计算机网络原理习题讲解

ChapterI1.Whatisthedifferencebetweenahostandanendsystem?Listthetypesofendsystems.IsaWebserveranendsystem?2.Whatisaclientprogram?Whatisaserverprogram?Doesaserverprogramrequestandreceiveservicesfromaclientprogram?3.Listsixaccesstechnologies.Classifyeachoneasresidentialaccess,companyaccess,ormobileaccess.4.Dial-upmodems,HFC,andDSLareallusedforresidentialaccess.Foreachoftheseaccesstechnologies,providearangeoftransmissionratesandcommentonwhetherthetransmissionrateissharedordedicated.5.DescribethemostpopularwirelessInternetaccesstechnologiestoday.Compareandcontrastthem.6.Whatadvantagedoesacircuit-switchednetworkhaveoverapacket-switchednetwork?WhatadvantagesdoesTDMhaveoverFDMinacircuit-switchednetwork?7.Considersendingapacketfromasourcehosttoadestinationhostoverafixedroute.Listthedelaycomponentsintheend-to-enddelay.Whichofthesedelaysareconstantandwhicharevariable?8.Howlongdoesittakeapacketoflength2,000bytestopropagateoveralinkofdistance2,000km,propagationspeed8102m/s,andtransmissionrate2Mbps?Moregenerally,howlongdoesittakeapacketoflengthLtopropagateoveralinkofdistanced,propagationspeeds,andtransmissionrateRbps?Doesthisdelaydependonpacketlength?Doesthisdelaydependontransmissionrate?9.WhatarethefivelayersintheInternetprotocolstack?Whataretheprincipalresponsibilitiesofeachoftheselayers?10.WhichlayersintheInternetprotocolstackdoesarouterprocess?Whichlayersdoesalink-layerswitchprocess?Whichlayersdoesahostprocess?11.Whatisanapplication-layermessage?Atransport-layersegment?Anetwork-layerdatagram?Alink-layerframe?12.Thiselementaryproblembeginstoexplorepropagationdelayandtransmissiondelay,twocentralconceptsindatanetworking.Considertwohosts,AandB,connectedbyasinglelinkofrateRbps.Supposethatthetwohostsareseparatedbymmeters,andsupposethepropagationspeedalongthelinkissmeters/sec.HostAistosendapacketofsizeLbitstoHostB.a.Expressthepropagationdelay,propd,intermsofmands.b.Determinethetransmissiontimeofthepacket,transd,intermsofLandR.c.Ignoringprocessingandqueuingdelays,obtainanexpressionfortheend-to-enddelay.d.SupposeHostAbeginstotransmitthepacketattimet=0.Attimetransdt,whereisthelastbitofthepacket?e.Supposepropdisgreaterthantransd.Attimet=transd,whereisthefirstbitofthepacket?f.Supposepropdislessthantransd.Attimet=transd,whereisthefirstbitofthepacket?g.Suppose8105.2s,L=100bits,andR=28kbps.Findthedistancemsothatpropdequalstransd.13.Inmodernpacket-switchednetworks,thesourcehostsegmentslong,application-layermessages(forexample,animageoramusicfile)intosmallerpacketsandsendsthepacketsintothenetwork.Thereceiverthenreassemblesthepacketsbackintotheoriginalmessage.Werefertothisprocessasmessagesegmentation.Figure1.24illustratestheend-to-endtransportofamessagewithandwithoutmessagesegmentation.Consideramessagethatis6108bitslongthatistobesentfromsourcetodestinationinFigure1.24.Supposeeachlinkinthefigureis2Mbps.Ignorepropagation,queuing,andprocessingdelays.a.Considersendingthemessagefromsourcetodestinationwithoutmessagesegmentation.Howlongdoesittaketomovethemessagefromthesourcehosttothefirstpacketswitch?Keepinginmindthateachswitchusesstore-and-forwardpacketswitching,whatisthetotaltimetomovethemessagefromsourcehosttodestinationhost?b.Nowsupposethatthemessageissegmentedinto4,000packets,witheachpacketbeing2,000bitslong.Howlongdoesittaketomovethefirstpacketfromsourcehosttothefirstswitch?Whenthefirstpacketisbeingsentfromthefirstswitchtothesecondswitch,thesecondpacketisbeingsentfromthesourcehosttothefirstswitch.Atwhattimewillthesecondpacketbefullyreceivedatthefirstswitch?c.Howlongdoesittaketomovethefilefromsourcehosttodestinationhostwhenmessagesegmentationisused?Comparethisresultwithyouranswerinpart(a)andcomment.d.Discussthedrawbacksofmessagesegmentation.14.下列说法中,正确的是()。A.在较小范围内布置的一定是局域网,币在较大范围内布置的一定是广域网B.城域网是连接广域网而覆盖园区的网络C.城域网是为淘汰局域网和广域网而提出的一种网络技术D.局域网是基于广播技术发展起来的网络,广域网是基于交换技术发展起来的向络解答:D。通常而言,局域网的覆盖范围较小,而广域网的覆盖范围较大,但这并不绝对。有时候在一个不大的范围内采用广域网,这取决于应用的需要和是否采用单一网络等多种因素。特别是局域网技术的进步,使得其覆盖范围越来越大,达到几十千米的范围。城域网是利用广域网技术、满足一定区域需求的一种网络,事实上,城域网的范围弹性非常大。最初的局域网采用广播技术,这种技术一直被沿用,而广域网最初使用的是交换技术,也一直被沿用。・15.相对于o⒏的7层参考模型的低4层,TCP/IP协议集内对应的层次有()。A.传输层、互联网层、网络接口层和物理层B.传输层、互联网层、网络接口层C.传输层、互联网层、ATM层和物理层D.传输层、网络层、数据链路蜃和物理层解答:B。根据TCP/P分层模型可知,其对应OSI低4层的分别是传输层、互联网层、网络接口层。16.在C/S模式的网络中,最恰当的是()。A.客户机提出请求,服务器响应请求、进行处理并返回结果B.服务器有时可以同时为多个客户机服务C.客户机可以将服务器的资源各份在本地,以避免向服务器请求服务D.服务器永远是网络的瓶颈解答:A。根据C/S模式的定义,选项A描述了C/S模式的基本工作流程。服务器必须总能而不是有时可以同时为多个客户机服务,否则网络就没有了存在的价值。由于服务器的资源太庞大,而且很多资源因为知识产权、保密、管理复杂等一系列的原因,使客户机不可能都把服务器的资源备份到本地。从表面上看,服务器可能是网络的瓶颈,但事实上:在多数情况下网络的主要瓶颈不在服务器，而在通信线路。17.比较分组交换与报文交换,说明分组交换优越的原因。解答:报文交换网络与分组交换的原理都是:将用户数据加上源地址、自的地址、长度、校验码等辅助信息封装成PDU,发送给下一个节点。下一个节点收到后先暂存报文,待输出线路空闲时再转发给下一个节点,重复这一过程直到到达目的节点。每个PDU可单独选择到达目的节点的路径。这种方式也称为存储―转发方式。两者的不同之处是:分组交换所生成的PDU的长度较短,而且是固定的;而报文交换的PDU的长度不是固定的。正是这一差别,使得分组交换具有独特的优点:缓冲区易于管理;分组的平均延迟更小,网络中占用的平均缓冲区更少;更易标准化;更适合应用。所以现在的主流网络基本上都可以看成是分组交换网络。18.单顶选择题【1】第一个分组交换网是(A)。A.ARPAnetB.X.25C.以太网D.Internet【2】在大多数网络中,数据链路层都是用请求重发已损坏了帧的办法来解决发送出错问题。如果一个帧被损坏的概率是p,而且确认信息不会丢失,则发送一帧的平均发送次数是(d)A.1+pB.I-pC.1/(1+p)D.1/(1-p)【3】物理层的电气特性规定的特性包括（b）A.接插件的形式B.信号的电压值C.电缆的长度D.各