信息通信网络概论英文课件：Chapter 3 Switch Technology Fundamentals

Chapter3SwitchTechnologyFundamentalsRequiredreading:Garcia1.14.14.47.1&7.33.1CircuitSwitching3.2MessageSwitching3.3PacketSwitching

3.3.1virtualcircuit 3.3.2datagram3.4TheDelayAnalysisinNetworks

TheNetworkCoremeshofinterconnectedroutersthefundamentalquestion:howisdatatransferredthroughnet?circuitswitching:dedicatedcircuitpercall:telephonenetpacket-switching:datasentthrunetindiscrete“chunks”VirtualCircuitDatagram3.1CircuitSwitchingEnd-endresourcesreservedfor“call”●linkbandwidth,switchcapacity●dedicatedresources:nosharing●circuit-like(guaranteed)performance●callsetuprequiredCircuitSwitchingnetworkresources(e.g.,bandwidth)dividedinto“pieces”●piecesallocatedtocalls●resourcepieceidleifnotusedbyowningcall(nosharing)●dividinglinkbandwidthinto“pieces”frequencydivisiontimedivisionCircuitSwitching:FDMandTDMFDMfrequencytimeTDMfrequencytime4usersExample:TiminginCircuitSwitchingDATACircuit

EstablishmentData

Transmission

Circuit

TerminationHost1Host2Node1Node2propagationdelaybetweenHost1andNode1propagationdelaybetweenHost2andhost1processingdelayatNode1CircuitSwitchingAnode(switch)inacircuitswitchingnetworkincominglinksoutgoinglinksNodeCircuitSwitching:Multiplexing/DemultiplexingTimedividedinframesandframesdividedinslotsRelativeslotpositioninsideaframedetermineswhichconversationthedatabelongstoNeedssynchronizationbetweensenderandreceiverIncaseofnon-permanentconversationsneedstodynamicallybindaslottoaconservationhowtodothis?Ifaconversationdoesnotuseitscircuitthecapacityis

lost!3.2MessageSwitchingeachend-enddatastreamasmessagesuserA,Bmessagesharenetworkresourceseachmessageusesfulllinkbandwidthresourcesusedasneeded,Bandwidthdivisioninto“pieces”DedicatedallocationResourcereservationresourcecontention:

●aggregateresourcedemandcanexceedamountavailable●congestion:messagesqueue,waitforlinkuse●storeandforward:messagesmoveonehopatatimetransmitoverlinkwaitturnatnextlinkTiminginMessageSwitchingDATAHost1Host2Node1Node2DATADATA3.3PacketSwitchingeachend-enddatastreamdividedintopacketsuserA,Bpacketssharenetworkresourceseachpacketusesfulllinkbandwidthresourcesusedasneeded,resourcecontention:

●aggregateresourcedemandcanexceedamountavailable●congestion:packetsqueue,waitforlinkuse●storeandforward:packetsmoveonehopatatimetransmitoverlinkwaitturnatnextlinkBandwidthdivisioninto“pieces”DedicatedallocationResourcereservationPacketSwitchingAnodeinapacketswitchingnetworkincominglinksoutgoinglinksNodeMemoryPacket-switching:store-and-forwardCEFABDCsendpacketstoERouterpacket-switchednetworksPacket1Packet2Packet3Packet1Packet2Packet3TimingofDatagramPacketSwitchingPacket1Packet2Packet3processing

delayofPacket1atNode2Host1Host2Node1Node2propagationdelaybetweenHost1andNode2transmissiontimeofPacket1atHost1Packet-switching:store-and-forward●TakesL/Rsecondstotransmit(pushout)packetofLbitsontolinkorRbps●Entirepacketmustarriveatrouterbeforeitcanbetransmittedonnextlink:storeandforward●delay=3L/R(assumingzeropropagationdelay)Example:L=7.5MbitsR=1.5Mbpstransmissiondelay=15secRRRLPacketSwitching:MessageSegmentingNowbreakupthemessageinto5000packetsstoreandforwardbehaviorEachpacket1,500bits,1msectotransmitapacketononelinkpipelining:eachlinkworksinparallelDelayreducedfrom3*5=15secto5.002secPacketSwitching:Multiplexing/DemultiplexingDatafromanyconversationcanbetransmittedatanygiventimeAsingleconversationcanusetheentirelinkcapacityifitisaloneHowtotellthemapart?usemeta-data(header)todescribedataHeaderDataTrailerPacketsStructurePacketSwitching:StatisticalMultiplexingSequenceofA&Bpacketsdoesnothavefixedpattern,bandwidthsharedondemandstatisticalmultiplexing.TDM:eachhostgetssameslotinrevolvingTDMframe.ABC100Mb/sEthernet1.5Mb/sDEstatisticalmultiplexingqueueofpacketswaitingforoutputlink45MbsHowdolossanddelayoccur?packetsqueueinrouterbuffers

●packetsqueue,waitforturn

●packetarrivalratetolinkexceedsoutputlinkcapacityABpacketbeingtransmitted(delay)packetsqueueing(delay)free(available)buffers:arrivingpacketsdropped(loss)ifnofreebuffersDelayinpacket-switchednetworkspacketsexperiencedelayonend-to-endpath●foursourcesofdelayateachhopnodalprocessing:

checkbiterrorsdetermineoutputlinkqueuingtimewaitingatoutputlinkfortransmissiondependsoncongestionlevelofrouterABpropagationtransmissionnodalprocessingqueuingDelayinpacket-switchednetworksTransmissiondelay:R=linkbandwidth(bps)L=packetlength(bits)timetosendbitsintolink=L/RPropagationdelay:d=lengthofphysicallinks=propagationspeedinmedium(~2x108m/sec)propagationdelay=d/sABpropagationtransmissionnodalprocessingqueueingNote:sandRareverydifferentquantities!Nodaldelaytproc=processingdelaytypicallyafewmicrosecsorlesstqueue

=queuingdelaydependsoncongestionttrans=transmissiondelay=L/R,significantforlow-speedlinkstprop

=propagationdelayafewmicrosecstohundredsofmsecsQueueingdelay(revisited)R=linkbandwidth(bps)L=packetlength(bits)a=averagepacketarrivalratetrafficintensity=La/RLa/R~0:averagequeueingdelaysmallLa/R->1:delaysbecomelargeLa/R>1:more“work”arrivingthancanbeserviced,averagedelayinfinite!“Real”InternetdelaysandroutesWhatdo“real”Internetdelay&losslooklike?Tracerouteprogram:providesdelaymeasurementfromsourcetorouteralongend-endInternetpathtowardsdestination.Foralli:sendsthreepacketsthatwillreachrouterionpathtowardsdestinationrouteriwillreturnpacketstosendersendertimesintervalbetweentransmissionandreply.3probes3probes3probesPacketswitchingversuscircuitswitching1Mbpslinkeachuser:100Kbpswhen“active”active10%oftimecircuit-switching:10userspacketswitching:10usersorlessnoproblem>10usersqueuingdelaybutstillpossibletosupportwith35users,probability>10%activelessthan.004Nusers1MbpslinkPacketswitchingversuscircuitswitchingGreatforburstydataresourcesharingsimplercongestion:packetdelayandlossprotocolsneededforreliabledatatransfer,congestioncontrolQ:Howtoprovidecircuit-likebehavior?bandwidthguaranteesneededforaudio/videoappsVC(VirtualCircuit虚电路)所谓虚电路是面向连接的服务，在数据传输之前需要先建立一条逻辑链路；数据交换后，必须释放这个连接。同一呼叫的数据分组只需标识逻辑电路号，并沿着虚电路按序传输，提供可靠的报文服务。它不同于电路交换中的物理连接，而是逻辑连接。DG(Datagram数据报)是无连接的服务，无需要预先建立连接，网络随时接受主机发送的分组；每个数据报需要标识出完整的目标地址；每个分组独立的选择路由，因此不能防止报文的丢失、重复或失序，它提供“尽最大努力交付”的服务，是一种不可靠的服务。分组交换提供两种不同形式的服务:虚电路和数据报。3.3.1Virtualcircuitscallsetupforeachcallbeforedatacanflow,andteardownaftereachpacketcarriesVCidentifier(notdestinationhostID)everyrouteronsource-destpathmaintains“state”foreachpassingconnectiontransport-layerconnectiononlyinvolvedtwoendsystemslink,routerresources(bandwidth,buffers)maybeallocatedtoVCtogetcircuit-likeperf.“source-to-destpathbehavesmuchliketelephonecircuit”performance-wisenetworkactionsalongsource-to-destpathVirtualcircuits:signalingprotocolsusedtosetup,maintainteardownVCusedinATM,frame-relay,X.25notusedintoday’sInternetapplicationtransportnetworkdatalinkphysicalapplicationtransportnetworkdatalinkphysical1.Initiatecall2.incomingcall3.Acceptcall4.Callconnected5.Dataflowbegins6.Receivedata子网采用虚电路方式，只需在建立连接时做一次路由选择。VirtualCircuitDelayPacket1TransmissiontimeVirtualCircuit

EstablishmentPacket2Packet1Packet3Packet2Packet1Packet3Packet2Packet3VirtualCircuit

Termination3.3.2Datagramnetworks:theInternetmodelnocallsetupatnetworklayerrouters:nostateaboutend-to-endconnectionsnonetwork-levelconceptof“connection”packetsforwardedusingdestinationhostaddresspacketsbetweensamesource-destpairmaytakedifferentpathsapplicationtransportnetworkdatalinkphysicalapplicationtransportnetworkdatalinkphysical1.Senddata2.Receivedata子网采用数据报方式，每个包都要做路由选择DatagramnetworksH1H5H6H4H2H3H1send

packets

toH5H2sendpacketstoH6RouterABCDEpacket-switchednetworksThereisnoroundtriptimedelaywaitingforconnectionsetup;ahostcansenddataassoonasitisready.Sourcehosthasnowayofknowingifthenetworkiscapableofdeliveringapacketorifthedestinationhostisevenup.Sincepacketsaretreatedindependently,itispossibletoroutearoundlinkandnodefailures.Sinceeverypacketmustcarrythefulladdressofthedestination,theoverheadperpacketishigherthanfortheconnection-orientedmodel.Packet1Packet2Packet3Packet1Packet2Packet3DatagramDelayPacket1Packet2Packet3processing

delayofPacket1atNode2Host1Host2Node1Node2propagationdelaybetweenHost1andNode2transmissiontimeofPacket1atHost1DatagramorVCnetwork:why?Internet*dataexchangeamongcomputers“elastic”service,nostricttimingreq.*“smart”endsystems(computers)canadapt,performcontrol,errorrecoverysimpleinsidenetwork,complexityat“edge”*manylinktypesdifferentcharacteristicsuniformservicedifficultATMevolvedfromtelephonyhumanconversation:stricttiming,reliabilityrequirementsneedforguaranteedservice“dumb”endsystemstelephonescomplexityinsidenetworkVirtualCircuitPK.Datagram(1)分组头：DG方式的每个分组头中要包含详细的目的地址VC方式由于预先已建立逻辑连接，分组头中只要含有对应于所建立的VC的逻辑信道标识选路：VC方式预先有建立过程，但一旦虚电路建立，在端到端之间所选定的路由上的各个交换节点都具有映象表，存放出入逻辑信道的对应关系，每个分组到来时只要查找映象表，而不需要进行复杂的选路。DG方式则不需要有建立过程，但对每个分组都要独立地进行选路。分组顺序：VC方式中，属于同一呼叫的各个分组在同一条虚电路上传送，分组会按原有顺序到达终点，不会产生失序现象。DG方式中，各个分组由于是独立选路，可以从不同的路由转送，会引起失序VirtualCircuitPK.Datagram(2)故障敏感性VC方式对故障较为敏感，当传输链路或交换节点发生故障时可能引起虚电路的中断，需要重新建立。（有些分组网具有再连接功能，出现故障时可自动建立新的虚电路，并做到不丢失用户数据）DG方式中各个分组可选择不同路由，对故障的防卫能力较强，从而可靠性较高。应用VC方式适用于较连续的数据流传送，其持续时间应显著地大于呼叫建立的时间，如文件传送、传真业务等。DG方式则适用于面向事务的询问／响应型数据业务。

DelayComparison采用存储转发方式的分组交换兼有电路交换和报文交换的优点，它与报文交换的不同在于：分组交换将用户要传送的信息分割为若干个分组（packet），每个分组中有一个分组头，含有可供选路的信息和其他控制信息。3.4ThedelayanalysisinNetworksDelay

inCircuitSwitchingAssume:Numberofhops=MPer-hopprocessingdelay=PLinkpropagationdelay=LTransmissionspeed=Wbit/sMessagesize=BbitsTotalDelay=totalpropagation

+totaltransmission

+totalprocessing=4ML+B/W+(M-1)PPLB/WTotalDelayDelayginDatagramPacketSwitchingAssume:Numberofhops=MPer-hopprocessingdelay=PLinkpropagationdelay=LPackettransmissiondelay=TMessagesize=NpacketsTotalDelay=totalpropagation

+totaltransmission

+

totalstore&forward+totalprocessing=ML+NT+(M-1)T+(M-1)PPTLTotalDelayPTDelayVirt.Circ.PacketSwitchingAssume:Numberofhops=MPer-hopprocessingdelay=PLinkpropagationdelay=LPackettransmissiondelay=TMessagesize=NpacketsTotalDelay=totalpropagation

+totaltransmission

+

totalstore&forward+totalprocessing=4ML+NT+(M