Unit61TwoApproachestoNetworkCommunication

Unit6NetworkCommunication6.1TwoApproachestoNetworkCommunication@Whethertheyprovideconnectionsbetweenonecomputerandanotherorbetweenterminalsandcomputers,communicationnetworkscanbedividedintotwobasictypes:circuit-switched(sometimescalledconnectionoriented)andpacket-switched(sometimescalledconnectionless).terminal:终端,晚期的;端子circuit-switched:电路交换式；回路交换；packet-switched:报文分组;数据包交换无论通信网络提供两台计算机还是终端和计算机之间的连接，可以把它分成两个基本类型：电路交换（有时叫面向连接）和包交换（有时也叫无连接）。Circuit-switchednetworksoperatebyformingadedicatedconnection(circuit)betweentwopoints.form:组成，构成；dedicated:专用的,专门的电路交换网络通过构成通信两端之间的专用连接（回路）来操作。TheU.S.telephonesystemusescircuitswitchingtechnology—atelephonecallestablishesacircuitfromtheoriginatingphonethroughthelocalswitchingoffice,acrosstrunklines,toaremoteswitchingoffice,andfinallytothedestinationtelephone.originating：起始，发起；switchingoffice：中继局；交换局trunkline：干线，中继线；主管道美国电话系统采用电路交换技术——一通电话建立一条从拔号方经过本地交换局，再通过干线到达远方的交换局最终连接到接听方。Whileacircuitisinplace,thephoneequipmentsamplesthemicrophonerepeatedly,encodesthesamplesdigitally,andtransmitsthemacrossthecircuittothereceiver.inplace：准备就绪;在合适的位置；就位，microphone：麦克风；话筒一旦回路准备就绪，电话机重复地将话筒声音取样，数字化地将取样编码，然后将它们通过回路发送到接听方。Thesenderisguaranteedthatthesamplescanbedeliveredandreproducedbecausethecircuitprovidesaguaranteeddatapathof64Kbps(thousandbitspersecond),therateneededtosenddigitizedvoice.guarantee：保证，担保保证发送方的是取样可以被传送并恢复，因为回路提供了发送数字声音所需的速率每秒64Kbps（每秒千比特）的数据路径。Theadvantageofcircuitswitchingliesinitsguaranteedcapacity:onceacircuitisestablished,noothernetworkactivitywilldecreasethecapacityofthecircuit.liein：在于，位于capacity：容量电路交换的优点在于它确保了传输容量：一条回路一建立，就没有其它的活动降低该回路的容量。Onedisadvantageofcircuitswitchingiscost:circuitcostsarefixed,independentoftraffic.Forexample,onepaysafixedrateforaphonecall,evenwhenthetwopartiesdonottalk.cost:v.花费；n.费用traffic:流量rate:费率;比率,等级电路交换的缺点是费用高：回路的花费是固定的，和流量无关。例如，你要为每通电话付固定的话费，即使双方一句话都没说。@Packet-switchednetworks,thetypeusuallyusedtoconnectcomputers,takeanentirelydifferentapproach.通常用于连接计算机包交换网络类型，采用完全不同的方法。Inapacket-switchednetwork,datatobetransferredacrossanetworkisdividedintosmallpiecescalledpacketsthataremultiplexedontohighcapacityintermachineconnections.packet:包，分组multiplex：多路复用，多路传输分组交换网中的通过网络传输的数据被分成叫做包的小数据片，包被多路传输到高容量的机器互连上。Apacket,whichusuallycontainsonlyafewhundredbytesofdata,carriesidentificationthatenablesthenetworkhardwaretoknowhowtosendittothespecifieddestination.identification:标识,鉴别,身份证每个分组都含有仅几百字节的数据，它携带着使网络硬件可以知道如何将包发送到指定目标的标识。Forexample,alargefiletobetransmittedbetweentwomachinesmustbebrokenintomanypacketsthataresentacrossthenetworkoneatatime.transmit:发送;传递发射传染oneatatime:一次一个的例如，一个有两台机器间传输的大文件须分割成许多包，然后通过网络一次一个地发送出去。Thenetworkhardwaredeliversthepacketstothespecifieddestination,wheresoftwarereassemblesthemintoasinglefileagain.reassemble:重新组装重新装配;网络硬件将这些包传递到指定的目标机器，在那里软件将它们重新组装成单一的文件。Thechiefadvantageofpacket-switchingisthatmultiplecommunicationsamongcomputerscanproceedconcurrently,withintermachineconnectionssharedbyallpairsofmachinesthatarecommunicating.concurrently：并发地，同时地包交换的优点是通过每对处于通信中的机器所共享的机器连接，计算机间多路通信可以并发地进行。Thedisadvantage,ofcourse,isthatasactivityincreased,agivenpairofcommunicationcomputersreceiveslessofthenetworkcapacity.缺点自然是随着活动的增加，给定的一对计算机间的通信将获得更少的网络容量。Thatis,wheneverapacketswitchednetworkbecomesoverloaded,computersusingthenetworkmustwaitbeforetheycansendadditionalpackets.overload:超负荷也就是说，每当包交接网络超负荷时，使用网络的计算机在发送更多包之前必须等待。@Despitethepotentialdrawbackofnotbeingabletoguaranteenetworkcapacity,packet-switchednetworkshavebecomeextremelypopular.drawback缺点，劣势尽管有潜在的不能够保障网络容量的缺陷，但是包交换网络已经非常普及了。Themotivationsforadoptingpacketswitchingarecostandperformance.motivation:动机;动力采用包交换方式的推动力是费用和性能。Becausemultiplemachinescansharethenetworkhardware,fewerconnectionsarerequiredandcostiskeptlow.因为多机器可共享网络硬件，无需太多连接，而且费用低廉。Becauseengineershavebeenabletobuildhighspeednetworkhardware,capacityisnotusuallyaproblem.因为工程师已经有能力设置高速网络硬件，容量正常不是问题。Somanycomputerinterconnectionsusepacket-switchingthat,throughouttheremainderofthistext,thetermnetworkwillreferonlytopacket-switchednetworks.remainder:剩余部分，剩余物;剩余的,留存下的如此多的计算机互连使用包交换技术，所以本教材剩余的所有部分，网络这个术语仅代表包交换网络。