计算机网络第一章作业参考答案.docx

计算机网络第一章作业参考答案.docx

《计算机网络第一章作业参考答案.docx》由会员分享，可在线阅读，更多相关《计算机网络第一章作业参考答案.docx（9页珍藏版）》请在冰豆网上搜索。

计算机网络第一章作业参考答案

计算机网络第一章作业参考答案

2.

Theperformanceofaclient-serversystemisstronglyinfluencedbytwomajornetworkcharacteristics:

thebandwidthofthenetwork（thatis,howmanybits/secitcantransport）andthelatency（thatis,howmanysecondsittakesforthefirstbittogetfromtheclienttotheserver）.Giveanexampleofanetworkthatexhibitshighbandwidthbutalsohighlatency.Thengiveanexampleofonethathasbothlowbandwidthandlowlatency.

Answer:

1,、Ifthedistanceistoofarifbandwidthisverybig,butthedelayisalsoveryobvious,suchasthetransatlanticcable.

2,、Ontheotherhand,ifjusttransportinabuildingevenifbandwidthisverylow,wecanreceivemessageimmediately.

3.

Besidesbandwidthandlatency,whatotherparameterisneededtogiveagoodcharacterizationofthequalityofserviceofferedbyanetworkusedfor（i）digitizedvoicetraffic?

（ii）videotraffic?

（iii）financialtransactiontraffic?

Answer:

P55~57,

i,、Soundtransmissionrequiresacorrespondingfixedtimeslot,sothearrivalofinformationisveryimportant。

ii,、Videorequestissmooth,thereforethepacketlossrateisveryimportant

iii,、Financialbusinessflowneedstobereal-timeandhighsafetyrequirement,soneedtochecktheerrorrate。

10.

Whataretworeasonsforusinglayeredprotocols?

Whatisonepossibledisadvantageofusinglayeredprotocols?

Answer:

Throughthelayeredprotocolcanmakethedesignproblemisdividedintosmallermanageableparts.Alayerprotocolchangehasnoimpactontheotherlayerprotocol.

Disadvantage:

Eachlayerwillprocesstheinformation,finallymaycausedelayordistortionofinformation.Therearesomeduplicatedfunctionsindifferentlayers,forexample,DLLandTL.11.

whatistheprincipaldifferencebetweenconnectionlesscommunicationandconnection-orientedcommunication,Giveoneexampleofaprotocolthatuses

（i）connectionlesscommunication;

（ii）connection-orientedcommunication.

Answer:

P57-59,

First:

connectionorientedcommunicationisdividedintothreephases,thefirstistoestablishaconnection,inthisstage,sendsarequestforestablishingaconnection.Onlywhentheconnectionissuccessfullyestablished,tobegintransmissionofdata,whichisthesecondphase.Then,whenthedatahasbeentransmitted,toreleasetheconnection.Withouttheconnectionofcommunicationnotsomuchstage,itdirectdatatransmission.

Second:

connection-orientedcommunicationwithdataofkeepingorder,andconnectionlesscommunicationscannotbeguaranteedtoreceivedataandsenddatainorder.

Example,

Videocallisaconnectionorientedcommunication,

Mailisaconnectionlesscommunication。

14.

InFig.1-19,aserviceisshown.Areanyotherservicesimplicitinthisfigure?

Ifso,where?

Ifnot,whynot?

Answer:

ServiceisprovidedbytheklayertoK+1layer.ServicesmusthavethelowerKisprovided,namely,theKserviceisprovidedbyk-1layer.

16.

whichoftheOSIlayersandTCP/IPlayershandleseachofthefollowing:

（a）Dividingthetransmittedbitstreamintoframes.

（b）Determiningwhichroutethroughthesubnettouse.

Answer,

（a）、DatalinklayerinOSImodelandLinkLayerinTCPmodel

（b）、NetworklayerinOSImodelandInternetLayerinTCPmodel

17.

Iftheunitexchangedatthedatalinkleveliscalledaframeandtheunitexchangedatthenetworkleveliscalledapacket,doframesencapsulatepacketsordopacketsencapsulateframes?

Explainyouranswer.

Answer,

Framesencapsulatepackets.Whenapacketarrivesatthedatalinklayer,theentirepacket,includingpackethead,andallofthecontentdata,areusedastheframedataarea.Or,thewholepackageputinanenvelope（frame）,（ifcanbeloaded.）.

18.

Asystemhasann-layerprotocolhierarchy.ApplicationsgeneratemessagesoflengthMbytes.Ateachofthelayers,anh-byteheaderisadded.Whatfractionofthenetworkbandwidthisfilledwithheaders?

Answer:

hn/（hn+m）*100%.

19.

ListtwowaysinwhichtheOSIreferencemodelandtheTCP/IPreferencemodelarethesame.Nowlisttwowaysinwhichtheydiffer.

Answer:

Similarities:

Bothareindependentoftheprotocolstackconcept;layer’sfunctionsaresimilartoo.

Differences:

OSIbetterdistinguishbetweentheservice,interfaceandtheconceptofagreement,sothanTCP/IPhasbetterconceal,canmakeiteasiertoreplace;OSIisfirsttheconceptofsomemodel,andthentotherealizationoftheagreement,andTCP/IPisthefirstagreement,andthenestablishthedescriptionoftheagreementofthemodel.

Numberleveldifference:

TCP/IPdoesn’thavesessionlayerandpresentationlayer,OSIdoesnotsupportnetworkinterconnection.OSIinnetworklayersupportconnectionlessandconnection-orientedcommunication,andinthetransportlayeronlyconnection-orientedcommunication,andTCP/IPnetworklayerinonlyonekindofcommunicationmode（connectionless）,butinthetransportlayersupporttwomodes.

20.

WhatisthemaindifferencebetweenTCPandUDP?

Answer:

TCPisaconnection-oriented,whileUDPisadatagramservice.

TCPisreliable,whileUDPisunreliable.

25.

Animageis1024×768pixelswith3bytes/pixel.Assumetheimageisuncompressed.Howlongdoesittaketotransmititovera56-kbpsmodemchannel?

Overa1-Mbpscablemodem?

Overa10-MbpsEthernet?

Over100-MbpsEthernet?

OvergigabitEthernet?

Answer:

Theimageis1024×768×3bytes=2359296bytes=18874368bits.AccordingtoT=C/BW

In56000bits/secspeed,transmissionrequires337.042seconds.In1000000bits/sec,about18.874seconds.

In10000000bits/sec,about1.8874seconds.

In100000000bits/sec,about0.18874second.

28.

Listtwoadvantagesandtwodisadvantagesofhavinginternationalstandardsfornetworkprotocols.

Answer:

Advantages:

1、ifeveryoneusesstandard,soeveryonecanwithanyotherpeople;2、extensiveuseofstandardswillleadtoeconomiesofscale,suchastheproductionoflarge

scaleintegratedcircuitchip.

Disadvantages,

1、inordertoachievestandardizationoftenleadtopoorstandards;2、oncethestandardiswidelyused,itwillbeverydifficulttomakeachange,eveniffoundnew

andbettermethodsortechnologies,alsohardtoreplace.

29.

Supposethealgorithmsusedtoimplementtheoperationsatlayerkischanged.Howdoesthisimpactoperationsatlayersk?

1andk+1?

Answer:

Noeffect.

Theinternalprocessingofeachlayerisindependent.

30.

Supposethereisachangeintheservice（setofoperations）providedbylayerk.Howdoesthisimpactservicesatlayersk-1andk+1?

Answer:

TheK+1layerWillbeaffected,becausetheservicesareprovidedbythelowertoupperlayer,suchasklayerprovidedservicestok+1layer,andklayer’sserviceisprovidedbythek-1layer.

33.

Makealistofactivitiesthatyoudoeverydayinwhichcomputernetworksareused.Howwouldyourlifebealteredifthesenetworksweresuddenlyswitchedoff?

Answer:

1、Chatonthenet;

2、Querydata;

3、Transactionprocessing;

4、ReceiveandsendE-mail;

5、Listeningtomusic、Seeamovie

Ifthesenetworksweresuddenlyswitchedoff,IthinkIwillhavetodoalotofthingsfacetoface,andcan’tdomanythings.

37.

TheInternetismadeupofalargenumberofnetworks.TheirarrangementdeterminesthetopologyoftheInternet.AconsiderableamountofinformationabouttheInternettopologyisavailableonline.UseasearchenginetofindoutmoreabouttheInternettopologyandwriteashortreportsummarizingyourfindings.

Answer:

1、Computerconnectionwayiscalled"networkTopology"（Topology）.Networktopologyisreferswiththetransmissionmediainterconnectionallsortsofequipmentofthephysicallayout,

especiallycomputerdistributionpositionsoastohowandcablethroughthem.Designanetwork,shouldbebasedontheactualconditionsofchoosingthecorrecttopologymode.Eachtopologyhasitsownadvantagesanddisadvantages.

2、classification,

1,、Starstructure

StarstructureistopointtoeachworkstationBywayofstarconnectionintonetwork.networkA

centralnode,othernode（workstation,server）andthecentralnodedirectlyconnected,thiskindofstructurewiththecentralnodeasthecenter,soalsoknownasCentralizednetwork.Ithasthefollowingcharacteristics:

simplestructure,easytomanagement;Controlissimpleandconvenient,easytomeshing;Networkdelaytimeislesser,transmissionerrorisrelativelylow.Butthe

Abilityispoor.shortcomingisobvious:

thehighcostandlowreliability,Resourcessharing

StarTopologicalstructureThemainadvantagesare:

1.Easytomanagementmaintenance;

2.Reconfigureflexible;

3.Convenientfaultdetectionandisolation

2,、Ringstructure

RingstructurebynetworkInsomenodeThroughthepoint-to-pointlinkendtoformaclosedring,

thiskindofstructuremakepublictransmissioncablecompositionringtypejoint,thedataintheloop,alongtheadirectionineachnodetransmission,informationfromonenodetoanothernode.

RingstructureHasthefollowingfeatures:

informationflowinthenetisalongthefixeddirection,twonodesonlyaroad,thesimplifiedpathChoiceofcontrol;Looponthenodearebootstrap

control,thecontrolsoftwareSimple;Becauseinformationsourceintheloopisserialthrougheachnode,whenringnodetoomuch,willbeaffectedInformationtransmissionrate,andmakethenetworkresponsetimeextension;Loopisclosed,noteasytoexpand,Reliabilityislow,anodefailure,willcauseparalysiswhich;Maintenancedifficult,tobranchnodefaultlocationMoredifficult.

Theadvantagesanddisadvantagesofringtopologyis:

advantageisbecauseeachnodeareat