Packet Loss in Mobile Ad Hoc Networks(2)

Abstract — We investigate packet loss in mobile ad hoc networks via simulation. Ad-hoc on-demand distance vector (AODV) and destination-sequenced distance vector (DSDV) are chosen as representatives of the on-demand and proactive routing protocols respect

pendingpacketsisfull.Itmayalsobedroppedatanintermediatehostifthelinktothenexthophasbroken.Westudytheeffectofcongestionandmobilityonpacketlossinvariousnetworkcontexts.AODVandDSDV[9]arechosenasrepresentativesofon-demandandproactiveroutingprotocolsrespectively.

Thisworkcanbene tthedesignofroutingand owcontrolalgorithms,thedimensioningofbuffers,identify-ingandavoidingtheperformancebottleneckofcurrentroutingprotocols,andchoosingproperparametersinfuturesimulationandanalyticstudies.

Therestofthepaperisorganizedasfollows.SectionIIintroducestherelatedwork.Thesimulationmodel,includingthesimulationenvironment,mobility,traf- c,routingprotocols,congestion-relatedandmobility-relatedpacketloss,arediscussedinsectionIII.SectionIVpresentstwosetsofexperimentsandtheresults.Therelationsbetweenshortestpathandcongestion,andpacketlossdistributionarediscussedinsectionV.SectionVIconcludesthepaper.

II.RELATEDWORK

Therehasbeensomerecentworkonaddressingpacketlossissuesinwirelessnetworks.S.BiazandN.H.Vaidyainvestigatetheabilityofthreelosspredictorstodistinguishcongestionlossesfromwirelesstransmissionlosses[10].Theyuseawirelesslinkwithtransmissionlossraterwinthesimulations.F.AnjumandL.TassiulasanalyticallystudytheperformanceofdifferentTCPalgorithmsoverawirelesschannelwithcorrelatedpacketlosses[11]kshmanetal.alsoanalyzetheimpactofrandompacketlossatawirelesslinkontheperformanceofTCP/IPin[12].TheyindicatethatbidirectionalcongestionincreasesTCP’ssensitivitytoloss.TheseeffortsassumetransmissionlossesonasinglewirelesslinkfollowasimplemodelandfocusonhowlosseseffecttheperformanceofTCP.Evenifwirelesstransmissionisloss-free,packetlossstillexistsinadhocnetworks.Ourworkistounderstandthemajorcausesforpacketlossandtocaptureitscharacteristics.

III.SIMULATIONMODEL

Weusethenetworksimulatorns-2(version2.1b9)foroursimulationstudy.A.Environment

Eachmobilehostusesanomni-directionalantennahavingunitygain.Thewirelessinterfaceworkslike

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the914MHzLucentWaveLANdirect-sequencespread-spectrum(DSSS)radiointerface[13].WaveLANismodeledasashared-mediaradiowithanominalbitrateof2Mb/s,andanominalradiorangeof250m[4].TheIEEE802.11distributedcoordinationfunction(DCF)isusedastheMAClayerprotocol.Aunicastdatapacketdestinedtoaneighborissentoutafterhandshakingwithrequest-to-send/clear-to-send(RTS/CTS)exchangesandfollowedbyanacknowledgement(ACK)packet.Thebroadcastpacketsaresimplysentoutwithouthand-shakeandacknowledgement.Theimplementationusescarriersensemultipleaccesswithcollisionavoidance(CSMA/CA).B.Mobility

Weusetherandomwaypointmodel[14]togeneratemovementsofmobilehosts.Atthebeginningofasimulation,mobilehostsarerandomlyplacedon1000mx1000masquare eld.Eachhostrandomlychoosesitsdestinationinthe eld,andamovingspeedthatrangesfrom0to20m/s.Alldestinationsandspeedsareinde-pendentandidenticallydistributed.Everyhostrepeatstheabovestepafterithasreachedthedestinationandwaitedaspeci edtime(thepausetime).Accordingtothismodel,thespeedanddirectionofthenextmovementhavenorelationtothoseofthepreviousmovement.Asindicatedin[15],thepausetimeandthemaximumspeedhavesimilarimpactsonthemobilitywithrespecttolinkchangeorroutechange.Thusthemobilityisvariedbychangingthepausetimeinthesimulation.C.Traf c

Toinvestigatetheimpactoftraf cloadandcongestioncontrolmechanismsonpacketlosses,bothunresponsivetraf candresponsivetraf carestudied.

Unresponsivetraf conlyconsistsofUDPcon-nections,eachofwhichisspeci edasasource-destination(S-D)pair.Everysourceisassociatedwithaconstantbitrate(CBR)traf cgenerator,whichsendsoutpacketsatthegivenrate.ThesourceofeachS-Dpairisrandomlychosenfromallhosts,andthedestinationisrandomlychosenfromallhostsotherthanthesource.AllS-Dpairsaremutuallyindependent.Thepacketsizeis xedat512bytes.Thestarttimeofeachconnectionisuniformlydistributedbetween0to100seconds. Responsivetraf ciscomprisedofTCP1connections.EachconnectionhasaTahoeTCPsenderanda

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TheTCPperformscongestioncontrolandround-trip-timeestima-tioninawaysimilartotheversionofTCPreleasedwiththe4.3BSDTahoeUNIXsystemfromUCBerkeley,soitiscalledTahoeTCP.