Internel I -Data & Communications

8/3/2019 Internel I -Data & Communications

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1)DefineAnalogSignalswithanexample?Thetermanalogdatareferstoinformationthatiscontinuous; Forexample,ananalogclockthathashour,minute,andsecondhandsgivesinformationinacontinuousform;the

movementsofthehandsarecontinuous.

Analogdata,suchasthesoundsmadebyahumanvoice,takeoncontinuousvalues. When

someonespeaks,ananalogwaveiscreatedintheair.Thiscanbecapturedbyamicrophone

andconvertedtoananalogsignalorsampledandconvertedtoadigitalsignal.

PeriodicandNonperiodicSignals

Aperiodicsignalcompletesapatternwithinameasurabletimeframe,calledaperiod,and

repeatsthatpatternoversubsequentidenticalperiods.Thecompletionofonefullpatternis

calledacycle.

A

non

periodic

signal

changes

without

exhibiting

apattern

or

cycle

that

repeats

overtime.

Bothanaloganddigitalsignalscanbeperiodicornonperiodic.Indatacommunications,we

commonlyuseperiodicanalogsignals(becausetheyneedlessbandwidth,andnonperiodic

digitalsignals(becausetheycanrepresentvariationindata.

Examplesofanalogtechnology:

1. photocopiers2. telephones3. audiotapes4. televisions(intensityandcolorinformationperscanline)5. VCRs(sameasTV)


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2)ExplainInterconnectionNetworks?InterconnectionofNetworks:InternetworkToday,

it

is

very

rare

to

see

aLAN,

aMAN,

or

aLAN

in

isolation;

they

are

connected

to

one

another.

Whentwoormorenetworksareconnected,theybecomeaninternetwork,orinternet.Asan

example,assumethatanorganizationhastwooffices,oneontheeastcoastandtheotheronthe

westcoast.TheestablishedofficeonthewestcoasthasabustopologyLAN;thenewlyopened

officeontheeastcoasthasastartopologyLAN.Thepresidentofthecompanylivessomewherein

themiddleandneedstohavecontroloverthecompanyfromherhorne.Tocreateabackbone

WANforconnectingthesethreeentities(twoLANsandthepresident'scomputer),aswitchedWAN

(operatedbyaserviceprovidersuchasatelecomcompany)hasbeenleased.ToconnecttheLANs

tothisswitchedWAN,however,threepointtopointWANsarerequired.Thesepointtopoint

WANscanbeahighspeedDSLlineofferedbyatelephonecompanyoracablemodernlineoffered

byacableTVproviderasshownin

Figure:AheterogeneousnetworkmadeoffourWANsandtwoLANs


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DIGITAL~TO~DIGITALCONVERSIONAsweknowthatdatacanbeeitherdigitaloranalog.Andthesignalsthatrepresentthedata

canalso

be

digital

or

analog.

Here,

we

can

represent

digital

data

by

using

digital

signals.

The

DigitalToDigitalconversioninvolvesthreetechniques:

1) Linecoding2) Blockcoding,3) Scrambling.

Linecodingisalwaysneeded~blockcodingandscramblingmayormaynotbeneeded.

LineCodingLinecodingistheprocessofconvertingdigitaldatatodigitalsignals.Weassumethatdata,in

theform

of

text,

numbers,

graphical

images,

audio,

or

video,

are

stored

in

computer

memory

as

sequencesofbits.Linecodingconvertsasequenceofbitstoadigitalsignal.Atthesender,

digitaldataareencodedintoadigitalsignal;atthereceiver,thedigitaldataarerecreatedby

decodingthedigitalsignal.

Figure:Linecodinganddecoding

LineCodingCharacteristics:SignalElementVersusDataElementLetusdistinguishbetweenadataelementandasignalelement.Indatacommunications,ourgoalistosenddataelements.Adataelementisthe

smallestentity

that

can

represent

apiece

of

information:

this

is

the

bit.

In

digital

data

communications,asignalelementcarriesdataelements.Asignalelementistheshortestunit

(timewise)ofadigitalsignal.Inotherwords,dataelementsarewhatweneedtosend;signal

elementsarewhatwecansend.Dataelementsarebeingcarried;signalelementsarethe

carriers.

Wedefinearatiorwhichisthenumberofdataelementscarriedbyeachsignalelement.


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Figure:SignalelementversusdataelementShowsseveralsituationswithdifferentvaluesofr.

DataRateVersusSignalRateThedataratedefinesthenumberofdataelements(bits)

sent

in

Is.

The

unit

is

bits

per

second

(bps).

The

signal

rate

is

the

number

of

signal

elementssentinIs.Theunitisthebaud.Thereareseveralcommonterminologies

usedintheliterature.Thedatarateissometimescalledthebitrate;thesignalrateis

sometimescalledthepulserate,themodulationrate,orthebaudrate.Wecanformulate

therelationshipbetweendatarateandsignalrateas


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LineCodingSchemes:Wecanroughlydividelinecodingschemesintofivebroadcategories,asshowninFigure

UnipolarScheme

Inaunipolarscheme,allthesignallevelsareononesideofthetimeaxis,eitheraboveor

below.NRZ(NonReturntoZero)Traditionally,aunipolarschemewasdesignedasanon

returntozero(NRZ)schemeinwhichthepositivevoltagedefinesbitIandthezerovoltage

definesbitO.ItiscalledNRZbecausethesignaldoesnotreturntozeroatthemiddleofthebit.

Figure:showaunipolarNRZscheme.

PolarSchemes

Inpolar

schemes,

the

voltages

are

on

the

both

sides

of

the

time

axis.

For

example,

the

voltagelevelfor0canbepositiveandthevoltagelevelforIcanbenegative.

NonReturntoZero(NRZ)InpolarNRZencoding,weusetwolevelsofvoltageamplitude.We

canhavetwoversionsofpolarNRZ:NRZLandNRZI,asshowninFigure:Thefigurealsoshows

thevalueofr,theaveragebaudrate,andthebandwidth.


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Inthefirstvariation,NRZL(NRZLevel),thelevelofthevoltagedeterminesthevalueofthebit.

Inthesecondvariation,NRZI(NRZInvert),thechangeorlackofchangeinthelevelofthe

voltagedeterminesthevalueofthebit.Ifthereisnochange,thebitis0;ifthereisachange,

thebitis1.

BipolarSchemes

Inbipolarencoding(sometimescalledmultilevelbinary),therearethreevoltagelevels:

positive,negative,andzero.Thevoltagelevelforonedataelementisatzero,whilethevoltage

levelfor

the

other

element

alternates

between

positive

and

negative.

MultilevelSchemes

Thedesiretoincreasethedataspeedordecreasetherequiredbandwidthhasresultedinthe

creationofmanyschemes.Thegoalistoincreasethenumberofbitsperbaudbyencodinga

patternofmdataelementsintoapatternofnsignalelements.

MultilineTransmission:MLT3

NRZIanddifferentialManchesterareclassifiedasdifferentialencodingbutusetwotransitionrules

toencodebinarydata(noinversion,inversion).Ifwehaveasignalwithmorethantwolevels,we

candesignadifferentialencodingschemewithmorethantwotransitionrules.MLT3isoneof

them.Themultilinetransmission,threelevel(MLT3)schemeusesthreelevels(+v,0,and V)andthreetransitionrulestomovebetweenthelevels.

1.Ifthenextbitis0,thereisnotransition.

2.Ifthenextbitis1andthecurrentlevelisnot0,thenextlevelis0.

3.Ifthenextbitis1andthecutTentlevelis0,thenextlevelistheoppositeofthelastnonzero

level.


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BlockCodingWeneedredundancytoensuresynchronizationandtoprovidesomekindofinherent

errordetecting.Blockcodingcangiveusthisredundancyandimprovetheperformance

oflinecoding.Ingeneral,blockcodingchangesablockofmbitsintoablockofnbits,wherenis

largerthanm.BlockcodingisreferredtoasanmB/nBencodingtechnique.

BlockcodingisnormallyreferredtoasmBlnBcoding;itreplaceseachmbitgroupwithannbitgroup.

4B/5B

Thefourbinary/fivebinary(4B/5B)codingschemewasdesignedtobeusedincombination

withNRZI.RecallthatNRZIhasagoodsignalrate,onehalfthatofthe

biphase,butithasasynchronizationproblem.


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8RI1OR

Theeightbinary/tenbinary(SBIlOB)encodingissimilarto4B/5Bencodingexcept

thatagroupof8bitsofdataisnowsubstitutedbyalObitcode.Itprovidesgreater

errordetectioncapabilitythan4B/5B.The8BIlOBblockcodingisactuallyacombination

of5B/6Band3B/4Bencoding.

Scrambling

BiphaseschemesthataresuitablefordedicatedlinksbetweenstationsinaLANarenotsuitable

forlongdistancecommunicationbecauseoftheirwidebandwidthrequirement.

ThecombinationofblockcodingandNRZlinecodingisnotsuitableforlongdistanceencoding

either,becauseoftheDCcomponent.BipolarAMIencoding,ontheotherhand,hasanarrow

bandwidthanddoesnotcreateaDCcomponent.However,alongsequenceofOsupsetsthe

synchronization.IfwecanfindawaytoavoidalongsequenceofOsintheoriginalstream,we

canuse

bipolar

AMI

for

long

distances.

We

are

looking

for

atechnique

that

does

not

increase

thenumberofbitsanddoesprovidesynchronization. Wearelookingforasolutionthat

substituteslongzerolevelpulseswithacombinationofotherlevelstoprovidesynchronization.

Onesolutioniscalledscrambling.


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R8ZS

BipolarwithSzerosubstitution(BSZS)iscommonlyusedinNorthAmerica.Inthistechnique,

eightconsecutivezerolevelvoltagesarereplacedbythesequenceOOOVBOVB.

HDB3

Highdensitybipolar3zero(HDB3)iscommonlyusedoutsideofNorthAmerica.Inthis

technique,whichismoreconservativethanB8ZS,fourconsecutivezerolevelvoltagesare

replacedwithasequenceofOOOVorBOOV.

2.ErrorDetectionandCorrectionI.INTRODUCTION

Networksmust

be

able

to

transfer

data

from

one

device

to

another

with

acceptable

accuracy.

Formostapplications,asystemmustguaranteethatthedatareceivedareidenticaltothedata

transmitted.Anytimedataaretransmittedfromonenodetothenext,theycanbecome

corruptedinpassage.Manyfactorscanalteroneormorebitsofamessage.Someapplications

requireamechanismfordetectingandcorrectingerrors.

Someapplicationscantolerateasmallleveloferror.Forexample,randomerrorsinaudioor

videotransmissionsmaybetolerable,butwhenwetransfertext,weexpectaveryhighlevelof

accuracy.

II.TypesofErrorsWheneverbitsflowfromonepointtoanother,theyaresubjecttounpredictablechanges

becauseofinterference.Thisinterferencecanchangetheshapeofthesignal.Inasinglebit

error,a0ischangedtoa1ora1toaO.Inabursterror,multiplebitsarechanged.For

example,a11100sburstofimpulsenoiseonatransmissionwithadatarateof1200bpsmight

changeallorsomeofthe12bitsofinformation.

SingleBitError

Thetermsinglebiterrormeansthatonly1bitofagivendataunit(suchasabyte,character,or

packet)ischangedfrom1to0orfrom0to1.


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BurstError

Thetermbursterrormeansthat2ormorebitsinthedataunithavechangedfrom1to0orfrom0

to1.

III. RedundancyThecentralconceptindetectingorcorrectingerrorsisredundancy.Tobeabletodetectorcorrect

errors,weneedtosendsomeextrabitswithourdata.Theseredundantbitsareaddedbythe

senderandremovedbythereceiver.Theirpresenceallowsthereceivertodetectorcorrect

corruptedbits.

IV. DetectionVersusCorrectionThecorrectionoferrorsismoredifficultthanthedetection.Inerrordetection,wearelookingonly

toseeifanyerrorhasoccurred.Theanswerisasimpleyesorno.Wearenoteveninterestedinthe

numberoferrors.Asinglebiterroristhesameforusasabursterror.

Inerrorcorrection,weneedtoknowtheexactnumberofbitsthatarecorruptedandmore

importantly,theirlocationinthemessage.Thenumberoftheerrorsandthesizeofthemessage

areimportantfactors.Ifweneedtocorrectonesingleerrorinan8bitdataunit,weneedto

considereightpossibleerrorlocations;ifweneedtocorrecttwoerrorsinadataunitofthesame

size,weneedtoconsider28possibilities.Youcanimaginethereceiver'sdifficultyinfinding10

errorsinadataunitof1000bits.

V. ForwardErrorCorrectionVersusRetransmissionTherearetwomainmethodsoferrorcorrection.Forwarderrorcorrectionistheprocessinwhich

thereceiver

tries

to

guess

the

message

by

using

redundant

bits.

This

is

possible,

as

we

see

later,

ifthenumberoferrorsissmall.Correctionbyretransmissionisatechniqueinwhichthereceiver

detectstheoccurrenceofanerrorandasksthesendertoresendthemessage.Resendingis

repeateduntilamessagearrivesthatthereceiverbelievesiserrorfree(usually,notallerrorscan

bedetected).


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VI. CodingRedundancyisachievedthroughvariouscodingschemes.Thesenderaddsredundantbitsthrougha

processthatcreatesarelationshipbetweentheredundantbitsandtheactualdatabits.The

receivercheckstherelationshipsbetweenthetwosetsofbitstodetectorcorrecttheerrors.The

ratioofredundantbitstothedatabitsandtherobustnessoftheprocessareimportantfactorsin

anycoding

scheme.

Figure

10.3

shows

the

general

idea

of

coding.

Wecandividecodingschemesintotwobroadcategories:blockcodingandconvolutioncoding.In

thisbook,weconcentrateonblockcoding;convolutioncodingismorecomplexandbeyondthe

scopeofthisbook.

VII. ModularArithmeticInmodulararithmetic,weuseonlyalimitedrangeofintegers.Wedefineanupperlimit,called

amodulusN.Wethenuseonlytheintegers0toN I,inclusive.ThisismoduloNarithmetic.For

example,ifthemodulusis12,weuseonlytheintegers0to11,inclusive.Anexampleofmoduloarithmeticisourclocksystem.Itisbasedonmodulo12arithmetic,substitutingthenumber12

forO.InamoduloNsystem,ifanumberisgreaterthanN,itisdividedbyNandtheremainderis

theresult.Ifitisnegative,asmanyNsasneededareaddedtomakeitpositive.Considerour

clocksystemagain.Ifwestartajobat11A.M.andthejobtakes5h,wecansaythatthejobisto

befinishedat16:00ifweareinthemilitary,orwecansaythatitwillbefinishedat4P.M.

(theremainderof16/12is4).

Additionandsubtractioninmoduloarithmeticaresimple.Thereisnocarrywhenyouaddtwo

digitsinacolumn.Thereisnocarrywhenyousubtractonedigitfromanotherinacolumn.

Modulo2Arithmetic

Ofparticularinterestismodulo2arithmetic.Inthisarithmetic,themodulusNis2.Wecanuse

only0and1.Operationsinthisarithmeticareverysimple.Thefollowingshowshowwecan

addorsubtract2bits.


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Noticeparticularlythatadditionandsubtractiongivethesameresults.Inthisarithmeticweuse

theXOR(exclusiveOR)operationforbothadditionandsubtraction.TheresultofanXOR

operation

is

0

if

two

bits

are

the

same;

the

result

is

I

if

two

bits

are

different.

Figure

10.4

shows

thisoperation.

OtherModuloArithmetic

Wealsouse,moduloNarithmeticthroughthebook.Theprincipleisthesame;weusenumbers

between0andN 1.Ifthemodulusisnot2,additionandsubtractionaredistinct. Ifwegeta

negativeresult,weaddenoughmultiplesofNtomakeitpositive.

FLOWANDERRORCONTROLDatacommunicationrequiresatleasttwodevicesworkingtogether,onetosendandtheother

toreceive.Evensuchabasicarrangementrequiresagreatdealofcoordinationforan

intelligibleexchangetooccur.Themostimportantresponsibilitiesofthedatalinklayerareflow

controlanderrorcontrol.Collectively,thesefunctionsareknownasdatalinkcontrol.

FlowControlFlowcontrolcoordinatestheamountofdatathatcanbesentbeforereceivingan

acknowledgmentandisoneofthemostimportantdutiesofthedatalinklayer.Inmost

protocols,flowcontrolisasetofproceduresthattellsthesenderhowmuchdataitcan

transmitbeforeitmustwaitforanacknowledgmentfromthereceiver.Theflowofdatamust

notbeallowedtooverwhelmthereceiver.Anyreceivingdevicehasalimitedspeedatwhichit

canprocessincomingdataandalimitedamountofmemoryinwhichtostoreincomingdata.

Thereceivingdevicemustbeabletoinformthesendingdevicebeforethoselimitsarereached

andtorequestthatthetransmittingdevicesendfewerframesorstoptemporarily.Incoming

datamustbecheckedandprocessedbeforetheycanbeused.Thenrateofsuchprocessingis

oftenslowerthantherateoftransmission.Forthisreason,eachreceivingdevicehasablockof

memory,calledabuffer,reservedforstoringincomingdatauntiltheyareprocessed.Ifthe

bufferbeginstofillup,thereceivermustbeabletotellthesendertohalttransmissionuntilitis

onceagainabletoreceive. Flowcontrolreferstoasetofproceduresusedtorestricttheamount

ofdatathatthesendercansendbeforewaitingforacknowledgment.

ErrorControlErrorcontrolisbotherrordetectionanderrorcorrection.Itallowsthereceivertoinformthesenderofanyframeslostordamagedintransmissionandcoordinatestheretransmissionof

thoseframesbythesender.Inthedatalinklayer,thetermerrorcontrolrefersprimarilyto

methodsoferrordetectionandretransmission.Errorcontrolinthedatalinklayerisoften

implementedsimply:Anytimeanerrorisdetectedinaexchange,specifiedframesare

retransmitted.Thisprocessiscalledautomaticrepeatrequest(ARQ). Errorcontrolinthedata

linklayerisbasedonautomatirepeatrequest,whichistheretransmissionofdata.


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Internel I -Data & Communications