Packet Loss in Mobile Ad Hoc Networks(3)

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

baseTCPSinkreceiver.Thesenderwindowsizeisdecreasedbyhalfwhenpacketlossesaredetected.Theretransmissionstartsfromthe rstlostpacket.TahoeTCPenterstheslowstartwhenanACKforanewpacketisreceived.AllTCPpacketshavethesamesizeof512bytes.Theinitialsenderwindowsizeis1andthemaximumboundonthewindowsizeis32.TCPSinkisresponsibleforreturningACKstothesender.ItgeneratesoneACKperpacketreceived.TheACKpacketsizeis40.ThedataofeachconnectionisgeneratedbyanattachedFTPapplication,whichsimulatesabulkdatatransfer.EveryFTPapplicationstartsatatimerandomlychosenfrom0to100seconds.D.RoutingProtocols

Theroutingprotocolgreatlyaffectspacketlossbesidesmobilityandtraf c.Allpropertiesofaroutingprotocol,suchaswhatroutinginformationismaintained,thewayinwhichtheinformationisobtained,howtochoosearoute,etc.,mayhavedifferenteffects.TheexperimentsareconductedbyusingDSDVandAODVroutingpro-tocols.Thesetwoprotocolssharealotofproperties.ThelargestdifferencebetweenthemisthatDSDVisaproactivewhileAODVison-demand.

DSDVextendsthebasicBellman-Fordmechanismbyattachingasequencenumber,whichisoriginatedbythedestination,toeachdistance.Thisdestinationsequencenumberisusedtodeterminethe“freshness”ofaroute.Routeswithmorerecentsequencenumbersarepreferredformakingpacketforwardingdecisionsbyahost,butnotnecessarilyadvertisedtootherhosts.Forrouteswiththeequalsequencenumber,theonewiththesmallestdistancemetricischosen.Eachtimeahostsendsanupdatetoitsneighbors,itscurrentsequencenumberisincrementedandincludedintheupdate.Thesequencenumberisdisseminatedthroughoutanetworkviaupdatemessages.TheDSDVprotocolrequireseachhosttoperiodicallyadvertiseitsownroutingtabletoitsneighbors.Updatesaretransmittedimmediatelywhensigni cantnewroutinginformationisavailable.Routesreceivedinbroadcastsareusedtoupdatetheroutingtable.Thereceiveraddsanincrementtothemetricofeachreceivedroutebeforeupdating.

AODVroutingprotocolisalsobasedupondistancevector,andusesdestinationsequencenumberstode-terminethefreshnessofroutes.Itoperatesintheon-demandfashion,asopposedtotheproactivewayoftheDSDVprotocol.AODVrequireshoststomaintainonlyactiveroutes.Anactiverouteisarouteusedtoforwardatleastonepacketwithinthepastactivetimeoutperiod.

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TABLEI

PACKETLOSSATMACANDNETWORKLAYERS

Whenahostneedstoreachadestinationanddoesnothaveanactiveroute,itbroadcastsaRouteRequest(RREQ),whichis oodedinthenetwork.AroutecanbedeterminedwhenRREQisreceivedeitherbythedestinationitselforbyanintermediatehostwithanactiveroutetothatdestination.ARouteReplay(RREP)isunicastbacktotheoriginatorofRREQtoestablishtheroute.EachhostthatreceivesRREQcachesaroutebacktotheoriginatoroftherequest,sothatRREPcanbesentback.Everyrouteexpiresafterapredeterminedperiodoftime.Sendingapacketviaaroutewillresettheassociatedexpirytime.E.DifferentiatedPacketLosses

Packetlossismeasuredatallmobilehosts.EveryhostmonitorsthenetworkinglayerandtheMAClayerforallkindsofpacketlosses.Thelayersoftheprotocolstackandthemodulesthatareresponsibleformobility-relatedandcongestion-relatedpacketlossareidenti ed,asshownintableI.

Mobility-relatedpacketlossmayoccuratboththenetworklayerandtheMAClayer.Whenapacketarrivesatthenetworklayer,theroutingprotocolforwardsthepacketifavalidroutetothedestinationisknown.Oth-erwise,thepacketisbuffereduntilarouteisavailable.Apacketisdroppedintwocases:

Thebufferisfullwhenthepacketneedstobebuffered.

Thetimethatthepackethasbeenbufferedexceedsthelimit.(TheAODVimplementationinns-2posesa30-secondlimitonthetimeapacketcanbebuffered.TheDSDVimplementationdoeshavealimit.)

TheMAClayermobility-relatedpacketlossoccurswhenthenexthopofapacketisoutofrangeatthemomentthepacketissentbytheMACprotocol.Thereasonisthattheroutinginformationisobsoleted.Itoccursfrequentlyinahighmobilitynetworkthaninalowmobilitynetwork.

Congestion-relatedpacketlossonlyoccursattheMAClayer.BecauseCSMA/CAisusedinthesimu-lation,apacketmaybedroppedduetocongestionfortworeasons: