计算机网络自顶向下方法答案(英文第六版)

ComputerNetworking:ATop-DownApproach,6thEditionSolutionstoReviewQuestionsandProblemsVersionDate:May2012Thisdocumentcontainsthesolutionstoreviewquestionsandproblemsforthe5theditionofComputerNetworking:ATop-DownApproachbyJimKuroseandKeithRoss.ThesesolutionsarebeingmadeavailabletoinstructorsONLY.PleasedoNOTcopyordistributethisdocumenttoothers(evenotherinstructors).Pleasedonotpostanysolutionsonapublicly-availableWebsite.We’llbehappytoprovideacopy(up-to-date)ofthissolutionmanualourselvestoanyonewhoasks.Acknowledgments:Overtheyears,severalstudentsandcolleagueshavehelpeduspreparethissolutionsmanual.SpecialthanksgoestoHongGangZhang,RakeshKumar,PrithulaDhungel,andVijayAnnapureddy.Alsothankstoallthereaderswhohavemadesuggestionsandcorrectederrors.Allmaterial©copyright1996-2012byJ.F.KuroseandK.W.Ross.AllrightsreservedChapter1ReviewQuestions1.Thereisnodifference.Throughoutthistext,thewords“host”and“endsystem”areusedinterchangeably.EndsystemsincludePCs,workstations,Webservers,mailservers,PDAs,Internet-connectedgameconsoles,etc.2.FromWikipedia:Diplomaticprotocoliscommonlydescribedasasetofinternationalcourtesyrules.Thesewell-establishedandtime-honoredruleshavemadeiteasierfornationsandpeopletoliveandworktogether.Partofprotocolhasalwaysbeentheacknowledgmentofthehierarchicalstandingofallpresent.Protocolrulesarebasedontheprinciplesofcivility.3.Standardsareimportantforprotocolssothatpeoplecancreatenetworkingsystemsandproductsthatinteroperate.4.1.Dial-upmodemovertelephoneline:home;2.DSLovertelephoneline:homeorsmalloffice;3.CabletoHFC:home;4.100MbpsswitchedEthernet:enterprise;5.Wifi(802.11):homeandenterprise:6.3Gand4G:wide-areawireless.5.HFCbandwidthissharedamongtheusers.Onthedownstreamchannel,allpacketsemanatefromasinglesource,namely,theheadend.Thus,therearenocollisionsinthedownstreamchannel.6.InmostAmericancities,thecurrentpossibilitiesinclude:dial-up;DSL;cablemodem;fiber-to-the-home.7.EthernetLANshavetransmissionratesof10Mbps,100Mbps,1Gbpsand10Gbps.8.Today,Ethernetmostcommonlyrunsovertwisted-paircopperwire.Italsocanrunoverfibersopticlinks.9.Dialupmodems:upto56Kbps,bandwidthisdedicated;ADSL:upto24Mbpsdownstreamand2.5Mbpsupstream,bandwidthisdedicated;HFC,ratesupto42.8Mbpsandupstreamratesofupto30.7Mbps,bandwidthisshared.FTTH:2-10Mbpsupload;10-20Mbpsdownload;bandwidthisnotshared.10.TherearetwopopularwirelessInternetaccesstechnologiestoday:a)Wifi(802.11)InawirelessLAN,wirelessuserstransmit/receivepacketsto/fromanbasestation(i.e.,wirelessaccesspoint)withinaradiusoffewtensofmeters.ThebasestationistypicallyconnectedtothewiredInternetandthusservestoconnectwirelessuserstothewirednetwork.b)3Gand4Gwide-areawirelessaccessnetworks.Inthesesystems,packetsaretransmittedoverthesamewirelessinfrastructureusedforcellulartelephony,withthebasestationthusbeingmanagedbyatelecommunicationsprovider.Thisprovideswirelessaccesstouserswithinaradiusoftensofkilometersofthebasestation.11.Attimet0thesendinghostbeginstotransmit.Attimet1=L/R1,thesendinghostcompletestransmissionandtheentirepacketisreceivedattherouter(nopropagationdelay).Becausetherouterhastheentirepacketattimet1,itcanbegintotransmitthepackettothereceivinghostattimet1.Attimet2=t1+L/R2,theroutercompletestransmissionandtheentirepacketisreceivedatthereceivinghost(again,nopropagationdelay).Thus,theend-to-enddelayisL/R1+L/R2.12.Acircuit-switchednetworkcanguaranteeacertainamountofend-to-endbandwidthforthedurationofacall.Mostpacket-switchednetworkstoday(includingtheInternet)cannotmakeanyend-to-endguaranteesforbandwidth.FDMrequiressophisticatedanaloghardwaretoshiftsignalintoappropriatefrequencybands.13.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)is0.008.14.IfthetwoISPsdonotpeerwitheachother,thenwhentheysendtraffictoeachothertheyhavetosendthetrafficthroughaproviderISP(intermediary),towhichtheyhavetopayforcarryingthetraffic.Bypeeringwitheachotherdirectly,thetwoISPscanreducetheirpaymentstotheirproviderISPs.AnInternetExchangePoints(IXP)(typicallyinastandalonebuildingwithitsownswitches)isameetingpointwheremultipleISPscanconnectand/orpeertogether.AnISPearnsitsmoneybychargingeachofthetheISPsthatconnecttotheIXParelativelysmallfee,whichmaydependontheamountoftrafficsenttoorreceivedfromtheIXP.15.Google'sprivatenetworkconnectstogetherallitsdatacenters,bigandsmall.TrafficbetweentheGoogledatacenterspassesoveritsprivatenetworkra