I nt e r s ys t e mLocat i onUpdat e andPagi ngSchemesforMul t i t i erWi rel essNet works * WenyeWangIanF.Akyildiz Br oadband&Wi rel essNet wor ki ng Labor at or y Schoolof El ect ri calandComput er Engi neer i ng Georgi a I nst i t ut e of Technol ogy At l ant a, GA30332 {wenye,ian}@ece.gatech.edu ABSTRACT Gl obalwi r el essne t wor ks e na bl emobi l e user st ocom-muni c a t e r egar dl es s of t hei r l ocat i ons, Oneoft he mos t i mpor t a nt i ssuesisl ocat i onma na ge me nt i na hi ghl ydyna mi c e nvi r onme nt becaus emobi l euser s ma yr oa mbe t we e ndi f f er ent wi r el essnet works~net -wor koper at or s~a ndgeogr aphi cal r egi ons, I nt hi s paper~al ocat i ont r a c ki ngme c ha ni s misi nt r oduced~ whi chconsi st sofi nt e r s ys t e ml ocat i onupda t e s us i ng t heconcept of bounda r yl ocat i onar ea( BLA) a nd pagi ngus i ngt heconcept of bounda r yl ocat i onreg- i st er ( BLR) . The BLAisde t e r mi ne dbyadyna mi cl ocat i onupda t e pol i cyi nwhi cht hevel oci t ya ndt he qual i t yof ser vi ce(QoS)ar et a ke ni nt oaccount ona pe r - us e r basi s, The BLRisus edt oma i nt a i nt he r ecor ds of mobi l euser scr ossi ngt hebounda r yof net wor ks . Thi s me c ha ni s mnot onl yr educes l oca- t i ont r a c ki ngcost sbut al sosi gni f i cant l ydecr eases cal l lossr at es a ndaver agepagi ngdel ays. The per -f or ma nc e e va l ua t i on oft hepr opos e ds chemes ispr o-vi dedt ode mons t r a t e t hei r ef f ect i venessi nmul t i t i e rwi r el essnet wor ks .1.I NTRODUCTI ON Fut urewirelessnetworksaxeenvisioned asseamlessworld- wideradiosystems,includingdiversenetworkbackbones rangingfromhighcapacitypicocellnetworkstoterrestrial microandmacronetworksaswellaslargesatellitesys- tems[1,7,18].Theincreasing demandsforheterogeneous servicesnecessitatehighqualitylocationmanagementt hatallowsthewirelessnetworkstolocatemobile usersroaming *This workis support ed by NSFunder grant NCR-97-04393 and by Korea Telecom -Wireless Communications Research Laboratory(WCRL). Permission to make digital or bard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. 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MOBICOM 2000BostonMAUSA Copyright ACM 2000 1-58113-197-6/00/08...$5.00 acrossdifferent networks.Location managementis also crit- ical tomai nt ai n calls in progressas mobile usersmove into a new service area.In general,location managementcontains twoprocesses:location updat eandpaging.Forstand-alone wireless networks, bot hof t hem have been investigated com- prehensively over thepastdecade[2,5,6,21].In thispaper wefocusonthelocationmanagement schemesforthemo- bileterminals(MTs)roamingacrossmultitier wirelessnet- workswithdifferenttechnologiesorprotocols.In[13], the registration,calldelivery,andhandsetidentity forthehet- erogeneous personal communication services(PCS)systems werediscussed.Theregistration protocolsweremodified to accommodatenetworksusing differenttechnologiessuchas GSMandIS-41.In[23], theauthorssuggestedto"homoge- nize"the service areasof two adjacent networks by filling the biggersizecellswithsmallerones.Sothelocationupdat e andpaging would beimplemented asinonenetwork. Also, it has been demonst rat ed in [8] t hat the roaming across networksimposesasignificantincreaseinsignalingtraf- fic.Thetrafficanalysiswasbasedontheinterworkingof PCS1900and IS-41 systems by using a dual mode home loca- tion registration.In[17], theinterworking between cordless and cellular systems,e.g.,DECTandGSM,wasthoroughly investigated.Thesignalingtrafficofintersystemlocation registration and paging wasanalyzed based on different net- workarchitectures.In[27], theintersystemlocationregis- t rat i onwasstudiedonGSM/ PDCroamingandadditional interworking uni t s wereproposedtocarryoutsignaling for- mat transformationandauthentication. However,ineach of theabovepapers,thesignaling costswerenotcomput ed basedonany specific location updat eandpaging algorithm andthequalityofservice(QoS)requirementssuchascall lossandpagingdelaywerenotconsidered. Inthispaper,anewlocationmanagement mechanismis introduced, whichisapplicable totheintegration of hetero- geneousnetworks.InSection2,thesystemmodelof multi- tierwirelessnetworksisdescribed,basedontheconceptof boundarylocationarea(BLA)andboundarylocationregis- ter(BLR).Then, inSection3,theproceduresofintersys- tern location updat eand paging axepresented,whichutilize theBLAsandBLRstosupportregistration requestsand to locatemobile users.InSection 4,theprobabilities of inter- 99 syst em roami ngarederived fordifferent mobi l i t y models.In Section5,signalingcosts,calllossrates, andpagingdelays, i nt roducedbyi nt ersyst emlocationupdat eandpaging,are demonst rat ed. Finally, inSection6numericalresultsare provided, followedbyt heconclusioninSection7. 2.SYSTEMDESCRI PTI ON Themul t i t i erwirelessnetworkweconsideriscomprisedof manynetworksusingdifferentst andardssuchasGSM, IS- 95,IS-54/136andsoon.Eachnetworkinagivengeograph- icalareawillt ypi cal l ybesuppor t edbyaspecificnetwork operat or. Inthissectionwei nt roduceasyst emarchi t ect ure t hat capt urest hehierarchicalst ruct ureofmicrocellsand macrocells. 2.1Syst emArchi t ect ure Inamul t i t i erwirelessserviceareaconsistingofdissimi- larnetworks,itisdesirabletoconsidersomefactorswhich willinfluencet heradi oconnectionsof t hemobilet ermi nal s (MTs)roami ngbetweendifferentnetworks.Forexample,t hesignalingformat sformicrocellandmacrocelltiersare different.Evenint hesametier, t hesignalingformat, user i nformat i onandi dent i fi cat i on aut hori zat i onaxe different for networksusingdifferentprotocols.As showninFig.1,t here aretwonetworks, YandWint hemicrocell tier,whichmay usedifferent prot ocol s suchas DCS1800andPCS1900.Each hexagonrepresent sal ocat i onarea(LA)wi t hi nast and- alonenet workandeachLAiscomposedofaclusterofmi- crocells.Thet ermi nal sare requi red toupdat etheirlocation i nformat i on wi t h t henetwork whenever t hey enter anew LA; therefore,t henet workknowst heresidingLAofat ermi nalallt hetime. Int hemacrocelltiert herearealsotwonet- works,XandZinwhichdifferentprotocols(e.g.,GSMand IS-41)areapplied. Formacrocellnetworks,oneLAcanbe onemacrocell.It ispossiblet hat networksXandW, al- t houghindifferenttiers,mayempl oysimilarprotocolssuch asIS-95,GSMoranyot herprotocol. Weconsidert hetwo- t i ermodel t hroughout ouranalysis;however,t heproposed scheme is appl i cabl e tot hemul t i t i er scenario by t aki ng pairs of adj acent networks.-- IntrasystemRoamingI-- Intersystem Roaming Figure1:Systemarchitectureofintrasystemand i ntersystemroaming.Thereare twot ypes of roami ngshowninFig.1:intrasystem orintersystemroaming. Int rasyst emroami ngreferstoan MT' smovementbetweent heLAswi t hi nanetworksuchas YandZ.Int ersyst era roami ngreferstot heMTst hat move betweendifferentnetworks.Forexample, mobileusersmay travelfromamacrocellnetworkwi t hi nanIS-41networkto aregiont hat usesGSMst andard.2.2BoundaryLocationArea(BLA)and BoundaryLocationRegister(BLR) Concepts Inourcontext, t hel ocat i onmanagement inmul t i t i ernet- worktacklest hei nt ersyst emroami ng, whichincludesin- t ersyst emlocationupdat eandpaging.Thei nt ersyst emlo- cat i onupdat eisconcernedwi t hupdat i ngt hel ocat i onin- format i onofanMTperformi ngi nt ersyst emroami ng. The i nt ersyst em paging is concerned wi t hsearching for t hecalled t ermi nal roami ngbetweendifferentserviceareas.Thegoal of i nt ersyst emlocationmanagement istoreducet hesignal- ingcostwhilemai nt ai ni ngQoSrequirements. Forexampl e,reducingcall lossisoneof t hekeyissuesinmai nt ai ni ngt he callconnection;decreasingt hepagingdel ayiscriticalt o reducet hecallset -uptime.Fori nt ersyst eml ocat i onupdat e, weconsideraboundar y regioncalledboundarylocationarea (BLA)existingatt he boundar ybetweentwonetworksindifferenttiers.Asillus- t r at edinFig.2,networksXandYareint hemacrocell andmicrocelltiers,respectively.Thereisahomel ocat i on register(HLP~)foreachnetworkandauserisper manent l y associated withanHLRinhi s/ hersubscri bednetwork. The BLAiscontrolledbyaboundar yi nt erworki nguni t (BIU), whichisconnect edtomobileswitchingcenters(MSCs)and visitor l ocat i on registers(VLRs)in bot hnetworks. TheBIU is responsible forret ri evi ngauser' sservicei nformat i onand t ransformi ngmessageformats. Also,t heBIUisassumed tohandl esomeot herissuessuchast hecompat i bi l i t y of air interfacesandt heaut hent i cat i onof mobileusers.Thecon- figurationofaBIUdependsont hetwoadj acent networks t hat thisBIUiscoordi nat i ng. Moredet ai l scanbefound in[4,8,17,27]. (Macroccll~c~Tier)~ ~ 0 ~(MicrocellTier) LA 3.~.' LA. . . . . . . .."" "MSC andVLR inX/MSC and VLR inY BoundaryLocationArea(BLA) Figure2:Boundary locationarea(BLA)and bound- arylocationregister(BLR).100 TheBLAisconsideredasadynami cregiondependi ngon eachMT' sprofile suchas speed andbandwi dt hrequirement.WhenanMTent erst heBLA, itsendsregi st rat i onrequest tot heBIUandt heBIUforwards this requesttot henetwork t owardswhicht heMTismoving.Byusingt heBLAcon- cept,anMTisallowedtoregisterandupdat eitslocation beforet heMTreceivesormakescallsint henewnetwork. Inaddi t i ontot heconceptof BLA, wedesignateaboundary locationregister(BLR)tobeembeddedint heBIU.ABLR isadat abasecachetomai nt ai nt heroami nginformation ofMTsmovingbetweendifferentnetworks.Theroami ng i nformat i oniscapt ur edwhent heMTrequestsalocation regi st rat i onint heBLA. TheBLRsenablet hei nt ersyst em pagingtobei mpl ement edwi t hi nt heappropri at enetwork t hat anMTis current l y residing in,t hus reducing t he paging costs.Therefore,t heBLI% andt heBIUareaccessibleto t hetwoadj acent networksandarecollocated tohandl ethe i nt ersyst emroami ngof MTs.Ont hecontrary, t heVLRand t heMSCprovide roami ng i nformat i onwithinanetworkand dealwitht hei nt rasyst emroami ngofMTs.Besides,t here isonlyoneBLRandoneBIUbetweenapai rof neighboring networks,but t heremaybemanyVLRsandMSCswithin ast and-al onenetwork.3.I NTERSYSTEMLOCATI ON UPD ATE AND PAG I NG SCHEMES WhenanMTismovingacrossdifferentnetworks,itmustperformi nt ersyst eml ocat i onupdat e, whichisbasedont he conceptofBLA. Tolocatet heMTforcalldelivery,inter- syst empagingisrealizedt hroughBLR. 3 . 1I nt e r s ys t e mLocat i onUpda t e Usi ng BLAConc e ptWedefinet heBLAofanMTtobet heregioninwhicht he MTsendsal ocat i on regi st rat i on requesttot henewnetwork t owardswhicht hat MTismoving.Anewlocationupdat e mechani smisusedsucht hat t heMTwill report its location whenitsdi st ancefromt heboundar yis lesst hanupdatedis- tance,d~v,det ermi nedby(1).Thislocationupdat escheme guarant eest hat t heMTupdat esitslocationinformationin anareat hat iswi t hi nadi st ancet hreshol dawayfromt he boundar yoftwonetworks(tiers),XandY.Forexample,Fig.3showst hemovement pat hofanMTmovingfrom networkXtoY.Assumet hi sMTperformeditslastloca- tionupdat eint heLAwi t hi nXanditsupdat edistanceis d~. TheBLAof t hi sMTist heregionwithind~fromt he boundar yinXandt heregionwithind~inY. InorderforanMTtosendregi st rat i onrequestfromXto Y,t heprocedure of i nt ersyst em location updat eincludes t he followingsteps: 1.InnetworkX, t hebasest at i ons(BSs)int heLAcon- tiguous toYbroadcast t hei r distances from t hebound- aryof twonetworks(tiers)periodically.Thus,theMT candet ermi ne its l ocat i on wi t h regard tot he boundar y byreceivingt hei nformat i onfromt heBSs. 2.If t heMTfinds t hat it is wi t hi nt heBLA by compari ng t hel ocat i oni nformat i onwithupdat et hreshol dd~, it sendsani nt ersyst eml ocat i onupdat erequesttot he BIUont heboundary. Ne t wor kX "~........." ~" ~' / "z"7~: ~Yt ~"-.~Mobi l e. ,' : dY x", , .[ , ~ .terminal( M T ) , , , I- . - , . . , . . - . . - . - - . . - . - - . - ( Mi e r oc e l l Ti e r )Fi gur e 3:Locat i onupdat edi st anceandt heconceptof BLA.3.TheBIUt ransformst hesignalingformat , aut hent i - catest heuseridentity, sendsregi st rat i onrequeststo Y,andupdat est heMT' sprofileint heBLR. Ift hi s MTneedscall connectionwhile roami ng, its request is put int hecallconnectionqueueinY. Theaboveprocessdependsont heBLAt hat , int urn, isde- pendent ont heMT' supdat edistance. Adynami cl ocat i on updat epolicy is developed todet ermi ne t heupdat edi st ance onaper-user basis.Underthis scheme,t heupdat edistance,d~for t heMTmoving from Xto Yis based on t hefollowing considerations: Velocity~. IfanMTismovingveryfast,itmayar- riveatt henewnetworkinashort time. Thus, t he updat edi st anceshouldbelongsot hat t heMTcan sendt hel ocat i onupdat emessageandfinishlocation regi st rat i onbeforeitenterst henewnet workY.The velocityratiov~isdefinedasv~=G/v=,wherev~is t heaveragespeedofallMTsinmacrocellnetworkX.QoSfactor71.~1 ist herat i ooft hebandwi dt hre- quiredbyanMTtot hebandwi dt havailableint he newnetworkwhent hemul t i medi a serviceis provided.Ifr/0)ofaMarkovchainast he di st ancebet weent hecurrent locationoft heMTandt he centeroft heLA. Thi sst at eisequivalenttot heindexof aringinwhicht heMTislocated. Asaresult,t heMTis saidtobeinst at ekifitiscurrent l yresidinginringrk. Thet ransi t i onprobabi l i t i es ak, k+land/3k,k-1representt he probabi l i t i es at which t hedi st ance of t heMTfrom t he center cellof anLAincreasesanddecreases,whicharegivenas: ( l - q) if k= 0,ak, k+l =( 1- - q)(+~) i f l < k< K, (6) /~k,k-1=(1-q)(-~-~)if1