计算机网络LKJ22fallHomework-第4次习题

Note:Onlytheanswerswritteninthissheetwillbescored!

、Answersheet

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第82题图片缺少了一个重要信息：E1端口ip是❷54

chapter1

WhichofthefollowingdescriptionaboutOSIlayersisincorrect?

A.Theapplicationlayercontainsavarietyofprotocolsthatarccommonlyneededbyusers

B.Thetransportlayerisconcernedwiththesyntaxandsemanticsoftheinformationtransmitted.

C.Thenetworklayercontrolstheoperationofthesubnetanddetermineshowpacketsareroutedfrom

sourcetodestination

D.Thedatalinklayeristotransformarawtransmissionfacilityintoalinethatappearsfreeof

undetectedtransmissionerrors.

•Thesessionlayer

-Tliesessionlayeroffersvariousseivices.includingdialogcontrol(keepingtrack

ofwhosetumitistotransmit),tokenmanagement(preventingtvvopartiesfiom

attemptingthesamecriticaloperationsimultaneously),andsyiiclironization

(checkpoiiHmglongtransmissionstoallowthemtopickupfromwheretheyleft

offintheeventofacrashandsubsequentrecoveiy)

•Thepiesentationlayer_____________________________________

-Thepresentationlayei|isconcealedwiththesyntaxandsemanticsofthe

informationtransmitted._________________________________________

—Tomakeitpossiblefbicomputeiswithdifferentiiiteinaldatalepiesentationsto

communicate.【hedatastructurestobeexchanJcedcanbedefinedinanabstiact

way.

•Thetransportlayer

-Tlietranspoillayerisatmeend-to-endlayer

•Inthelowerlayers,eachprotocolsisbetweenamachineandirs

inunediateneighbors,andnotbetweentheultimatesourceand

destinationmachines.

,Thedifierencebetweenlayers1tluoiigh3.whicharechained,

andlayers4tlnough7whichareend-to-end.

-ConnectionlessTransportProtocol:LDP

-Connection-OrientedTranspoitProtocol:TCP

•TheNetworkLayer

—isconcernedwithgettingpacketfiomsoiucetodestination

-isthelowestlayerthatdealswithend-to-endtransmission

2

TheReferenceModel：OSI:PhysicalLayer

•Thephysicallayer

-Thelowestlayerinourprotocolmodel,isthefbmidationonwhich

thenetworkisbuilt.

-Thesedesignissueslargelydealwithmechanical,electrical.and

timinginterfaces,aswellasthephysicaltransmissionmedium.

•Wewilltalkabout

-Thetheoreticalbasisfordataconununication(Fourieranalysis,

bandwidth-liniitedsignals)

—Physicalmediafallintotwocategories:guidedmedia(wired,

hvisted-paii'wiie.fiber)andunguideclmedia(wiieless)

—Digitalmodulationandmultiplexing

•TDM,FDM,CDM

-Introducetwotypicalconunimicationsystems:PSTNandmobile

phonesystem

TheReferenceModel:OSI：DataLinkLayer

•Tlueemainfunctionsofthedatalinklayer

-Franinig:thesenderbreaksuptheuiputdatamtodataframes(typically

afewhundredorafewthousandbytes)andtransmittheframes

sequentially.

-Flowcontrol:howtokeepafastertransmitterfromdrowningaslow

receiverindata.

•ExamplesoflinklayerprotocolsincludeEthemet.WiFiandthe

Point-to-PointProtocol(PPP)

•Broadcastnetwoikshaveanadditionalissueinthedatalink

layer:howtocontrolaccesstothesharedcliamiel.

-Aspecialsublayerofthedatalinklayer—themediumaccesscontiol

sublayerwilldealwiththisproblem.

2.ThethreecentralconceptsoftheOSImodelare.

A.services,interfacesandprotocolsB.architecture,modelandswitching

C.subnet,layeringandportD.protocols,layersandinterfaces

3.Onceuponatime,peoplethoughtthattheOSImodelanditsprotocolsweregoingtotakeoverthe

worldandpusheveiylhingelseoutoflliviiway.Thisdidnothappen.Why?Alookbackatsomeof(he

reasonsmaybeuseful.Theycanbesummarizedasfollowingexceptfor:

A.Badtiming.B.Badtechnology.C.Badprice.D.Badimplementations.

B

4.meansthattheswitchorroutermustreceivetheentirepacketbeforeitcanbegintotransmit

thefirstbitofthepacketontotheoutboundlink.

A.Queuingdelay

B.Store-and-forwardtransmission

C.Packetswitching

1).Propagation

3

5.Supposeasystemhasafourlayerprotocolhierarchy.Applicationsgeneratemessagesoflength

320bytes.Ateachofthelayers(includingtopperandbottomlayers),an20byteheaderisadded.What

fractionofthenetworkbandwidthisfilledwithheaders?

A.0.20B.0.25C.0.30D.0.40

20*4/(20*4+320)

TheOSIvs.TCP/IP

•TheOSIreferencemodelwasdevisedbeforethe

correspondingprotocolswereinvented.WhileforTCP/IP.

theprotocolscamefiist.andthemodelasreallyjusta

desciiptionoftheexistingprotocols.

•The|OSImodelsupportJbothlcoiuiectionlessand______

connection-oiientedconununication卜1thenetwoikIryei]

budonlvIconnection-oHentedcommunicationinthe

Itiansportlayer]TheTCP/IPmodelsupportsonlyone

modeiuthenehvorklayer(counectiouless)butbothin

thetransportlayer.

chapter2

6.Inthesystem,theuserstaketurns,eachoneperiodicallygettingtheentirebandwidthfora

littleburstoftime.

A.FDMB.TDMC.WDMD.CDM

7.Televisionchannelsare4MHzwide.Howmanybils/seccanbesentifsixteen-leveldigitalsignalsare

used?Assumeanoiselesschannel.

A.16MbpsB.24MbpsC.32MbpsD.40Mbps

2*4*log_2(16)

8.Ifabinarysignalissentovera3-kHzchannelwhosesignal-to-noiscratioS/Nis31,whatisthe

maximumachievabledatarate?

A.6kbpsB.12kbpsC.15kbpsD.18kbps

别选C.15kbps

4

1.Ifabinarysignalissentovera3-kHzchannelwhosesignal-to-noise

ratiois20dB,whatisthemaximumachievabledatarate?1

Nyquist2Wlog2V=2X3ZcXlog22=6kb/sQ

ShannonWlog2=3/cxlog2(l+100)=19.97kb/s^

•••maximumachievabledatarate=6kbisQ

I

9.Inpacketswitching,circuitswitching,andmessageswitching,whichonedoesnotutilizestore-and-

fonvardtransmissiontechnology?

A.packetswitchingB.circuitswitching

C.messageswitchingD.noneofabove

线路交换（Circuitswitching）也叫电路交换，最初用在公用电话系统中。电路交换就是由交换机负

责在两部通信站点（如两部电话机）之间建立一条专用的物理线路分配给双方传输数据使用。

用户线是电话用户到所连接的市话交换机的连接线路，是用户专用的线路，而交换机之间拥有大量话

路的中继线则是许多用户共享的，正在通话的用户只占用了其中一个话路。一旦双方挂断电话，即表

示数据交换完毕，A和B用户之间建立的物理连接也将释放。

在诵信子网中.不采用“存储一转发”的交换技术星（二A.电…

A.电路交换B报文交换C点电路D数竞报相关知识点解析A此敏考官的是电路交焕知识点.电路

交换过程中通信双方独占物理线路不存在“存储T专发二因此答案为从反覆收藏

百度题库©悻；

10.Whichprotocoldoesnotbelongtothedatalinklayer?

A.HDLCB.ICMP(networklayer)C.PPPD.SDLC

11.Whichofthefollowingfactorsdoesnotaffectthechanneldatatransferrate()?

A.Signaltonoiserat沁B.FrequencyBandwidth

C.ModulationraleD.Signalpropagationspeed

你用网线把一点点数据很快地传输过去

和用大货车拉很多硬盘慢悠悠地开过去

hups:〃www.iavatD/computcr-nctwork-mcq

12.Anoisychannelhasabandwidthof4KHZ,itsS/Nratiois511,thenitsmaximumdataratewillbe

5

A.36kbpsB.32kbpsC.63kbpsD.NONEOFABOVE

13.Whichonecanbeusedasakeycomponentofopticaltransmissionsystem?

A.UTPB.semiconductorlaserdevice

C.HUBD.WiFirouter

GuidedTransmissionMedia:TwistedPairs(III)

TwistedPairs

-Fullduplexlinks(1-GbpsEthernet,allfourpairsinboth

directions).

-Halfduplexlinks(100-MbpsEthernet,onepairfbreachdirection)

-Simplexlinks(one-waystreet)

UTP一UnshieldedTwisted

Pair

Category7cableshave

shieldingontheindividual

twistedpairtoreducethe

susceptibilitytoexternal

interferenceandcrosstalk

Figure2-3.Category5UTPcablewithfourtwistedpairs,withothernearbycables.

14.Thecablebetweentollofficeandtheendofficeoftelephonecompanyarcknownasthe

A.localloopB.trunkC.microwavelineD.coaxialcable

6

ExampleI:PSTN(II)

本地长途中心交换局

交换局电话局Intermediate

TelephoneEndTollswitchingTollEndTelephone

officeoffice(s)officeoffice

G4

'Veryhigh

LocalTollTollLocal

bandwidth

loopconnectingconnectingloop

intertoll

trunktrunk

trunks

Figure2-3().Atypicalcircuitrouteforalong-distancecall.

Thetelephonesystemconsistsofthreemajorcomponents:

1.Localloops:telephonemodem,ADSL,fiber

2.Trunks(digitalfiberopticlinksconnectingswitchingoffices)—main

considerationproblemismultiplexing(FDMandTDM)

3.Switchingoffices(wherecallsaremovedfiomonetrunktoanother)——two

switchingways

15.Itisimpossiblefortocausetransmissionimpairmentsoftelephonelocalloop.

A.differentFouriercomponentspropagatingatdifferentspeed

B.thermalnoise

C.crosstalkbetweentwoclosewires

D.multipathfading

16.AnT1channelcontains24PCMsignals,itsdataraleis.

A.2.048MbpsB.1.544Mbps

C.64kbpsD.2.5Gbps

7

DetailedSolution±DownloadSolutionPDF

•TheT1digitalcarriersystemisaNorthAmericandigitalmultiplexing.

•T1standsfortransmissiononeandspecifiesadigitalcarriersystemusingPCM

encodedanalogsignal.

•WiththeT1carriersystem,thereissampling,encoding,andmultiplexingof24

voicebandchannels.Eachchannelcontainsan8-bitPCMcodeandsampled8000

timesasecond.

•Anadditionalbit(calledframingbit)isaddedtoeachframe.Theframingbitoccurs

onceperframe(8000bpsrate)andrecoveredinthereceiver.Itisusedtomaintain

frameandsamplesynchronizationbetweentheTDMtransmitterandreceiver.

嘴鬻x8缶x8,000-)+(1缥醇x8,000-)

1，536,000—+8,000―

1，544,000岳

1.544^7

second

FramerstructureofT1carrierPCMtelephonyis:

chapter3

17.Bitstring1110111111101willbecomeafterbitstuffing.

A.11101111011101B.1I10III1I0II0I

C.1110111111101D.11101111110101

11101111101101

分快M二ilag

Framing一FlagBytewithBitStuffing(I)

•Framingcanbealsobedoneatthebitlevel

•Example:HDLC(High-LevelDataLinkControl)protocol

-Eachframebeginsandendswithaspecialbitpattern01111110or

0x7Einhexadecimal.Thispatternisaflagbyte.

-Wheneverthesender'sdatalinklayerencounters行veconsecutive

Isinthedata.itautomaticallystuffea0bitintotheoutgoingbit

stream.

•Theadvantageofbitstuffingisthatitensuresaminimum

densityoftransitionthathelpthephysicallayermaintain

syncliionization.

•ApplicationExample:USB(UniversalSerialBus)usesbit

stuffing.

8

18.WhichfieldofPPPframecanbeomittedtoimproveefficiencyduringframetransmission?

A.checksumB.controlC.protocolD.length

PPPusesbytestuffing

19.Whatistheremainderobtainedbydividingx7+x>+lbythegeneratorpolynomialx3+l?

A.110B.OilC.111D.Noneofabove

20.WhichisnotacorrectmethodtobuildVLANs?

A.EveryportofswitchisassignedaVLANID;

B.EveryportofswitchisassignedaTCPportnumber;

C.EveryMACaddressisassignedaVLANID;

D.Switchportssendingandreceivingpayloadofthesamelayer3protocolarcassignedthesame

VLANID;

21.WithHammingcode,(hecodewhichcancorrect3biterrorsalmostmaydetectalmost

error(s).

A.5C.7D.8

7位和8位都只能纠错3bii

ErrorControl:Error-Correcting

•Theerror-detectinganderror-correctingpropertiesofa

blockcodedependonitsHammingdistance.

-Toreliablydelectderrors,youneedadistanced+1code

—Toccrrectdeircr),youneedadistance2d+1code.

-Example——consideracodewithonlyfourvalidcodewords:

0000000000,0000011111,11111OOOOO,llllllllll.Thiscodehas

adistanceof5,whichmeansthatitcancorrectdoubleerrorsor

detectquadmpleerrors.codewords的距离为5,只能纠2bit的差错

•Ifacodeword0000000111arrivesandweexpectonlysingle-or

double-biterrors,thereceiverwillknowthattheoriginalmusthave

been0000011111.若发生3bit差错，则纠错失败(会错误地纠错)

•However,atripleerrorchanges0000000000into0000000111.the

errorwillnotbecorrectedproperly(since0000011111hasthe

smallerHanuningdistances(2)tothearrivedcodeword,whilethe

Hammingdistancebetween0000000000and0000000111is3).

9

ErrorControl:Error-Correcting

•Imasinethatwewanttodesignacodewithmmessagebitsandr

checkbitsthatwillallowallsingleerrorstobecorrected.

•Eachofthe2Z,/legalmessageshasnillegalcodewordsatadistanceof

1fromit.Thus,eachofthe2Wlegalmessagesrequiresn+1bit

patternsdedicatedtoit.

•Sincethetotalnumberofbitpatternsis2〃，wemusthave(〃+1)2W<

2”.Usingn=m+r,thisrequirementbecomes

(〃？+尸+1)<2r

•Givenm,thisputsalowerlimitonthenumberofcheckbitsneededto

coixectsingleerrors.

22.Whatistheremainderobtainedforaframe1101011111usingthegeneratorpolynomialG(x)=x4+x+

1?

A.0101B.0110C.0010D.1100

最高此项是4,那么被陡数要补4个0

23.Whatisthemaximumsendingwindowsizeoftheselectiverepeatprotocolwhenuse3bitsforframe

serialnumber?

A.4B.5C.6D.7

Selectiverepeatprotocol:Max.size=(MAXSEQ-I)2

10

ASlidingWindowProtocolUsing

SelectiveRepeat

•Selectiverepeatprotocol:Max.size=（MAXSEQ+I）/2

3

-SendersendsFo...F3orFo°...F3

24.Thetechniqueoftemporarilydelayingoutgoingacknowledgementssothattheycanbehookedonto

thenextoutgoingdataframeisknownas.

A.acknowledgingB.piggybackingC.go-backingD.hooking

chapter4

25.Whatisthebaudrateofclassic10-MbpsEthernet?

A.10MB.15MC.20MD.25M

因为采用了曼彻斯特编码

26.AccordingtoCSMA/CD,ifthepropagationtimeofthelineis100ms,thetransmissiontimeofthe

framemustnotlessthan:

A.100msB.200msC.400msD.500ms

27.TheEthernetusesanalgorithmcalledbinaryexponentialback-off,after3collisions,thestationwill

chosearandomnumberbetween0and.

A.7B.8C.15D.16

11

CSMA/CDwithBinaryExponential

Backoff

Final可能考

•ClassicEthernetusesthe1-persistentCSMACDalgorithm.

-Astationsensesthechannelwhenithasaframetosendandsendthe

frameassoonasthechannelbecomesidle.

等待陋机时间，如何随机？

•Howtherandomintervalisdetemiinedwhenacollisionoccurs?

-Afteracollision,timeisdividedintodiscreteslotswhoselengthisequal

totheworst-caseround-trippropagationtimeontheEthernet(2r).

•ToaccouunodatethelongestpathallowedbyEtheniet,theslottimehas

beensetto512bittimes,oi51.2pisec-七行田”出“川巴…

-After/collisions,arandomnumberkbetween&and2^-1ischosen.

-After10collisionshavebeenreached,therandomizationintervalis

frozenatamaximumof1023slots.

-After16collisions,thecontrollerthrowsinthetowelandreportsfailure

backtothecomputer.

28.Thehostsconnectedbyasinglenewroutermaybelongto

A.thesamecollisiondonainandthesamebroadcastdomain

B.(hesamecollisiondomainbutdifferentbroadcastdomains

C.(hesamebroadcastdomainbutdifferentcollisiondomains

D.differentcollisiondomainsanddifferentbroadcastdomains

https:〃blog.csdn.nct/ytra:stcr/articlc/dctails/l26915713

29.Afterthesenderfirstsendsframesfrom0to8andattheendoftimeoutreceivestheacknowledgments

forframe1,3,and5,thenextframeilwillretransmitisframe.(assumetheprotocolisgo-

back-nandtheacknowledgmentnumberindicatesthelastframenumberreceivedcorrectly.)

A.2B.4C.6D.7

30.WhichisnottheCSMA/CAruleof802.11?

A.IfstationXreceivedRTSofstationA,XmustremainsilentforashorttimesothatXwillnot

interferewithA'sreceiptofCTS.

B.IfstationXreceivedRTS,butdidnotreceiveCTS,thenXcantransmititsdataandwillnotinterfere

withotherstations.。

C.IfstationXhasnotreceivedRTS,butreceivedCTS,thenXmaynottransmititsdata..

D.IfstationXhasreceivedbothRTSandCTS,thenXmaytransmititsdata.

总之，收到CTS就不能发。

12

AWhichisnottheCSMA/CA(CarrierMultipleAccms/CoUisionAvoidance)ruleof80211

a)IfstationXreceivedRTS(RequesttoSend)ofstationA,Xmustremainsilentforashorttime

(backoff).

b)IfstationXreceivedRTS,butdidnotreceiveCTS(CleartoSend),thenXmaynottransmitits

data(Y<A-X>W)exposeproblen

c)IfstationXhasnotreceivedRTS,butreceivedCTS,thenXmaynottransmititsdata,«A)

d)IfstationXhasreceivedbothRTSandCTS,thenXmaynottransmititsdata(!2A什'■.V

发送message.A发个RTS给Y.然知Y网应•个CTS给A.内为X收能接收到RTS也能

报4%CTS»说明X限GA的尢传F0*他I内.也在Y侪J无，k8的曲内・那

巧法X只能静默)“

无线局域网

目前最广发使用的局域网。

CSMA/CA协议(载波监听多路访问/冲突避免)有以下3个规则：

如果站X收到RTS,但没收到CTS帧，贝!JX可以发送自己的数据而不会干扰其他站。

如果站X没收到RTS,但收到CTS帧，则站X不可以发送自己的数据。

如果站X能收到RTS和CTS,贝11站X不可以发送自己的数据。

31.Whenaswitchissetupport-basedVLANs,whichfeatureisimpossibletoachieve?

A.AportbelongstotwodifferentVLANs

B.PortsondifferentswitchesbelongstoasameVLAN

C.IP-Secencryption

D.Multicastfunction

32.AnEthernettopologyandthecurrentforwardingtableoftheswitchareshowninthefollowingfigure.

Hosts()()-el-d5-(X)-23-alsendadataframetohost00-e1-d500-23-c1.Afterreceivingthisframe,host

00-el-J5-OO-23-C1sendshost00-e1-d5-00-23-alaconfirmationframe.Theforwardingportsofthe

twoframesontheswitcharc().

00-el-d5-00-23-al00-el-d5-00-23-blOO-el-d5-OO-23-Cl

A.{3}and{I}B.{2,3}and{1}C.{2,3}and{1,2}D.{1,2,3}and{1}

第一次ARP广播

chapter5

33.WhichisnottheprivateaddressthatwillnotappearinInternetdatagram?

A.2B.

13

C.D.

私有地址(Privateaddress)属于非注册地址，专门为组织机构内部使用.

以下表列出留用的内部寻址地址

A类-55

B类-55

C类-55

34.Whichprotocolisusedincommand“ping”？

A.ARPB.ICMP

C.RARPD.ECHO

35.isnotalegalIPV6address.

A.2A00::1345:A367:892B:24E0B.1382:4567:89AB:CDEF

C.::8D.2A43:0000:0000:0000:0123:4567:89AB:CDEF

128位

36.RIPisa.

A.InteriorGatewayProtocolB.ExteriorGatewayProtocol

C.staticroutingprotocolD.linkstateroutingprotocol

14

AutonomousSystem

•Boththescaleandadministrativeautonomycanbesolved

byorganizingroutersintoautonomoussystems.

•Routerswithinthesameautonomoussystem(AS)allrunthe

sameroutingalgorithm(forexample,anLSorDV

algorithm)andhaveinformationabouteachother.

-Inlia-aulonoinoussystemroutingprotocol

•ToconnectASstoeachother,oneormoreoftheroutersin

anASwillhavetheaddedtaskofbeincresDonsiblefor

fonvaidincpacketstodestinationoulsidvtheAS.

-Theseroutersarecalledgatewayrouters.

Inthefollowing,wewillexaminetwointra-ASroutine

protocols(RIPandOSPF)andtheinter-ASroutingDroJocol

(BGP)thatareusedintoday'sInternet.

37.WhichofthefollowingdevicesisneededforapackettohepassedfromoneLANtoInternet?

A.BridgeB.Router

C.SwitchD.Hub

38.Whichoneisnotapartoflink-staterouting?

A.Arouterdiscoversitsneighborsandlearnstheirnetworkaddresses.

B.Measurethedelayorcosttoeachofitsneighbors.

C.Exchangeroutingtablewithitsneighbors.

D.Constructapackettellingallithasjustlearned.

15

5.23LinkStatc|Routing

DistancevectorroutingwasusedintheARPANETuntil1979.whenitwasre­

placedbylinkstaterouting.Tlieprimaiyproblemtliatcauseditsdemisewasthat

thealgoritlunoftentooktoolongtoconvergeafterthenetworktopologychanged

(duetothecoimt-to-infinityproblem).Consequently,itwasreplacedbyanen-

tiielynewalgorithm.nowcalledlinkstaterouting.Variantsoflinkstaterouting

calledIS-ISandOSPFaietlieroutingalgoiitlunsthataiemostwidelyusedinside

largenetworksandtheIntemettoday.

Theideabehindlinkstateroutingisfaiilysimpleandcanbestatedasfive

parts.Eachroutermustdothefollowingthingstomakeituoik：

1.Discoveritsneighborsandleanitheirnetworkaddresses.

2.Setthedistanceorcostmetrictoeachofitsneighbors.

3.Constractapackettellingallithasjustleained.

4.Sendthispackettoandreceivepacketsfromallotherrouters.

5.Computetheshortestpathtoeveryotherrouter.

5.2.4DistanceVectorRouting

Computernetworksgenerallyusedyuainicroutingalgoiitlunsthataremore

complexthanflooding.butmoreefficientbecausetheyfindshortestpathsforthe

cuiienttopology.Twodynamicalgoiitlunsinparticular,distancevectorrouting

andlinkstaterouting,aiethemostpopular.Intliissection,wewilllookatthefor­

meralgoritlun.hithefollowingsection,wewillstudythelatteralgoritlun.

Adistancevectorroutingalgoritliinoperatesbyhavingeachroutermaintain

atable(i.e.,avector)givingthebestknowndistancetoeachdestinationandwhich

linktousetogetthere.Tliesetablesaieupdatedbyexchangingiiifonnatiouwith

theneighbors.Eventually,everyrouterknowsthebestlinktoreacheachdestina­

tion.

Thedistancevectorroutingalgorithmissometimescalledbyothernames,

mostcommonlythedistiibutedBellman-Fordroutingalgoritlun,afterthere-

seaicherswhodevelopedit(Bellman.1957:andFordandFulkerson.1962).Itwas

theorisinalARPANETroutinealsoritlunandwasalsousedintheInternetunder

thenameRIP.

Indistancevectorrouting,eachroutermaintainsaroutingtableindexedby.

andcoutainingoneentiyfor.eachrouterintheuetuoik.Tliisentryhastwopaits:

thepreferred