计算机网络_自顶向下方法_第四版_课后习题答案

ComputerNetworking:ATop-DownApproachFeaturingtheInternet,4thEdition计算机网络：自顶向下方法（第四版）SolutionstoReviewQuestionsandProblems课后习题解答VersionDate:October29,2007Thisdocumentcontainsthesolutionstoreviewquestionsandproblemsforthe4theditionofComputerNetworking:ATop-DownApproachFeaturingtheInternetbyJimKuroseandKeithRoss.ThesesolutionsarebeingmadeavailabletoinstructorsONLY.PleasedoNOTcopyordistributethisdocumenttoothers(evenotherinstructors).Pleasedonotpostanysolutionsonapublicly-availableWebsite.We’llbehappytoprovideacopy(up-to-date)ofthissolutionmanualourselvestoanyonewhoasks.Chapter1ReviewQuestions1.Thereisnodifference.Throughoutthistext,thewords“host”and“endsystem”areusedinterchangeably.EndsystemsincludePCs,workstations,Webservers,mailservers,Internet-connectedPDAs,WebTVs,etc.2.SupposeAlice,anambassadorofcountryAwantstoinviteBob,anambassadorofcountryB,overfordinner.Alicedoesn’tsimplyjustcallBobonthephoneandsay,“cometoourdinnertablenow”.Instead,shecallsBobandsuggestsadateandtime.Bobmayrespondbysayinghe’snotavailablethatparticulardate,butheisavailableanotherdate.AliceandBobcontinuetosend“messages”backandforthuntiltheyagreeonadateandtime.Bobthenshowsupattheembassyontheagreeddate,hopefullynotmorethan15minutesbeforeoraftertheagreedtime.DiplomaticprotocolsalsoallowforeitherAliceorBobtopolitelycanceltheengagementiftheyhavereasonableexcuses.3.Anetworkingprogramusuallyhastwoprograms,eachrunningonadifferenthost,communicatingwitheachother.Theprogramthatinitiatesthecommunicationistheclient.Typically,theclientprogramrequestsandreceivesservicesfromtheserverprogram.4.1.Dial-upmodemovertelephoneline:residential;2.DSLovertelephoneline:residentialorsmalloffice;3.CabletoHFC:residential;4.100MbpsswitchedEtherent:company;5.WirelessLAN:mobile;6.Cellularmobileaccess(forexample,WAP):mobile5.HFCbandwidthissharedamongtheusers.Onthedownstreamchannel,allpacketsemanatefromasinglesource,namely,theheadend.Thus,therearenocollisionsinthedownstreamchannel.6.Currentpossibilitiesinclude:dial-up;DSL;cablemodem;fiber-to-the-home.7.EthernetLANshavetransmissionratesof10Mbps,100Mbps,1Gbpsand10Gbps.ForanXMbpsEthernet(whereX=10,100,1,000or10,000),ausercancontinuouslytransmitattherateXMbpsifthatuseristheonlypersonsendingdata.Iftherearemorethanoneactiveuser,theneachusercannotcontinuouslytransmitatXMbps.8.Ethernetmostcommonlyrunsovertwisted-paircopperwireand“thin”coaxialcable.Italsocanrunoverfibersopticlinksandthickcoaxialcable.9.Dialupmodems:upto56Kbps,bandwidthisdedicated;ISDN:upto128kbps,bandwidthisdedicated;ADSL:downstreamchannelis.5-8Mbps,upstreamchannelisupto1Mbps,bandwidthisdedicated;HFC,downstreamchannelis10-30MbpsandupstreamchannelisusuallylessthanafewMbps,bandwidthisshared.10.TherearetwomostpopularwirelessInternetaccesstechnologiestoday:a)WirelessLANInawirelessLAN,wirelessuserstransmit/receivepacketsto/fromabasestation(wirelessaccesspoint)withinaradiusoffewtensofmeters.ThebasestationistypicallyconnectedtothewiredInternetandthusservestoconnectwirelessuserstothewirednetwork.b)Wide-areawirelessaccessnetworkInthesesystems,packetsaretransmittedoverthesamewirelessinfrastructureusedforcellulartelephony,withthebasestationthusbeingmanagedbyatelecommunicationsprovider.Thisprovideswirelessaccesstouserswithinaradiusoftensofkilometersofthebasestation.11.Acircuit-switchednetworkcanguaranteeacertainamountofend-to-endbandwidthforthedurationofacall.Mostpacket-switchednetworkstoday(includingtheInternet)cannotmakeanyend-to-endguaranteesforbandwidth.12.Inapacketswitchednetwork,thepacketsfromdifferentsourcesflowingonalinkdonotfollowanyfixed,pre-definedpattern.InTDMcircuitswitching,eachhostgetsthesameslotinarevolvingTDMframe.13.Attimet0thesendinghostbeginstotransmit.Attimet1=L/R1,thesendinghostcompletestransmissionandtheentirepacketisreceivedattherouter(nopropagationdelay).Becausetherouterhastheentirepacketattimet1,itcanbegintotransmitthepackettothereceivinghostattimet1.Attimet2=t1+L/R2,theroutercompletestransmissionandtheentirepacketisreceivedatthereceivinghost(again,nopropagationdelay).Thus,theend-to-enddelayisL/R1+L/R2.14.Atier-1ISPconnectstoallothertier-1ISPs;atier-2ISPconnectstoonlyafewofthetier-1ISPs.Also,atier-2ISPisacustomerofoneormoretier-1.15.a)2userscanbesupportedbecauseeachuserrequireshalfofthelinkbandwidth.b)Sinceeachuserrequires1Mbpswhentransmitting,iftwoorfeweruserstransmitsimultaneously,amaximumof2Mbpswillberequired.Sincetheavailablebandwidthofthesharedlinkis2Mbps,therewillbenoqueuingdelaybeforethelink.Whereas,ifthreeuserstransmitsimultaneously,thebandwidthrequiredwillbe3Mbpswhichismorethantheavailablebandwidthofthesharedlink.Inthiscase,therewillbequeuingdelaybeforethelink.c)Probabilitythatagivenuseristransmitting=0.2d)Probabilitythatallthreeusersaretransmittingsimultaneously=333133pp=(0.2)3=0.008.Sincethequeuegrowswhenalltheusersaretransmitting,thefractionoftimeduringwhichthequeuegrows(whichisequaltotheprobabilitythatallthreeusersaretransmittingsimultaneously)is0.008.16.Thedelaycomponentsareprocessingdelays,transmissiondelays,propagationdelays