Chapter 4 Digital Transmission Part1

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Chapter Chapter Digital TransmissionDigital Transmission

(Part 1)(Part 1).

Chapter Chapter 44Digital TransmissionDigital Transmission

(Part 1)(Part 1).

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Chapter 4 TopicsChapter 4 Topics

1.1. IntroductionIntroduction2.2. DigitalDigital toto DigitalDigital ConversionConversion

a)a) LineLine CodingCodingb)b) BlockBlock codingcodingc)c) ScramblingScrambling

33.. AnalogAnalog toto DigitalDigital ConversionConversiona)a) PulsePulse CodeCode Modulation(PCM)Modulation(PCM)b)b) DeltaDelta Modulation(DM)Modulation(DM)

44.. TransmissionTransmission ModesModes55.. DTEDTE andand DCEDCE InterfacesInterfaces

We have three main topics:We have three main topics:

ConversionConversion

ConversionConversionModulation(PCM)Modulation(PCM)

InterfacesInterfaces

We have three main topics:We have three main topics:

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1. Introduction1. Introduction

InIn thisthis chapterchapter wewe discussdiscuss digitaldigital

WeWe havehave twotwo casescasesa)a) IfIf wewe havehave DigitalDigital DataData

digitaldigital transmissiontransmission

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InIn thisthis chapterchapter wewe discussdiscuss digitaldigital

WeWe havehave twotwo casescasesb)b) IfIf wewe havehave AnalogAnalog SignalSignal

digitaldigital transmissiontransmission

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BeforeBefore wewe discussdiscuss thesethese twotwodefinitionsdefinitions

Data ElementData Elementisis thethe smallestsmallest entityentity thatthatcancan representrepresent aa piecepiece ofofinformationinformation (bit)(bit)..isis whatwhat wewe needneed toto sendsend

isis beingbeing carriedcarried

twotwo casecase let’slet’s introduceintroduce somesome

Signal ElementSignal Elementisis thethe shortestshortest unitunit ofof aadigitaldigital signalsignal (voltage(voltagelevel)level)isis whatwhat wewe cancan sendsend

isis thethe carriercarrier

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Ratio (r) Ratio (r) which is the number of data elements which is the number of data elements carried by each signal elementcarried by each signal element

which is the number of data elements which is the number of data elements carried by each signal elementcarried by each signal element

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Real Example:Real Example:SupposeSuppose eacheach datadata elementelementbebe carriedcarried fromfrom oneone placeplace totoaa signalsignal elementelement asas aa carcar thatthatWhenWhen rr == 11,, itit meansmeans eacheach personpersonWhenWhen rr >> 11,, itit meansmeans thatthattravellingtravelling inin aa carcar..WhenWhen rr ==11//22,, itit meansmeans thatthattrailertrailer..

elementelement isis aa personperson whowho needsneeds totototo anotheranother.. WeWe cancan thinkthink ofof

thatthat cancan carrycarry peoplepeoplepersonperson isis drivingdriving aa carcar..

thatthat moremore thanthan oneone personperson

thatthat oneone personperson drivedrive carcar andand

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Data RateData RateIsIs thethe numbernumber ofof datadataelementselements (bits)(bits) sentsent inin llsecsec..TheThe unitunit isis bitsbits perper secondsecond

sometimessometimes calledcalled thethe bitbitraterate

Signal RateSignal RateIsIs thethe numbernumber ofof signalsignalelementselements sentsent inin 11 secsec..

TheThe unitunit isis thethe baudbaud

sometimessometimes calledcalled thethe pulsepulseraterate oror baudbaud raterate

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OneOne goalgoal inin TelecommunicationsTelecommunicationsdatedate raterate ((increasesincreases thethe speedspeeddecreasingdecreasing thethe signalsignal rateraterequirementrequirement))..

RealReal ExampleExample::InIn ourour CarCar--peoplepeople analogy,analogy,peoplepeople inin fewerfewer carscars toto preventpreventaa limitedlimited bandwidthbandwidth inin ourour

TelecommunicationsTelecommunications isis toto increaseincrease thethespeedspeed ofof transmissiontransmission)) whilewhile

((decreasesdecreases thethe bandwidthbandwidth

analogy,analogy, wewe needneed toto carrycarry moremorepreventprevent traffictraffic jamsjams.. WeWe havehave

transportationtransportation systemsystem

TheThe relationshiprelationship betweenbetween DataDatacancan bebe representedrepresented byby::

WhereWhereSS == NoNo.. ofof signalsignal elements,elements,cc == casecase factorfactor (worst,(worst, bestbestNN == datadata raterate (bps),(bps),r r = ratio between data and signal elements.= ratio between data and signal elements.

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DataData RateRate andand SignalSignal RateRate

oror average),average),

= ratio between data and signal elements.= ratio between data and signal elements.

ExampleExample::AA signalsignal isis carryingcarrying datadata ininencodedencoded asas oneone signalsignal elementelement100100 kbps,kbps, whatwhat isis thethe averageaverageifif cc isis betweenbetween 00 andand 11??

SolutionSolution::

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inin whichwhich oneone datadata elementelement isiselementelement (r(r ==11)).. IfIf thethe bitbit raterate isisaverageaverage valuevalue ofof thethe baudbaud raterate

BandwidthBandwidth::DigitalDigital SignalSignal thatthatnonperiodicnonperiodicBandwidthBandwidth ofof aa nonperiodicnonperiodicwithwith anan infiniteinfinite rangerange..DigitalDigital SignalsSignals wewe encounterencounterbandwidthbandwidth withwith finitefinite valuesvaluesTheThe BandwidthBandwidth isis theoreticallytheoreticallyofof thethe componentscomponents havehavecancan bebe ignoredignored.. SoSo TheThefinitefinite

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carriescarries informationinformation isis

nonperiodicnonperiodic signalsignal isis continuouscontinuous

encounterencounter inin realreal lifelife havehave aavaluesvalues..

theoreticallytheoretically infinite,infinite, butbut manymanysmallsmall amplitudeamplitude thatthat theythey

TheThe effectiveeffective bandwidthbandwidth isis

BandwidthBandwidth::

WeWe cancan saysay thatthat thethe baudbauddeterminesdetermines thethe requiredrequiredsignalsignal..

IfIf wewe useuse thethe transportationtransportationcarscars affectsaffects thethe traffic,traffic, notnotbeingbeing carriedcarried..

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baudbaud raterate,, notnot thethe bitbit rate,rate,bandwidthbandwidth forfor aa digitaldigital

transportationtransportation analogy,analogy, thethe numbernumber ofofnotnot thethe numbernumber ofof peoplepeople

BandwidthBandwidth::TheThe BandwidthBandwidth (range(rangeproportionalproportional toto thethe signalsignal raterate

TheThe minimumminimum bandwidthbandwidth cancan

TheThe maximummaximum datadata rateratechannelchannel isis givengiven..

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(range(range ofof frequencies)frequencies) isisraterate (baud(baud rate)rate)..

cancan bebe givengiven asas

ifif thethe bandwidthbandwidth ofof thethe

ExampleExampleTheThe maximummaximum datadata raterate ofofisis (defined(defined byby thethe NyquistNyquist formula)formula)withwith thethe previousprevious formulaformula forforSolutionSolutionAA signalsignal withwith LL levelslevels actuallyactually cancanIfIf eacheach levellevel correspondscorresponds totoassumeassume thethe averageaverage casecase (c(c == 11//

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ofof aa channelchannel (see(see ChapterChapter 33))formula)formula).. DoesDoes thisthis agreeagree

forfor NNmaxmax??

cancan carrycarry loglog22 LL bitsbits perper levellevel..oneone signalsignal elementelement andand wewe

//22),), thenthen wewe havehave

ConsiderationsConsiderationsBeforeBefore discussingdiscussing differentdifferent LineLine CodingCodingconsiderationsconsiderations affectingaffecting onon thethe choicechoice ofof

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a)a) DCDC ComponentsComponents

WhenWhen thethe voltagevoltage levellevel inin aa digitaldigitalthethe frequencyfrequency spectrumspectrum (according(accordingveryvery lowlow frequenciesfrequencies (around(around zero),zero),componentscomponents,, thesethese frequenciesfrequenciesthatthat cannotcannot passpass lowlow frequenciesfrequencies

CodingCoding SchemesSchemes,, wewe willwill discussdiscuss thethe mainmainofof lineline codingcoding schemescheme::

digitaldigital signalsignal isis constantconstant forfor aa while,while,(according(according toto FourierFourier analysis)analysis) createscreates

zero),zero), calledcalled DCDC (direct(direct--current)current)presentpresent problemsproblems forfor aa systemsystem

frequenciesfrequencies

a)a) DCDC ComponentsComponents

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b)b) BaselineBaseline WanderingWandering

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22-- A long string of 0s or 1s can cause a drift in the baseline (baseline wandering) and make it difficult for the receiver to decode correctly

11-- the receiver calculates a running average of the received signal power.(baseline)

A long string of 0s or 1s can cause a drift in the baseline baseline wandering) and make it

difficult for the receiver to decode correctly

the receiver calculates a running average of the received signal power.(baseline)

c)c) TransmissionTransmission powerpower andand bandwidthbandwidth

TheThe transmittedtransmitted powerpower shouldshould bebetransmissiontransmission bandwidthbandwidth needsneedscomparedcompared toto thethe channelchannel bandwidthbandwidth

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bandwidthbandwidth efficiencyefficiency

bebe asas smallsmall asas possible,possible, andand thetheneedsneeds toto bebe sufficientlysufficiently smallsmall

bandwidthbandwidth..

d)d) SelfSelf--synchronizationsynchronizationTheThe receiver‘sreceiver‘s bitbit intervalsintervals mustmustsender'ssender's bitbit intervalsintervals.. IfIf thethe receiverreceiverthethe bitbit intervalsintervals areare notnot matchedmatchedmisinterpretmisinterpret thethe signalssignals..

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mustmust correspondcorrespond exactlyexactly toto thethereceiverreceiver clockclock isis fasterfaster oror slower,slower,

matchedmatched andand thethe receiverreceiver mightmight

d)d) SelfSelf--synchronizationsynchronization

A self-synchronizing digital signal includes timing information in the data being transmitted. This can be achieved if there are transitions in the signal that alert the receiver to the beginning, middle, or end of the pulse. If the receiver's clock is out of synchronization, these points can

reset the clock.

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synchronizing digital signal includes timing information in the data being transmitted. This can be achieved if there are transitions in the signal that alert the receiver to the beginning, middle, or end of the pulse. If the receiver's clock is out of synchronization, these points can

reset the clock.

Line Coding SchemesLine Coding SchemesWeWe cancan dividedivide lineline codingcoding schemesschemesasas shownshown inin thethe figurefigure belowbelow..

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2. Digital to Digital Conversion2. Digital to Digital Conversion

schemesschemes intointo fivefive broadbroad categories,categories,


Unipolar SchemeUnipolar SchemeInIn aa unipolarunipolar scheme,scheme, allall thethe signalsignalthethe timetime axis,axis, eithereither aboveabove oror belowbelow

NRZ (NonNRZ (Non--ReturnReturn--toto--Zero)Zero)

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signalsignal levelslevels areare onon oneone sideside ofofbelowbelow..


Polar SchemesPolar SchemesInIn polarpolar schemes,schemes, thethe voltagesvoltagestimetime axisaxis..NRZ (NonNRZ (Non--ReturnReturn--toto--Zero)Zero)NRZNRZ--LL (NRZ(NRZ--Level)Level) andand NRZNRZ--II

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voltagesvoltages areare onon thethe bothboth sidessides ofof thethe

II (NRZ(NRZ--Invert)Invert)..


RZ (ReturnRZ (Return--toto--Zero)Zero)ItIt usesuses threethree valuesvalues:: positive,positive, negative,negative,signalsignal changeschanges notnot betweenbetween bitsbits

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negative,negative, andand zerozero.. InIn RZ,RZ, thethebitsbits butbut duringduring thethe bitbit..


Biphase: Manchester and Differential ManchesterBiphase: Manchester and Differential ManchesterItIt usesuses threethree valuesvalues:: positive,positive, negative,negative,

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Biphase: Manchester and Differential ManchesterBiphase: Manchester and Differential Manchesternegative,negative, andand zerozero..


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In bipolar encoding, we use three levels: positive, zero, and negative.

Bipolar EncodingBipolar Encoding


In bipolar encoding, we use three levels: positive, zero, and negative.

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Bipolar Bipolar schemes: AMI and schemes: AMI and pseudoternarypseudoternary


pseudoternarypseudoternary

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Bipolar Bipolar schemes: AMI and schemes: AMI and pseudoternarypseudoternaryThe advantages:The advantages:

11-- There is no DC component and no baseline wandering.There is no DC component and no baseline wandering.

22-- singlesingle--errorerror--detection capability .detection capability .

33-- less bandwidth .less bandwidth .

44-- Immunity to noise & other interferences.Immunity to noise & other interferences.



There is no DC component and no baseline wandering.There is no DC component and no baseline wandering.

detection capability .detection capability .

Immunity to noise & other interferences.Immunity to noise & other interferences.

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Bipolar Bipolar schemes: AMI and schemes: AMI and pseudoternarypseudoternaryThe disadvantages:The disadvantages:

11-- There is a synchronization problem .There is a synchronization problem .(It can be solved by Scrambling technique)(It can be solved by Scrambling technique)

22-- Some Complexity.Some Complexity.

Finally, AMI is commonly used for longFinally, AMI is commonly used for long



There is a synchronization problem .There is a synchronization problem .(It can be solved by Scrambling technique)(It can be solved by Scrambling technique)

Finally, AMI is commonly used for longFinally, AMI is commonly used for long--distance communication.distance communication.

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Scrambling:Scrambling: is a technique used in digitalconversion, means modifying part of the rules in line coding scheme to provide bit synchronization.

We have two common scrambling techniques B8ZS and HDB3.


is a technique used in digital-to-digital conversion, means modifying part of the rules in line coding scheme to provide bit synchronization.

We have two common scrambling techniques B8ZS and HDB3.

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B8ZS (Bipolar with 8B8ZS (Bipolar with 8--zero substitution)zero substitution)

B8ZS substitutes eight consecutive zeros with


zero substitution)zero substitution)

B8ZS substitutes eight consecutive zeros with OOOVBOVB.

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B8ZS (Bipolar with 8B8ZS (Bipolar with 8--zero substitution)zero substitution)2. Digital to Digital Conversion2. Digital to Digital Conversion

zero substitution)zero substitution)

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HDB3 (HighHDB3 (High--density bipolar 3 density bipolar 3

HDB3 substitutes four consecutive zeros with 000V or B00V depending

on the number of nonzero pulses after the last substitution.


density bipolar 3 density bipolar 3 --zero)zero)

HDB3 substitutes four consecutive zeros with 000V or B00V depending

on the number of nonzero pulses after the last substitution.

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Chapter 4 Digital Transmission Part1

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