陈婵的英语.docx
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陈婵的英语.docx
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陈婵的英语
I.J.ComputerNetworkandInformationSecurity,2013,4,1-7
PublishedOnlineApril2013inMECS(http:
//www.mecs-press.org/)
DOI:
10.5815/ijcnis.2013.04.01
FormalVerificationofCongestionControl
AlgorithminVANETs
MohamadYusofDarus
FacultyofComputerScience&InformationSystems,UniversitiTeknologiMalaysia(UTM),Malaysia
yusof@tmsk.uitm.edu.my
KamalrulnizamAbuBakar
FacultyofComputerScience&InformationSystems,UniversitiTeknologiMalaysia(UTM),Malaysia
knizam@utm.my,kamarul@fsksm.utm.my
Abstract—AVehicularAd-HocNetworks(VANETs)isthetechnologythatusesmovingcarsasnodesinanetworktocreateamobilenetwork.VANETsturneveryparticipatingcarintoawirelessrouter,allowingcarsofeachothertoconnectandcreateanetworkwithawiderange.VANETsaredevelopedforenhancingthedrivingsafetyandcomfortofautomotiveusers.TheVANETscanprovidewidevarietyofservicesuchasIntelligentTransportationSystem(ITS)e.g.safetyapplications.ManyofsafetyapplicationsbuiltinVANETsarerequiredreal-timecommunicationwithhighreliability.Oneofthemainchallengesistoavoiddegradationofcommunicationchannelsindensetrafficnetwork.Manyofstudiessuggestedthatappropriatecongestioncontrolalgorithmsareessentialtoprovideefficientoperationofthenetwork.However,mostofcongestioncontrolalgorithmsarenotreallyapplicabletoevent-drivensafetymessages.InthispaperweproposecongestioncontrolalgorithmassolutiontopreventcongestioninVANETsenvironment.Weproposeacompletevalidationmethodandanalysetheperformanceofourcongestioncontrolalgorithmsforevent-drivensafetymessagesindifferencecongestedscenarios.TheeffectivenessoftheproposedcongestioncontrolalgorithmisevaluatedthroughthesimulationusingVeinssimulator.
IndexTerms—VANETs,Congestioncontrol,Event-
drivensafetymessage,Beaconmessages,CCHchannel
I.INTRODUCTION
WithVANETs(VehicleAd-HocNetworks)arecomposedofvehiclesequippedwithadvancedwirelesscommunicationdeviceswithoutanybasestations.EachvehicleequippedwithVANETsdevicewillbeanodeinthead-hocnetworkandcanreceiveandrelayothersmessagesthroughthewirelessnetwork.ThistypeofnetworkcanprovidewidevarietyofservicessuchasIntelligentTransportationSystem(ITS)[1,2].ThesafetyapplicationisoneofthemostcrucialapplicationinITS.Thesafetymessagesmustdelivertoeachneighbouringnodewithoutanydelay.Basically,wecandividesafetyapplicationsintwo
(2)categories;periodic(beacon)messagesandevent-drivensafetymessages.
Theperiodicsafetymessageexchangeispreventiveinnature,andtheobjectiveistoavoidtheoccurrenceofdangeroussituations.Theperiodicsafetymessagemaycontaininformationregardingtheposition,direction,andspeedofvehicles.Theevent-drivensafetymessagesmaybegeneratedasaresultofadangeroussituationorwhenanabnormalconditionisdetectedsuchasroadaccident[1,3].Theevent-drivensafetymessagesdisseminatedwithinacertainareawithhighpriority.Theevent-drivensafetymessagesusuallyhavestrongreliabilityandneedtobedeliveredtoeachneighborwithalmostnodelays.BothofthesesafetymessageswillsendthroughonesinglechannelkwonasControlChannel(CCH),whichisrestrictedtosafetycommunicationsonly.
Inhightraffic,alargenumberofvehiclesbroadcastbeaconsafetymessagesatahighfrequencyorevent-drivensafetymessagesarebroadcastmultipletimes;theCCHchannelwilleasilycongested.Thissituationwilldecreaseathroughputwhiledelayisincreasingsignificantly.Ontheotherhand,thesimplestwayofbroadcastingbeaconsafetymessagestoallnodesisusingblindflooding.Eachnodethatreceivedthepacketswillrebroadcastthispacketandwillleadthebroadcaststorm.InVANETs,itisbasedontheCarrierSenseMultipleAccesswithCollisionAvoidance(CSMA/CA)scheme[3,4,5].InCSMA/CA,whenanodereceivedapacketthatistransmitted,itsensestomakesurethechannelisclear(noothernodeistransmittedatthetime).Ifthechannelisidle,thenthepacketissent.Ifthechannelisbusy,thenodemustdeferitsaccessandduringhighutilizationperiodsthiscouldleadtounbounddelayed,itwillaffectthesafetyapplications[4].Furthermore,blindlybroadcastingthepacketmayleadtofrequentcontentionandcollisionsintransmissionamongneighbouringnodes.Moreover,anodecanexperienceverylongchannelaccessdelayduetotheriskofthechannelbeingbusyduringitslisteningperiodmessages[6].ItwillaffecttheperformanceofsafetyapplicationsinVANETs.
Thispaperaimstodevelopcongestioncontrolalgorithmtoprovidereliabilityfordisseminatingevent-drivensafetymessages.ThenouralgorithmisvalidatedthroughformalverificationsimulationusingUPPAALintegratedtool.Finally,theperformanceofproposedcongestioncontrolalgorithmevaluatedthroughVeinssimulatorbasedonwarningpacketdelay.
Therestpartofthepaperisorganizedasfollows:
InsectionII,wepresentrelatedworkconcerningcongestioncontrolalgorithmswithinVANETs.SectionIII,wediscussedourcongestioncontrolalgorithms.TheverificationandperformanceevaluationsarepresentedinsectionIV.Finally,sectionVconcludesthepaperwithoutlooksonthefuturework.
II.RELATEDWORKS
TherehavebeenseveralworksaddressingthecongestionprobleminVANET.Inresearch[7],theydevelopedacongestioncontrolapproachbasedontheconceptofdynamicpriorities-basedscheduling.Theyevaluateddynamicpriorityfactorbasedon:
nodespeedconsideration,messageutilityconsiderationandmessagevalidityconsideration.Thisapproachrequiredcontextexchangebetweenneighbournodes,whichgeneratesacommunicationoverhead.Onotherhand,thecongestioncontrolalgorithmforevent-drivensafetymessagesisproposedin[8].ThiscongestioncontrolevaluatedtheperformanceoftheSafetyElectronicBrakeLightwithForwarding(EEBL-F).Researchin[8]setthepredefinedthresholdintheircongestioncontrolalgorithmbasedonchannelusagelevel.Eachdeviceperiodicallysensesthechannelusagelevel,anddetectsthecongestionwheneverthemeasuredchannelusagelevelexceedsthepredefinedthreshold.Measuringthechannelusagelevelistoodifficulttoanalyseunderrealisticenvironmentdueofthedifferenttrafficload.Inasimilarstudyin[9],theyproposedcongestioncontrolalgorithmforDSRCbasedonsafetyapplications.However,theyjustassumedtheCCHchannelissuccessfullyreservedforevent-drivenapplicationswithouttestingthesuccessfullyrateforevent-drivensafetymessages.Inresearch[9],theysetthechanneloccupancytimeasthreshold.IfchanneloccupancytimemeasuredatanodeinCCHintervalislongerthanagiventhreshold,allbeaconsafetymessageswillbeblockedimmediatelyintheremainderofthatCCHintervalandtheCCHintervalfollowedtoreducechannelloadandreservespaceforevent-drivensafetymessages.Measuringthechannelwiththechanneloccupancytimeistoodifficulttoanalyseandneedstheproperratecontroldesign.
III.THEPROPOSEDCONGESTIONCONTROLALGORITHM
Inthisresearch,weproposedcongestioncontrol
algorithmthatcanensurehighreliabilityandtimely
deliveryofdisseminatingevent-drivensafetymessages.
Ourproposecongestioncontrolalgorithmcanbedivided
intotwomainparts:
event-drivendetectionand
measurement-baseddetection.InFigure1showsthe
flowchartstepsoftheproposedcongestioncontrol
algorithm.DetailsonMeasurement-BasedDetectionand
Event-DrivenDetectionthathasbeenusedinthe
flowchartwillbeexplainedlater.
A.Measurement-BasedDetection
Themeasurement-basedcongestiondetectionwill
monitorCCHchannelbasedonpacketschannelqueue.
TheCCHchanneliscongestedifthenumberofmessages
inthequeueexceededadefinedthreshold.Basedon
researchin[1]isconcludedthataqueuewithalengthof
fivebeaconmessagesissufficienttobeusedfor802.11p
beaconing.Inourproposedcongestioncontrolalgorithm,
thecongestioncontrolwilldiscardabeaconsafety
messageswheneverthelengthofpacketqueuemorethan
fivebeaconsafetymessages.
B.Event-DrivenDetection
Theevent-drivendetectionmethodmonitorstheevent-
drivensafetymessageanddecidestostartthecongestion
controlalgorithmwheneverevent-drivensafetymessage
isdetectedorgenerated.Thecongestioncontrolwill
launchimmediatelythequeuefreezingmethodforall
MACtransmissionqueuesexceptfortheevent-driven
safetymessage.Inordertosendevent-drivensafety
messagewiththeminimumdelay,thelowerpriority
messagessuchasbeaconmessagesemissionisfreeze.
Currently,theevent-drivendetectionmethodhasbeen
usedintheexistingofcongestioncontrolalgorithm[8].
Figure.1.Flowchartstepsoftheproposedcongestioncontrolalgorithm
V.SIMULATIONANDEXPERIMENT
A.Formalverification
Inordertoverifyourcongestioncontrolalgorithm,we
proposetospecifyitusingtheUPPAALintegratedtool.
Thistoolwasdevelopedincollaborationbetweenthe
DepartmentofInformationTechnologyatUppsala
University,SwedenandtheDepartmentofComputer
ScienceatAalborgUniversityinDenmark.Theformal
verificationhasbeenextensivelyusedtoanalyze
congestioncontrolalgorithminvehicularnetworks[2,7,
10].
Wedividedourcongestioncontrolalgorithmintofour
independentsub-systems:
messagemanager,message
queue,messagedequeueandengine.
a.TheMessageManager
Thissub-systemisresponsibleforgeneratingmessages
andtransferringmessagestothecong
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