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.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(广泛用于楼内布线)
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).
2.However,ithaslargelybeenreplacedwithfiber-opticcable.2.WaswidelyusedincableTVnetworks
3.However,cableTVprovidersreplacedmostofcoaxwithfiber-opticcable.
4.3.WaswidelyusedinearlytraditionalEthernetLANs.
5.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.机内信号与线路类型不一样;机内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.
(Def
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