1.-第一章課后習(xí)題及答案

1.-第一章課后習(xí)題及答案LtD第一章(Q1)Whatisthedifferencebetweenahostandanendsystem?Listthetypesofendsystems.IsaWebserveranendsystem?Answer:Thereisnodifference.Throughoutthistext,thewords“host”and“endsystem”areusedinterchangeably.EndsystemsincludePCs,workstations,Webservers,mailservers,Internet-connectedPDAs,WebTVs,etc.(Q2)Thewordprotocolisoftenusedtodescribediplomaticrelations.Giveanexampleofadiplomaticprotocol.Answer:SupposeAlice,anambassadorofcountryAwantstoinviteBob,anambassadorofcountryB,overfordinner.Alicedoesn’tsimplyjustcallBobonthephoneandsay,cometoourdinnertablenow”.Instead,shecallsBobandsuggestsAnswer:Currentpossibilitiesinclude:dial-up(upto56kbps);DSL(upto1Mbpsupstream,upto8Mbpsdownstream);cablemodem(upto30Mbpsdownstream,2Mbpsupstream.(Q7)WhataresomeofthephysicalmediathatEthernetcanrunover?Answer:Ethernetmostcommonlyrunsovertwisted-paircopperwireand“thin”coaxialcable.Italsocanrunoverfibersopticlinksandthickcoaxialcable.(Q8)Dial-upmodems,HFC,andDSLareallusedforresidentialaccess.Foreachoftheseaccesstechnologies,providearangeoftransmissionratesandcommentonwhetherthetransmissionrateissharedordedicated.Answer:Dialupmodems:upto56Kbps,bandwidthisdedicated;ISDN:upto128kbps,bandwidthisdedicated;ADSL:downstreamchannelis.5-8Mbps,upstreamchannelisupto1Mbps,bandwidthisdedicated;HFC,downstreamchannelis10-30MbpsandupstreamchannelisusuallylessthanafewMbps,bandwidthisshared.(Q13)Whyisitsaidthatpacketswitchingemploysstatisticalmultiplexing?ContraststatisticalmultiplexingwiththemultiplexingthattakesplaceinTDM.Answer:Inapacketswitchednetwork,thepacketsfromdifferentsourcesflowingonalinkdonotfollowanyfixed,pre-definedpattern.InTDMcircuitswitching,eachhostgetsthesameslotinarevolvingTDMframe.(Q14)Supposeuserssharea2Mbpslink.Alsosupposeeachuserrequires1Mbpswhentransmitting,buteachusertransmitsonly20percentofthetime.(SeethediscussionofstatisticalmultiplexinginSection1.3.)Whencircuitswitchingisused,howmanyuserscanbesupported?Fortheremainderofthisproblem,supposepacketswitchingisused.Whywilltherebeessentiallynoqueuingdelaybeforethelinkiftwoorfeweruserstransmitatthesametime?Whywilltherebeaqueuingdelayifthreeuserstransmitatthesametime?Findtheprobabilitythatagivenuseristransmitting.Supposenowtherearethreeusers.Findtheprobabilitythatatanygiventime,allthreeusersaretransmittingsimultaneously.Findthefractionoftimeduringwhichthequeuegrows.Answer:2userscanbesupportedbecauseeachuserrequireshalfofthelinkbandwidth.Sinceeachuserrequires1Mbpswhentransmitting,iftwoorfeweruserstransmitsimultaneously,amaximumof2Mbpswillberequired.Sincetheavailablebandwidthofthesharedlinkis2Mbps,therewillbenoqueuingdelaybeforethelink.Whereas,ifthreeuserstransmitsimultaneously,thebandwidthrequiredwillbe3Mbpswhichismorethantheavailablebandwidthofthesharedlink.Inthiscase,therewillbequeuingdelaybeforethelink.Probabilitythatagivenuseristransmitting=0.2Probabilitythatallthreeusersaretransmittingsimultaneously=33p(Q16)Considersendingapacketfromasourcehosttoadestinationhostoverafixedroute.Listthedelaycomponentsintheend-to-enddelay.Whichofthesedelaysareconstantandwhicharevariable?Answer:Thedelaycomponentsareprocessingdelays,transmissiondelays,propagationdelays,andqueuingdelays.Allofthesedelaysarefixed,exceptforthequeuingdelays,whicharevariable.(Q19)SupposeHostAwantstosendalargefiletoHostB.ThepathfromHostAtoHostBhasthreelinks,ofratesR1=250kbps,R2=500kbps,andR3=1Mbps.Assumingnoothertrafficinthenetwork,whatisthethroughputforthefiletransfer.Supposethefileis2millionbytes.Roughly,howlongwillittaketotransferthefiletoHostB?Repeat(a)and(b),butnowwithR2reducedto200kbps.Answer:250kbps64seconds200kbps;80seconds(P2)Considerthecircuit-switchednetworkinFigure1.8.Recallthattherearencircuitsoneachlink.Whatisthemaximumnumberofsimultaneousconnectionsthatcanbeinprogressatanyonetimeinthisnetwork?Supposethatallconnectionsarebetweentheswitchintheupper-left-handcornerandtheswitchinthelower-right-handcorner.Whatisthemaximumnumberofsimultaneousconnectionsthatcanbeinprogress?Answer:Wecannconnectionsbetweeneachofthefourpairsofadjacentswitches.Thisgivesamaximumof4nconnections.Wecannconnectionspassingthroughtheswitchintheupper-right-handcornerandanothernconnectionspassingthroughtheswitchinthelower-left-handcorner,givingatotalof2nconnections.(P4)Reviewthecar-caravananalogyinSection1.4.Assumeapropagationspeedof50km/hour.Supposethecaravantravels150km,beginninginfrontofonetollbooth,passingthroughasecondtollbooth,andfinishingjustbeforeathirdtollbooth.Whatistheend-to-enddelay?Repeat(a),nowassumingthattherearefivecarsinthecaravaninsteadoften.Answer:Tollboothsare150kmapart,andthecarspropagateat50km/hr,Atollboothservicesacaratarateofonecarevery12seconds.Therearetencars.Ittakes120seconds,ortwominutes,forthefirsttollboothtoservicethe10cars.Eachofthesecarshasapropagationdelayof180minutesbeforearrivingatthesecondtollbooth.Thus,allthecarsarelinedupbeforethesecondtollboothafter182minutes.Thewholeprocessrepeatsitselffortravelingbetweenthesecondandthirdtollbooths.Thusthetotaldelayis364minutes.Delaybetweentollboothsis5*12secondsplus180minutes,i.e.,181minutes.Thetotaldelayistwicethisamount,i.e.,362minutes.(P5)Thiselementaryproblembeginstoexplorepropagationdelayandtransmissiondelay,twocentralconceptsindatanetworking.Considertwohosts,AandB,connectedbyasinglelinkofrateRbps.Supposethatthetwohostsareseparatedbymmeters,andsupposethepropagationspeedalongthelinkissmeters/sec.HostAistosendapacketofsizeLbitstoHostB.Expressthepropagationdelay,dprop,intermsofmands.Determinethetransmissiontimeofthepacket,dtrans,intermsofLandR.Ignoringprocessingandqueuingdelays,obtainanexpressionfortheend-to-enddelay.SupposeHostAbeginstotransmitthepacketattimet=0.Attimet=dtrans,whereisthelastbitofthepacket?Supposedpropisgreaterthandtrans.Attimet=dtrans,whereisthefirstbitofthepacket?Supposedpropislessthandtrans.Attimet=dtrans,whereisthefirstbitofthepacket?Supposes=2.5*108,L=100bits,andR=28kbps.Findthedistancemsothatdpropequalsdtrans.Answer:dprop=m/sseconds.dtrans=L/Rseconds.dend-to-end=(m/s+L/R)seconds.ThebitisjustleavingHostA.ThefirstbitisinthelinkandhasnotreachedHostB.ThefirstbithasreachedHostB.Wantm=(P6)Inthisproblemweconsidersendingreal-timevoicefromHostAtoHostBoverapacket-switchednetwork(VoIP).HostAconvertsanalogvoicetoadigital64kbpsbitstreamonthefly.HostAthengroupsthebitsinto56-Bytepackets.ThereisonelinkbetweenHostAandB;itstransmissionrateis500kbpsanditspropagationdelayis2msec.AssoonasHostAgathersapacket,itsendsittoHostB.AssoonasHostBreceivesanentirepacket,itconvertsthepacket’sbitstoananalogsignal.Howmuchtimeelapsesfromthetimeabitiscreated(fromtheoriginalanalogsignalatHostA)untilthebitisdecoded(aspartoftheanalogsignalatHostB)?Answer:Considerthefirstbitinapacket.Beforethisbitcanbetransmitted,allofthebitsinthepacketmustbegenerated.Thisrequires56*864*10Thetimerequiredtotransmitthepacketis 56*8Propagationdelay=2msec.Thedelayuntildecodingis7msec+896μsec+2msec=9.896msecAsimilaranalysisshowsthatallbitsexperienceadelayof9.896msec.(P9)ConsiderapacketoflengthLwhichbeginsatendsystemA,travelsoveronelinktoapacketswitch,andtravelsfromthepacketswitchoverasecondlinktoadestinationendsystem.Letdi,si,andRidenotethelength,propagationspeed,andthetransmissionrateoflinki,fori=1,2.Thepacketswitchdelayseachpacketbydproc.Assumingnoqueuingdelays,intermsofdi,si,Ri,(i=1,2),andL,whatisthetotalend-to-enddelayforthepacket?SupposenowthepacketLengthis1,000bytes,thepropagationspeedonbothlinksis2.5*108m/s,thetransmissionratesofbothlinksis1Mbps,thepacketswitchprocessingdelayis2msec,thelengthofthefirstlinkis6,000km,andthelengthofthelastlinkis3,000km.Forthesevalues,whatistheend-to-enddelay?Answer:ThefirstendsystemrequiresL/R1totransmitthepacketontothefirstlink;thepacketpropagatesoverthefirstlinkind1/s1;thepacketswitchaddsaprocessingdelayofdproc;afterreceivingtheentirepacket,thepacketswitchrequiresL/R2totransmitthepacketontothesecondlink;thepacketpropagatesoverthesecondlinkind2/s2.Addingthesefivedelaysgivesdend-end=L/R1+L/R2+d1/s1+d2/s2+dprocToanswerthesecondquestion,wesimplyplugthevaluesintotheequationtoget8+8+24+12+2=54msec.(P10)Intheaboveproblem,supposeR1=R2=Randdproc=0.Furthersupposethepacketswitchdoesnotstore-and-forwardpacketsbutinsteadimmediatelytransmitseachbititreceiversbeforewaitingforthepackettoarrive.Whatistheend-to-enddelay?Answer:Becausebitsareimmediatelytransmitted,thepacketswitchdoesnotintroduceanydelay;inparticular,itdoesnotintroduceatransmissiondelay.Thus,dend-end=L/R+d1/s1+d2/s2ForthevaluesinProblem9,weget8+24+12=44msec.(P11)SupposeNpacketsarrivesimultaneouslytoalinkatwhichnopacketsarecurrentlybeingtransmittedorqueued.EachpacketisoflengthLandthelinkhastransmissionrateR.WhatistheaveragequeuingdelayfortheNpackets?Answer:Thequeuingdelayis0forthefirsttransmittedpacket,L/Rforthesecondtransmittedpacket,andgenerally,(n-1)L/Rforthenthtransmittedpacket.Thus,theaveragedelayfortheNpacketsis(L/R+2L/R+.......+(N-1)L/R)/N=L/RN(1+2+.....+(N-1))=LN(N-1)/(2RN)=(N-1)L/(2R)Notethathereweusedthewell-knownfactthat1+2+.......+N=N(N+1)/2(P14)Considerthequeuingdelayinarouterbuffer.LetIdenotetrafficintensity;thatis,I=La/R.SupposethatthequeuingdelaytakestheformIL/R(1-I)forI<1.Provideaformulaforthetotaldelay,thatis,thequeuingdelayplusthetransmissiondelay.PlotthetotaldelayasafunctionofL/R.Answer:ThetransmissiondelayisL/R.ThetotaldelayisILLetx=L/R.Totaldelay=(P16)PerformaTraceroutebetweensourceanddestinationonthesamecontinentatthreedifferenthoursoftheday.Findtheaverageandstandarddeviationoftheround-tripdelaysateachofthethreehours.Findthenumberofroutersinthepathateachofthethreehours.Didthepathschangeduringanyofthehours?TrytoidentifythenumberofISPnetworksthattheTraceroutepacketspassthroughfromsourcetodestination.Routerswithsimilarnamesand/orsimilarIPaddressesshouldbeconsideredaspartofthesameISP.Inyourexperiments,dothelargestdelaysoccuratthepeeringinterfacesbetweenadjacentISPs?Repeattheaboveforasourceanddestinationondifferentcontinents.Comparetheintra-continentandinter-continentresults.Answer:Experiments.(P18)Supposetwohosts,AandB,areseparatedby10,000kilometersandareconnectedbyadirectlinkofR=2Mbps.Supposethepropagationspeedoverthelinkis2.5?108meters/sec.Calculatethebandwidth-delayproduct,R?dprop.Considersendingafileof400,000bitsfromHostAtoHostB.Supposethefileissentcontinuouslyasonelargemessage.Whatisthemaximumnumberofbitsthatwillbeinthelinkatanygiventime?Provideaninterpretationofthebandwidth-delayproduct.Whatisthewidth(inmeters)ofabitinthelink?Isitlonger