ch21第二章講義課件

COMPUTERNETWORKS

Chapter02

ThePhysicalLayerPart12023/2/61ComputerNetworks-02-1Chapter2ThePhysicalLayerEssence:Physicallayerdefinesthemechanical,electrical,andtiminginterfacestothenetwork,providesthemeanstotransmitbitsfromsendertoreceiver,thatis,involvesalotonhowtouse(analog)signalsfordigitalinformation.物理層在點到點連接的兩個網絡設備之間提供透明的比特流傳輸。

提供機械，電氣，功能及規(guī)程特性2023/2/62ComputerNetworks-02-1Chapter2ThePhysicalLayerWhatwillbetalkedabout?TheoreticalanalysisofdatatransmissionTransmissionmedia(wiresandnowires)Threecommunicationsystem:telephone,mobilephone,andcabletelevisionsystem本章介紹的主要內容包括數據通信的基本知識、傳輸介質以及三種主要的通信系統(tǒng)2023/2/63ComputerNetworks-02-12023/2/65ComputerNetworks-02-1

Tounderstandwhat’sgoingon,weneedFourierAnalysis.AperiodicfunctionwithperiodT(andfrequencyf=1/T),g(t)canbewrittenas:將電壓或電流表示成為時間的單值函數f(t)。這樣就可以用數學的方法來描述信號的變化，并對其進行數學分析。19世紀中葉，法國數學家傅立葉證明，任何正常的周期為T的函數g(t)，都可以由無限個正弦和余弦函數組成。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衰減,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編碼效率＝比特率/波特率2023/2/613ComputerNetworks-02-12.1.3ThemaximumdatarateofachannelNyquisttheory（noiselesschannels）Nyquistshowedthatifthecut-offfrequencyisHHz,thefilteredsignalcanbereconstructedbymaking2Hsamples.Nomore,noless.Consequence:Maximumtransmissionrate=2Hlog2Vbps(whereVisthenumberofsignalvalues)有限帶寬、無噪聲信道的最大數據傳輸率信號的最佳采樣頻率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，香農把尼奎斯特的工作擴大到信道受到隨機（熱）噪聲干擾的情況。噪聲分為：系統(tǒng)內和系統(tǒng)外（人為及非人為）噪聲。2023/2/615ComputerNetworks-02-1模擬信號和數字信號的發(fā)送

模擬信號發(fā)送： 模擬數據（聲音） 模擬信號 數字數據（二進制脈沖） 模擬信號

數字信號發(fā)送： 模擬數據 數字信號 數字數據（二進制脈沖） 數字信號

電話系統(tǒng)調制解調器MODEM編碼解碼器CODEC數字編碼解碼器2023/2/617ComputerNetworks-02-12.2GuidedTransmissionMediaMagneticMedia磁介質NeverunderestimatethebandwidthofastationwagonfulloftapeshurtlingdownthehighwayExampleCosts:roughly$5000toship200TB,3centsfor1GB2023/2/618ComputerNetworks-02-1CopperWiresTwistedpair雙絞線Twoinsulatedcopperwires,twistedlikeaDNAstring(reduceselectricalinference).Often,twistedpairsgobythebundle.Comparabletotelephonewiringathome.Shielded(STP)TwistedPair屏蔽雙絞線Unshielded(UTP)TwistedPair無屏蔽雙絞線

Category3:nottouseanymoreCategory5/SuperCategory5Category6/7:forGigabittransmission2023/2/619ComputerNetworks-02-1CoaxialCable同軸電纜ExactlyliketheoneyouuseforyourTVSet:BasebandCoax基帶50-ohmcablefordigitaltransmission10Base-2,BNC,Thin-LAN,185m/persegment10Base-5,AUI,Thick-LAN,500m/persegmentAtmost5segments,upto945m/2500m.BroadbandCoax頻帶75-ohmcableforanalogtransmission,likecableTV（CATV）.2023/2/621ComputerNetworks-02-12023/2/622ComputerNetworks-02-1Principle:Ratherthanusingelectricalsignals,weuseopticalonesthatarepassedthroughopticalfiber.Principalworkingisbasedontherefractionpropertyoflight:(a)三個例子：一束光按照不同角度從氧化硅內部射到空氣/氧化硅的界面上.(b)通過完全的內部反射，光線在光纖內部傳播.FiberOptics光纖2023/2/623ComputerNetworks-02-1ST：插入鎖定光耦合器（ST）SC：568A標準，方形，插入鎖定2023/2/625ComputerNetworks-02-1FiberConnectionsObservation:Aninterfaceconsistsofareceiver(photodiode)whichtransformslightintoelectricalsignals,and/oratransmitter(LEDorlaser-diode)Passiveinterface:無源接口AcomputerisdirectlyconnectedtotheopticalfiberActiveinterface:有源接口

There’sanordinaryelectricalrepeaterconnectedtotwofibersegmentsandthecomputer:2023/2/626ComputerNetworks-02-12023/2/627ComputerNetworks-02-12.3WirelessTransmissionTheelectromagneticspectrum電磁波譜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).電磁波可以承載的信息量與其波長有關。Observation:Fiberopticsoperateinthehighfrequencyrange,whichexplainsthetransmissionratesofgigabitspersecond.光纖的優(yōu)勢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，無線電波沿著地面?zhèn)鞑?(b)在HF波段，無線電波被電離層折射回來.Observation:Microwavetransmissionisalsopopularandisgoodforlongdistances,aslongasit’sdirected.2023/2/633ComputerNetworks-02-1MicrowaveTransmissionWavestravelinstraightlinesandcanbenarrowlyfocusedabove100MHz,

concentratingenergyintoasmallbeamusingdish-likeantenna.Repeatersareneededperiodically,for100mhightowers,repeaterscanbespaced80kmapart.Multipath-fadingisweatherandfrequencydependent.Frequencyrange:2.4GHz-2.484GHz,noFCC(federalcommunicationcommission)licensingneeded.微波沿直線傳輸，每隔一定距離需要中繼器，存在多徑衰減問題。頻率范圍：2.4GHz-2.484GHz。2023/2/634ComputerNetworks-02-1InfraredandMillimeterWaves

LightWaveTransmissionFig.2-14.Convectioncurrentscaninterferewithlasercommunicationsystems.Abidirectionalsystem,with