期末考试复习计算机网络英文版.docx

期末考试复习计算机网络英文版.docx

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期末考试复习计算机网络英文版

第一章计算机网络概述

●Identifythefivecomponentsofadatacommunicationssystem.

（数据通信系统5个组成部分：

报文发送方接受方传输介质协议）

message,sender,receiver,transmissionmediumandprotocol.

●Whatarethethreecriterianecessaryforaneffectiveandefficientnetwork?

performance,reliability,andsecurity.

●Whatisaninternet（互联网）?

WhatistheInternet（因特网）?

★Aninternetisaninterconnectionofnetworkswhichisacollectionofnetworksconnectedbyinternetworkingdevicessuchasrouters.

★TheInternetisthenameofaspecificworldwidenetworkthatusestheTCP/IPprotocolsuiteandisthelargestinternetintheworld.

第二章数据和信号

●Whichpracticalmethodsareusedforanalysisofasignal?

Andwhichissuitablefornetworktechniques?

研究电磁信号的方法是哪两种？

适用于网络技术的？

①Timedomainandfrequencydomain②frequencydomain

●Whycan'tanydigitalsignalbetransferredwithnodistortionincommunicationtransmission?

（数字信号为什么不能无失真传输？

）

1.Becausetoreceiveanexactreplicaofthedigitalsignal,allofthefrequencycomponentsmustbefaithfullythroughthetransmissionmedium,ifcertainharmoniccannottransfersuccessful,thesignalwereceivewillbedistorted.However,anymediumtransferssignalsonlywithincertainfrequencyranges,thatis,hasalimitedbandwidth.

2.Transmissionimpairmentsofsignals（attenuation,distortionandnoise）alwaysexist,sosignalerrorsunavoidable.

●What'sthebandwidth（带宽）?

Explainbrieflywhatcorrelationdobandwidth,datarateandcosthaveincommunicationprocess?

带宽、数据率、成本之间的关系？

Therangeoffrequenciescontainedinacompositesignalisitsbandwidth.

Innetworking,weusethetermbandwidthintwocontextswithtwodifferentmeasuringvalues:

BandwidthinHertzistherangeoffrequenciesinacompositesignalortherangeoffrequenciesthatachannelcanpass,usuallyusedtopresenttheperformanceofanalogchannelsandtransmissionmedia.（Anotheris）Bandwidthinbitspersecondisthespeedofbittransmissioninachannel,alink,orevenanetwork,usuallyusedtopresenttheperformanceoflinksandcommunicationdevicesofanetwork.（课本P84）

Thegreaterthetransmissionbandwidth,thegreaterthedatarate,andthehigherthecost.

●Q3-3:

Howcanacompositesignalbedecomposedintoitsindividualfrequencies?

复位信号如何分解成单独的频率成分？

Ifthecompositesignalisperiodic,wecanuesFourierSeriesdecomposeitintoaseriesofsignalswithdiscretefrequencies;

ifthecompositesignalisnonperiodic,wecanuesFourierTransformdecomposeitintoacombinationofsinewaveswithcontinuousfrequencies.

●Q3-4:

Namethreetypesoftransmissionimpairment.三种传输减损？

Attenuation,distortion,andnoise.衰减、失真和噪声

●Q3-7:

WhatdoestheNyquisttheoremhavetodowithcommunications?

奈奎斯特公式在通信中的作用？

Estimatethechannelcapacityofnoise-freedigitalchannels,,whenthechannelisgreaterthanthedataratetransmittedonchannelcapacity,itwillbefailureduetoseveredistortion.

☆TheNyquisttheoremdefinesthetheoreticalmaximumbitrateofanoiselesschannel.

●Q3-8:

WhatdoestheShannoncapacityhavetodowithcommunications?

香农容量原理在通信中的作用？

Estimatethethermalnoiseinterferencechannelcapacity（theoreticalupperlimit）,thedatarateishigher,damagecausedbynoisewillbemoreserious.

☆TheShannoncapacitydefinesthetheoreticalhighestdataratefornoisychannel.

第三章传输介质

●Brieflydescribethemainapplicationsofthreetypesofguidedtransmissionmedia.三种重要的有向传输介质的应用

总的说：

Twisted-paircableisusedforvoiceanddatacommunications.CoaxialcableisusedincableTVnetworksandtraditionalEthernetLANs.Fiber-opticcableisusedinbackbonenetworks,cableTVnetworks,andFastEthernetnetworks.

分别说：

twisted-paircable:

双绞线（广泛用于楼布线）

1.Usedastelephonelinestoprovidevoiceanddatachannels–connectingsubscriberstothecentraltelephoneoffice.

2.UsedasDSL（digitalsubscriberline）looptoprovidehigh-data-rateInternetconnection–thesocalledFamilyBroadBand.

3.UsedinLANstoprovidehigh-data-ratebasebandtransmission（10and100Mbps）.

coaxialcable:

同轴电缆（不用了，目前主要用于有线电视）

1.Waswidelyusedinanalogtelephonenetworks（10000voicesignals）andindigitaltelephonenetworks（datarateupto600Mbps）.

However,ithaslargelybeenreplacedwithfiber-opticcable.2.WaswidelyusedincableTVnetworks

However,cableTVprovidersreplacedmostofcoaxwithfiber-opticcable.

3.WaswidelyusedinearlytraditionalEthernetLANs.

However,ithasbeenreplacedwithUTPwhichhasdatarateupto100Mbps.

☆Now,mainlyusedforCATV

fiber-opticcable:

光纤（广泛用于楼间布线和长途传输）

widelyusedforwiringsbetweenbuildingsorlong-distancetransmission.suchas：

1.High-speedbackbonenetworksbecauseitswidebandwidthiscost-effective.

2.CableTVnetworksuseusuallyacombinationofopticalfiberandcoaxialcable.

3.BackbonechannelsinLANs,CANsandWANs

4.Lightning-proofnetworks

5.Secrecynetworksagainstwiretapping

●Whattypesofopticalfibertransmissionmodesarethere?

Whichofthemissuitableforlongdistancetransmission?

Types:

sigalmodeandmultimode,andthemultimodeincludetwotypes:

Step-indexandGraded-index.

Thesigalmodeissuitableforlongdistancetransmission.

第四章

●WhatisEncoding?

Whymustencodingbeusedfordatatransmission?

（1）【什么是编码？

】Convertinternaldataofasenderintolinesignalssuitablefortransmissioniscalledencoding.

（2）【数据发送时为什么要编码？

】

1）Electromagneticsignalsininformationprocessdevicesarenotallowedtransmittingdirectlyovercommunicationlines.

2）Thesignaltypeofadevicediffersfromthesignaltypeofacommunicationline.

3）Theparameterrequirementsofdevicesignalsdiffersfromtheoneoflinesignals.

4）Specificrequirementsinsynchronization,efficiency,errorcontrol,etc..

★Becauseelectromagneticsignalsininformationprocessdevicesarenotallowedtransmittingdirectlyovercommunicationlines,soencodingmustbeusedfordatatransmissiontoconvertsinternaldataofasenderintolinesignalssuitablefortransmission.

2.3.4.机信号与线路类型不一样；机信号参数与线路传输的要求不一样；同步、效率、纠错等方面的特殊要求

●Whichtwotechniquesareusedwhenanalogdatatransmittedbydigitalsignals?

Whichofthemisthemostimportant?

Why?

用数字信号传输模拟数据常用技术有哪两种？

最常用的是？

为什么？

（1）PulseCodeModulation（PCM）andDeltaModulation（DM）.

（2）PulseCodeModulation（PCM）isthemostimportant.

（3）TheprincipaladvantageofDMoverPCMisthesimplicityofitsimplementation.Ingeneral,PCMexhibitsbetterSNRcharacteristicsatthesamedatarate.

●WhatprocesseswillbeinvolvedwhenanalogdataaredigitizedbyPCMtechnique?

用PCM技术将模拟数据数字化时，要经过哪些处理步骤？

Sampling（PAM）---->quantization---->binaryencoding---->digitaltodigitalcoding

●Q4-1Listthreetechniquesofdigital-to-digitalconversion.

Theyarelinecoding,blockcodingandscramblingcoding.

●Q4-4Definebaselinewanderinganditseffectondigitaltransmission.

（1）Indecodingadigitalsignal,thereceivercalculatesarunningaverageofthereceivedsignalpower.Thisaverageiscalledthebaseline.

（2）Theincomingsignalpowerisevaluatedagainstthisbaselinetodeterminethevalueofthedataelement.

（3）Alongstringof0sor1scancauseadriftinthebaseline（baselinewandering）andmakeitdifficultforthereceivertodecodecorrectly.

●Q4-5DefineaDCcomponentanditseffectondigitaltransmission.

（Define）Whenthevoltagelevelinadigitalsignalisconstantforawhile,thespecturmcreatesverylowfrequencies（resultsofFourieranalysis）.Thesefrequencesaroundzero,calledDC（derect-current）components,thatpresentproblemsforasystemthatcannotpasslowfrequenciesorasystemthatuseselectricalcoupling.

（Effect）DCcomponentmeans0/1paritythatcancausebase-linewondering.直流分量的影响不仅仅是基线偏移，还有更重要的原因。

看看教材和课件，或看看原书作业答案。

●Q4-9Definescramblingandgiveitspurpose.

（Define）Scramblingisawaytoavoidalongsequenceof0sintheoriginalstream,Scramblingisatechniquethatdoesnotincreasethenumberofbitsanddoesprovidesynchronization,Scramblingisasolutionthatsubstituteslongzero-leverpulsewithacombinationofotherlevertoprovidesynchronization.

（Purpose）substituteslongzero-leverpulsewithacombinationofotherlevertoprovidesynchronization.

☆Itspurposeistoprovidesynchronizationwithoutincreasingthenumberofbits.

●Q4-10CompareandcontrastPCMandDM.

♬PCMisthemostcommontechniquetochangeananalogsignaltodigitaldataeditisverycomplex.AlthoughDMisnotperfect,thequantizationerrorofDMismuchlessthanthatforPCM.Ingeneral,PCMexhibitsbetterSNRcharacteristicsatthesamedatarate.

☆Bothofthemareusesamplingtoconverttheanalogsignaltodigitalsignal.PCMfindsthevalueofthesignalamplitudeforeachsample;DMfindsthechangefromtheprevioussample（betweentwoconsecutivesamples）.

第五章

●Whichmodulationtechniquesareusedinanalogtransmission?

Whichofthemisthemostsusceptibletonoise?

Whichofthemisthemechanismcommonlyusedinallmodernmodems.

1.Amplitudeshiftkeying（ASK）

Frequencyshiftkeying（FSK）

Phaseshiftkeying（PSK）

QuadratureAmplitudeModulation（QAM）

2.Themostsusceptibletonoiseofthefourdigital-to-analogconversiontechniquesisASK.

3.ThemechanismcommonlyusedinallmodernmodemsofthemisQAM.

●Q5-1Defineanalogtransmission.

（1）Analogtransmissionreferstothetransmissionofanalogsignalsusingaband-passchannel.Analogtransmissionistheonlychoiceifwehaveabandpasschannel.

（2）Whiledigitaltransmissionisverydesirable,alow-passchannelisneeded.Basebanddigitaloranalogsignalsareconvertedtoacomplexanalogsignalwitharangeoffrequenciessuitableforthechannel.

●Q5-2Definecarriersignalandexplainitsroleinanalogtransmission.

（Define）Asenderproducesananalogs