An Enhanced Intersystem Location Management Scheme. Based On User Profile Replication

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1 An Enhanced Intersystem Location Management Scheme Based On User Profile Replication HyeJeong Lee and Dong-Ho Cho Division of Electrical Engineering, Dept. of Electrical Engineering & Computer Science, KAIST, 373-1, Guseong-dong, Yuseong-gu, Daejeon, , Korea Abstract Given the increasing demand for international roaming, it is very important to manage the location of roaming subscribers efficiently and cost-effectively. When a user requires location registration in the roaming network, many signaling messages and database accesses are involved in the location registration between its home network and the roaming network. Therefore, we propose a new intersystem location management scheme to replicate roaming user s service profile at mobility gateway. The analysis results show that the proposed scheme achieves lower signaling cost, database access cost and registration latency compared with the previous schemes. I. INTRODUCTION As business and personal life styles become increasingly international, the need to communicate anytime and anywhere increases critically. The demand of roaming calls has been more than 900 million in 2001, and will be expected to grow over 1 billion (20% of worldwide mobile service revenues) in the near future. Currently, the networks such as GSM, CDMA and TDMA/AMPS are installed tremendously over the world. Also, several standards like IS-95A/B, cdma2000 1x and 1xEV- DO/EV are adopted together. But those standards have no compatibility each other and each system has a different mobility management method, based on different Mobility Application Part (MAP) protocols such as IS-41 [1] and GSM-MAP [2]. Therefore, to realize global roaming, intersystem and interstandard roaming capabilities are required. Unlike fixed static networks, the current location of a Mobile Terminal (MT) cannot be obtained from its identification number in the mobile communication systems. Therefore, a location management scheme is needed to effectively keep track of the location of the MT and to locate a called MT when an incoming call is initiated. Previous research activities for location management have concentrated mainly on efficient location update and paging algorithm in stand-alone system. But for the roaming users moving among different networks, new efficient location management schemes are required. Since there is no centralized common database, many signaling messages in association with authentication, retrieval of the subscriber profile, and intersystem registration/call delivery, are exchanged between the home network and the roaming network. This process can be very costly because such activity consumes communication resources including radio bandwidth and frequent database accesses. Other possible problems exist such as the need of signal conversion between the different mobility protocol, billing, numbering and fraud management. Several mobility gateways, such as Interworking Services Gateway (ISG)[3], Jambala Mobility Gateway [4] and Gateway Location Register (GLR) [5] of 3GPP, have been developed for intersystem mobility management. Those mobility gateways perform location registration after an MT arrives at the new system and provide interworking functionality such as signaling conversion and forwarding traffics into another networks. Therefore, there exist severe overhead for intersystem location registration and large registration latency when retrieving subscriber profiles from the remote HLR. A new signaling protocol called the (Boundary Location Register -Mobility Application Part) is proposed in [6] and [7], which focuses on the active location registration for ongoing services when the MTs are moving. In the, the MT is allowed to update its location information prior to reaching the roaming network, i.e., when it enters a Boundary Location Area (BLA). By using the, the intersystem location registration latency and call losses after roaming can be reduced. However, if the roaming probability of the MT in a BLA is relatively small, there exist an unnecessary prior intersystem registration process and a waste of database resource. Also, the does not consider the effect of user s mobility and the prior registration to the roaming network for ongoing sessions is somewhat inefficient because location registration is performed during the idle state of MT. In this paper, we propose a new intersystem location management scheme for intersystem roaming to solve the above problems. The rest of this paper is organized as follows. We first present a new intersystem location management scheme in Section 2. In Section 3, we analyze the performance of the intersystem location management schemes. Based on some typical model parameters, several numerical results of the location management schemes are compared in Section 4. Finally, Section 5 presents conclusions and further study area. II. PROPOSED INTERSYSTEM LOCATION MANAGEMENT SCHEME We consider the system model shown in Fig. 1. It is assumed that two systems X and Y use different protocols. We designate the location areas that are adjacent to the system boundary to Intersystem Location Areas (ILAs). For convenience, it is assumed that the MTs can move from system X to system Y only through these ILAs. Because each system X and Y uses a different protocol, a mobility gateway is required that is responsible for signal conversion, maintenance of user profile, and internetwork location registration. We call this mobility gateway the Intersystem Roaming Gateway (IRG) and represent proposed intersystem location management scheme as scheme.

2 HLR Fig. 1. non-ila System X case 1 case 2 ILA MSC/VLR case 5 case 3 system boundary IRG Roaming ILA MSC/VLR case 4 System model of the proposed System Y non-ila registration request registration confirm registration inform HLR The functional process of the IRG is similar to that of the existing GLR and BLR. The proposed operates according to the procedure of IS-41 or GSM-MAP for the location registration in an ordinary location area (i.e., nonila). When an MT enters an ILA, the MT performs a regular location update to the serving MSC/VLR and HLR through a nearby base station (BS). Following this process, the serving MSC/VLR replicates the received service profile to the IRG, thus the IRG maintains the profiles of the users located in the ILAs. If the MT moves into the adjacent roaming system Y, it requests the location registration. During first registration process of a roaming user upon arriving at a roaming network, the serving MSC/VLR in the roaming network recognizes that the MT is not one of its own subscriber and forwards the request to the IRG. Upon receiving the location registration request, the IRG searches its database to find the roaming user s profile. Since the IRG operates as the HLR of the roaming user, the subscriber s service profile is obtained at the IRG. Thus, the location registration process can be completed by the IRG and the serving MSC/VLR of roaming network Y alone. Once the intersystem location registration process has been completed, the IRG simply notifies the HLR of the roaming user s information such as location data and other signaling data. The proposed has following advantages. Saving of signal exchange and database access during location registration: Because the profile of the roaming subscriber can be obtained at the IRG, there is no need to access to the HLR for intersystem location registration. Also, since the overhead of signal message conversion at the mobility gateway is not required, load at the mobility gateway can be reduced. Reduction of registration latency: Due to the replicated subscriber profile, the intersystem location registration is performed between the IRG and the serving MSC/VLR in the roaming network. Therefore, the latency for intersystem location registration can be reduced. Compatibility with existing mobility protocol: By simply adding HLR functionality to the IRG, the can be used based on the existing HLR-VLR architecture unchanged. As a result, the proposed, in conjunction with the IS-41 and GSM-MAP standards, can support mobility of the roaming users efficiently. III. PERFORMANCE ANALYSIS A. System Description In this section, we investigate the overall performance in view of signaling cost, database access cost and latency of location registration. Signaling cost is defined as the message traffic exchanged among the databases, such as the HLR, Gateway and VLR during location registration and call delivery. The database access cost is defined as the database overhead generated during the update, deletion and retrieval of the user data. Also, the location registration latency is defined as time gap from the time that an MT sends a request for the registration to the time that a confirmation of the registration arrives. To analyze the performance of three comparative intersystem location management schemes such as GLR-MAP, and, following parameters are defined. For convenience, we use the same word Gateway for the GLR, BLR and IRG. We also use the subscripts h forthehlr, g forthe Gateway and v forthevlr. s hg, s hv, s vg : transmission cost of a signal between the HLR and the Gateway, between the HLR and a VLR, and between a VLR and the Gateway, respectively. d h, d v, d g : access cost for the HLR, the VLR and the Gateway η: user density in the ILA (users/km 2 ) w: width of the ILA (km) S: size of the ILA (km 2 )(i.e.,s = w 2 ) L: perimeter of the ILA (km) (i.e., L =4w) λ c : average incoming call arrival rate for the users in the ILA (calls/hour) λ m : average LA boundary crossing rate (updates/hour) v ave : average velocity of the MTs in the ILA (km/hour) We consider square-shaped location areas in this work as shown in Fig. 1. Since the location update procedures operate according to IS-41 or GSM-MAP in normal LAs, we consider only the following situation, in which an MT moves around the system boundary and location registration procedure is initiated in the ILA. case 1: The MT moves from a nonila into an ILA. case 2: The MT moves from an ILA into a nonila. case 3: The MT moves from the ILA in which it is currently staying into another ILA. case 4: The MT moves from the ILA of its home network into the ILA of an adjacent system. case 5: The MT stays in the currently staying ILA. The signaling cost s r i and sd i, which are involved in the location registration and call delivery for the case i (i =1,, 5) respectively, are calculated using the parameter s hg, s hv and s vg. Similarly, the database access cost d r i for the location registration and d d i for the call delivery in each case i (i =1,,5) are obtained using the parameter d h, d v and d g.pagingcostis determined according to specific paging method and is the same for all location management schemes. So, we do not consider paging cost in the calculation of call delivery cost.

3 The movement of a group of users in a cellular system can be adequately modeled by a uniform mobility model [8]. Then, the number of subscribers can be assumed to be constant with a density of η users per unit area. Also, the users move independently and the direction of these movements for each user is uniformly distributed in [0, 2]. Therefore, the average number of users crossing the location area borders per unit time, E[m], can be calculated as E[m] =ηw 2 λ m = ηw 2 (v ave L)/(S) =4v ave ηw/. (1) B. Signaling Cost According to the assumption in the Section III-A, the number of users crossing the boundary of one LA during unit time are 1 4E[m], and the number of incoming users and outgoing users at the ILA are the same, because the user density in the ILA is assumed to be statistically constant. Therefore, the average signaling cost of one ILA for location registration per unit time, CS R, can be obtained by C R S = 1 2 [1 4 4v aveηw (s r 1 + sr 2 +4sr 3 +2sr 4 )]. (2) Given that the average incoming call arrival rate for the users in the ILA is λ c, the average signaling cost of call delivery for all the users in the ILA, CS D, can be calculated as C D S = η S λ c s d 5 = ηw2 λ c s d 5, (3) where s d 5 is the unit signaling cost of call delivery for the users in the ILA. Therefore, the total signaling cost for the location registration and the call delivery, CS T, is given as follows. C. Database Access Cost C T S = C R S + CD S (4) We can evaluate the database access cost based on the procedure used to derive the signaling cost in the Section III-B. The database access cost in one ILA for location registration per unit time, CD R, is given as C R D = 1 2 [1 4 4v aveηw (d r 1 + dr 2 +4dr 3 +2dr 4 )]. (5) The average database access cost of the call delivery for the users in the ILA, CD D, is obtained as C D D = η S λ c d d 5 = ηw 2 λ c d d 5. (6) Therefore, the total database access cost for one ILA, which is related with the location registration and the call delivery procedures, CD T, can be expressed as C T D = CR D + CD D. (7) D. Registration Latency The signal transmission delay can be ignored relative to the database service time due to high transmission rates, thus the number of database accesses is major factor in the registration latency. We assume that all databases are single exponential servers with an infinite queue where requests for queries and updates are processed on a First-Come First-Served (FCFS) basis. Therefore, the databases can be modeled as M/G/1 queueing model [9]. The system time consists of service time and waiting time in the queue. To evaluate the system time, following parameters and notations are defined. t h, t v, t g : system time for the HLR, the VLR and the Gateway database λ h, λ v, λ g : average arrival rate of service request for the HLR, the VLR and the Gateway database µ h, µ v, µ g : average processing rate for the HLR, the VLR and the Gateway database σh 2, σ2 v, σg: 2 variance of service time for the HLR, the VLR and the Gateway database By using the Pollaczek-Khinchin formula [9], the average system time spent in the HLR, t h, is given as t h =1/µ h + λ h (1/µ 2 h + σ 2 h)/(2 λ h /µ h ). (8) Similarly, we can obtain the service time for the VLR and the Gateway database by substituting the corresponding parameters into (8). The registration latency li r, which is involved in the location registration for the case i (i =1,, 5) can be obtained using the system time parameter t h, t v and t g Therefore, we can calculate the average registration latency for the users crossing the boundary of ILA per unit time, L R, as following. L R = 1 2 [1 4 4v aveηw (l1 r + l2 r +4l3 r +2l r 4)] (9) IV. NUMERICAL RESULTS AND DISCUSSION We consider an ILA of length 50km and with a user density of 16 users/km 2. We assume that the average velocity of users in the selected ILA is 30 km/hour, therefore, the average call arrival rate, λ c is 0.72 calls/hour and the average location update rate, λ m is set to 0.72 updates/hour. The signaling costs s hv and s hg areassumedtobemoreexpensive than s vg, because the VLR and the gateway are located closer to each other than the other components. For the sake of simplicity, it can be assumed that s hg = s hv = α s vg, (α 1). (10) For the comparison of the number of messages exchanged during the location registration and the call delivery, the signaling cost parameter s vg is set to 1 and α is set to two values of 1 and. Fig. 2 and Fig. 3 illustrate the effect of call arrival rate and location update rate on the total signaling cost related with the location registration and the call delivery in one ILA. In order to represent the signaling cost reduction produced by the proposed, the relative costs such as CT S,IRG MAP C T S,GLR MAP and CT S,BLR MAP C T S,GLR MAP are shown. It can be seen that the average

4 NormalizedTotal Signaling Cost ( =1) ( =1) ( =) ( =) NormalizedTotal Signaling Cost ( =0.25) ( =0.35) ( =0.45) ( =0.25) ( =0.35) ( =0.45) Fig. 2. Normalized total signaling cost vs. call arrival rate for ILA (λ m = 0.72 updates/hour) Fig. 4. Normalized total signaling cost vs. call arrival rate for ILA (α = and λ m =0.72 calls/hour) NormalizedTotal Signaling Cost ( =1) ( =1) ( =) ( =) CD,IRG MAP T CD,GLR MAP T 1.5 LocationArea Update Rate (number of update/hour) Fig. 3. Normalized total signaling cost vs. location update rate for ILA (λ c = 0.72 calls/hour) signaling cost of the is smaller than that of the other two schemes, regardless of λ c and λ m.whenα is, the signaling cost of the is similar to or less than that of the for low call arrival rate or high user mobility. Using the, the user in the ILA updates its location information only to the BLR, thus the location registration cost increases slowly compared to the other schemes according to the increase of location update rate. However, the total signaling cost for the is very high compared with the other two schemes when the call arrival rate is high, because all incoming calls to the users in the ILA arrive at the serving MSC/VLR via gateway. Now we define β as the probability that users adjacent to the boundary move straightly between the ILA of their home network and the ILA of the roaming network, in order to investigate the effect of the movement tendency. From Fig. 4, the total signaling cost of the decreases as the value of β increases. Therefore, we can see that the is more effective than the in case that the user roaming probability is large. We also consider the database access cost for location registration and call delivery in the ILA. The database access cost increases in proportion to the number of service requests arriving at the database. It should be noted that the HLR receives the greatest number of service requests. Therefore, it is assumed as following. d h >d g >d v (11) The average processing rate of the databases is set to 1000 (i.e., the average processing time is 1 ms) and the variance of service time for the databases is set to The average normalized database access cost such as and CT D,BLR MAP, are represented in the Fig. 5 CD,GLR MAP T and Fig. 6. A database access occurs when a signaling message is exchanged between databases, so the tendency of the database access cost is similar to that of the signaling cost. Also, the results in the lowest database access cost for most cases. Fig. 7 represents the average registration latency for the varying call arrival rates. As explained in the Section III-D, waiting time in the database increases as the service request arrives frequently. Because the signaling cost for the GLR-MAP and the becomes higher when the call arrival rate or the location update rate increases, the registration latency of those schemes increases rapidly compared with the. We can get similar result for the graph of the average registration latency versus location update rates. V. CONCLUDING REMARKS The proposed manages the locations of roaming users efficiently, using simple procedures that replicate the service profiles of the users entering the ILA to the gateway. We have analyzed the performance of the and compared the with existing GLR-MAP and in view of signaling cost, database access cost and registration latency. The numerical results show that the proposed scheme has the smallest signaling exchange and database access among them. Although the signaling and database cost for the HLR increases slightly, the performs better than the other schemes. The proposed outperforms conventional schemes in

5 0.03 Normalized Total Database Access Cost Average Registration Latency (sec) GLR-MAP Fig. 5. Normalized total database access cost vs. call arrival rate for ILA (d h =3, d g =1.5, d v =1and λ m =0.72 updates/hour) Fig. 7. Average registration latency vs. call arrival rate for ILA (λ m =0.72 updates/hour) Normalized Total Database Access Cost Location Area Update Rate (number of update/hour) Fig. 6. Normalized total database access cost vs. location update rate for ILA (d h =3, d g =1.5, d v =1and λ c =0.72 calls/hour) [10] I. F. Akyildiz and W. Wang, A dynamic location management scheme for next-generation multitier PCS systems, IEEE Trans. on Wireless Commun., vol. 1, no. 1, pp , Jan [11] V. Garg and J. E. Wilkes, Interworking and interoperability issues for North American PCS, IEEE Commun. Mag., vol. 34, no. 3, pp , Mar [12] I. Han, Location management based on group management for subscribers on transportation systems in mobile communication networks, Doctorial Dissertation in KAIST, Nov case that most users located in an ILA move into an adjacent roaming network. Thus, may be very suitable to transportation systems such as trains, that move along a predetermined route with many users. REFERENCES [1] EIA/TIA IS-4, Cellular radio-telecommunications intersystem operations, Jul [2] M. Rahnema, Overview of the GSM system and protocol architecture, IEEE Commun. Mag., vol. 31, no. 4, pp , Apr [3] U. Black, Second generation mobile and wireless networks, Prentice Hall PTR, New Jersey, [4] A. Bertrand, Jambala Mobility Gateway -Convergence and inter-system roaming, Ericsson Review, no. 2, pp , [5] 3GPP TS, Gateway Location Register (GLR) -Stage2 (Release 4), 3GPP TS , v4.0.0, Mar [6] W. Wang and I. F. Akyildiz, A new signaling protocol for intersystem roaming in next-generation wireless systems, IEEE J. Selected Areas Commun., vol. 19, no. 10, pp , Oct [7] I. F. Akyildiz, W. Wang and Y. J. Lee, Location Management in 3G/4G Wireless Systems, Proc. of SPIE - The Convergence of Information Technologies and Communications Conference, vol. 4531, pp. 1-13, Aug [8] R. Thomas, H. Gilbert and G. Maziotto, Influence of the moving of the mobile stations on the performance of a radio mobile cellular network, Proc. of the Third Nordic Seminar on Digital Land Mobile Radio Communications, pp. 1-9, Sep [9] L. Kleinrock, Queueing Systems Volume 1: Theory : Weley, 1975.