第一章课后习题及答案.docx

第一章课后习题及答案.docx

《第一章课后习题及答案.docx》由会员分享，可在线阅读，更多相关《第一章课后习题及答案.docx（8页珍藏版）》请在冰点文库上搜索。

第一章

1.（Q1）Whatisthedifferencebetweenahostandanendsystem?

Listthetypesofendsystems.IsaWebserveranendsystem?

Answer:

Thereisnodifference.Throughoutthistext,thewords“host”and“endsystem”areusedinterchangeably.EndsystemsincludePCs,workstations,Webservers,mailservers,Internet-connectedPDAs,WebTVs,etc.

2.（Q2）Thewordprotocolisoftenusedtodescribediplomaticrelations.Giveanexampleofadiplomaticprotocol.

Answer:

SupposeAlice,anambassadorofcountryAwantstoinviteBob,anambassadorofcountryB,overfordinner.Alicedoesn’tsimplyjustcallBobonthephoneandsay,cometoourdinnertablenow”.Instead,shecallsBobandsuggestsadateandtime.Bobmayrespondbysayinghe’snotavailablethatparticulardate,butheisavailableanotherdate.AliceandBobcontinuetosend“messages”backandforthuntiltheyagreeonadateandtime.Bobthenshowsupattheembassyontheagreeddate,hopefullynotmorethan15minutesbeforeoraftertheagreedtime.DiplomaticprotocolsalsoallowforeitherAliceorBobtopolitelycanceltheengagementiftheyhavereasonableexcuses.

3.（Q3）Whatisaclientprogram?

Whatisaserverprogram?

Doesaserverprogramrequestandreceiveservicesfromaclientprogram?

Answer:

Anetworkingprogramusuallyhastwoprograms,eachrunningonadifferenthost,communicatingwitheachother.Theprogramthatinitiatesthecommunicationistheclient.Typically,theclientprogramrequestsandreceivesservicesfromtheserverprogram.

4.（Q4）Listsixaccesstechnologies.Classifyeachoneasresidentialaccess,companyaccess,ormobileaccess.

Answer:

1.Dial-upmodemovertelephoneline:

residential;2.DSLovertelephoneline:

residentialorsmalloffice;3.CabletoHFC:

residential;4.100MbpsswitchedEtherent:

company;5.WirelessLAN:

mobile;6.Cellularmobileaccess（forexample,3G/4G）:

mobile

5.（Q5）Listtheavailableresidentialaccesstechnologiesinyourcity.Foreachtypeofaccess,providetheadvertiseddownstreamrate,upstreamrate,andmonthlyprice.

Answer:

Currentpossibilitiesinclude:

dial-up（upto56kbps）;DSL（upto1Mbpsupstream,upto8Mbpsdownstream）;cablemodem（upto30Mbpsdownstream,2Mbpsupstream.

6.（Q7）WhataresomeofthephysicalmediathatEthernetcanrunover?

Answer:

Ethernetmostcommonlyrunsovertwisted-paircopperwireand“thin”coaxialcable.Italsocanrunoverfibersopticlinksandthickcoaxialcable.

7.（Q8）Dial-upmodems,HFC,andDSLareallusedforresidentialaccess.Foreachoftheseaccesstechnologies,providearangeoftransmissionratesandcommentonwhetherthetransmissionrateissharedordedicated.

Answer:

Dialupmodems:

upto56Kbps,bandwidthisdedicated;ISDN:

upto128kbps,bandwidthisdedicated;ADSL:

downstreamchannelis.5-8Mbps,upstreamchannelisupto1Mbps,bandwidthisdedicated;HFC,downstreamchannelis10-30MbpsandupstreamchannelisusuallylessthanafewMbps,bandwidthisshared.

8.（Q13）Whyisitsaidthatpacketswitchingemploysstatisticalmultiplexing?

ContraststatisticalmultiplexingwiththemultiplexingthattakesplaceinTDM.

Answer:

Inapacketswitchednetwork,thepacketsfromdifferentsourcesflowingonalinkdonotfollowanyfixed,pre-definedpattern.InTDMcircuitswitching,eachhostgetsthesameslotinarevolvingTDMframe.

9.（Q14）Supposeuserssharea2Mbpslink.Alsosupposeeachuserrequires1Mbpswhentransmitting,buteachusertransmitsonly20percentofthetime.（SeethediscussionofstatisticalmultiplexinginSection1.3.）

a.Whencircuitswitchingisused,howmanyuserscanbesupported?

b.Fortheremainderofthisproblem,supposepacketswitchingisused.Whywilltherebeessentiallynoqueuingdelaybeforethelinkiftwoorfeweruserstransmitatthesametime?

Whywilltherebeaqueuingdelayifthreeuserstransmitatthesametime?

c.Findtheprobabilitythatagivenuseristransmitting.

d.Supposenowtherearethreeusers.Findtheprobabilitythatatanygiventime,allthreeusersaretransmittingsimultaneously.Findthefractionoftimeduringwhichthequeuegrows.

Answer:

a.2userscanbesupportedbecauseeachuserrequireshalfofthelinkbandwidth.

b.Sinceeachuserrequires1Mbpswhentransmitting,iftwoorfeweruserstransmitsimultaneously,amaximumof2Mbpswillberequired.Sincetheavailablebandwidthofthesharedlinkis2Mbps,therewillbenoqueuingdelaybeforethelink.Whereas,ifthreeuserstransmitsimultaneously,thebandwidthrequiredwillbe3Mbpswhichismorethantheavailablebandwidthofthesharedlink.Inthiscase,therewillbequeuingdelaybeforethelink.

c.Probabilitythatagivenuseristransmitting=0.2

d.Probabilitythatallthreeusersaretransmittingsimultaneously=33p3（1-p）0=0.23=0.008.Sincethequeuegrowswhenalltheusersaretransmitting,thefractionoftimeduringwhichthequeuegrows（whichisequaltotheprobabilitythatallthreeusersaretransmittingsimultaneously）is0.008.

10.（Q16）Considersendingapacketfromasourcehosttoadestinationhostoverafixedroute.Listthedelaycomponentsintheend-to-enddelay.Whichofthesedelaysareconstantandwhicharevariable?

Answer:

Thedelaycomponentsareprocessingdelays,transmissiondelays,propagationdelays,andqueuingdelays.Allofthesedelaysarefixed,exceptforthequeuingdelays,whicharevariable.

11.（Q19）SupposeHostAwantstosendalargefiletoHostB.ThepathfromHostAtoHostBhasthreelinks,ofratesR1=250kbps,R2=500kbps,andR3=1Mbps.

a.Assumingnoothertrafficinthenetwork,whatisthethroughputforthefiletransfer.

b.Supposethefileis2millionbytes.Roughly,howlongwillittaketotransferthefiletoHostB?

c.Repeat（a）and（b）,butnowwithR2reducedto200kbps.

Answer:

a.250kbps

b.64seconds

c.200kbps;80seconds

12.（P2）Considerthecircuit-switchednetworkinFigure1.8.Recallthattherearencircuitsoneachlink.

a.Whatisthemaximumnumberofsimultaneousconnectionsthatcanbeinprogressatanyonetimeinthisnetwork?

b.Supposethatallconnectionsarebetweentheswitchintheupper-left-handcornerandtheswitchinthelower-right-handcorner.Whatisthemaximumnumberofsimultaneousconnectionsthatcanbeinprogress?

Answer:

a.Wecannconnectionsbetweeneachofthefourpairsofadjacentswitches.Thisgivesamaximumof4nconnections.

b.Wecannconnectionspassingthroughtheswitchintheupper-right-handcornerandanothernconnectionspassingthroughtheswitchinthelower-left-handcorner,givingatotalof2nconnections.

13.（P4）Reviewthecar-caravananalogyinSection1.4.Assumeapropagationspeedof50km/hour.

a.Supposethecaravantravels150km,beginninginfrontofonetollbooth,passingthroughasecondtollbooth,andfinishingjustbeforeathirdtollbooth.Whatistheend-to-enddelay?

b.Repeat（a）,nowassumingthattherearefivecarsinthecaravaninsteadoften.

Answer:

Tollboothsare150kmapart,andthecarspropagateat50km/hr,Atollboothservicesacaratarateofonecarevery12seconds.

a.Therearetencars.Ittakes120seconds,ortwominutes,forthefirsttollboothtoservicethe10cars.Eachofthesecarshasapropagationdelayof180minutesbeforearrivingatthesecondtollbooth.Thus,allthecarsarelinedupbeforethesecondtollboothafter182minutes.Thewholeprocessrepeatsitselffortravelingbetweenthesecondandthirdtollbooths.Thusthetotaldelayis364minutes.

b.Delaybetweentollboothsis5*12secondsplus180minutes,i.e.,181minutes.Thetotaldelayistwicethisamount,i.e.,362minutes.

14.（P5）Thiselementaryproblembeginstoexplorepropagationdelayandtransmissiondelay,twocentralconceptsindatanetworking.Considertwohosts,AandB,connectedbyasinglelinkofrateRbps.Supposethatthetwohostsareseparatedbymmeters,andsupposethepropagationspeedalongthelinkissmeters/sec.HostAistosendapacketofsizeLbitstoHostB.

a.Expressthepropagationdelay,dprop,intermsofmands.

b.Determinethetransmissiontimeofthepacket,dtrans,intermsofLandR.

c.Ignoringprocessingandqueuingdelays,obtainanexpressionfortheend-to-enddelay.

d.SupposeHostAbeginstotransmitthepacketattimet=0.Attimet=dtrans,whereisthelastbitofthepacket?

e.Supposedpropisgreaterthandtrans.Attimet=dtrans,whereisthefirstbitofthepacket?

f.Supposedpropislessthandtrans.Attimet=dtrans,whereisthefirstbitofthepacket?

g.Supposes=2.5*108,L=100bits,andR=28kbps.Findthedistancemsothatdpropequalsdtrans.

Answer:

a.dprop=m/sseconds.

b.dtrans=L/Rseconds.

c.dend-to-end=（m/s+L/R）seconds.

d.ThebitisjustleavingHostA.

e.ThefirstbitisinthelinkandhasnotreachedHostB.

f.ThefirstbithasreachedHostB.

g.Want

m=LRS=10028*1032.5*108=893km.

15.（P6）Inthisproblemweconsidersendingreal-timevoicefromHostAtoHostBoverapacket-switchednetwork（VoIP）.HostAconvertsanalogvoicetoadigital64kbpsbitstreamonthefly.HostAthengroupsthebitsinto56-Bytepackets.ThereisonelinkbetweenHostAandB;itstransmissionrateis500kbpsanditspropagationdelayis2msec.AssoonasHostAgathersapacket,itsendsittoHostB.AssoonasHostBreceivesanentirepacket,itconvertsthepacket’sbitstoananalogsignal.Howmuchtimeelapsesfromthetimeabitiscreated（fromtheoriginalanalogsignalatHostA）untilthebitisdecoded（aspartoftheanalogsignalatHostB）?

Answer:

Considerthefirstbitinapacket.Before