电子信息科学专业英语En-Unit4

电子信息科学专业英语UNIT4Informationnetwork,protocolsandapplications4-1ComputernetworksKEYWORDSTRANSLATIONEXERCISESNOTESReadingKEYWORDSchannel通道,信道hierarchy层次，等级结构routingproblem路由问题vendor厂商，自动售货机mainframe主机，大型机autonomy自主权，自治protocol协议duplicate复制、复写facility设备，工具standardize使标准化incompatible不兼容的TRANSLATIONInthenineteen-sixties,almostimmediatelyaftercomputersbecamewidespread,thedesiregrewtoconnectcomputerstogether.在上世纪六十年代，几乎立即在计算机变得普遍之后，把计算机相互连接起来的需求就开始形成了。Originallytheaimwastoallowtheexchangeofinformation(programs,datafiles)betweentheusersofdifferentmainframes.最初的目地是为了在不同主机的使用者之间信息(包括程序，数据文件)的交换。广泛分布Whensmallercomputersbecameavailableandspreadinlargernumberswithinasingleorganization,connectionsbetweenthesesmallcomputersbecameattractivefortheshareduseofresources(processingtime)aswell.当小型计算机在一个机构内的大量分布变得可用时，这些小型计算机之间的连接因资源(处理时间)的共享使用变得引人注目。Theresultingsystems,inwhichcomputersofpossiblydifferenttypeandsizeareconnectedbutfullyretaintheirautonomy,arecalledcomputernetworks.所产生的系统称为计算机网络，在这个系统中那些可能不同类型和大小的计算机被互相连接并完全保持了他们各自的独立性。Computernetworks,world-widenetworksof(large)computersarereferredtoaswideareanetworks,andcampus-widenetworksof(small)computersarereferredtoaslocalareanetworks.计算机网络中，大型计算机的网络称为广域网，小型计算机组成的校园网称为局域网。Thecompletedesignofacomputernetworkandthenetworkingsoftwarerequiresthesolutionofmanyproblemsandisaverycomplicatedmatter.计算机网络和网络软件的完全设计需要解决许多问题并且是一件非常复杂的事情。把……称为称……为……Toreducethedesigncomplexityanetworkarchitectureisviewedasahierarchyoflayers.为了减少设计复杂度，网络结构被视为一个多层结构。Eachlayer(orlevel)consistsofprotocolsthatusefunctionsofthelowerlayersandprovideservicestothehigherlayers.每层(或级)由协议组成，这些协议使用较低层的功能并且为较高层提供服务。Eachlayerperformswell-definedfunctions,andcanbeoflimitedcomplexity.Togetherthelayersimplementthefacilitiesofthenetwork.每层运行明确的功能,而且复杂度是有限的。所有的层一起实现网络设备的运行。复杂度TheapproachtonetworkarchitectureandimplementationwasstandardizedintheInternationalStandardsOrganization'sOSIreferencemodelforOpenSystemsInterconnection.网络体系结构的实现方法及其执行，已被国际标准化组织标准化为开放系统互连叁考模型。Themodelcontainssevenlayersandwasdesignedsoastopreventthenetworkingsoftwareofdifferentvendorsbeinglargelyincompatible.该模型包含七层并且是为了防止不同计算机销售商的网络软件不兼容而被设计的。Thefollowinglayersaredistinguished.Thephysicallayerprovidesprotocolsfortransmittingbitsviaaline.Thedatalinklayerimplementsmessagetransfersandhandlestransmissionerrors.下列的层是相互区别的。物理层提供经由一条线传输位的协议。数据链路层实现信息传输以及处理传输误差。Inthenetworklayerconfiguration-dependentfunctionsareimplemented,including,e.g.,theroutingofmessages.在网络层中配置相关的功能被实现,包括,例如,消息的路由。Theroutingproblemrequiresthatapathisfoundtotransportinformationbetweensitesthatarenotconnecteddirectlybyachannel.路由问题需要找到一条在不被一个信道直接连接的站之间传输信息的路径。Asthenetworklayermayroutesubsequentmessagesviadifferentpaths,messagesmayarriveinanorderdifferentfromtheorderinwhichtheyweresent.由于网络层可以为后来的消息经过不同的路径配置路由,因而消息可能以不同于发送时的次序抵达。配置相关Also,duetothepossibilityofintermediatecomputersinthenetworkgoingdown,messagesmaybelostorduplicated(astheresultofaretransmission).同时由于在下面的网络中存在中间计算机的可能性，信息可能被丢失或复制。(正如转播的结果)Anextlayerinthehierarchy,thetransportlayer,servestodealwiththeseproblems.Thesessionlayerconcernstheproblemofreliablyopeningandclosingconnectionsbetweenprocessesinthenetwork.分层组织的下个层——传输层,负责处理这些问题。会话层关注的是网络进程之间中可靠地打开和关闭联接的问题。Thepresentationlayerperformsthenecessarydatacompressionandconversion.Theapplicationlayercontainsprotocolsfortheexchangeofinformationbetweendistributeduserprograms(applications).表示层运行必需的数据压缩和变换。应用层包含在分布式的用户程序(应用)之间进行信息交换的协议。Inahierarchicalarchitecturethemodelofdistributedsystemscanbeusedforthedesignofprotocolsineachofthelevels.Onlytheassumptionsaboutthesystemandtheproblemstobesolveddifferfromleveltolevel.在分层结构中，分布式系统的模型可以用来为每一层设计协议。只有关于系统的假设和待解决的问题在层与层之间相互不同。Forexample,inthedatalinklayerthepossibilityoftransmissionerrorsisconsidered,butinthehigherlevelprotocolsthispossibilityisignored,inthenetworklayeritisassumedthattheprocessesarenotalldirectlyconnectedbyachannel(otherwisetheroutingproblemwouldbetrivial).例如，在数据链路层传输误差的可能性被考虑，但是在更高层的协议中这个可能性被忽略；在网络层假设进程不是全部被一个信道直接连接（否则路由问题将没有意义)Inhigherlevels,however,theassumptioncanbemade,becausemessagepassingbetweeneverypairsofprocessesisimplementedbythenetworklayer.然而，在更高层这个假设仍然可以做，因为在每对进程之间消息的传输是靠网络层来实现的。Inthetransportlayeronlythetwoendpointsofachannelareconsidered;thedistributedsystemitappliestoconsistoftwoprocessesonly.在传输层，只有一个信道的两个终点被考虑，采用的分布式系统只包含两个进程。Matchthefollowingtermstotheappropriatelayerdefinition(1)_______providesprotocolsfortransmittingbitsviaaline.(2)_______implementsmessagetransfersandhandlestransmissionerrors.(3)_______implementsconfiguration-dependentfunctions,including,e.g.,theroutingofmessages.(4)_______servestodealwiththeleastorretransmissionproblems.(5)_______concernstheproblemofreliablyopeningandclosingconnectionsbetweenprocessesinthenetwork.(6)_______performsthenecessarydatacompressionandconversion.(7)_______containsprotocolsfortheexchangeofinformationbetweendistributeduserprograms(applications).a.Theapplicationlayerb.Thesessionlayerc.Thepresentationlayerd.Th