1.计算机网络原理第一章课后习题及答案

第一章

1.(Q1)Whatisthedifferencebetweenahostandanendsystem?Listthetypesofend

systems.IsaWebserveranendsystem?

Answer:Thereisnodifference.Throughoutthistext,thewords“host”and“endsystem”are

usedinterchangeably.EndsystemsincludePCs,workstations,Webservers,mailservers,

Internet-connectedPDAs,WebTVs,etc.

2.(Q2)Thewordprotocolisoftenusedtodescribediplomaticrelations.Giveanexampleofa

diplomaticprotocol.

Answer:SupposeAlice,anambassadorofcountryAwantstoinviteBob,anambassadorof

countryB,overfordinner.Alicedoesn’tsimplyjustcallBobonthephoneandsay,cometo

ourdinnertablenow”.Instead,shecallsBobandsuggestsadateandtime.Bobmayrespond

bysayinghe’snotavailablethatparticulardate,butheisavailableanotherdate.Aliceand

Bobcontinuetosend“messages”backandforthuntiltheyagreeonadateandtime.Bob

thenshowsupattheembassyontheagreeddate,hopefullynotmorethan15minutes

beforeoraftertheagreedtime.DiplomaticprotocolsalsoallowforeitherAliceorBobto

politelycanceltheengagementiftheyhavereasonableexcuses.

3.(Q3)Whatisaclientprogram?Whatisaserverprogram?Doesaserverprogramrequest

andreceiveservicesfromaclientprogram?

Answer:Anetworkingprogramusuallyhastwoprograms,eachrunningonadifferenthost,

communicatingwitheachother.Theprogramthatinitiatesthecommunicationistheclient.

Typically,theclientprogramrequestsandreceivesservicesfromtheserverprogram.

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

mobileaccess.

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,up

to8Mbpsdownstream);cablemodem(upto30Mbpsdownstream,2Mbpsupstream.

6.(Q7)WhataresomeofthephysicalmediathatEthernetcanrunover?

Answer:Ethernetmostcommonlyrunsovertwisted-paircopperwireand“thin”coaxial

cable.Italsocanrunoverfibersopticlinksandthickcoaxialcable.

7.(Q8)Dial-upmodems,HFC,andDSLareallusedforresidentialaccess.Foreachofthese

accesstechnologies,providearangeoftransmissionratesandcommentonwhetherthe

transmissionrateissharedordedicated.

Answer:Dialupmodems:upto56Kbps,bandwidthisdedicated;ISDN:upto128kbps,

bandwidthisdedicated;ADSL:downstreamchannelis.5-8Mbps,upstreamchannelisupto

1Mbps,bandwidthisdedicated;HFC,downstreamchannelis10-30Mbpsandupstream

channelisusuallylessthanafewMbps,bandwidthisshared.

8.(Q13)Whyisitsaidthatpacketswitchingemploysstatisticalmultiplexing?Contrast

statisticalmultiplexingwiththemultiplexingthattakesplaceinTDM.

Answer:Inapacketswitchednetwork,thepacketsfromdifferentsourcesflowingonalinkdo

notfollowanyfixed,pre-definedpattern.InTDMcircuitswitching,eachhostgetsthesame

slotinarevolvingTDMframe.

9.(Q14)Supposeuserssharea2Mbpslink.Alsosupposeeachuserrequires1Mbpswhen

transmitting,buteachusertransmitsonly20percentofthetime.(Seethediscussionof

statisticalmultiplexinginSection1.3.)

a.Whencircuitswitchingisused,howmanyuserscanbesupported?

b.Fortheremainderofthisproblem,supposepacketswitchingisused.Whywilltherebe

essentiallynoqueuingdelaybeforethelinkiftwoorfeweruserstransmitatthesame

time?Whywilltherebeaqueuingdelayifthreeuserstransmitatthesametime?

c.Findtheprobabilitythatagivenuseristransmitting.

d.Supposenowtherearethreeusers.Findtheprobabilitythatatanygiventime,all

threeusersaretransmittingsimultaneously.Findthefractionoftimeduringwhichthe

queuegrows.

Answer:

a.2userscanbesupportedbecauseeachuserrequireshalfofthelinkbandwidth.

b.Sinceeachuserrequires1Mbpswhentransmitting,iftwoorfeweruserstransmit

simultaneously,amaximumof2Mbpswillberequired.Sincetheavailablebandwidthof

thesharedlinkis2Mbps,therewillbenoqueuingdelaybeforethelink.Whereas,if

threeuserstransmitsimultaneously,thebandwidthrequiredwillbe3Mbpswhichis

morethantheavailablebandwidthofthesharedlink.Inthiscase,therewillbequeuing

delaybeforethelink.

c.Probabilitythatagivenuseristransmitting=0.2

d.Probabilitythatallthreeusersaretransmittingsimultaneously=33(1−)0=

3

0.23=0.008.Sincethequeuegrowswhenalltheusersaretransmitting,thefractionof

timeduringwhichthequeuegrows(whichisequaltotheprobabilitythatallthreeusers

aretransmittingsimultaneously)is0.008.

10.(Q16)Considersendingapacketfromasourcehosttoadestinationhostoverafixedroute.

Listthedelaycomponentsintheend-to-enddelay.Whichofthesedelaysareconstantand

whicharevariable?

Answer:Thedelaycomponentsareprocessingdelays,transmissiondelays,propagation

delays,andqueuingdelays.Allofthesedelaysarefixed,exceptforthequeuingdelays,which

arevariable.

11.(Q19)SupposeHostAwantstosendalargefiletoHostB.ThepathfromHostAtoHostB

hasthreelinks,ofratesR1=250kbps,R2=500kbps,andR3=1Mbps.

a.Assumingnoothertrafficinthenetwork,whatisthethroughputforthefiletransfer.

b.Supposethefileis2millionbytes.Roughly,howlongwillittaketotransferthefileto

HostB?

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

Answer:

a.

b.

c.

250kbps

64seconds

200kbps;80seconds

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

eachlink.

a.Whatisthemaximumnumberofsimultaneousconnectionsthatcanbeinprogressat

anyonetimeinthisnetwork?

b.Supposethatallconnectionsarebetweentheswitchintheupper-left-handcorner

andtheswitchinthelower-right-handcorner.Whatisthemaximumnumberof

simultaneousconnectionsthatcanbeinprogress?

Answer:

a.Wecannconnectionsbetweeneachofthefourpairsofadjacentswitches.Thisgivesa

maximumof4nconnections.

b.

Wecannconnectionspassingthroughtheswitchintheupper-right-handcorner

andanothernconnectionspassingthroughtheswitchinthelower-left-handcorner,

givingatotalof2nconnections.

13.(P4)Reviewthecar-caravananalogyinSection1.4.Assumeapropagationspeedof50

km/hour.

a.

Supposethecaravantravels150km,beginninginfrontofonetollbooth,passing

throughasecondtollbooth,andfinishingjustbeforeathirdtollbooth.Whatisthe

end-to-enddelay?

b.Repeat(a),nowassumingthattherearefivecarsinthecaravaninsteadoften.

Answer:Tollboothsare150kmapart,andthecarspropagateat50km/hr,Atollbooth

servicesacaratarateofonecarevery12seconds.

a.

Therearetencars.Ittakes120seconds,ortwominutes,forthefirsttollboothtoservice

the10cars.Eachofthesecarshasapropagationdelayof180minutesbeforearrivingat

thesecondtollbooth.Thus,allthecarsarelinedupbeforethesecondtollboothafter

182minutes.Thewholeprocessrepeatsitselffortravelingbetweenthesecondand

thirdtollbooths.Thusthetotaldelayis364minutes.

b.Delaybetweentollboothsis5*12secondsplus180minutes,i.e.,181minutes.Thetotal

delayistwicethisamount,i.e.,362minutes.

14.(P5)Thiselementaryproblembeginstoexplorepropagationdelayandtransmissiondelay,

twocentralconceptsindatanetworking.Considertwohosts,AandB,connectedbya

singlelinkofrateRbps.Supposethatthetwohostsareseparatedbymmeters,and

supposethepropagationspeedalongthelinkissmeters/sec.HostAistosendapacketof

sizeLbitstoHostB.

a.

Expressthepropagationdelay,d

prop,intermsofmands.

d

b.Determinethetransmissiontimeofthepacket,trans,intermsofLandR.

c.

Ignoringprocessingandqueuingdelays,obtainanexpressionfortheend-to-end

delay.

d.SupposeHostAbeginstotransmitthepacketattimet=0.Attimet=d

thelastbitofthepacket?

trans,whereis

Supposed

packet?

dd

propisgreaterthantrans.Attimet=trans,whereisthefirstbitofthe

e.

dd

propislessthantrans.Attimet=trans,whereisthefirstbitofthepacket?

d

f.

Supposed

g.

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

propequalsd

trans.

Answer:

d

d

d

a.

b.

c.

d.

e.

f.

prop=m/sseconds.

trans=L/Rseconds.

end-to-end=(m/s+L/R)seconds.

ThebitisjustleavingHostA.

ThefirstbitisinthelinkandhasnotreachedHostB.

ThefirstbithasreachedHostB.

g.

Want

m=LR

100

S=28∗103

2.5∗108=893km.

15.(P6)Inthisproblemweconsidersendingreal-timevoicefromHostAtoHostBovera

packet-switchednetwork(VoIP).HostAconvertsanalogvoicetoadigital64kbpsbit

streamonthefly.HostAthengroupsthebitsinto56-Bytepackets.Thereisonelink

betweenHostAandB;itstransmissionrateis500kbpsanditspropagationdelayis2msec.

AssoonasHostAgathersapacket,itsendsittoHostB.AssoonasHostBreceivesan

entirepacket,itconvertsthepacket’sbitstoananalogsignal.Howmuchtimeelapses

fromthetimeabitiscreated(fromtheoriginalanalogsignalatHostA)untilthebitis

decoded(aspartoftheanalogsignalatHostB)?

Answer:Considerthefirstbitinapacket.Beforethisbitcanbetransmitted,allofthebitsin

thepacketmustbegenerated.Thisrequires

56∗8

64∗103sec=7

Thetimerequiredtotransmitthepacketis

56∗8

500∗103sec=896

Propagationdelay=2msec.

Thedelayuntildecodingis

7msec+896μsec+2msec=9.896msec

Asimilaranalysisshowsthatallbitsexperienceadelayof9.896msec.

16.(P9)ConsiderapacketoflengthLwhichbeginsatendsystemA,travelsoveronelinktoa

packetswitch,andtravelsfromthepacketswitchoverasecondlinktoadestinationend

system.Letdi,si,andRidenotethelength,propagationspeed,andthetransmissionrateof

linki,fori=1,2.Thepacketswitchdelayseachpacketbydproc.Assumingnoqueuing

delays,intermsofdi,si,Ri,(i=1,2),andL,whatisthetotalend-to-enddelayforthe

packet?SupposenowthepacketLengthis1,000bytes,thepropagationspeedonboth

linksis2.5*108m/s,thetransmissionratesofbothlinksis1Mbps,thepacketswitch

processingdelayis2msec,thelengthofthefirstlinkis6,000km,andthelengthofthelast

linkis3,000km.Forthesevalues,whatistheend-to-enddelay?

Answer:ThefirstendsystemrequiresL/R1totransmitthepacketontothefirstlink;the

packetpropagatesoverthefirstlinkind1/s1;thepacketswitchaddsaprocessingdelayof

dproc;afterreceivingtheentirepacket,thepacketswitchrequiresL/R2totransmitthepacket

ontothesecondlink;thepacketpropagatesoverthesecondlinkind2/s2.Addingthesefive

delaysgives

dend-end=L/R1+L/R2+d1/s+d2/s+dproc

1

2

Toanswerthesecondquestion,wesimplyplugthevaluesintotheequationtoget8+8+

24+12+2=54msec.

17.(P10)Intheaboveproblem,supposeR1=R2=Randdproc=0.Furthersupposethepacket

switchdoesnotstore-and-forwardpacketsbutinsteadimmediatelytransmitseachbitit

receiversbeforewaitingforthepackettoarrive.Whatistheend-to-enddelay?

Answer:Becausebitsareimmediatelytransmitted,thepacketswitchdoesnotintroduceany

delay;inparticular,itdoesnotintroduceatransmissiondelay.Thus,

dend-end=L/R+d1/s+d2/s2

1

ForthevaluesinProblem9,weget8+24+12=44msec.

18.(P11)SupposeNpacketsarrivesimultaneouslytoalinkatwhichnopacketsarecurrently

beingtransmittedorqueued.EachpacketisoflengthLandthelinkhastransmissionrateR.

WhatistheaveragequeuingdelayfortheNpackets?

Answer:Thequeuingdelayis0forthefirsttransmittedpacket,L/Rforthesecond

transmittedpacket,andgenerally,(n-1)L/Rforthenthtransmittedpacket.Thus,theaverage

delayfortheNpacketsis

(L/R+2L/R+.......+(N-1)L/R)/N=L/RN(1+2+.....+(N-1))=LN(N-1)/(2RN)=(N-1)L/(2R)

Notethathereweusedthewell-knownfactthat

1+2+.......+N=N(N+1)/2

19.(P14)Considerthequeuingdelayinarouterbuffer.LetIdenotetrafficintensity;thatis,I=

La/R.SupposethatthequeuingdelaytakestheformIL/R(1-I)forI<1.

a.Provideaformulaforthetotaldelay,thatis,thequeuingdelayplusthetransmission

delay.

b.PlotthetotaldelayasafunctionLof/R.

Answer:

a.ThetransmissiondelayisL/R.Thetotaldelayis

/

(1−)+=1−

b.

Letx=L/R.

Totaldelay=1−

20.(P16)PerformaTraceroutebetweensourceanddestinationonthesamecontinentatthree

differenthoursoftheday.

a.Findtheaverageandstandarddeviationoftheround-tripdelaysateachofthethree

hours.

b.Findthenumberofroutersinthepathateachofthethreehours.Didthepaths

changeduringanyofthehours?

c.TrytoidentifythenumberofISPnetworksthattheTraceroutepacketspassthrough

fromsourcetodestination.Routerswithsimilarnamesand/orsimilarIPaddresses

shouldbeconsideredaspartofthesameISP.Inyourexperiments,dothelargest

delaysoccuratthepeeringinterfacesbetweenadjacentISPs?

d.Repeattheaboveforasourceanddestinationondifferentcontinents.Comparethe

intra-continentandinter-continentresults.

Answer:Experiments.

21.(P18)Supposetwohosts,AandB,areseparatedby10,000kilometersandareconnected

byadirectlinkofR=2Mbps.Supposethepropagationspeedoverthelinkis2.5•108

meters/sec.

a.Calculatethebandwidth-delayproduct,R•dprop.

b.Considersendingafileof400,000bitsfromHostAtoHostB.Supposethefileissent

continuouslyasonelargemessage.Whatisthemaximumnumberofbitsthatwillbe

inthelink