ch21第二章讲义课件

COMPUTERNETWORKS

Chapter02

ThePhysicalLayerPart12023/2/61ComputerNetworks-02-1Chapter2ThePhysicalLayerEssence:Physicallayerdefinesthemechanical,electrical,andtiminginterfacestothenetwork,providesthemeanstotransmitbitsfromsendertoreceiver,thatis,involvesalotonhowtouse(analog)signalsfordigitalinformation.物理层在点到点连接的两个网络设备之间提供透明的比特流传输。

提供机械，电气，功能及规程特性2023/2/62ComputerNetworks-02-1Chapter2ThePhysicalLayerWhatwillbetalkedabout?TheoreticalanalysisofdatatransmissionTransmissionmedia(wiresandnowires)Threecommunicationsystem:telephone,mobilephone,andcabletelevisionsystem本章介绍的主要内容包括数据通信的基本知识、传输介质以及三种主要的通信系统2023/2/63ComputerNetworks-02-12023/2/65ComputerNetworks-02-1

Tounderstandwhat’sgoingon,weneedFourierAnalysis.AperiodicfunctionwithperiodT(andfrequencyf=1/T),g(t)canbewrittenas:将电压或电流表示成为时间的单值函数f(t)。这样就可以用数学的方法来描述信号的变化，并对其进行数学分析。19世纪中叶，法国数学家傅立叶证明，任何正常的周期为T的函数g(t)，都可以由无限个正弦和余弦函数组成。2023/2/66ComputerNetworks-02-1Example:01100010,8bitforASCII“b”an

=1/πn[cos(πn/4)-cos(3πn/4)+cos(6πn/4)-cos(7πn/4)]bn

=1/πn[sin(3πn/4)-sin(πn/4)+sin(7πn/4)-sin(6πn/4)]cn

=3/4Therootmeansquareamplitudes

is(an2+bn2)1/Bandwidth-LimitedSignals2023/2/67ComputerNetworks-02-1BandwidthWhatdoesthisallmean?Digitalsignaltransmissioncanbethoughtofasbeingconstructedasaninfinitenumberofperiodicanalogsignals.Thequalityoftransmissionisfrequencydependent,notallpartsofthedigitalsignalgetthroughthewireasyouwouldexpect.Digitalsignaltransmissionissubjecttoattenuation衰减,distortion失真,etc.Thisispartlycausedbydisallowinghigh-frequencycomponentstopassthrough.Sotherangeoffrequencyorthenumberofbitsofatransmissionmediumiscalledbandwidth.2023/2/69ComputerNetworks-02-1Example:(Wearetryingtotransmitasinglebyte):Withabitrateofbbits/sec,ittakes8/bsecondstosendabyte.Thefrequencyf1ofthefirstharmonicisb/8HzAssumption:Weareusingasimpleencodingtechniquebasedonthefactthatthelinesupportsonlytwosignalvalues.bpsT(msec)Firstharmonic(Hz)#harmonicssent30026.6737.58060013.3375.04012006.67150.02024003.33300.01048001.67600.0596000.831200.02192000.422400.01384000.214800.002023/2/610ComputerNetworks-02-1Whatisbaudrate:Thenumberchangesinasignalpersecondiscalledthebaudrate,B=1/T.Whatisbitrate:Thenumberofbitstransmittedpersecondiscalledthebitrate,bitspersecond,b/sorbps.

Whatistherelationshipbetweenbitrateandbaudrate:s=B*log2v=1/T*log2v

wheresisbitrate,Bisbaudrate,Tisfrequencyperiod,visnumberofsignalvalues.Ex:B=9600,v=2,s=9600bps;B=2400,v=16,s=9600bpsBaud-rate波特率andBit-rate比特率2023/2/611ComputerNetworks-02-1Wheredowetalkaboutbitrateandbaudrate?DTEDCEDCEDTETransmissionmediumBaudrateBitrateinterfaceinterface编码效率＝比特率/波特率2023/2/613ComputerNetworks-02-12.1.3ThemaximumdatarateofachannelNyquisttheory（noiselesschannels）Nyquistshowedthatifthecut-offfrequencyisHHz,thefilteredsignalcanbereconstructedbymaking2Hsamples.Nomore,noless.Consequence:Maximumtransmissionrate=2Hlog2Vbps(whereVisthenumberofsignalvalues)有限带宽、无噪声信道的最大数据传输率信号的最佳采样频率2H2023/2/614ComputerNetworks-02-1Shannon（noisychannel）showedthatanoisychannelwithasignal-to-noiseratioS/N,hasalimitwithrespecttothebitrate:Maximumtransmissionrate=Hlog2(1+S/N)Inpractice,weusedB(decibel)torepresentsignal-to-noiseratioinsteadofS/N:10log10(S/N)=?dB

1948年，香农把尼奎斯特的工作扩大到信道受到随机（热）噪声干扰的情况。噪声分为：系统内和系统外（人为及非人为）噪声。2023/2/615ComputerNetworks-02-1模拟信号和数字信号的发送

模拟信号发送： 模拟数据（声音） 模拟信号 数字数据（二进制脉冲） 模拟信号

数字信号发送： 模拟数据 数字信号 数字数据（二进制脉冲） 数字信号

电话系统调制解调器MODEM编码解码器CODEC数字编码解码器2023/2/617ComputerNetworks-02-12.2GuidedTransmissionMediaMagneticMedia磁介质NeverunderestimatethebandwidthofastationwagonfulloftapeshurtlingdownthehighwayExampleCosts:roughly$5000toship200TB,3centsfor1GB2023/2/618ComputerNetworks-02-1CopperWiresTwistedpair双绞线Twoinsulatedcopperwires,twistedlikeaDNAstring(reduceselectricalinference).Often,twistedpairsgobythebundle.Comparabletotelephonewiringathome.Shielded(STP)TwistedPair屏蔽双绞线Unshielded(UTP)TwistedPair无屏蔽双绞线

Category3:nottouseanymoreCategory5/SuperCategory5Category6/7:forGigabittransmission2023/2/619ComputerNetworks-02-1CoaxialCable同轴电缆ExactlyliketheoneyouuseforyourTVSet:BasebandCoax基带50-ohmcablefordigitaltransmission10Base-2,BNC,Thin-LAN,185m/persegment10Base-5,AUI,Thick-LAN,500m/persegmentAtmost5segments,upto945m/2500m.BroadbandCoax频带75-ohmcableforanalogtransmission,likecableTV（CATV）.2023/2/621ComputerNetworks-02-12023/2/622ComputerNetworks-02-1Principle:Ratherthanusingelectricalsignals,weuseopticalonesthatarepassedthroughopticalfiber.Principalworkingisbasedontherefractionpropertyoflight:(a)三个例子：一束光按照不同角度从氧化硅内部射到空气/氧化硅的界面上.(b)通过完全的内部反射，光线在光纤内部传播.FiberOptics光纤2023/2/623ComputerNetworks-02-1ST：插入锁定光耦合器（ST）SC：568A标准，方形，插入锁定2023/2/625ComputerNetworks-02-1FiberConnectionsObservation:Aninterfaceconsistsofareceiver(photodiode)whichtransformslightintoelectricalsignals,and/oratransmitter(LEDorlaser-diode)Passiveinterface:无源接口AcomputerisdirectlyconnectedtotheopticalfiberActiveinterface:有源接口

There’sanordinaryelectricalrepeaterconnectedtotwofibersegmentsandthecomputer:2023/2/626ComputerNetworks-02-12023/2/627ComputerNetworks-02-12.3WirelessTransmissionTheelectromagneticspectrum电磁波谱Wirelesstransmissionstravelatthespeedoflight(c),usesafrequency(f)whichhasawavelength(l).Therelationisthat:

c=l*f

cisaconstant,approximately3x108m/sec,thatistheoreticalinvacuum.Inpractice,cisabout2x108m/seceitherinfiberorcopper.Thatis200m/μsec.2023/2/629ComputerNetworks-02-1Thelargerthewavelengthis,thelongerthedistanceitcantravelwithoutattenuation.Also,thedispersionofhigherfrequenciesismuchlower.Fig.2-11Theelectromagneticspectrumanditsusesforcommunication2023/2/630ComputerNetworks-02-1Note:WecanencodeonlyafewbitsperHertzinthelowfrequencyrange,butmuchmoreinthehigherranges.Thismeansthatwirelesstransmissionwillgenerallyhaveamuchlowerbandwidth(inpractice:1-2Mbps).电磁波可以承载的信息量与其波长有关。Observation:Fiberopticsoperateinthehighfrequencyrange,whichexplainsthetransmissionratesofgigabitspersecond.光纤的优势Ifgiventhewidthofawavelengthband∆λ,wecancomputethecorrespondingfrequencyband∆f,andfromthatthedataratethebandcanproduce.2023/2/631ComputerNetworks-02-1Conclusion:thewidertherange,andtheshorterthewavelength,thehigherthebandwidth.

Example:Fiberopticsoftenworkatl=1.3*10-6withDl=0.16*10-6leadingto30THzbandwidth!Observation:Radiotransmission(VLF–VHF)isextremelypopularforitscheapnessandrange.Also,wavesjustgoallovertheplace.2023/2/632ComputerNetworks-02-1Fig.2-12.(a)在VLF,VF,MF波段，无线电波沿着地面传播.(b)在HF波段，无线电波被电离层折射回来.Observation:Microwavetransmissionisalsopopularandisgoodforlongdistances,aslongasit’sdirected.2023/2/633ComputerNetworks-02-1MicrowaveTransmissionWavestravelinstraightlinesandcanbenarrowlyfocusedabove100MHz,

concentratingenergyintoasmallbeamusingdish-likeantenna.Repeatersareneededperiodically,for100mhightowers,repeaterscanbespaced80kmapart.Multipath-fadingisweatherandfrequencydependent.Frequencyrange:2.4GHz-2.484GHz,noFCC(federalcommunicationcommission)licensingneeded.微波沿直线传输，每隔一定距离需要中继器，存在多径衰减问题。频率范围：2.4GHz-2.484GHz。2023/2/634ComputerNetworks-02-1InfraredandMillimeterWaves

LightWaveTransmissionFig.2-14.Convectioncurrentscaninterferewithlasercommunicationsystems.Abidirectionalsystem,with