《网路概论》PPT课件

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1、網路概論Class4–DataLinkControlPartII授課老師楊人順2001/10/16-17--PresentationOutlinesErrorDetectionTechniquesCyclicredundancycheckErrorControlErrorControlTechniqueErrorControlMechanismDataLinkProtocolsMultiplexingWhatismultiplexing?FrequencyDivisionMultiplexingTimeDivisionMultiplexingC

2、odeDivisionMultipleAccessDigitalSubscriberLine--CyclicredundancycheckCRCisthemostcommonandpowerfulerror-detectingcodes.AlgorithmSendercalculatesaFrameCheckSequence(FCS)fromapattern.Sendersendsaframewith(Message+FCS).Receiverthendividestheincomingframebythesamepatternand,ifthe

3、reisnoremainder,assumetherewasnoterror.Modulo2Arithmetic:anexclusion-ORoperationtoreplacethe“Add”and“Subtract”DefinitionT=(k+n)-bitframetobetransmitted,withn

4、leTargetsinreceiversiteNoerrordetected:T/PnoremainderErrorsdetected:T/Pwithasleastonebiterror.Atrue:

5、P

6、=

7、F

8、+1FCSgeneration--CRCPrinciple(con’t)ErrorDetectionExample:pp.204AnotherviewofCRC:expressallvalueaspolynomialsinadummyvariableXE.g.,M=110011,M(X)=X5+X4+X+1,P=11001,P(X)=X

9、4+X3+1--CapacityofCRCAnerrorE(X)willonlybeundetectableifitisdivisiblebyP(X).Thefollowingerrortypesaredetectable:Allsingle-biterrorsAlldouble-biterrors,aslongasP(X)hasasleastthree1sAnyoddnumberoferrors,aslongasP(X)containsafactor(X+1)Anybursterrorforwhichthelengthoftheburstisl

10、essthanthelengthofthedivisorpolynomial;thatis,lessthanorequaltothelengthoftheFCSMostlargerbursterrorsUndetectableerrorprobability(risthelengthofFCS)Abursterroroflengthr+1,undetectableerrorprobabilityis1/2r–1Alongerburst,undetectableerrorprobabilityis1/2r--CurrentCRCVersionsCR

11、C-12,P(X)=X12+X11+X3+X2+X+1Usedforthestreamof6bitscharacters,FCSis12bitsCRC-16,P(X)=X16+X15+X2+1Usedforthestreamof8bitscharacters,FCSis16bitsPopularformostapplicationsUnitedStatesCRC-CCITT,P(X)=X16+X12+X5+1Usedforthestreamof8bitscharacters,FCSis16bitsPopularformostapplication

12、sEuropeCRC-32,P(X)=X32+X26+X23+X22+X16+X12+X11+X10+X8+X7+X5+X4+X2+X+