计算机网路基础第三章习题.docx

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1、2.Thefollowingcharacterencodingisusedinadatalinkprotocol:A:B:FLAG:ESC:Showthebitsequencetransmitted(inbinary)forthefour-characterframeABESCFLAGwheneachofthefollowingframingmethodsisused:(a)Bytecount.(b)Flagbyteswithbytestuffing.(c)Startingandendingflagbyteswithbitstuffing.(a)(b)(c)7.

2、Inthetextbook,theauthorsshowthatforachannelwitherrorrate10^-6,errordetectingbasedretransmissionismoreefficientthanerrorcorrecting(see27thslide).Pleasegiverangesofthechannelerrorrateinwhicherrorcorrectingismoreefficient,consideringonlyforblocks(1000bits)withatmost1biterror.Supposethee

3、rrorrateisx.N=1/(x*1000);Forerrorcorrecting,itneed10*Nbitofcheckbits.Forerrordetectingitneed2001bit10*N<2001,10/(x*1000)<2001,x=4.9*10^-68.Hammingcodeisaneffectivewayforerrorcorrecting.Showthatthenumberofcheckbits(i.e.r)intheHammingcodesdescribedinthetextbook(e.g.,Fig.3-6)(almost)ach

4、ievesthelowboundofEq(3-1).Eq(3-1):(m+r+1)≤2^rInfigure3-6,m+r=11So12≤2^r,r=4.Sothenumberofcheckbitsis4.9.Supposeyouhavethefollowing12-bitmessage:1(a)Numberingbitsfromrighttoleft(ieleast-significantbitontheright),insertcheckbitsaccordingtotoHamming’s1-biterrorcorrectionsystem.Indicatew

5、hichbitsarecheckbitsandwhicharemessagebits.(b)Hamming’sschemeonlycorrects1-biterrors.Sinceit’sadistance3code,itcouldalsobeusedtodetect2-biterrors.Describea3-biterror(3*1-biterrors)intheabovecodewordaffectingonlymessagebits(notcheckbits)thatwouldbeundetected(andofcourseuncorrected).Be

6、suretodescribehowandwhythealgorithmfails.(a)cherkbitswillbeinsertinthe124816.messagebitsare3567910111213141517.(b)todetect3-biterror,thehammingdistanceneedtobe4.tocorrect3-biterror，thehammingdistanceneedtobe7.Sothealgorithmfails.16.Consideranoriginalframe1.Thegeneratorpolynomialx^4+x

7、+1,showtheconvertedframeafterappendingtheCRC.Theremainderisx^2+x+1.22.A3000-km-longT1trunkisusedtotransmit64-byteframes.Howmanybitsshouldthesequencenumbersbeforprotocol5andprotocol6respectively?Thepropagationspeedis6usec/km.T1:1.536Mbps.Sotransmit64-byteframesneed0.3ms.Thepropogation

8、timeis3000*6