Evaluation of Distributed and Replicated HLR for Location Management in PCS Network

Transcription

1 JOURNAL OF INFORMATION SCIENCE AND ENGINEERING 9, 85-0 (2003) Evaluatio of Distributed ad Replicated HLR for Locatio Maagemet i PCS Network Departmet of Computer Sciece ad Iformatio Egieerig Natioal Chiao Tug Uiversity Hsichu, 300 Taiwa {gcche, sylee}@csie.ctu.edu.tw Persoal Commuicatio Service (PCS) etwork is the itegratio of cellular Network ad covetioal wired etwork. To support user mobility, the locatios of the mobile statios (MSs) should be costatly tracked usig a database. The widespread deploymet of PCS will lead to a tremedous icrease i the umber of updates ad queries to the locatio database. The cetralized, stad-aloe Home Locatio Register (HLR) i GSM ca be a potetial bottleeck. I case of the locatio database failure, icomig calls may be lost. Accordig to the aalysis of the load i HLR, we suggest replicatig the HLR i order to icrease its capacity ad reliability. I this paper we evaluate the effects of the umber of replicas i terms of query respose time, misroutig probability of icomig calls ad average cost per query. Aother issue icurred i the replicated databases is data cosistecy. A cocurrecy cotrol protocol called preemptive Read-Oe-Write-All (PROWA) protocol is also proposed. Keywords: mobile computig, locatio maagemet, HLR, replicated databases, PCS. INTRODUCTION To support user mobility, the PCS etworks have to store ad maitai locatio iformatio of mobile statios (MSs) so that a icomig call ca be delivered to the target MS. The operatios o locatio iformatio cosist of locatio updates ad locatio queries. A update occurs whe a MS chages locatio. A query occurs whe a MS eeds to be located, e.g., to deliver a icomig call to this MS. The widespread deploymet of PCS will lead to a tremedous icrease i the umber of updates ad queries to the locatio database. Thus, a key challege to locatio maagemet is to develop a efficiet database architecture so that the locatio data ca be readily available for sigalig such as call setup ad routig. The curret approach to support user mobility requires a two-level database []. This architecture cosists of a home locatio database (or Home Locatio Register, HLR) ad a visitor locatio database (or Visitor Locatio Register, VLR). This HLR-VLR architecture, has bee established as the de facto, ad is widely used i idustry, such as Global System for Mobile Telecommuicatio (GSM) for Europe ad the IS-4 recommedatios for North America. We will describe locatio maagemet i GSM ad IS-4 i more detail i sectio 2. I such systems, HLR is cetralized ad stad-aloe Received September 28, 2000; revised April 8, 200; accepted Jauary 25, Commuicated by Mig-Sya Che. 85

2 86 i the etwork. Some queries ad updates have to travel log distaces whe the icomig calls or the MSs are far away from the HLR. Aother drawback of covetioal HLR-VRL architecture is that HLR teds to be the bottleeck. A typical GSM mobile customer traffic profile is [2] 9 2 mi/day usage 60/40 split (60% of traffic is from wireless etwork to fixed etwork) average call duratio approximately 50 sec. switchig the phoe o/off approximately 4 times per day HLR implemetatios are commercially available to support approximately 300,000 customers. Each user with the above metioed traffic profile will submit approximately 20 HLR operatios per day (locatio updatig, routig, autheticatio, etwork attachmet). For a GSM with 300,000 subscribers, the load o HLR will thus be approximately 6,000,000 operatios per day. From experiece with SONOFON GSM, 2-3% of the operatios are durig busy hours, i.e. about 800,000 queries per hour (or 220 trasactio per secod). The peak value may be 50% higher [3]. Such a heavy load caot be supported by stadard relatioal databases eve o the most powerful processig equipmet available today. As a cosequece, HLR has the potetial to be a bottleeck ad caot guaratee the quality of service (e.g., call setup time). Aother class of mobility databases is based o the tree structures [4-6]. I such schemes, the user is assumed to be located at oe of the leave odes of a tree etwork. Some iteral odes i this tree may cotai a database which maitais a list of all users curretly located at odes i its associated subtree. For a give user, a poiter is maitaied at each database i the path from the root to the ode i which the user is located. The lowest database i the path poits to the cell i which the user is located. Ay other database o the path poits to the databases i the lower level o the path. Whe a MS moves to aother cell, oly the databases i the path betwee the two cells eed to be updated. I this architecture, a call setup is by search upward to a commo ode which cotais a locatio database ad the follows the poiters dowward to get the address of the cell where the target MS is located. This may ivolve may database queries whe the path betwee the icomig call ad the target MS is log. Therefore, the call setup time is relatively loger whe compared with HLR-VLR architecture. I additio to proposig ew database architectures, some researchers try to reduce the locatio registratio cost by dyamically adaptig the pagig area. These ca be classified ito three categories: distace-based [7, 8], movemet-based [9, 0] ad time-based []. The basic idea behid these schemes is that the user does ot make ay locatio update uless it has exceeded the boudary of a dyamically determied pagig area. These schemes ca efficietly reduce the umber of locatio registratios. However, sice the system has o exact locatio iformatio for the mobile hosts, there will be some pagig overhead ad the call setup time will be loger tha that i GSM ad IS-4. To guaratee the quality of service, we suggest adoptig the HLR-VLR architecture. However, the problem of the overloaded HLR must be take ito accout. From a aalysis of the load source i HLR, we suggest replicatig the HLR to avoid the performace bottleeck i HLR. A model to evaluate the effect of the umber of replicas is developed i this paper. The performace metrics cosidered i this model are the

3 REPLICATED HLR FOR LOCATION MANAGEMENT IN PCS NETWORK 87 average call setup time, the misroutig probability of icomig calls, ad average cost per query. Aother issue icurred i the replicated databases is data cosistecy. A modified Read-Oe-Write-All (ROWA) protocol referred to as Preemptive ROWA (PROWA) is also proposed. The rest of this paper is orgaized as follows. I ext sectio, we briefly describe the locatio maagemet i GSM ad IS-4. The effect of the umber of HLR copies is evaluated i sectio 3. Experimetal results ad coclusios are preseted i sectios 4 ad 5, respectively. 2. LOCATION MANAGEMENT IN GSM AND IS-4 I order to uderstad the locatio maagemet i PCS etwork, we briefly describe the HLR-VLR architecture used i GSM ad IS-4. The procedures of locatio update ad call delivery are also described. 2. HLR-VLR Architecture The commuicatio area covered by a base statio is called a cell. The locatio of a MS is thus the address of the cell i which this MS is located. The major task of mobility maagemet is to update the locatio of a MS whe it moves from oe cell to aother. The update procedure is referred to as registratio. We use GSM as a example to illustrate the HLR-VLR architecture ad the iformatio stored i HLR ad VLR. Fig. shows the etwork architecture of mobility maagemet i GSM []. Fig.. The etwork architecture i GSM. I this architecture, the base statio i each cell is coected to a Mobile Switch Ceter (MSC), which is a telephoe switch tailored for PCS etwork. Thus a MSC covers several cells. Oe (or several) MSC is coected to a VLR ad exchages the locatio iformatio with the correspodig VLR through Sigalig System Number 7 (SS7) etwork [3]. Similarly, the VLRs commuicate with the HLR to exchage the locatio iformatio usig SS7 messages.

4 88 The locatio maagemet i PCS etwork is achieved by the cooperatio of HLR ad VLR. The locatio iformatio of a MS stored i HLR are the ISDN umber (address) of the VLR visited by the MS ad the ISDN umber (address) of the MSC coected by the cell i which the MS is located. Similarly, the locatio data stored i VLR are the address of the correspodig MSC ad the cell idetity. 2.2 Registratio ad Call Delivery I GSM the locatio update is referred to as registratio ad occurs whe a MS moves from oe cell to aother. Accordig to the coectio relatioship betwee these two cells, the locatio update occurs i the followig three situatios []. Case. Iter-Cell movemet: These two cells are coected to the same MSC. Because the address of the cell i which the MS is located is stored i VLR, the registratio message should be set to VLR to update the cell idetity. However, sice the address of MSC ad VLR do ot chage i this situatio, o HLR update eed be executed. Case 2. Iter-MSC movemet: These two cells are coected to differet MSCs of the same VLR. Because the VLR stores the address of cells ad MSC, the registratio message should be set to VLR to update the record to idicate the curret locatio. I additio, the MSC address is also stored i HLR, the registratio message should also be set to the HLR to update the field of MSC address of this record. Case 3. Iter-VLR movemet: These two cells are coected to differet MSCs ad differet VLRs. Sice the MS moves to a differet VLR, this VLR does ot have a VLR record of the MS. The VLR creates a VLR record for the MS, ad seds a registratio message to update the HLR as described i Case 2 except that the VLR field is also updated. The record i the old VLR is deleted whe the de-registratio message from the ew VLR arrived or whe the old BS does ot receive the respose from the MS for a pre-defied time period. To deliver a icomig call to the target MS, routig iformatio (MSC address ad cell address) must be obtaied. Whe a icomig call arrives (from mobile or covetioal PSTN phoe), it is routed to a Gateway MSC (GMSC). The GMSC submits a query to the HLR to get the locatio of the target MS. Whe a query arrives, the HLR searches the database to get the address of the VLR i which the MS record is stored. After gettig the VLR address, the HLR submits a query to the correspodig VLR to get the MSC address ad cell address ad the returs them to the Gateway MSC. Based o the MSC address ad cell address, a icomig call ca be correctly delivered to the target MS. From the above descriptio, we ca fid that whe a icomig call arrives, both the HLR ad the correspodig VLR will be queried. However, whe a MS chages its locatio, the HLR will be updated oly whe Case 2 ad Case 3 occur. Because a MSC covers several cells, the frequecy of Case occurrig is far more tha that of Case 2 ad Case 3. From this observatio, we have to take differet cosideratios i the desig of HLR ad VLR.

5 REPLICATED HLR FOR LOCATION MANAGEMENT IN PCS NETWORK 89 The major tasks i HLR are 3. REPLICATED HLR locatio updatig routig (directig icomig calls to the target MSs) autheticatig (validatig a MS ) supplemetary services Except for locatio updatig, most processig ivolves oly a query operatio i HLR. I other words, i most situatios, the frequecy of a HLR beig queried is far greater tha that of a HLR beig updated. O the basis of this observatio, we propose replicatig the HLR (represeted as RHLR) to resolve the problem of HLR beig the performace bottleeck ad to icrease the reliability of HLR. The most importat issue i HLR replicatio is how may copies a HLR must be replicated ad where to place them. Replicatig a HLR ad distributig the replicas i the etwork ca icrease the reliability of HLR ad reduce the commuicatio cost as well as call setup time. O the other had, replicatig a HLR will icrease the cost to setup ad maitai database copies. Also, the database load will icrease due to the extra overhead of update operatios, makig the cotets of the databases cosistet. I this sectio, we evaluate the effects of the umber of replicas i terms of query respose time, misroutig probability of icomig calls, ad average cost per query. Before we evaluate the effect of the umber of replicas, we first describe the cocurrecy cotrol protocol used i this model. 3. Cocurrecy Cotrol A importat issue i replicated databases is to guaratee the cosistecy of data amog all the replicates. If the locatio data i the replicas of HLR are ot cosistet, some queries may get obsolete data ad cause the icomig calls to be misrouted or lost. May cocurrecy cotrol protocols have bee proposed to guaratee the serializability i replicated databases [2]. However, the characteristics of locatio data stored i HLR are differet from those of the data stored i covetioal databases. First, the query ad update i locatio maagemet i PCS etwork ofte ivolve oly oe record. Secod, the time to execute trasactios i locatio maagemet is differet from that i covetioal database service. I covetioal database services, e.g., the bakig system, the evet i the real world ca happe oly whe the correspodig trasactio commits. For example, whe oe wats to withdrawal moey from a accout, oe ca get the moey oly whe the withdraw trasactio commits. If the withdrawal trasactio fails, he/she must resubmit it or he/she caot get the moey. However, i PCS system, the update procedure is executed after a MS moves to a differet cell. No matter whether or ot a update trasactio ca commit, the movemet of the MS has occurred. Furthermore, the MS has bee i the ew cell eve durig the processig of the update procedure. Some researchers proposed cocurrecy cotrol strategies which guaratee the serializability of the executio of query ad update operatio i mobility databases. Some researchers proposed cocurrecy cotrol strategies which guaratee the serializa-

6 90 bility of the executio of query ad update operatio i mobility databases. For example, i 997 K. K. Leug proposed a update algorithm called Primary-Writer Protocol (PWP) usig a multi-versio approach [3]. I PWP each record is replicated ad distributed to may sites, ad oe of the replicas is assiged to be the primary site (PS) of this record. Queries are routed to the PS or other replicas accordig to the call distributio algorithm. A update must be set to the primary site, ad the, if committed, set to other replicas. PWP keep multiple versios for a record. A sequece of update operatios is executed i the replicas accordig to the order of their versio umbers i the PS. A query may eed to read the old versio of a record to prevet violatig serializability. However, sice a mobile uit has oly oe curret locatio, it is meaigless to keep multiple versios of the locatio iformatio ad let a query read old versios of locatio data. Although the executio order of query ad update operatio i PWP guaratees serializability, the MS has already bee i a ew cell. As a cosequece, the query will get a obsolete locatio data ad cause the icomig call to be misrouted or lost. I this paper we preset the observatio that the update procedure i a locatio database is executed after the movemet of a MS. No matter whether or ot a update trasactio ca commit, the movemet of the MS has occurred. Therefore, it is meaigless to let a query read old versios of locatio data. From this observatio, we suggest abortig the queries o a record if a update request o the same record arrives. This is what we called preemptive. The Read-Oe-Write-All (ROWA) protocol is employed i may replicated database systems to guaratee cosistet cotet [2]. I ROWA protocol, a query is routed to oe of the replicas accordig to the routig fuctio, ad a update message is set to all replicas to guaratee the cosistecy. I order to reduce the probability of a icomig call beig misrouted or lost, we modify the Read-Oe-Write-All (ROWA) protocol ad propose the Preemptive Read-Oe-Write-All (PROWA) protocol. The mai idea of PROWA is based o the above observatio. Whe a update operatio arrives at the database, if there is a query operatio executig o the same record, cocurrecy cotrol aget will abort the query operatio ad restart it after a certai time (log eough to complete a update operatio) i order to get the correct locatio data. fter the descriptio of the cocurrecy cotrol protocol, we ca ow preset the proposed model to evaluate the effect of the umber of replicas. 3.2 System Model We partitio the service area of the PCS etwork ito several service regios (SRs). Each SR is covered by a GMSC. This partitio reflects the situatio that a query submitted to HLR is iitiated from a GMSC; Table lists the otatios ad symbols used. The loads icurred by autheticatio ad supplemetary service are ot discussed here. The capacity of a database is defied as the umber of queries that a database ca process per uit time. The mai idea of the RHLR scheme is to replicate the HLR ad distribute the replicas to the SRs. However, how may replicas a HLR should be replicated ad where to locate the replicas are the mai issues of the RHLR scheme. I the followig sectios, we describe a liear programmig model to decide the placemet of the replicas.

7 REPLICATED HLR FOR LOCATION MANAGEMENT IN PCS NETWORK 9 Table. Defiitios of symbols used. Symbol S i λu i λq i C ij M P B α β σ ω ρ K q D i R ij Defiitio a service regio umber of service regios arrival rate of locatio updates submitted from S i to the HLR arrival rate of queries submitted from S i to the HLR commuicatio cost to trasfer a SS7 message betwee S i ad S j average cost to setup ad maitai a database per uit time capacity of a database (umber of operatios the database ca execute per secod) badwidth of a sigalig lik (bits per secod) load to process a query operatio i HLR load to process a update operatio i HLR cost to process a query operatio i HLR cost to process a update operatio i HLR legth of a SS7 message for query ad update (bytes) umber of replicas of a HLR query-to-update ratio umber of HLR replicas located i regio S i ratio of queries submitted from S i to the database located i S j over all queries submitted from S i I additio, the relatioships betwee the umber of replicas ad system performace (query respose time, average cost per query ad the probability of a icomig call is misrouted) are also discussed i this paper. The systems egieer ca determie the umber of replicas accordig to the quality-of-service the system should provide ad the available budget. 3.3 Query Respose Time The time to set up a icomig call is affected by the time eeded to get the locatio iformatio of the target MS (by submittig a query to the HLR). The time to complete a query ca be divided ito three parts: the time to trasfer SS7 messages betwee the GMSC ad the HLR, the time to complete a query i the HLR ad the time to get the MSC address ad cell address from VLR. Sice our mai objective here is to gai a geeral uderstadig of the relative performace with respect to the umber of replicas, we make the followig assumptios. () Sice the umber of queries ad updates submitted from a SR, S i, is large eough, they ca be approximated by a Poisso process with mea λq i ad λu i respectively []. (2) The databases ad sigalig liks are modeled as a sigle-server queue. Queries ad updates are processed ad trasmitted o a First-Come-First-Serve (FCFS) basis. I order to maximize the utilizatio of each replica, we balace the load amog all replicas. This ca be achieved by defiig a routig fuctio for queries. We will discuss how to determie the routig fuctio for queries i later discussio. The time to trasfer a SS7 message betwee GMSC ad HLR cosists of message delay ad propagatio delay. Whe a HLR is replicated K times, the load i

8 92 each replica is ( α λqi ) / K + β λ ui. Accordig to Little s theory [4], the message delay is i= i= T m = B /(8ρ) ( λq ) / K i i= i= λu i. () I Eq. (), B is the badwidth of the lik betwee two adjacet SRs. Similarly, if there are K replicas of a HLR, the time to process a query i HLR is Td = P ( α λq ) / K β i u i i= i= λ. (2) The average propagatio delay is affected by the placemet of the K replicas. The optimal value of the average propagatio delay ca be determied by liear programmig. We describe the model to determie the optimal placemet of the replicas as follows: (A) Objective Fuctio for Miimizig the Propagatio Delay Our objective is to miimize the average propagatio delay of a query. We use R ij to represet the ratio of queries which are submitted by S i to S j over all queries submitted by S i. F ij is the propagatio delay of the lik betwee S i ad S j. The average propagatio delay of a query is thus Z 2 = λ qi Rij Fij. i= j= (B) Costraits (i) Query Satisfactio Costrait To esure that every query will have a defied route, the followig costrait must be satisfied. j= R = for all i, i ij (ii) Load Balacig Costrait The load is balaced amog all replicas. If there are D j replicas allocated ito S j, the followig costrait must be satisfied.

9 REPLICATED HLR FOR LOCATION MANAGEMENT IN PCS NETWORK 93 ( λ R ) = (( λ ) / K) D, for all j, j. qi ij i= i= qi (iii) Costrait o Number of Replicas The sum of the umber of replicas i all SRs must equal K. D i = K i= The resultig model is: Mi λ R i= j= qi ij F ij j s.t. j= R ij =, ( i= λ R ) = (( λ ) / K) D, for all j, j. qi ij i= qi j We do ot modify the architecture of VLR i GSM ad IS-4. The time to get the MSC address ad cell address from VLR does ot chage whe a HLR is replicated. For simplicity, we assume that the time to get the MSC ad cell addresses from VLR is costat ad is referred to as T qv. As discussed i sectio 2.2, the queried result must be set back to the Gateway MSC to route the icomig call to the target MS. The average time to complete a query whe the umber of replicas equals K is ( λ R (2F ) / λ ) + T + T + T (3) qi ij ij i= j= i= qi d m qv If the umber of replicas, K, equals, the system is reduced to GSM. The average completio time of a query is determied by the etwork delay as well as the time to complete the query i HLR. I GSM, because there is oly oe database, all queries are routed to the same regio. However, whe the HLR is replicated ad distributed i the etwork, the query ca be routed to the database i a earby regio. I additio, the time to complete a query i HLR will decrease as the umber of replicas icreases. It is obvious that the completio time of a query is shorter tha that of GSM i most situatios.

10 Cost of Replicated HLR The cost of Replicated HLR cosists of the commuicatio cost, operatig cost ad the cost to set up ad maitai a database. We aalysis the cost of RHLR as follows: (A) Commuicatio cost The PROWA protocol is implemeted i the replicated HLR. All updates must be set to all the database copies to make the cotet cosistet. The total commuicatio cost for update ad query operatio per uit time is Update commuicatio cost: Z = λ uid jcij i= j = Query commuicatio cost: Z2 = λ qirijcij i= j = (B) Database cost The cost to setup ad maitai a database per time uit is deoted by M. The total cost to set up ad maitai K replicas is Z 3 = MK (C) Cost of query ad update operatios I order to maitai data cosistecy, i replicated HLR, some extra update operatios eed to be executed whe compared with the cetralized, stad-aloe HLR. I the cetralized, stad-aloe HLR architecture, a update procedure eeds to oly update oe database. However, i replicated HLR, the update procedure eeds to update all replicas. The cost to process query ad update operatios is D i i= i= i= λqi) + ω( ui)( Z 4 = σ ( λ ). The overall cost is the sum of all the costs ad is equal to Z = Z + Z 2 + Z 3 + Z 4 (4) The value of Eq. (4) varies depedig o database cost, operatio cost, as well as the commuicatio cost. With advaces i computer systems, the cost to set up a database will steadily decrease. However, the cost to trasfer a message i the etwork is ot expected to rapidly decrease i the ear future.

11 REPLICATED HLR FOR LOCATION MANAGEMENT IN PCS NETWORK Probability of Icomig Calls Beig Misrouted or Lost To evaluate the probability of a icomig call beig misrouted or lost due to obsolete data from HLR, we defie the vulerable period to be the time iterval durig which, if a query completes, it may get a obsolete locatio data ad thus cause a call to be misrouted or lost. We use the misroutig probability of icomig calls to represet the probability of a icomig call beig misrouted or lost due to obsolete data from HLR. Fig. 2 illustrates the vulerable period. I Fig. 2, a MS movemet of Case 2 or Case 3 (defied i sectio 2., that is, iter-msc movemet or iter-vlr movemet) occurs (i S i whe a icomig call arrives i S k ). S j is the SR with a HLR replica to which a query i S k is routed. I PROWA, if a query is executed o the same record whe a update arrives, sice a locatio update arrived meas that correspodig MS has already moved, the cocurrecy cotrol aget will abort this query ad restart it after a certai period of time. However, eve i PROWA protocol, there will be a chace that a icomig call is misrouted or lost. A icomig call will be misrouted or lost if it receives the cell address; S i i Fig. 2, after the MS s movemet or the query arrives i S j before the correspodig update arrives. This time iterval is the vulerable period as illustrated i Fig. 2. Fig. 2. Vulerable period for icomig calls beig misrouted or lost. Now we evaluate the probability that a icomig call is misrouted or lost. Assume that the HLR update of a HLR is Possio distributed with mea µ. The average time iterval betwee two updates is /µ. The probability of a icomig call to this MS beig misrouted or lost is the vulerable period over the average time iterval betwee two updates. As show i Fig. 2, the vulerable period is (t 2 t ). Therefore, the probability of a icomig call to this MS beig misrouted or lost is (F ij +F kj +T m )/(/µ), where F ij ad F kj are the propagatio delays of S i -S j ad S j -S k, respectively, ad T m is the message delay of HLR.

12 96 4. EXPERIMENTAL RESULTS The sigalig etwork used i the simulatio is a 5 5 mesh etwork, which has 25 SRs. The queries ad updates submitted to HLR are uiformly distributed i each SR. The commuicatio cost betwee the mobile or covetioal PSTN phoe ad the Gateway MSC i the same SR is ormalized to. The commuicatio cost betwee two SRs is set to the legth of the shortest path betwee them. Each database ca support 220 queries per secod. Each query costs to be processed i the database. The cost of a update processed i database is set to.. For demostratio purposes, the average cost, M, to set up ad maitai a database is set to,000 i the experimets. The trasmissio speed of a lik i each directio is set to 28k bits per secod, which is the curret ISDN stadardized trasmissio rate. The propagatio delay i the same SR is set to 5 msec. The propagatio delay betwee two SRs is i proportio to the legth of the shortest path betwee them. The delay, T qv, caused by gettig the result from VLR is set to 20 msec. 4. Load-Related Experimets I these experimets we evaluate the effects of the umber of replicas, K, i various loads. We vary the umber of queries submitted from a service regio from 00 to 600. A large umber of queries submitted from a service regio may represet the load at peak times. Fig. 3 shows the average completio time of a query. Let q represets the query-to-update ratio. I load-related experimets, q is set to 5. We will discuss the effects of the query-to-update ratio i the followig sectios. I Fig. 3 ad the followig figures, K represets the result obtaied from the aalytic model ad SK represets the result obtaied from the simulatio model. From Fig. 3, as expected, we fid that the average completio time of a query is shorter whe a large umber of replicas is used. However, it will be ifiitely log if the load i each replica exceeds the capacity of the database. The umbers o top of each curve i Fig. 3 give the maximum load that ca be supported. Sice the results obtaied from the simulatio ad the aalytic model are quite close, for clarity of illustratio, the lies for the simulatio results are ot draw i the figures i the followig. I additio to reducig the average completio time of a query, replicatig a HLR ca also reduce the misroutig probability of icomig calls. Accordig to the discussio i sectio 3.5, the more copies a HLR replicates, the lower the misroutig probability of icomig calls is. The experimetal result is show i Fig. 4, where P misrouted represets the misroutig probability of icomig calls. Although icreasig the umber of replicas ca reduce both the average completio time ad the misroutig probability of icomig calls, the cost will icrease whe the umber of replicas icreases. He result of cost-related experimets is show i Fig.5. Here the average cost is the total cost (commuicatio cost, database cost ad cost of queries ad updates) divided by the umber of icomig calls. There is o sigificat differece amog the average costs of differet umbers of replicas. The reaso is as follows. Whe the umber of replicas icreases, the cost caused by the updates also icreases. However, the commuicatio cost of a query will decrease if the query ca be routed to a earer site.

13 REPLICATED HLR FOR LOCATION MANAGEMENT IN PCS NETWORK Fig. 3. Average query respose time for query-to-update ratio q = 5. Fig. 4. Probability of call beig misrouted for query-to-update ratio q = 5. Fig. 5. Average costs for q = 5. 97

14 Access-Patter-Related Experimets The performace of RHLR will be affected by the ratio of updates over all operatios. We use q to represet the query-to-update ratio which meas that there are q queries arrivig betwee two cosecutive updates. If the total load is fixed, a large value of q meas fewer updates ad the load i each replica will be less. As a cosequece, the time to process a query i HLR will be shorter. As we see i Figs. 6 ad 7, whe the value of q icreases, the query respose time ad the misroutig probability of icomig calls decrease. However, the decrease i respose time for a fix value of K is ot sigificat. This is because that the load of a update i a database is set to.. If the load to process a update were set to a larger value, the decrease would be greater. Fig. 6. Query respose time for load =00. Fig. 7. Misroutig probability of icomig calls for load = 00. Similar to the query respose time, there are fewer updates whe the query-to-update ratio is high, the cost resultig from updates is smaller tha i low query-to-update ratios. The differece betwee the average cost with differet umbers of replicas decreases as the value of q grows because there are fewer updates for larger q. Experimetal results are show i Fig. 8.

15 REPLICATED HLR FOR LOCATION MANAGEMENT IN PCS NETWORK 99 Fig. 8. Average cost for load = CONCLUSIONS I this paper we preset two observatios i mobility maagemet of PCS etwork ad propose correspodig mechaisms to improve the system performace. The first observatio is that the frequecy with which a HLR is queried is greater tha that of it beig updated. Based o this observatio, we propose replicatig the HLR. The systems egieer ca determie the umber of replicas depedig o the desired quality of ad the available budget. The optimal placemet of the replicas is determied usig the liear programmig method give i this paper. Aother observatio is that the update procedure i the locatio database is executed after the movemet of a MS. From to this observatio, we suggest abortig the queries i a record if a update arrives i the same record. This is what we called preemptive. The property preemptive ca be applied to other cocurrecy cotrol protocols. Sice the read-oe-write-all (ROWA) protocol is the most commoly used protocol i existig commercial replicated databases, we modify ROWA to be Preemptive ROWA (PROWA) i this paper. PROWA protocol ca reduce the misroutig probability of icomig calls by abortig the query operatios whe a update operatio arrives i the same record. The cost of replicated HLR icreases as the umber of replicas icreases. However, the call setup time is shorter ad the misroutig probability of icomig calls is lower whe we add more replicas. I other words, RHLR provides better quality of service tha GSM by payig a extra cost. Whe the query-to-update ratio is high, the average cost per icomig call i RHLR is slightly higher tha that i GSM ad IS-4. However, the respose time of RHLR is much shorter tha i GSM ad IS-4. I additio, RHLR ca solve the problem that sigle HLR teds to be the performace bottleeck. Updatig all the replicated databases to maitai cosistecy of cotet icurs extra cost. I the future we will try to develop a ew cocurrecy cotrol protocol to reduce the cost of makig the cotets i the replicated HLR cosistet. REFERENCES. Y. -B. Li ad I. Chlamtac, Wireless ad Mobile Network Architectures, Wiley, New York, B. Gabelgaard, The (GSM) HLR advatages ad challeges, i Proceedigs of

16 00 the 3rd IEEE Iteratioal Coferece o Uiversal Persoal Commuicatios, 994, pp S. Moha ad R. Jai, Two user locatio strategies for persoal commuicatio services, IEEE Persoal Commuicatios, Vol., 994, pp S. Dolev, D. K. Pradha, ad J. L. Welch, Modified tree structure for locatio maagemet i mobile eviromet, i Proceedigs of IEEE INFOCOM 95, 995, pp A. Hac ad C. Sheg, User mobility maagemet i PCS etwork: hierarchical databases ad their placemet, i Proceedigs of the 5th IEEE Iteratioal Coferece o Uiversal Persoal Commuicatios (ICUPC), 996, pp H. -C. Li ad S. L. Lee, A presettig locatio strategy for persoal commuicatio usig hierarchical locatio database, i Proceedigs of the 7th Iteratioal Coferece o Parallel ad Distributed Systems, 2000, pp M. Verkama, A simple implemetatio of distace-based locatio updates, i Proceedigs of the 6th IEEE Iteratioal Coferece o Uiversal Persoal Commuicatios (ICUPC), 997, pp J. H. Zhag ad J. W. Mark, A local VLR cluster approach to locatio maagemet for PCS etworks, i Proceedigs of 999 IEEE Wireless Commuicatios ad Networkig Coferece, 999, pp I. F. Akyildiz, J. S. M. Ho, ad Y. -B. Li, Movemet-based locatio update ad selective pagig for PCS etworks, IEEE/ACM Trasactios o Networkig, Vol. 4, 996, pp V. Casares-Gier ad J. Mataix-Oltra, O movemet-based mobility trackig strategy-a ehaced versio, IEEE Commuicatio Letters, Vol. 2, 998, pp S. H. Ryu, K. H. Lee, Y. Y. Oh, J. Y. Lee, ad S. B. Lee, Adaptive time-based locatio update scheme usig fuzzy logic, i Proceedigs of 999 Iteratioal Coferece o Cosumer Electroics (ICCE), 999, pp A. K. Elmagarmid, Database Trasactio Models for Advaced Applicatios, Morga Kaufma, K. K. Leug, A update algorithm for replicated sigalig databases i wireless ad advaced itelliget etworks, IEEE Trasactios o Computers, Vol. 46, 997, pp L. Kleirock, Queueig Systems, Wiley, New York, 975. Gua-Chi Che ( 陳冠棋 ) received the B.S. degree i Maagemet Iformatio System from Natioal Cetral Uiversity, Taiwa, i 993, the M.S. degree i Maagemet Iformatio System from Natioal Taiwa Istitute of Techology, Taiwa, i 995, ad the Ph.D. i Computer Sciece ad Iformatio Egieerig from Natioal Chiao Tug Uiversity, Taiwa, i 200. Curretly, he is a seior egieer at Ambit Microsystems Co., Taiwa. His research iterests iclude wireless etwork, database systems, mobile computig ad multimedia iformatio systems.

17 REPLICATED HLR FOR LOCATION MANAGEMENT IN PCS NETWORK 0 Suh-Yi Lee ( 李素瑛 ) received the B.S. degree i electrical egieerig from Natioal Chiao Tug Uiversity, Taiwa, i 972, the M.S. degree i computer sciece from Uiversity of Washigto, U.S.A., i 975, ad the Ph.D. degree i computer sciece from Istitute of Electroics, Natioal Chiao Tug Uiversity. Her research iterests iclude multimedia iformatio system, computer etwork, mobile computig, data miig, ad itelliget aget o web.