《计算机专业英语教程》-第5章

Ⅰ.GettingstartedThinkingaboutthesequestionswhilereading:Whatisatopology?Whatarecommonlyusedtopologiesincomputernetworks?Andwhatshouldbeconsideredwhenplanningatopology?Ⅱ.TextWhatisaTopology?Thephysicaltopologyofanetworkreferstotheconfigurationofcables,computers,andotherperipherals.Physicaltopologyshouldnotbeconfusedwithlogicaltopologywhichisthemethodusedtopassinformationbetweenworkstations.Topologycanbeconsideredasavirtualshapeorstructureofanetwork.Thisshapeactuallydoesnotcorrespondtotheactualphysicaldesignofthedevicesonthecomputernetwork.Thecomputersonthehomenetworkcanbearrangedinacircleshapebutitdoesnotnecessarilymeanthatitpresentsaringtopology.LessonAAnOverviewofComputerNetworkTopologies下一页返回Computernetworktopologiescanbecategorizedinthefollowingcategories:①Bus,②Star,③Ring,④Mesh,⑤Tree.Hybridnetworksarethecomplexnetworks,whichcanbebuiltoftwoormoreabovementionedtopologies.BusTopologyAbustopologyusesacommonbackbonetoconnectallthenetworkdevicesinanetworkinalinearshape.Asinglecablefunctionsasthesharedcommunicationmediumforallthedevicesattachedwiththiscablewithaninterfaceconnector.LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回Thedevice,whichwantstocommunicate,sendsthebroadcastmessagetoallthedevicesattachedwiththesharedcablebutonlytheintendedrecipientactuallyacceptsandprocessesthatmessage.Ethernetbustopologiesareeasytoinstallanddon’trequiremuchcablingandonlyamainsharedcableisusedfornetworkcommunication.10Base-2and10BaseTaretwopopulartypesoftheEthernetcablesusedinthebustopology.However,thebusnetworkworkswithverylimiteddevices.Performanceissuesarelikelytooccurinthebustopologyifmorethan12-15computersareaddedinabusnetwork.Additionally,ifthebackbonecablefails,thenthewholenetworkbecomesuseless,unlikeinthestartopologyinwhichifonecomputerisdetachedfromanetworkthenthereisnoteffectontheothercomputersinanetwork.AdvantagesofaBusTopology(1)Easytoconnectacomputerorperipheraltoalinearbus.(2)Requireslesscablelengththanastartopology.LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回DisadvantagesofaBusTopology(1)Entirenetworkshutsdownifthereisabreakinthemaincable.(2)Terminatorsarerequiredatbothendsofthebackbonecable.(3)Difficulttoidentifytheproblemiftheentirenetworkshutsdown.Notmeanttobeusedasastand-alonesolutioninalargebuilding.RingTopologyInaringnetwork,everycomputerordevicehastwoadjacentneighborsforcommunication.Inaringnetwork,allthecommunicationmessagestravelinthesamedirectionwhetherclockwiseoranti-clockwise.Anydamageofthecablebetweenanytwocomputerscanresultinthebreakdownofthewholenetwork.Ringtopologynowhasbecomealmostobsolete.FDDI,SONETorTokenRingTechnologycanbeusedtoimplementRingTechnology.Ringtopologiescanbefoundintheoffice,schoolorsmallbuildings.LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回Advantagesofaringtopology(1)Veryorderlynetworkwhereeverydevicehasaccesstothetokenandtheopportunitytotransmit.(2)Performsbetterthanastartopologyunderheavynetworkload.(3)Doesnotrequirethenetworkservertomanagetheconnectivitybetweenthecomputers.Disadvantagesofaringtopology(1)Onemalfunctioningworkstationcancreateproblemsfortheentirenetwork.(2)Moves,addsandchangesofdevicescanaffectthenetwork.(3)NetworkadaptercardsaremuchmoreexpensivethanEthernetcardsandhubs.(4)MuchslowerthananEthernetnetworkundernormalload.LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回StarTopologyInthecomputernetworkingworld,themostcommonlyusedtopologyintheLANisthestartopology.Startopologiescanbeimplementedinhome,officesoreveninabuilding.Allthecomputersinthestartopologiesareconnectedtocentraldeviceslikethehub,switchorrouter.Thefunctionalityofallthesedevicesisdifferent.Computersinastarnetworkareusuallyconnectedwiththehub,switchorrouterwiththeUnshieldedTwistedPair(UTP)orShieldedTwistedPair(STP)cables.Ascomparedtothebustopology,astartopologyrequiresmoredevicesandcablestocompleteanetwork.Thefailureofonenodeorcableinastarnetworkwillnotcausedisruptionstotheentirenetworkascomparedtothebustopology.However,ifthecentralconnectingdevice,suchasthehub,switchorrouter,failsduetoanyreason,thenultimatelythewholenetworkcancollapse.LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回AdvantagesofaStarTopology(1)Easytoinstallandwire.(2)Nodisruptionstothenetworkwhenconnectingorremovinganycomputerexceptthecentraldevice.(3)Easytodetectfaults.DisadvantagesofaStarTopology(1)Requiresmorecablelengththanabustopology.(2)Ifthehub,switchorrouterfails,nodesattachedaredisabled.(3)Moreexpensivethanthebustopologybecauseofthecostofthehubs,etc.TreeTopologyThetreetopologyiscomposedofthemultiplestarnetworksonabus.Inotherwords,itintegratesmultiplestartopologiestogetherontoabus.Onlythehubdevicescanconnectdirectlywiththetreebusandeachhubfunctionsasarootofatreeofthenetworkdevices.LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回Thiscombinationofstarandringnetworkssupportsfutureexpandabilityofthecomputernetworks,muchbetterthanabusorstar.

AdvantagesofaTreeTopology(1)Point-to-pointwiringforindividualsegments.(2)Supportedbyseveralhardwareandsoftwarevenders.DisadvantagesofaTreeTopology(1)Overalllengthofeachsegmentislimitedbythetypeofcableused.(2)Moredifficulttoconfigureandwirethanothertopologies.MeshTopologyThemeshtopologyworksontheconceptofroutes.Inameshtopology,themessagesenttothedestinationcantakeanypossibleshortest,easiestroutetoreachitsdestination.Intheprevioustopologies,messagesareusuallybroadcastedtoeverycomputerinthenetwork,especiallyinthebustopology.LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回TheInternetemploysthemeshtopologyandthemessagefindsitsrouteforitsdestination.Therouterworksinfindingtheroutesforthemessagesandinreachingthemtotheirdestinations.Thetopologyinwhicheverydevicedirectlyconnectstoeveryotherdeviceiscalledafullmeshtopologyunlikeinthepartialmeshinwhicheverydeviceisindirectlyconnectedtootherdevices.Advantagesofameshtopology(1)Providesredundantpathsbetweendevices.(2)Nodisruptionstocurrentuserswhenthenetworkisexpanded.Disadvantagesofameshtopology(1)Requiresmorecablesthanothertopologies.(2)Complicatedimplementation.LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回SummaryTopologiesaretheimportantpartofthenetworkdesigntheory.Abetternetworkcanbebuiltifyouhavetheknowledgeofthesetopologiesandifyouknowthedifferencesbetweendifferenttopologies.Similarly,youshouldhavetheknowledgeofeachnetworkdevicesothatyoucanproperlyusethemaccordingtoyournetworkneeds.Amis-configurednetworkcanresultinawasteoftimeandenergyaswellasalotoftroubleshootingmethodstoresolvetheissue.Sothebasicunderstandingofthenetworktopologiesandnetworkdevicesisamusttobuildagoodnetwork.Whenchoosingatopology,thefollowingitemsshouldbetakenintoconsideration:(1)Money.Abusnetworkmaybetheleastexpensivewaytoinstallanetwork;youdonothavetopurchaseconcentrators.(2)Lengthofcableneeded.Thelinearbusnetworkusesshorterlengthsofcable.(3)Futuregrowth.Withastartopology,expandinganetworkiseasilydonebyaddinganotherconcentrator.LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回(4)Cabletype.Themostcommoncableinschoolsisunshieldedtwistedpair,whichismostoftenusedwithstartopologies.

Ⅲ.Exercises(1)WritedownthemeaningofthefollowingnounsorabbreviationsinChineseEthernet ()LAN ()UTP ()STP ()FDDI ()Topology ()Node ()Load ()Hub ()Switch ()Router ()LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回(2)IdentificationandMatching.Choosethecorrectnamesforthefollowingpictures:Whattopologiesarethey?A.StarTopologyB.MeshTopologyC.BusTopologyD.TreeTopologyLessonAAnOverviewofComputerNetworkTopologies上一页下一页返回Ⅳ.AnswerstoExercises（1）Ethernet（以太网）LAN（局域网）UTP（非屏蔽双绞线）STP（屏蔽双绞线）FDDI（光纤分布式数据接口）Topology（拓扑）Node（节点）Load（负载）Hub（集线器）Switch（交换机）Router（路由器）（2）1.B2.A3.D4.CⅤ.PositionKnowledge我们知道局域网（LocalAreaNetwork，LAN）是将小区域内的各种通信设备互联在一起所形成的网络，覆盖范围一般局限在房间、大楼或园区内。局域网的特点是：距离短、延迟小、数据速率高、传输可靠。LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回目前常见的局域网类型包括：以太网（Ethernet）、光纤分布式数据接口（FDDI）、异步传输模式（ATM）、令牌环网（TokenRing）、交换网（Switching）等，它们在拓扑结构、传输介质、传输速率、数据格式等多方面都有许多不同。其中应用最广泛的是以太网，这种总线结构的LAN，是目前发展最迅速、也最经济的局域网。这里简单对以太网（Ethernet）、光纤分布式数据接口（FDDI）、异步传输模式（ATM）进行介绍。1.以太网Ethernet是Xerox、DigitalEquipment和Intel三家公司开发的局域网组网规范，并于20世纪80年代初首次出版，称为DIX1.0。1982年修改后的版本为DIX2.0。这三家公司将此规范提交给IEEE（电子电气工程师协会）委员会，变成了IEEE的正式标准，编号为IEEE802.3。Ethernet和IEEE802.3虽然有很多规定不同，但通常认为与802.3是兼容的。IEEE将802.3标准提交国际标准化组织（ISO）第一联合技术委员会（JTC1），再次经过修订变成了国际标准ISO8802.3。LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回以太网根据不同的媒体可分为：10Base-2、10Base-5、10Base-T及10Base-FL。10Base-2以太网采用细同轴电缆组网，最大的网段长度是200m，每网段节点数是30，它是相对最便宜的系统；10Base-5以太网采用粗同轴电缆，最大网段长度为500m，每网段节点数是100，它适合用于主干网；10Base-T以太网采用双绞线，最大网段长度为100m，每网段节点数是1024，它的特点是易于维护；10Base-FL以太网采用光纤连接，最大网段长度是2000m，每网段节点数为1024，此类网络最适于在楼间使用。2.FDDI网络光纤分布数据接口（FDDI）是目前成熟的LAN技术中传输速率最高的一种。这种传输速率高达100Mbps的网络技术所依据的标准是ANSIX3T9.5。该网络具有定时令牌协议的特性，支持多种拓扑结构，传输媒体为光纤。使用光纤作为传输媒体具有多种优点：（1）较长的传输距离，相邻站间的最大长度可达2km，最大站间距离为200km。（2）具有较大的带宽，FDDI的设计带宽为100Mbps。LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回（3）具有对电磁和射频干扰抑制能力，在传输过程中不受电磁和射频噪声的影响，也不影响其设备。（4）光纤可防止传输过程中被分接偷听，也杜绝了辐射波的窃听，因而是最安全的传输媒体。光纤分布式数据接口FDDI是一种使用光纤作为传输介质的、高速的、通用的环形网络。它能以100Mbps的速率跨越长达100km的距离，连接多达500个设备，既可用于城域网络也可用于小范围局域网。然而FDDI与以太网一样，其本质仍是介质共享、无连接的网络，这就意味着它仍然不能提供服务质量保证和更高的带宽利用率。在少量站点通信的网络环境中，它可达到比共享以太网稍高的通信效率，但随着站点的增多，效率会急剧下降，这时候无论从性能和价格都无法与交换式以太网、ATM网相比。据相关部门统计，现在各大型院校、教学院所、政府职能机关建立局域或城域网络的设计倾向较为集中的在ATM和快速以太网这两种技术上，原先建立较早的FDDI网络，也在向星型、交换式的其他网络技术过渡。LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回3.ATM网络随着人们对集话音、图像和数据为一体的多媒体通信需求的日益增加，特别是为了适应今后信息高速公路建设的需要，人们又提出了的宽带综合业务数字网（B-ISDN）这种全新的通信网络，而B-ISDN的实现需要一种全新的传输模式，即异步传输模式（ATM）。1990年，国际电报电话咨询委员会（CCITT）正式建议将ATM作为实现B-ISDN的一项技术基础，这样，以ATM为机制的信息传输和交换模式也就成为电信和计算机网络操作的基础和2l世纪通信的主体之一。ATM是目前网络发展的最新技术，它采用基于信元的异步传输模式和虚电路结构，根本上解决了多媒体的实时性及带宽问题。实现面向虚链路的点到点传输，它通常提供155Mbps的带宽。既汲取了话务通信中电路交换的“有连接”服务和服务质量保证，又保持了以太、FDDI等传统网络中带宽可变、适于突发性传输的灵活性，从而成为迄今为止适用范围最广、技术最先进、传输效果最理想的网络互联手段。ATM技术具有如下特点：（1）实现网络传输有连接服务，实现服务质量保证（QoS）。（2）交换吞吐量大、带宽利用率高。LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回（3）具有灵活的组网拓扑结构和负载平衡能力，伸缩性、可靠性极高。（4）ATM是现今唯一可同时应用于局域网、广域网两种网络应用领域的网络技术，它将局域网与广域网技术统一。4.其他局域网令牌环是IBM公司于20世纪80年代初开发成功的一种网络技术。之所以称为环，是因为这种网络的物理结构具有环的形状。环上有多个站逐个与环相连，相邻站之间是一种点对点的链路，因此令牌环与广播方式的Ethernet不同，它是一种顺序向下一站广播的LAN。与Ethernet不同的另一个诱人的特点是，即使负载很重，仍具有确定的响应时间。令牌环所遵循的标准是IEEE802.5，它规定了三种操作速率：1Mbps、4Mbps和16Mbps。开始时，UTP电缆只能在1Mbps的速率下操作，STP电缆可操作在4Mbps和16Mbps的速率下，现已有多家厂商的产品突破了这种限制。LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回Ⅵ.Translation1.计算机网络拓扑概述网络的物理拓扑结构指的是电缆、计算机以及其他外设的配置情况。不要混淆物理拓扑和逻辑拓扑，因为逻辑拓扑指的是不同工作站之间信息传输的方式。我们可以把拓扑看成网络的虚拟形状或虚拟结构，但这个形状实际上和计算上网络上各种设备的物理设计并不一致。家庭网络里的多台计算机可以排列成一个圆圈，但并不代表这个网络是环形拓扑。计算机网络拓扑结构可以分为以下几类：①总线形拓扑；②星形拓扑；③环形拓扑；④网形拓扑；⑤树形拓扑。混合型网络相对复杂，可由上述两种或多种拓扑结构组成。LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回2.总线形拓扑总线形拓扑用一根主干电缆将网络内的所有设备连接起来，呈线形形状。这样一根电缆通过接口连接器将所有设备连接起来，而电缆本身作为这些设备共享的通信媒介。想要通信的设备会向共享这根电缆的所有设备发送广播消息，但接收并处理信息的实际上只有预期的接收设备。使用总线形拓扑的以太网易于安装，无需大量布缆工作，并且用于网络通信的只有一根主干的共享电缆。总线形拓扑中常用的以太网电缆有10Base-2和10Base-T两种。但是，总线形网络所带的设备数量非常有限，如果这种拓扑结构中的计算机数量超过12～15台，那网络的工作就可能受到影响。此外，如果主干电缆出了问题，整个网络就会随之瘫痪。这与星形拓扑不同，从星形网络中拆除一台计算机对网络中的其他计算机并无影响。（1）总线形网络的优势①易于添加计算机或者外设。②所需电缆比星形拓扑少。LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回（2）总线型网络的不足①主干电缆出了问题会导致整个网络瘫痪。②主干电缆两端须安装终结器。③网络瘫痪后不易查找问题。总线形网络不宜作为大型建筑物单独的网络解决方案。3.环形拓扑在环形网络中，每台计算机或设备都有两个相邻的通信设备。环形网络中所有的通信消息均按一个方向传送（顺时针方向或逆时针方向）。任何两台计算机之间的电缆中断都会造成网络瘫痪。目前星形拓扑几乎已经过时了。FDDI、SONET或者令牌环技术可用来运行环形技术。环形拓扑可用于办公室、学校或小型建筑。（1）环形拓扑的优势①网络结构有序，每个设备都可以获取令牌、传输数据。②负载重的情况下网络效果比星形拓扑好。③无需借助网络服务器管理计算机之间的互联。LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回（2）环形拓扑的不足①一台工作站运行不正常会造成整个网络出现故障。②设备的移动、添加和变更会影响整个网络。③网络适配器比以太网卡和集线器的价格昂贵得多。④负载正常的情况下比以太网的速度慢。4.星形拓扑在计算机互联方面，局域网中使用最为广泛的拓扑结构是星形拓扑。家中、办公室、甚至建筑物里都可以使用星形拓扑。星形拓扑中的所有计算机都连接到集线器、交换机或路由器等中心设备上。这些设备的功能不尽相同。星形网络中的多台计算机通常通过非屏蔽双绞线或屏蔽双绞线连接到集线器、交换机或路由器上。与总线形拓扑相比，按照星形拓扑组网需要更多的设备和电缆，但星形网络中某个节点或某段电缆出现问题不会引起网络中断。然而如果集线器、交换机或路由器等中心连接设备由于某种原因出了问题，那整个网络就会瘫痪了。LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回（1）星形拓扑的优势①易于安装和布线。②除了中心设备外，连接或拆除任何一台计算机不会引起网络中断。③易于判断故障。（2）星形拓扑的不足①所需电缆比总线形拓扑多。②如果集线器、交换机或路由器出现故障，所连接的节点全部瘫痪。③由于涉及集线器等费用，星形拓扑比总线型拓扑的造价高。5.树形拓扑树形拓扑由总线上的多个星形网络组成。换言之，树形拓扑在总线上结合了多个星形拓扑。与树形总线直接连接的只有集线器等设备，每个集线器相当于树形网络设备的一个根。和总线型以及星形网络比较起来，这样一个由多个星形网络结合起来的网络为计算机网络提供了更好的扩展性。LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回（1）树形网络的优势①每个网段的点对点布线。②可获得不同硬件商和软件商的支持。（2）树形网络的不足①每个网段的总长度受所用电缆类型的限制。②配置和布线要比其他拓扑结构困难。6.

网形拓扑网形拓扑建立在路由的概念上。在网状拓扑中，发往目的地的消息可以采用最短、最容易的路由进行传送，而在上述各种拓扑结构里，尤其是总线形网络里，消息通常是广播给网络里所有的计算机。互联网采用的是网状拓扑，其中的消息寻找自己的目的地。路由器为消息寻找路由并将消息传送到目的地。网络中任何一台设备都与其他任何一台设备互联的拓扑结构称为全网形拓扑，这与每台设备与其他设备不直接互联的部分网形拓扑不同。LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回（1）网形拓扑的优势①设备之间有冗余通路。②扩展网络时不会造成现有用户的中断。（2）网形拓扑的不足①所需电缆比其他拓扑多。②操作复杂。7.总结拓扑是网络设计理论中的一个重要部分。具备了关于拓扑的知识并知晓不同拓扑之间的区别就可以构架更好的网络。同样，了解每个网络设备的知识才能根据网络需要恰当地使用这些设备。配置不当的网络既费时又费力并且解决问题时需要大量解决问题的方法。因此，网络拓扑和网络设备的基础知识是构架优秀网络所必需的。选择拓扑时，需考虑以下事项。①资金：构架总线形网络也许是造价最低的，因为你无需购买集线器。LessonAAnOverviewofComputerNetworkTopologies上一页下一页返回②所需电缆长度：总线形网络使用的电缆长度较短。③未来的扩展：星形拓扑扩展容易，增加集线器即可。④电缆类型：学校里最常见的电缆是非屏蔽双绞线，这种双绞线通常用于星形拓扑。

LessonAAnOverviewofComputerNetworkTopologies上一页返回Ⅰ.TaskIntroductionThingaboutthesequestionswhilereading:WhatistheOSIreferencemodel?Howmanylayersarethereinthemodelandwhataretheyrespectively?Trytorememberallofthemwhilereadingandwritethenamesofalltheselayers.TheOpenSystemsInterconnection(OSI)referencemodeliscommonlyusedtodescribeinanabstractmannerthefunctionsinvolvedindatacommunication.Thismodel,originallyconceivedintheInternationalOrganizationforStandardization(ISO),definesdatacommunicationsfunctionsintermsoflayers.IntheOSIreferencemodel,eachlayerisresponsibleforcertainbasicfunctions,suchasgettingdatafromonedevicetoanotherorfromoneapplicationonacomputertoanother.TheOSIreferencemodelisdepictedasthefollowingpicture.Successfulcommunicationbetweentwoapplicationsdependsonsuccessfulfunctionsatallsevenlayers.LessonBTheOSIReferenceModel下一页返回Wemustemphasizethatthedefinitionofalayeredarchitectureofdatacommunicationisonlyanabstraction.Itismeanttodescribethefunctionsinvolvedindatacommunicationinawaywhichallowsdifferentsystemstobecompared.TheOSIreferencemodeldefinitionisintendedtoneitherimplynorconstraintheimplementationofanycommunicationsystem.WewillnowreviewthefunctionsoftheOSIlayersandsomeoftheprimaryprotocolsateachlayer.Layer1-ThePhysicalLayerThephysicallayerfunctionsincludeallphysicalaspectsofcommunicatingbetweentwodirectly-connectedphysicalentities.Typicallythesephysicalpropertiesincludeelectromechanicalcharacteristicsofthemediumorlinkbetweenthecommunicatingphysicalentitiessuchasconnectors,voltages,transmissionfrequencies,etc..Thislayersummarizesthephysicswhichunderliethecommunicationpath.LessonBTheOSIReferenceModel上一页下一页返回Theessentialserviceprovidedbythephysicallayerconsistsofanunstructuredbitstream,whichcanbeusedbyhigherlayerstoprovidethebasisforhigherlayercommunicationservices.Anexampleofaphysicallayeristheinkonpaperusedbythisbooktoconveyinformation.Anotherexampleistheradiofrequenciesusedinawirelesscommunicationssystem.Layer2-TheDataLinkLayerThedatalinklayeracceptsthebitstreamprovidedbythephysicallayerandprovidesreliabletransferofdatabetweentwodirectly-connectedLayer2entities.“Directly-connected”meansthattheLayer2entities'communicationpathdoesnotrequireanotherLayer2entity.However,thisdoesnotimplyadedicatedpath;inthecaseofEthernet,manyLayer2entitiescanbesharingacommon(physical)mediumsuchasacoaxialcableora10BASE-Thub.LessonBTheOSIReferenceModel上一页下一页返回Layer2functionalityislimitedinscope.Itcouldbelikenedtoanintra-officecorrespondencebetweenco-workers;thereisaneedforreliabilitybutaddressingisrelativelysimple.Localareanetworks(LANs)operateatLayer2.Thedatalinklayerisitselfconceptuallysubdividedintotwosub-layers,mediumaccesscontrolandlogicallinkcontrol,whichmorespecificallydefinetheprimaryaspectsofdatalinklayerfunctionality.Layer3-TheNetworkLayerThenetworklayerdefinesthefunctionsnecessarytosupportdatacommunicationbetweenindirectly-connectedentities.ItprovidesthecapabilityofforwardingmessagesfromoneLayer3entitytoanotheruntilthefinaldestinationisreached.Thenetworklayerintroducesanotherlayertothemodel.Networklayerfunctionsincluderoutingandforwardingofpacketstotheirfinaldestinations.LessonBTheOSIReferenceModel上一页下一页返回Inordertoforwardapackettoitsdestinationhost,routinginformationmustbeprovidedtotheintermediatesystems(ISs)orroutersresponsibleforforwardingpacketstotheirrespectivedestinations.Thisroutinginformationincludestheaddressofthedestination,whichiscontainedineachpacket.Thenexthoptobetraversedbythepacketisdeterminedprimarilybythisdestinationaddress.Layer4-TheTransportLayerThetransportlayerisconcernedwithgettingLayer4messagesfromsourcetodestinationinareliablemanner.TheperspectiveofLayer4isofend-to-endcommunicationsratherthanthehop-by-hopperspectiveofLayer3.Layer4assumesthatpacketscanbemovedfromnetworkentitytonetworkentity,eventuallygettingtothefinaldestinationhost.HowthisisaccomplishedisofnoconcerntoLayer4.LessonBTheOSIReferenceModel上一页下一页返回Likeotherlayers,transportlayerprotocolscanbeeitherconnection-orientedorconnectionless,dependingontheservicesrequiredbyhigherlayers.AcommonimplementationofLayers3and4involvesaconnection-orientedtransportlayerprotocolrunningoveraconnectionlessnetworklayerprotocol,suchastheubiquitousTCP/IP(TransportControlProtocol/InternetProtocol)protocol.Inthisinstance,thecommunicatinghostsmaintainstateinformationoncommunicationswitheachothertodeterminewhenandwhattosend.ThisstateinformationdefinestheconnectionbetweenthecommunicatingLayer4entities.Layer5-TheSessionLayerThesessionlayerprovidesacontrolstructureforcommunicationbetweenapplicationsonhosts.Thecommunicationatlayer5iscalledasession,whichdefinesthetimingofcommunicationsbetweenthehosts'applications.Synchronizationofcommunicatingapplicationscomesintoplaywhencoordinatedtimingofcorrespondingeventsattheendpointsisimperative,suchasinfinancialtransactions.LessonBTheOSIReferenceModel上一页下一页返回Remember,layersdefinecommunicationfunctions,notimplementations.Itisunlikelythatasessionlayerwouldbeexplicitlyimplementedasastand-aloneprogram,althoughitsfunctionswouldbeimplementedsomewhere.Sessionlayerfunctionsdependonthereliabilityofcommunicationsbetweentheendpoints,andsessionlayerfunctionsmustthereforebeimplementedaboveLayer4.Layer6-ThePresentationLayerThepresentationlayerperformsanynecessarydatatransformationsorformattingrequiredbytheendapplications.Functionsprovidedbythepresentationlayerincludedatacompression,fileformattingandencryption.Commondataformattingisimportantbecauseitallowsthesameapplicationfiletobeaccessedbytheapplicationrunningondifferentcomputerplatforms.Thisbookisitselftheproductofanapplicationrunningondifferentplatforms,withcommonfilesbeingmodifiedviathesedifferentplatforms.LessonBTheOSIReferenceModel上一页下一页返回Layer7-TheApplicationLayerTheapplicationlayerprovidestheserviceswhichdirectlysupportanapplicationrunningonahost.Theseservicesaredirectlyaccessiblebyanapplicationviacommonwell-knownapplicationprograminterfaces(APIs),whichcanactuallyoccuratmanylayers.Examplesoflayer7servicesincludeFTP

(filetransferprotocol),TelnetandSNMP(simplenetworkmanagementprotocol).Mostnetworkmanagementactivitiesarebasedontheservicesprovidedbylayer7applicationentities,whichinturnrelyonlowerlayerservicestobeabletoperformtheirfunctions.LessonBTheOSIReferenceModel上一页下一页返回Ⅱ.Haveatry(1)Thereareoftenanumberofacronymsappearinginarticlesoncomputerscience,memorizeasmanyaspossibleandtrytowritethefollowingonesinfull.OSI ()ISO ()FTP ()TCP/IP ()API ()LessonBTheOSIReferenceModel上一页下一页返回(2)ThefollowingisachartoftheOSIreferencemodel,pleasefilltheblanksappropriately.LessonBTheOSIReferenceModel上一页下一页返回Ⅲ.AnswertoHaveatry(1)OSI (OpenSystemInterconnection)ISO (InternationalStandardOrganization)FTP (FileTransferProtocol)TCP/IP (TransportControlProtocol/InternetProtocol)API (ApplicationProgramInterface)LessonBTheOSIReferenceModel上一页返回Asisknowntoall,Englishisprobablythemostimportantlanguageintheworld,especiallytothosewhomajorincomputerscience.ThatisbecausethecomputeritselfwasinventedinAmericaandalmostallimportantprogressinthisfieldhasalsobeenmadeinEnglish.Foryoutobetterunderstandtheabovepassage,ninesentencesfromLessonBhavebeensingledoutfortranslationtips.（1） TheOpenSystemsInterconnection(OSI)referencemodeliscommonlyusedtodescribeinanabstractmannerthefunctionsinvolvedindatacommunication.

解析：本句中describe的宾语是functions,但却在两词之间插入了介词短语inanabstractmanner用来修饰describe。而functions后面用过去分词作后置定语来说明functions.

参考译文：开放系统参考模型常用来以一种抽象的方式描述数据通信中所涉及的各种功能。LessonCTranslationTips下一页返回（2） Itismeanttodescribethefunctionsinvolvedindatacommunicationinawaywhichallowsd