东北大学计算机网络平时测验1

本试卷共3页，第1页东北大学考试试卷（测验一）2015—2016学年第一学期课程名称：计算机网络（ComputerNetworks）┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄总分一二三四五六七八学院班级学号姓名……………○……………密……………○……………封……………○…………线………………………………1.Circuitswitchingversuspacketswitching.(10’)a.Supposethatallofthenetworksourcessenddataataconstantbitrate.Wouldpacket-switchingorcircuit-switchingbemoredesirableinthiscase?Why?b.Supposethatallofthenetworksourcesarebursty—thattheyonlyoccasionallyhavedatatosend.Wouldpacket-switchingorcircuitswitchingbemoredesirableinthiscase?Why?a.Circuit-switchingismoredesirableherebecausetherearenostatisticalmultiplexinggainstobehad,andbyusingcircuits,eachconnectionwillgetaconstantamountofbandwidththatmatchesitsCBRrate.Ontheotherhand,circuit-switchinghasmoreoverheadintermsofsignalingneededtosetupthecall,sothereisanargumentthatpacket-switchingispreferableheresincethereisnocallsetupoverheadwithpacket-switching.Ifthiswereanexamquestioneitheranswerwouldbecorrect(aslongasyouprovidethecorrectreasoning!).b.Packet-switchingisbetterherebecausetherearestatisticalmultiplexinggains—whenasourcedoesnothavedatatosend,itwillnotbeallocatedbandwidth(itwouldbeunusedwhenthesourcehadnothingtosend).Withpacket-switching,thisbandwidthisavailableforusebyothersources.2.Circuitswitching.(15’)ConsidersendingapacketofFbitsoverapathofQlinks.EachlinktransmitsatRbps.Thenetworkislightlyloadedsothattherearenoqueuingdelays.Propagationdelayisalsonegligible.a.Supposethenetworkisapacket-switcheddatagramnetworkandaconnection-orientedserviceisused.Supposeeachpackethash*Fbitsofheaderwhere0h1.Assumingtssetuptime,howlongdoesittaketosendthepacket?b.Supposethatthenetworkisacircuit-switchednetwork.Furthermore,supposethatthetransmissionrateofthecircuitbetweensourceanddestinationisR/24bps.Assumingtssetuptimeandnobitsofheaderappendedtothepacket,howlongdoesittaketosendthepacket?c.Whenisthedelaylongerforpacketswitchingthanforcircuitswitchingassumingh=0.5.Interpretyourresult?a.Thetimerequiredtotransmitthepacketoveronelinkis(1+h)F/R.ThetimerequiredtotransmitthepacketoverQlinksisQ(1+h)F/RThus,thetotaldelayforpacketswitchingisQ(1+h)F/R+ts.b.Forcircuitswitching,bitsarenot“storeandforwarded”beforeeachlink.Thus,thereisonlyonetransmissiondelayofF/(R/24)=24F/R.Thetotaldelayis24F/R+ts.c.ThedelayislongerforpacketswitchingwhenQ(1+h)F/R+ts24F/R+tsorequivalentlywhenQ16.Thus,iftherearemorethan16links,packetswitchinghasalargerdelay,duetothestoreandforwarding.Iftherearefewerthan16links,circuitswitchinghasalargerdelay,duetoitsreducedtransmissionrate.本试卷共3页，第2页学院班级学号姓名……………○……………密……………○……………封……………○…………线………………………………5.Protocollayers.(10’)a.Whatarethefiveprotocollayers,fromtoptobottom,intheInternet?b.Foreachofthefivelayers,whatisthenameofthepacketsprocessedatthelayer?c.Anend-systemprocessesuptowhichlayer?d.Arouterprocessesuptowhichlayer?e.Alink-layerswitchprocessesuptowhichlayer?a.application,transport,network,link,physicalb.message,segment,datagram,frame,packetc.anend-systemprocessesupthroughtheapplicationlayerd.arouterprocessesupthroughthenetworklayere.alink-layerswitchprocessesupthroughthelinklayer6.Carriersenseandcollisiondetection.(10’)ConsidertwonodesAandBonthesameEthernetsegment,andsupposethepropagationdelaybetweenthetwonodesis225bittimes.Supposeattimet=0bothnodesAandBbegintotransmitaframe.Atwhattime(inbittimes)dotheydetectthecollision?Assumingbothnodestransmita48-bitjamsignalafterdetectingacollision,atwhattime(inbittimes)donodesAandBsenseanidlechannel?Howmanysecondsisthisfora10MbpsEthernet?BothnodesAandBdetectthecollisionattimet=225.Attimet=225+48=273bothnodesstoptransmittingtheirjamsignals.ThelastbitofthejamsignalfromBarrivesatAattimet=273+225=498bittimes.Similarly,thelastbitofthejamsignalfromBarrivesatAattimet=273+225=498bittimes.Fora10MbpsEthernet,thiscorrespondsto(498bits)/(107bits/sec)=49.8microseconds.3.Delayswithmultiplelinks.(15’)ConsiderapacketoflengthLwhichbeginsatendsystemA,travelsoveronelinktoapacketswitch,andtravelsfromthepacketswitchoverasecondlinktoadestinationendsystem.Letdi,siandRidenotethelength,propagationspeed,andtransmissionrateoflinki,fori=1,2.Thepacketswitchdelayseachpacketbydproc.Assumingnoqueuingdelays,intermsofdi,si,Ri,(i=1,2)andL,whatisthetotalend-to-enddelayforthepacket?Supposethepacketis1,000bytes,thepropagationspeedonbothlinksis2.5*108m/sthetransmissionratesofbothlinksis1Mbps,thepacketswitchprocessingdelayis1msec,thelengthofthefirstlinkis4,000km,andthelengthofthelastlinkis1,000km.Forthesevalues,whatistheend-to-enddelay?ThefirstendsystemrequiresL/R1totransmitthepacketontothefirstlink;thepacketpropagatesoverthefirstlinkind1/s1thepacketswitchaddsaprocessingdelayofdprocafterreceivingtheentirepacket,thepacketswitchrequiresL/R2totransmitthepacketontothesecondlink;thepacketpropagatesoverthesecondlinkind2/s2Addingthesefivedelaysgivesdend-end=L/R1+L/R2+d1/s1+d2/s2+dproc.Toanswerthesecondquestion,wesimplyplugthevaluesintotheequationtoget8+8+16+4+1=37msec.4.Storeandforwarding.(10’)InQuestion3,supposeR1=R2=Randdproc=0.Furthermore,supposethepacketswitchdoesnotstore-and-forwardp