Analog vs. Digital Transmission - WPI

1 Analogvs. DigitalTransmissionCompareat two levels:1. Data|continuous(audio) (text)2. Signaling|continuouslyvaryingelectromagn eticwave |transmitwithoutregardto signalcontent Di erencein how attenuationis handled, improvingdigitaltechnology dataintegrity. Repeaterstake outcumulative problemsin easierto multiplexlargechannelcapacitieswithdigit al easyto applyencryptionto digitaldata betterintegrationif allsignalsarein , theroleplayedby phonecompaniesin datatransmission, , everyoneagreesthat ber istheway to as we prefer ber,analogcommunicationswillbe withus is characterizedby:1. Low bandwidth:It carriesa bandwidthof about3 , thesystemonlyallowssignalsbetween0-3kHzt o passthrough|allhigherfrequenciesarechopp edo .The0-3kHzspectrumcoversthemostimportant frequenciesof humanvoice,which is preciselywhatthephonesystemhasbeendesign edto Higherrorrate:Relative to LANs,theerrorrateis roughly11 ordersof magnitudehigher!

2 Errorsdon'tmatteras much to analogcommunication,especiallywhenvoicei s digitalcommunications,of course,a 1-biterrorcanhave organizedinto a hierarchy: Localphonesareconnectedto acentral o ceover a , onlytwowiresareneededto connectionyourphoneto thephonesystem. The2-wirepathis calledthelocal loop, andis nolongerthan10kmin length. Anareacodeandthe rstthreedigitsof a telephonenumber uniquelyidentifyoraddressa centralo ce. Callsbetweenneighborsconnectedto thesamecentralo cego onlythroughthecentralo calltiesupa circuitin thecentralo smallfractionof thecircuitscanbe usedsimultaneously. Thatis, thephonecompany plansforexpecteduseratherthanworstcaseus e. Centralo cesconnecttotoll o cesovertoll connectingtrunks, which fact,higherbandwidthtrunkscarrymultiple\ voicegrade"linesover a singlephysicalchannel.

3 Tollo cesarethenconnectedin variousways. Fiber is increasinglyconnectingtollo cesandcentralo ces, 'snotcoste ective to replacewiringat trunks,however,many phonecustomersessentiallysharethelines, deviceis amodemto senddigitaldatausinganalogtechnology, thesendergeneratesacarriersignalat somecontinuoustone( phonecircuits)thatlookslike a sinewave. Thefollowingtechniquesareusedto encodedigitaldatainto analogsignals(Fig2-18)Resultingbandwidth is centeredonthecarrierfrequency. amplitude-shiftmodulation(keying): varytheamplitude( )of transmitdigitaldataover optical ber. frequency-shiftmodulation: two (ormoretones)areused,which arenearthecarrierfrequency. Usedin a full-duplexmodem(signalsin bothdirections). phase-shiftmodulation: systematicallyshiftthecarrierwave at instance,thewave couldbe shiftedby 45,135,225,315degreeat each thiscase,each timinginterval carries2 bitsof notshiftby 0, 90,180,270?

4 Shiftingzerodegreesmeansnoshift,andanext endedsetof noshiftsleadsto clock synchronizationdi ,calledQuadrature AmplitudeModulation, hasthefollowingcharacteristics:Lookat : phaseshifts|2-bitencoding.(Quadrature Phase-ShiftKeying) phaseshiftsplusfouramplitudes| getane ective datarateof 9600bps. (Fig2-26). a similarmannerwithamplitude-, reasons: Transmissionmediamay needto usea higherfrequencythanthatusedby thedata(such as voice) Lines\broadband"| phonecompaniesby connectingcustomersto a di erent kindof switchthatdoes notattenuatefrequenciesas is factorof thedistancefromthecustomerto bandwidthinto 4 KHzchannelswithmoreallocatedfordownstrea m(endo ceto home)thanupstream(hometo endo ce).Gethigherdataratesthanstandardphones ervice|upto con splitterandhave a lteroneach justa physicallayer standardallowinghigherspeedaccessto ber opticsinto analogtransmission:1.

5 Analogcircuitsrequireampli ers,andeach ampli eraddsdistortionandnoiseto In contrast,digitalampli ersregenerateanexactsignal, ( Analog )signalis sampled,itsvalueis determined,andthenodethengeneratesa newsignalfromthebitvalue;theincomingsign alis ,intermediatenodesamplifytheincomiingsig nal, Voice,data,video, carriedby analogcircuit?Themodemexampleshowsthedi cultiesincarryingdigitalover simpleencodingmethod is to useconstant voltagelevelsfora \1"anda \0".Canleadto longperiods wherethevoltagedoes SynchronizationWithdigitaltransmission,o neproblemthatcontinuallyarisesisclock mustbe ableto determinewhenonebittimeendsandthenextone starts,sothatit samplesonepulse,ratherthanpartof onepulseandpartof :quartzclocks , thesenderandreceiver' :1. includetiminginformationin thedatasignal2.

6 Usea separatechannel( ,wire)to transmittiminginformationManchesterencod ingis samplingunitinto 2 halves where: a binary\1"is sent as a high-low voltagesequence a \0"is sent as a low-highsequence becauseeach samplingtimecontainsonetransition,therec eiver caneasilysynchronizeitsclock to thesender' a relatedtechnique,di erentialManchesterencoding, a \1"bitis indicatedby theabsenceof a transitionat thestartof thebittime,whilea \0"is indicatedby thepresenceof a of Manchesterencoding: halfthebandwidthis wastedbecauseit takes two transitionsto represent onebitAdvantages: reducedcomplexity of transmitterandreceiver ,endo (coder/decoder)is a devicethatconvertsananalogsignalinto convertanalogsignalsto digitalsignals,many systemsusePulseCodeModulation(PCM):1. PCMsamplesthe4kHzsignal8,000timesper advantageof Nyquist'sresult,samplingthe4kHzbandwidth signalat 2H= 8 thousandtimesper second.

7 (Herewe assumetheuseof a standardvoicegradeline.)2. Each samplemeasurestheamplitudeof thesignal,convertingit into CarrierOnepopularproductis Bell'sT1carrier(Figure2-33)1. It multiplexes24 voicechannelsover , it carries24 voicechannelsat thesametimeover Each of the24 analoginputsis sampledin round-robinfashionanditsn-bitencodingis sent Each encodingconsistsof 7 bitsof sampleddata,plus1 bitof signalinginformation( ,out-of-bandinformation).4. Each voicegradesub-channelcarries(7 bitsX 8000samples)= 56kbpsof data,plus8000bpsof signalinginformation,requiringa digitaldatarateof Samplesaretransmittedin 193-bitunits(frames).6. Each 193-bitframeconsistsof 24 8 + 1 bitsof information;theextrabitofinformationcarr iessynchronizationinformationIt alternatesbetweena \0"and\1"allowingthereceiver to verifythatit is A T1channelhasanaggregatecarryingcapacity of ,CCITT feltthat8 kbpssignalingwas overkill,so theirstandardencodesdigitalsignalsdi erently:1.

8 InCommonChannelSignaling, all8 bitscarrydata,andtheextraframebitis usedto Signalingis yet , ve ofsixsamplescarries8 bitsof data,whileeverysixthsamplecarriessevenbi tof :OtherBellstandardsspecifyhow T1trunksareto be multiplexedover highercapacity trunks,such as:T2( ),T3( )andT4( ).EncodingSystemsIt turnsoutthat8 bitsof datacanbe reducedthroughcompression. For compression,theassumptionis thatthesignalchangesrelativelyslowlycomp aredto thesamplingfrequency:1. Indi erentialpulsecode modulation, each samplecontainsthe(signed)di erencebetweenthecurrent onlyrequires5 bits,andworkswellin practicewithvoicetra sampledi ersby either+/-1 relative to theprevioussample,requiringonlya singlebitto represent each predictwhatthenextsamplewilllooklike,tra nsmittingthedi SwitchingProblem:Givena channelof largecapacity, how does onesubdividethechannelintosmallerlogical channelsforindividualusers?

9 Multiplexmany conversationsover avorsof solution:Frequencydivisionmultiplexing(F DM):Dividethefrequencyspectruminto smallersubchannels,givingeach userexclusive useof a subchannel( ,radioandTV).OneproblemwithFDMis thata useris givenallof thefrequencyto use,andif theuserhasnodatato send,bandwidthis wasted|it cannotbe usedby (TDM):Usetimeslicingto give each userthefullbandwidth,butforonlya fractionof a secondat a time(analogousto timesharingin operatingsystems).Again,if theuserdoesn'thave datato sent duringhistimeslice,thebandwidthis notused( ,wasted).Statistical multiplexing:Allocatebandwidthto cient useof channelbandwidthbecauseit , channelbandwidthis allocatedto packetsthatarewaitingfortransmission,and ausergeneratingnopacketsdoesn'tuseany of we seeanotherreasonwhy usedonthetrunklines,allocating4 kHzto each kHzis consistentlyusable,with500 Hzofguard bandwidthoneach endof channelsis as follows: Bundle12 voicegradelinesinto a unitcalledagroup.

10 A groupcarriessignalsin the60-108kHzspectrum. Combine5 groupsintosupergroup. Combine5 supergroupsinto ,in which datais generatedat a constant rate( ).How welldoes it workforcomputertra c?Notso cis extremelybursty, characterizedby alternatingperiods of techniquecalledcircuitswitching(seeFigur e2-39).1. Oncea callhasbeencompleted,theuserseesa setof \virtualwires" Theusersendsa continuousstreamof data,which thechannelguaranteesto deliverat a DatatransmissionhandledelegantlyusingTDM or predictable|thevoicegradesignalis sampledusingPCMgeneratinga steadystreamof setuprequiredbeforeany datacanbe sent, allowingnetworkto setupthepath,allocatesubchannels, decidewhoto terminationrequiredwhenpartiescompleteca ll,allowingthenetworkto thispoint, a billingrecordis savedsomewherethatrecordswherethecallwas made,itsduration, circuitswitching:1.