DigitalTransmissionFundamentals.ppt

1 chapter4digitaltransmissionfundamentals 4 1digitalrepresentationofinformation4 2whydigitalcommunications 4 3digitalrepresentationofanalogsignals4 4characterizationofcommunicationchannels4 5fundamentallimitsindigitaltransmission4 6linecoding4 7modemsanddigitalmodulation4 8propertiesofmediaanddigitaltransmissionsystems4 9errordetectionandcorrection 2 theseven layerosireferencemodel applicationlayer presentationlayer sessionlayer transportlayer networklayer datalinklayer physicallayer applicationlayer presentationlayer sessionlayer transportlayer networklayer datalinklayer physicallayer networklayer application application datalinklayer physicallayer networklayer datalinklayer physicallayer communicatingendsystems oneormorenetworknodes end to endprotocols 3 physicallayer transfersbitsacrosslinkdefinition specificationofthephysicalaspectsofacommunicationslinkethernet dsl cablemodem telephonemodems twisted paircable coaxialcableopticalfiber radio infrared 4 chapter4digitaltransmissionfundamentals digitalrepresentationofinformation 5 blockvs streaminformation blockinformationthatoccursinasingleblocksize bits blockorbytes block1kbyte 210bytes1mbyte 220bytes1gbyte 230bytes streaminformationthatisproduced transmittedcontinuouslybitrate bits second1kbps 103bps1mbps 106bps1gbps 109bps 6 digitizationofanalogsignal sampleanalogsignalintimeandamplitudefindclosestapproximation originalsignal samplevalue approximation rs bitrate bits samplex samples second 3bits sample 7 bitrateofdigitizedsignal bandwidthwshertz higherbandwidth morefrequentsamplesminimumsamplingrate 2xwsrepresentationaccuracy rangeofapproximationerrorhigheraccuracy smallerspacingbetweenapproximationvalues morebitspersample 8 example voice audio telephonevoicews 4khz 8000samples sec8bits samplers 8x8000 64kbpscellularphonesusemorepowerfulcompressionalgorithms 8 12kbps cdaudiows 22khertz 44000samples sec16bits samplers 16x44000 704kbpsperaudiochannelmp3usesmorepowerfulcompressionalgorithms 50kbpsperaudiochannel 9 chapter4communicationnetworksandservices whydigitalcommunications 10 atransmissionsystem transmitter convertsinformationintosignalsuitablefortransmissioninjectsenergyintocommunicationsmediumorchannelreceiver receivesenergyfrommediumconvertsreceivedsignalintoformsuitablefordeliverytouser 11 transmissionimpairments communicationchannelpairofcopperwirescoaxialcableradiolightinopticalfiberlightinairinfrared transmissionimpairmentssignalattenuationsignaldistortionspuriousnoiseinterferencefromothersignals 12 analoglong distancecommunications eachrepeaterattemptstorestoreanalogsignaltoitsoriginalformrestorationisimperfectdistortionisnotcompletelyeliminatednoise interferenceisonlypartiallyremovedsignalqualitydecreaseswith ofrepeaterscommunicationsisdistance limited 13 analogvs digitaltransmission analogtransmission alldetailsmustbereproducedaccurately sent received received distortionattenuation digitaltransmission onlydiscretelevelsneedtobereproduced distortionattenuation simplereceiver wasoriginalpulsepositiveornegative 14 digitallong distancecommunications regeneratorrecoversoriginaldatasequenceandretransmitsonnextsegmentcandesignsoerrorprobabilityisverysmalltheneachregenerationislikethefirsttime communicationsispossibleoververylongdistancesdigitalsystemsvs analogsystemslesspower longerdistances lowersystemcostmonitoring multiplexing coding encryption protocols 15 digitalbinarysignal foragivencommunicationsmedium howdoweincreasetransmissionspeed howdoweachievereliablecommunications aretherelimitstospeedandreliability bitrate 1bit tseconds 16 pulsetransmissionrate objective maximizepulseratethroughachannel thatis maketassmallaspossible channel t t pulsefrequentlywithoutinterferingwitheachother 2xwcpulses secondforlowpasschannelwcpulses secondforbandpasschannelwcisthebandwidthofthechannel t 17 multilevelpulsetransmission ifpulsesamplitudesareeither aor a theneachpulseconveys1bit sobitrate 1bit pulsex2wcpulses sec 2wcbpsifamplitudesarefrom a a 3 a 3 a thenbitrateis2x2wcbpsbygoingtom 2mamplitudelevels weachievebitrate mbits pulsex2wcpulses sec 2mwcbps 18 noise reliablecommunications allphysicalsystemshavenoisepresenceofnoiselimitsaccuracyofmeasurementofreceivedsignalamplitudeerrorsoccurifsignalseparationiscomparabletonoiselevelbiterrorrate ber increaseswithdecreasingsignal to noiserationoiseplacesalimitonhowmanyamplitudelevelscanbeusedinpulsetransmission 19 snr averagesignalpower averagenoisepower snr db 10log10snr signal to noiseratio error noerrors 20 arbitrarilyreliablecommunicationsispossibleifthetransmissionraterc thenarbitrarilyreliablecommunicationsisnotpossible arbitrarilyreliable meansthebercanbemadearbitrarilysmallthroughsufficientlycomplexcoding ccanbeusedasameasureofhowcloseasystemdesignistothebestachievableperformance shannonchannelcapacity c wclog2 1 snr bps 21 example findtheshannonchannelcapacityforatelephonechannelwithwc 3400hzandsnr 10000c 3400log2 1 10000 3400log10 10001 log102 45200bpsnotethatsnr 10000correspondstosnr db 10log10 10001 40db 22 bitratesofdigitaltransmissionsystems 23 examplesofchannels 24 chapter4digitaltransmissionfundamentals digitalrepresentationofanalogsignals 25 interpolationfilter a b nyquist perfectreconstructionifsamplingrate1 t 2ws samplingtheorem 26 digitaltransmissionofanaloginformation 27 quantizationerror noise x nt y nt quantizermapsinputintoclosestof2mrepresentationvalues quantizationofanalogsamples 28 w 4khz sonyquistsamplingtheorem 2w 8000samples second8bits samplepcm pulsecodemodulation telephonespeech bitrate 8000 x8bits sec 64kbps example telephonespeech 29 chapter4digitaltransmissionfundamentals characterizationofcommunicationchannels 30 communicationschannel signalbandwidthtransferdatafaster asignalvariesmorequickly channelbandwidthachannelormediumhasaninherentlimitonhowfastthesignalsitpassescanvary transmissionimpairmentssignalattenuationsignaldistortionspuriousnoiseinterferencefromothersignals transmittedsignal receivedsignal receiver communicationchannel transmitter 31 ideallow passfilter idealfilter allsinusoidswithfrequencyf wcarepassedwithoutattenuationanddelayedbytseconds sinusoidsatotherfrequenciesareblocked amplituderesponse wc y t aincos 2 ft 2 ft aincos 2 f t t x t t 32 example low passfilter simplestnon idealcircuitthatprovideslow passfiltering 33 bandpasschannel somechannelspasssignalswithinabandthatexcludeslowfrequenciestelephonemodems radiosystems channelbandwidthisthewidthofthefrequencybandthatpassesnon negligiblesignalpower 34 chapter4digitaltransmissionfundamentals fundamentallimitsindigitaltransmission 35 noisedistribution noiseischaracterizedbyprobabilitydensityofamplitudesamplesnoisedistributionisgaussian bell shaped asbelow t pr x t x0 pr x t x0 areaundergraph x0 x0 s2 avgnoisepower 36 probabilityoferror erroroccursifnoisevalueexceedscertainmagnitudeprob oflargevaluesdropsquicklywithgaussiannoisetargetprobabilityoferrorachievedbydesigningsystemsoseparationbetweensignallevelsisappropriaterelativetoaveragenoisepower pr x t d 37 chapter4digitaltransmissionfundamentals linecoding 38 whatislinecoding mappingofbinaryinformationsequenceintothedigitalsignalthatentersthechannelex 1 mapsto asquarepulse 0 to apulselinecodeselectedtomeetsystemrequirements lowfrequencycontent somechannelsblocklowfrequencieslongperiodsof aorof acausessignalto droop waveformshouldnothavelow frequencycontent 39 linecodeselectedtomeetsystemrequirements transmittedpower powerconsumption bittiming transitionsinsignalhelptimingrecoverybandwidthefficiency excessivetransitionswastesbandwidth complexity cost iscodeimplementableinchipathighspeed errordetection abilitytodetecterrorshelps 40 unipolar polarnon return to zero nrz unipolarnrz 1 mapsto apulse 0 mapstonopulsehighaveragepower0 5 a2 0 5 02 a2 2longstringsofaor0poortiminglow frequencycontentsimple polarnrz 1 mapsto a 2pulse 0 mapsto a 2pulsebetteraveragepower0 5 a 2 2 0 5 a 2 2 a2 4longstringsof a 2or a 2poortiminglow frequencycontentsimple unipolarnrz polarnrz 41 bipolarcode threesignallevels a 0 a 1 mapsto aor ainalternation 0 mapstonopulseevery pulsematchedby pulsesolittlecontentatlowfrequenciesstringof1sproducesasquarewavespectrumcenteredatt 2longstringof0scausesreceivertolosesynchzero substitutioncodes bipolarencoding 42 manchestercode mbnbcodes 1 mapsintoa 2firstt 2 a 2lastt 2 0 mapsinto a 2firstt 2 a 2lastt 2everyintervalhastransitioninmiddletimingrecoveryeasyusesdoubletheminimumbandwidthsimpletoimplementusedin10 mbpsethernet otherlanstandards mbnblinecodemapsblockofmbitsintonbitsmanchestercodeis1b2bcode4b5bcodeusedinfddilan8b10bcodeusedingigabitethernet64b66bcodeusedin10gethernet manchesterencoding 43 differentialcoding errorsinsomesystemscausetranspositioninpolarity abecome aandviceversaallsubsequentbitsinpolarnrzcodingwouldbeinerrordifferentiallinecodingprovidesrobustnesstothistypeoferror 1 mappedintotransitioninsignallevel 0 mappedintonotransitioninsignallevelsamespectrumasnrzerrorsoccurinpairsalsousedwithmanchestercoding nrz inverted differentialencoding differentialmanchesterencoding 44 linecodingexamples 45 spectrumoflinecodes assume1s 0sindependent equiprobable nrzhashighcontentatlowfrequenciesbipolartightlypackedaroundt 2manchesterwastefulofbandwidth 46 chapter4digitaltransmissionfundamentals modemsanddigitalmodulation 47 bandpasschannels bandpasschannelspassarangeoffrequenciesaroundsomecenterfrequencyfcradiochannels telephone dslmodemsdigitalmodulatorsembedinformationintowaveformwithfrequenciespassedbybandpasschannelsinusoidoffrequencyfciscenteredinmiddleofbandpasschannelmodulatorsembedinformationintoasinusoid fc wc 2 fc 0 fc wc 2 48 information 1 1 amplitudeshiftkeying 1 1 frequencyshiftkeying t t amplitudemodulationandfrequencymodulation mapbitsintoamplitudeofsinusoid 1 sendsinusoid 0 nosinusoiddemodulatorlooksforsignalvs nosignal mapbitsintofrequency 1 sendfrequencyfc d 0 sendfrequencyfc ddemodulatorlooksforpoweraroundfc dorfc d 49 phasemodulation mapbitsintophaseofsinusoid 1 sendacos 2pft i e phaseis0 0 sendacos 2pft p i e phaseispequivalenttomultiplyingcos 2pft by aor a 1 sendacos 2pft i e multiplyby1 0 sendacos 2pft p acos 2pft i e multiplyby 1wewillfocusonphasemodulation information 50 modulator demodulator 51 information basebandsignal modulatedsignalx t exampleofmodulation acos 2pft acos 2pft 52 recoveredinformation basebandsignaldiscernableaftersmoothing aftermultiplicationatreceiverx t cos 2pfct a a 0 t 2t 3t 4t 5t 6t exampleofdemodulation a 1 cos 4pft a 1 cos 4pft 53 chapter4digitaltransmissionfundamentals propertiesofmediaanddigitaltransmissionsystems 54 fundamentalissuesintransmissionmedia informationbearingcapacityamplituderesponsen 2 0 x108m secinopticalfiber 55 electromagneticspectrum 56 electromagneticspectrum frequencyofcommunicationssignals analogtelephone dsl cellphone wifi opticalfiber 57 wirelessmedia signalenergypropagatesinspace limiteddirectionalityinterferencepossible sospectrumregulatedlimitedbandwidthsimpleinfrastructure antennas transmittersnophysicalconnectionbetweennetwork useruserscanmove 58 wiredmedia signalenergycontained guidedwithinmediumspectrumcanbere usedinseparatemedia wiresorcables morescalableextremelyhighbandwidthcomplexinfrastructure ducts conduits poles right of way 59 attenuation attenuationvarieswithmediawiredmediahasexponentialdependencereceivedpoweratdmetersproportionalto10 kdattenuationindb kd wherekisdb meterwirelessmediahaslogarithmicdependencereceivedpoweratdmetersproportionaltod nattenuationindb nlogd wherenispathlossexponent n 2infreespace 60 twistedpair twoinsulatedcopperwiresarrangedinaregularspiralpatterntominimizeinterferencelowcosttelephonesubscriberloopfromcustomertoco lowerattenuationrateanalogtelephone higherattenuationratefordsl 61 twistedpairbitrates dataratesof24 gaugetwistedpair 62 ethernetlans category3unshieldedtwistedpair utp ordinarytelephonewirescategory5utp tightertwistingtoimprovesignalqualityshieldedtwistedpair stp tominimizeinterference costly10base tethernet100base t4fastethernetcat5 stpprovideotheroptions 63 coaxialcable cylindricalbraidedouterconductorsurroundsinsulatedinnerwireconductorhighinterferenceimmunityhigherbandwidththantwistedpairhundredsofmhz 64 opticalfiber lightsources lasers leds generatepulsesoflightthataretransmittedonopticalfiberverylongdistances 1000km veryhighspeeds 40gbps wavelength nearlyerror free berof10 15 profoundinfluenceonnetworkarchitecture 65 geometryofopticalfiber totalinternalreflectioninopticalfiber transmissioninopticalfiber corehashigherindexofrefractionthancladdinglightraysincidentatlessthancriticalangleqciscompletelyreflectedbackintothecore 66 multimode thickercore raysondifferentpathsinterferecausingdispersion limitingbitratesinglemode verythincore moreexpensivelasers butachievesveryhighspeeds multimode single modefiber 67 opticalfiberproperties advantagesverylowattenuationnoiseimmunityextremelyhighbandwidthsecurity verydifficulttotapwithoutbreakingnocorrosionmorecompact lighterthancopperwiredisadvantagesnewtypesofopticalsignalimpairments dispersionlimitedbendradiusdifficulttosplice 68 verylowattenuation 850nmlow costledslans 1300nmmetropolitanareanetworks shorthaul 1550nmlong