCanpassframeoverexternalcommssysteme.g.WANlinkCaptureframeEncapsulateitForwarditacrosslinkRemoveencapsulationandforwardoverLANlink,51,ConnectionofTwoLANs,52,15.4.3FixedRouting,ComplexlargeLANsneedalternativeroutesLoadbalancingFaulttoleranceBridgemustdecidewhethertoforwardframeBridgemustdecidewhichLANtoforwardframeonRoutingselectedforeachsource-destinationpairofLANsDoneinconfigurationUsuallyleasthoprouteOnlychangedwhentopologychanges,53,BridgesandLANswithAlternativeRoutes102-105101-104or107？,54,15.4.4SpanningTree支撑树方法,BridgeautomaticallydevelopsroutingtableAutomaticallyupdateinresponsetochanges3mechanisms:Frameforwarding帧转发Addresslearning地址探索Loopresolution环路消除,55,Frameforwarding,MaintainforwardingdatabaseforeachportListstationaddressesreachedthrougheachportForaframearrivingonportX:SearchforwardingdatabasetoseeifMACaddressislistedforanyportexceptXIfaddressnotfound,forwardtoallportsexceptXIfaddresslistedforportY,checkportYforblockingorforwardingstateBlockingpreventsportfromreceivingortransmittingIfnotblocked,transmitframethroughportY,56,AddressLearning,CanpreloadforwardingdatabaseCanbelearnedWhenframearrivesatportX,ithascomefromtheLANattachedtoportXUsethesourceaddresstoupdateforwardingdatabaseforportXtoincludethataddressTimeroneachentryindatabaseEachtimeframearrives,sourceaddresscheckedagainstforwardingdatabase,57,15.4.4SpanningTreeAlgorithm,Addresslearningworksfortreelayouti.e.noclosedloopsForanyconnectedgraphthereisaspanningtreethatmaintainsconnectivitybutcontainsnoclosedloopsEachbridgeassigneduniqueidentifierExchangebetweenbridgestoestablishspanningtree,58,LoopofBridges,59,15.5Layer2andLayer3Switches,NowmanytypesofdevicesforinterconnectingLANsBeyondbridgesandroutersLayer2switchesLayer3switches,60,15.5.1Hubs集线器,ActivecentralelementofstarlayoutEachstationconnectedtohubbytwolinesTransmitandreceiveHubactsasarepeaterWhensinglestationtransmits,hubrepeatssignalonoutgoinglinetoeachstationLineconsistsoftwounshieldedtwistedpairsLimitedtoabout100mHighdatarateandpoortransmissionqualitiesofUTPOpticalfibermaybeusedMaxabout500mPhysicallystar,logicallybusTransmissionfromanystationreceivedbyallotherstationsIftwostationstransmitatthesametime,collision,61,HubLayouts,MultiplelevelsofhubscascadedEachhubmayhaveamixtureofstationsandotherhubsattachedtoitfrombelowFitswellwithbuildingwiringpracticesWiringclosetoneachfloorHubcanbeplacedineachoneEachhubservicesstationsonitsfloor,62,Two-LevelStarTopology,63,BusesandHubs,BusconfigurationAllstationssharecapacityofbus(e.g.10Mbps)OnlyonestationtransmittingatatimeHubusesstarwiringtoattachstationstohubTransmissionfromanystationreceivedbyhubandretransmittedonalloutgoinglinesOnlyonestationcantransmitatatimeTotalcapacityofLANis10MbpsImproveperformancewithlayer2switch,64,SharedMediumBusandHub,source,65,SharedMediumHubandLayer2Switch,66,Layer2Switches,CentralhubactsasswitchIncomingframefromparticularstationswitchedtoappropriateoutputlineUnusedlinescanswitchothertrafficMorethanonestationtransmittingatatimeMultiplyingcapacityofLAN,67,Layer2SwitchBenefits,NochangetoattacheddevicestoconvertbusLANorhubLANtoswitchedLANForEthernetLAN,eachdeviceusesEthernetMACprotocolDevicehasdedicatedcapacityequaltooriginalLANAssumingswitchhassufficientcapacitytokeepupwithalldevicesForexample(Figure15.13)ifswitchcansustainthroughputof20Mbps,eachdeviceappearstohavededicatedcapacityforeitherinputoroutputof10MbpsLayer2switchscaleseasilyAdditionaldevicesattachedtoswitchbyincreasingcapacityoflayer2,68,TypesofLayer2Switch,Store-and-forwardswitchAcceptsframeoninputlineBuffersitbriefly,ThenroutesittoappropriateoutputlineDelaybetweensenderandreceiverBoostsintegrityofnetworkCut-throughswitchTakesadvantageofdestinationaddressappearingatbeginningofframeSwitchbeginsrepeatingframeontooutputlineassoonasitrecognizesdestinationaddressHighestpossiblethroughputRiskofpropagatingbadframesSwitchunabletocheckCRCpriortoretransmission,69,Preamble,SFD,Dest.Address,SourceAddress,Length,Data,FCS,6bytes,6bytes,2bytes,Max1500bytes,4bytes,Cut-through交换机在收到以太网帧的目的MAC地址之后即开始转发整个数据帧,Storeandforward交换机收到整个数据帧后才开始转发数据帧,两种交换模式,70,Layer2SwitchvBridge,Layer2switchcanbeviewedasfull-duplexhubCanincorporatelogictofunctionasmultiportbridgeBridgeframehandlingdoneinsoftwareSwitchperformsaddressrecognitionandframeforwardinginhardwareBridgeonlyanalyzesandforwardsoneframeatatimeSwitchhasmultipleparalleldatapathsCanhandlemultipleframesatatimeBridgeusesstore-and-forwardoperationSwitchcanhavecut-throughoperationBridgesufferedcommerciallyNewinstallationstypicallyincludelayer2switcheswithbridgefunctionalityratherthanbridges,71,ProblemswithLayer2Switches(1),Asnumberofdevicesinbuildinggrows,layer2switchesrevealsomeinadequaciesBroadcastoverload广播超负荷Lackofmultiplelinks缺少多链路SetofdevicesandLANsconnectedbylayer2switcheshaveflataddressspace单调地址空间AlluserssharecommonMACbroadcastaddressIfanydeviceissuesbroadcastframe,thatframeisdeliveredtoalldevicesattachedtonetworkconnectedbylayer2switchesand/orbridgesInlargenetwork,broadcastframescancreatebigoverheadMalfunctioningdevicecancreatebroadcaststormNumerousbroadcastframesclognetwork,72,ProblemswithLayer2Switches(2),CurrentstandardsforbridgeprotocolsdictatenoclosedloopsOnlyonepathbetweenanytwodevicesImpossibleinstandards-basedimplementationtoprovidemultiplepathsthroughmultipleswitchesbetweendevicesLimitsbothperformanceandreliability.Solution:breakupnetworkintosubnetworksconnectedbyroutersMACbroadcastframelimitedtodevicesandswitchescontainedinsinglesubnetworkIP-basedroutersemploysophisticatedroutingalgorithmsAllowuseofmultiplepathsbetweensubnetworksgoingthroughdifferentrouters允许在子网间使用多条路径,73,ProblemswithRouters,RoutersdoallIP-levelprocessinginsoftwareHigh-speedLANsandhigh-performancelayer2switchespumpmillionsofpacketspersecondSoftware-basedrouteronlyabletohandlewellunderamillionpacketspersecondSolution:layer3switchesImplementpacket-forwardinglogicofrouterinhardware在硬件上实现了路由器的分组转发逻辑TwocategoriesPacketbypacketFlowbased,74,PacketbyPacketorFlowBased,OperatesinsamewayastraditionalrouterOrderofmagnitudeincreaseinperformancecomparedtosoftware-basedrouterFlow-basedswitchtriestoenhanceperformancebyidentifyingflowsofIPpacketsSamesourceanddestinationDonebyobservingongoingtrafficorusingaspecialflowlabel流标签inpacketheader(IPv6)Onceflowisidentified,predefinedroutecanbeestablished,75,TypicalLargeLANOrganization,ThousandstotensofthousandsofdevicesDesktopsystemslinks10Mbpsto100MbpsIntolayer2switchWirelessLANconnectivityavailableformobileusersLayer3switchesatlocalnetworkscoreFormlocalbackboneInterconnectedat1GbpsConnecttolayer2switchesat100Mbpsto1GbpsServersconnectdi