计算机网络课后习题2

第二次作业

提交日期：9月23日；课堂提交

Problem1Apacketswitchreceivesapacketanddeterminestheoutboundlinktowhichthepacketshouldbeforwarded.Whenthepacketarrives,oneotherpacketishalfwaydonebeingtransmittedonthisoutboundlinkandfourotherpacketsarewaitingtobetransmitted.Packetsaretransmittedinorderofarrival.Supposeallpacketsare1500bytesandthelinkrateis2Mbps.Whatisthequeueingdelayforthepacket?Moregenerally,whatisthequeueingdelaywhenallpacketshavelengthL,thetransmissionrateisR,xbitsofthecurrentlybeingtransmittedpackethavebeentransmitted,andnpacketsarealreadythequeue?

Problem2SkypeoffersaservicethatallowsyoutomakeaphonecallfromaPCtoanordinaryphone.ThismeansthatthevoicecallmustpassthroughboththeInternetandthroughatelephonenetwork.Discusshowthismightbedone.

Problem3Inamodernpacket-switchednetworks,includingtheInternet,thesourcehostsegmentslong,application-layermessages(forexample,animageoramusicfile)intosmallerpacketsandsendsthepacketsintothenetwork.Thereceiverthenreassemblesthepacketsbackintotheoriginalmessage.Werefertothisprocessasmessagesegmentation.Figure1.27illustratestheend-to-endtransportofamessagewithandwithoutmessagesegmentation.Consideramessagethatis8·106bitslongthatistobesentfromsourcetodestinationinFigure1.27.Supposeeachlinkinthefigureis2Mbps.Ignorepropagation,queuing,andprocessingdelays.

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(a)Considersendingthemessagefromsourcetodestinationwithoutmessagesegmentation.

Howlongdoesittaketomovethemessagefromthesourcehosttothefirstpacketswitch?Keepinginmindthateachswitchusesstore-and-forwardpacketswitching,whatisthetotaltimetomovethemessagefromsourcehosttodestinationhost?

(b)Nowsupposethatthemessageissegmentedinto800packets,witheachpacketbeing10,000

bitslong.Howlongdoesittaketotomovethefirstpacketfromsourcehosttothefirstswitch?Whenthefirstpacketisbeingsentfromthethefirstswitchtothesecondswitch,thesecondpacketisbeingsentfromthesourcehosttothefirstswitch.Atwhattimewillthesecondpacketbefullyreceivedatthefirstswitch?

(c)Howlongdoesittaketomovethefilefromsourcehosttodestinationhostwhenmessage

segmentationisused?Comparethisresultwithyouranswerinpart(a)andcomment.(d)Inadditiontoreducingdelay,whatarereasonstousemessagesegmentation?(e)Discussthedrawbacksofmessagesegmentation.