计算机网络复习ppt

ComputerNetworks计算机网络2020/1/29ComputerNetworks2时间安排答疑时间：18周周三、19周周一，上午10:00-12:00期末考试：19周周三(06月27日)14:00-16:00，厚209实验：19周周四~20周周二2020/1/29ComputerNetworks3WhatisaComputerNetwork?ComputerNetworkisaninterconnectedcollectionofautonomous(自治的)computersconnectedbyasingletechnology.TheInternetisanetworkofnetworks.2020/1/29ComputerNetworks4ClassificationofNetworksbyscaleLAN,WANandMANbytopologyBus,Star,RingandTreebyswitchingapproachCircuitswitchingandPacketswitchingbytransmissionmediaWirelessnetworkandWirednetwork2020/1/29ComputerNetworks5CircuitSwitching电路交换Linkbandwidth,switchcapacityDedicatedresourceswithnosharingGuaranteedtransmissioncapacityCallsetuprequiredEnd-to-endresourcesreservedfor“call”2020/1/29ComputerNetworks6CircuitSwitching电路交换•Capacityofmedium(媒体,介质)exceedsthecapacityrequiredfortransmissionofasinglesignal–Howcanweimprove“efficiency”?Let’smultiplex(复用).•Dividelinkbandwidthinto“pieces”:–FDM(FrequencyDivisionMultiplexing频分复用)–TDM(TimeDivisionMultiplexing时分复用)2020/1/29ComputerNetworks7CircuitSwitching:FDMandTDMFDM频分多路复用frequencytimeTDM时分多路复用frequencytime4usersExample:partiallinkbandwidthTimeslot(时隙)2020/1/29ComputerNetworks8PacketSwitching分组交换sourcebreakslongmessagesintosmaller“packets(数据包，数据分组)”packetssharenetworkresourceseachpacketusesfulllinkbandwidth“store-and-forward(存储转发)”transmission2020/1/29ComputerNetworks9ACEBDH1H3H5H6H4H2分组交换网示意图访问结点用户子网通信子网转接结点链路2020/1/29ComputerNetworks10PacketSwitching:StatisticalMultiplexingSequenceofA&Bpacketsdoesnothavefixedpatternstatisticalmultiplexing统计多路复用.ABC10MbsEthernet1.5MbsDEstatisticalmultiplexingQueue(队列)ofpacketswaitingforoutputlink2020/1/29ComputerNetworks11Howdolossanddelayoccur?packetsqueueinrouterbuffers•packetarrivalratetolinkexceedsoutputlinkcapacity•packetsqueue,waitforturn•ifqueueisfull,arrivingpacketsdropped(Drop-Tail)ABpacketbeingtransmitted(delay)packetsqueueing(delay)free(available)buffers:arrivingpacketsdropped(loss)ifnofreebuffers2020/1/29ComputerNetworks121.dproc=processingdelay(处理时延)typicallyafewmicrosecsorless2.dqueue=queuingdelay(排队时延)dependsoncongestion3.dtrans=transmissiondelay(发送时延)=L/R,significantforlow-speedlinks4.dprop=propagationdelay(传播时延)afewmicrosecstohundredsofmsecsFoursourcesofpacketdelay2020/1/29ComputerNetworks13Nodaldelay总时延=处理时延+排队时延+发送时延+传播时延proptransqueueprocnodalddddd2020/1/29ComputerNetworks14Packetswitchingvs.Circuitswitching•Packetswitchingisgreatforburstydata(突发性数据)–resourcesharing•Packetswitchingcausesexcessive(额外的)congestion,packetdelayandloss–protocolsneededforreliabledatatransfer,congestioncontrolIspacketswitchinga“slamdunk”winner?2020/1/29ComputerNetworks15NetworkArchitecture网络体系结构OSI(OpenSystemInterconnection)isanISOstandardforworldwidecommunicationsTheOSIReferenceModeldefinesanetworkingframeworkforimplementingprotocolsinsevenlayers.2020/1/29ComputerNetworks16TheISO/OSIReferenceModels2020/1/29ComputerNetworks17Aprotocolisthespecialsetofrulesusedtocontrolcommunicationbetweenentities.Basedonthecommunicationbetweentwoentitiescontrolledbyprotocols,onelayerprovidesservicestoanotherlayerabove.NetworkArchitecture网络体系结构2020/1/29ComputerNetworks18Relationshipbetweenaserviceandaprotocol.NetworkArchitecture网络体系结构2020/1/29ComputerNetworks19ApplicationPresentationSessionTransportNetworkDataLinkPhysicalApplicationPresentationSessionTransportNetworkDataLinkPhysicalPhysicalControlispassedfromonelayertothenext,startingattheapplicationlayerinonestation,proceedingtothebottomlayer,overthechanneltothenextstationandbackupthehierarchy.NetworkArchitecture网络体系结构2020/1/29ComputerNetworks20①PhysicalLayerhowtotransmitbitstothechannel；②DataLinkLayerhowtotransmitframestoadjacentnode(neighbour),overasinglelink；③NetworkLayerhowtoroutepacketstoahostontheotherside,acrossnetwork(s)NetworkArchitecture网络体系结构2020/1/29ComputerNetworks21④TransportLayerhowtosenddatasegmentstoaprocessrunningonanotherhost,acrossnetwork(s)⑤SessionLayermanageconnections⑥PresentationLayerencode/decodemessages,security,encryption⑦ApplicationLayereverythingelse!NetworkArchitecture网络体系结构2020/1/29ComputerNetworks22Comparison:OSIandTCP/IPPhysicalDataLinkNetworkTransportSessionPresentationApplicationNetworkInterfaceIPTransportApplication2020/1/29ComputerNetworks23TCP/IPArchitectureTheTCP/IPArchitecturewasdefinedbyInternetEngineeringTaskForce(IETF)TransparentDesign–EverythingoverIP;–IPoverEverything;–Best-effort.■■■FTPTCPUDPIPNET1NET2NETnHTTPMulti-mediaTFTP2020/1/29ComputerNetworks24EncapsulationApp.TCPIPEthernetdriverUserDataUserDataCom.ApplicationPDUApplicationPDUTcpHTcpHIpHApplicationPDUTcpHIpHEthHEth.TmessageTCPsegmentIPpacketEthernetframeEthernet2020/1/29ComputerNetworks25De-capsulationApp.TCPIPEthernetdriverUserDataUserDataCom.ApplicationPDUApplicationPDUTcpHTcpHIpHApplicationPDUTcpHIpHEthHEth.TmessageTCPsegmentIPpacketEthernetframeEthernet2020/1/29ComputerNetworks26EncapsulationandDemultiplexingApplicationdataTCPHeaderIPHeaderEthernetHeaderEthernetTrailerEthernetframedestinationaddresssourceaddresstype6bytesCRC4bytes2020/1/29ComputerNetworks2700:e0: