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SEP:AStableElectionProtocolforclustered

heterogeneouswirelesssensornetworks

GEORGIOSSMARAGDAKISIBRAHIMMATTAAZERBESTAVROSComputerScienceDepartment

BostonUniversity

gsmaragd,matta,best@cs.bu.edu

Abstract—Westudytheimpactofheterogeneityofnodes,intermsoftheirenergy,inwirelesssensornetworksthatarehierarchicallyclustered.Inthesenetworkssomeofthenodesbecomeclusterheads,aggregatethedataoftheirclustermembersandtransmitittothesink.Weassumethatapercentageofthepopulationofsensornodesisequippedwithadditionalenergyresources—thisisasourceofheterogeneitywhichmayresultfromtheinitialsettingorastheoperationofthenetworkevolves.Wealsoassumethatthesensorsarerandomly(uniformly)distributedandarenotmobile,thecoordinatesofthesinkandthedimensionsofthesensorfieldareknown.Weshowthatthebehaviorofsuchsensornetworksbecomesveryunstableoncethefirstnodedies,especiallyinthepresenceofnodeheterogeneity.Classicalclusteringprotocolsassumethatallthenodesareequippedwiththesameamountofenergyandasaresult,theycannottakefulladvantageofthepresenceofnodeheterogeneity.WeproposeSEP,aheterogeneous-awareprotocoltoprolongthetimeintervalbeforethedeathofthefirstnode(werefertoasstabilityperiod),whichiscrucialformanyapplicationswherethefeedbackfromthesensornetworkmustbereliable.SEPisbasedonweightedelectionprobabilitiesofeachnodetobecomeclusterheadaccordingtotheremainingenergyineachnode.WeshowbysimulationthatSEPalwaysprolongsthestabilityperiodcomparedto(andthattheaveragethroughputisgreaterthan)theoneobtainedusingcurrentclusteringprotocols.WeconcludebystudyingthesensitivityofourSEPprotocoltoheterogeneityparameterscapturingenergyimbalanceinthenetwork.WefoundthatSEPyieldslongerstabilityregionforhighervaluesofextraenergybroughtbymorepowerfulnodes.

I.INTRODUCTION

Motivation:WirelessSensorNetworksarenetworksoftiny,

batterypoweredsensornodeswithlimitedon-boardprocess-

ing,storageandradiocapabilities[1].Nodessenseandsend

theirreportstowardaprocessingcenterwhichiscalled“sink.”

Thedesignofprotocolsandapplicationsforsuchnetworks

hastobeenergyawareinordertoprolongthelifetime

ofthenetwork,becausethereplacementoftheembedded

batteriesisaverydifficultprocessoncethesenodeshave

beendeployed.ClassicalapproacheslikeDirectTransmission

andMinimumTransmissionEnergy[2]donotguaranteewell

balanceddistributionoftheenergyloadamongnodesofthe

sensornetwork.UsingDirectTransmission(DT),sensornodes

transmitdirectlytothesink,asaresultnodesthatarefar

awayfromthesinkwoulddiefirst[3].Ontheotherhand,

usingMinimumTransmissionEnergy(MTE),dataisrouted

ThisworkwassupportedinpartbyNSFgrantsITRANI-0205294,EIA-0202067,ANI-0095988,andANI-9986397.overminimum-costroutes,wherecostreflectsthetransmission

powerexpended.UnderMTE,nodesthatarenearthesinkact

asrelayswithhigherprobabilitythannodesthatarefarfrom

thesink.Thusnodesnearthesinktendtodiefast.Underboth

DTandMTE,apartofthefieldwillnotbemonitoredfora

significantpartofthelifetimeofthenetwork,andasaresult

thesensingprocessofthefieldwillbebiased.Asolution

proposedin[4],calledLEACH,guaranteesthattheenergy

loadiswelldistributedbydynamicallycreatedclusters,using

clusterheadsdynamicallyelectedaccordingtoapriorioptimal

probability.Clusterheadsaggregatereportsfromtheircluster

membersbeforeforwardingthemtothesink.Byrotatingthe

cluster-headroleuniformlyamongallnodes,eachnodetends

toexpendthesameenergyovertime.

MostoftheanalyticalresultsforLEACH-typeschemesare

obtainedassumingthatthenodesofthesensornetworkare

equippedwiththesameamountofenergy—thisisthecase

ofhomogeneoussensornetworks.Inthispaperwestudythe

impactofheterogeneityintermsofnodeenergy.Weassume

thatapercentageofthenodepopulationisequippedwith

moreenergythantherestofthenodesinthesamenetwork—

thisisthecaseofheterogeneoussensornetworks.Weare

motivatedbythefactthattherearealotofapplications

thatwouldhighlybenefitfromunderstandingtheimpactof

suchheterogeneity.Oneoftheseapplicationscouldbethe

re-energizationofsensornetworks.Asthelifetimeofsensor

networksislimitedthereisaneedtore-energizethesensor

networkbyaddingmorenodes.Thesenodeswillbeequipped

withmoreenergythanthenodesthatarealreadyinuse,which

createsheterogeneityintermsofnodeenergy.Notethatdue

topractical/costconstraintsitisnotalwayspossibletosatisfy

theconstraintsforoptimaldistributionbetweendifferenttypes

ofnodesasproposedin[5].

Therearealsoapplicationswherethespatialdensityofsen-

sorsisaconstraint.Assumingthatwiththecurrenttechnology

thecostofasensoristensoftimesgreaterthanthecostof

embeddedbatteries,itwillbevaluabletoexaminewhetherthe

lifetimeofthenetworkcouldbeincreasedbysimplydistribut-

ingextraenergytosomeexistingnodeswithoutintroducing

newnodes.1

1Wealsostudythecaseofuniformlydistributingsuchextraenergyoverallnodes.Inpractice,however,itmaybedifficulttoachievesuchuniformdistributionbecauseextraenergycouldbeexpressedonlyintermsofdiscretebatteryunits.Evenifthisispossible,weshowinthispaperthatsuchfairdistributionofextraenergyisnotalwaysbeneficial.

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