An ef cient location management scheme for PCS networks

Transcription

1 Computer Communications ) 1355±1369 Research An ef cient location management scheme for PCS networks Haidar Safa, Samuel Pierre*, Jean Conan Department of Electrical and Computer Engineering, LARIM, EÂ cole Polytechnique de MontreÂal, C.P. 6079, Succ. Centre-ville, MontreÂal, QueÂ., Canada H3C 3A7 Received 14 March 2000; revised 19 December 2000; accepted 20 December 2000 Abstract This paper investigates a new location management scheme for improving the basic location strategy proposed in the IS-41 standard. This scheme essentially consists of adding to the current network architecture a pointer table and a location data table. A pointer table is provided for each location area LA) where it tracks the MUs that have moved out of this LA. The location data table located on a local signal transfer point LSTP) node contains the data location of the MUs frequently called from the LAs connected to this LSTP. These two tables contribute to reduce signi cantly the cost of both location update and call delivery procedures. Computational results indicate that the amount of reduction obtained depends on the value of the call to mobility ratio of the MUs. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Personal communication system; 15±41; Mobility management; Location update; Location search; Wireless networks 1. Introduction In wireless personal communication systems PCSs), users carrying MUs must be able to move from one location to another while maintaining access capability to services regardless of their location. This problem is known in the literature as the mobility management problem [15]. In general mobility management integrates radio mobility which amounts to handover process, and network mobility which refers to location management [19]. In this paper, we address the location management aspect, which allows the network to keep location information on the MU's in order to nd their location when calls need be delivered to them. In most current networks, location management techniques consist of dividing the coverage area into many LAs which are sets of adjacent cells [14]. MUs within a cell communicate with a cell base station through wireless links which is connected to the wireline network through a mobile switching center MSC) that serves the LA. The signaling system no. 7 SS7) is used to carry users' information and signaling messages among the MSC's and the location databases [3,4,16]. In North America, IS-41 standard is used for both mobility and location management [3±5]. This standard employs a two-level database architecture consisting of home location registers HLRs) and visitor location * Corresponding author. Tel.: , ext. 4685; fax: address: samuel.pierre@polymtl.ca S. Pierre). registers VLRs). Each network comprises one HLR and several VLRs. The HLR of a network is a centralized database containing the pro les type of service, quality of service, billing information, current LA, etc.) of its assigned subscribers. The VLRs are distributed throughout the network. Generally a VLR may serve many MSCs. However, most of the current PCS manufacturers implement a combined MSC and VLR with one VLR per MSC. A VLR stores the pro les of the MUs that are currently residing in its associated LAs. Fig. 1 illustrates the basic architecture of the signaling network of a PCS. Location management implies location update or location registration when an MU joins the network) and location search for calls delivery) [3,4]. A location update procedure consists of updating the MU's location information in the HLR. It occurs when an MU moves from one LA to another. Location search procedure occurs when an MU is called. It amounts to identifying the MU's current LA before the connection can be established. These two procedures are well speci ed by the IS-41 standard and will be presented in Section 2. They both require network messages to be sent from a LA to the network database HLR every time the MU moves to a new LA or receives a call. Recent studies indicate that the overhead message traf c due to locating a MU or updating its pro le is signi cant [13]. In this context, the paper proposes a new location management strategy that optimizes both location update procedure and call delivery procedure costs. In Section 2 we present the IS-41 location procedures and some previous /01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S )

2 1356 H. Safa et al. / Computer Communications ) 1355±1369 Nomenclature Acronyms GTT global title translation HLR home location register LSTP local signal transfer point LA location area MSC mobile switching center MU mobile unit PCS personal communication system SS7 signaling system no. 7 SCP service control point SSP service switching point STP signal transfer point TLDN temporary location directory number VLR visitor location register related work. Section 3 describes the proposed location management scheme. A performance analysis and results of some computations are presented in Section 4 and nally some conclusions are drawn in Section The IS-41 location scheme and some variants In this section, we present the IS-41 location scheme as it is speci ed in revision C of IS-41 standard [4]. Fig. 2 illustrates the location update procedures which can be described as follows: 1. When a MU enters a new LA, it sends a registration message to the MSC/VLR through the base station. 2. The MSC of the new LA area registers the MU with its associated VLR and sends a registration noti cation message to the HLR via the signal transfer point STP). 3. Based on the MU's identi cation number, the STP executes the global title translation GTT) procedure to determine the HLR of the MU. The corresponding message is then forwarded to the HLR. 4. The HLR sends a registration cancellation message to the previous MSC/VLR. 5. The previous MSC deletes the MU's pro le in its associated VLR and sends a cancellation acknowledgment message regcanc) to the HLR. 6. The HLR acknowledges the location update by sending an acknowledgment to the new MSC/VLR. The HLR normally provides the pro le of the MU in this message. When the new MSC/VLR receives the message, it starts providing service to the MU. The location search procedure of the IS-41 standard [4] is illustrated in Fig. 3 which for the sake of simplicity does not show the STP entities. It can be described as follows: 1. Any incoming call from a mobile or xed unit identi ed as A) to a MU-B is received by an originating MSC/VLR called the source MSC/VLR). 2. The source MSC/VLR sends a location request message to the HLR of the called MU via the STP. 3. The STP uses the GTT procedure to determine which HLR the message must be forwarded to. 4. Upon receipt of the message, the HLR determines that the call is for MU-B and sends a routing request message to MU-B's current MSC/VLR destination MSC/VLR). 5. When the destination MSC/VLR of MU-B receives the message, it allocates a temporary location directory number TLDN) for the call which is sent to the HLR. 6. The HLR relays the TLDN to the source MSC/VLR. 7. Finally the source MSC/VLR routes the call to the destination MSC/VLR. During the execution of both previous procedures, it is seen that the HLR is accessed every time the MU moves Fig. 1. Basic architecture of the signaling network of a PCS.

3 H. Safa et al. / Computer Communications ) 1355± caching location strategy may cost more than the IS-41 location search procedure. In Ref. [11], a two-locations' algorithm was proposed to reduce the location update cost. According to this strategy, the MU and the HLR know the two most recently visited LAs. Hence whenever a user moves to a LA known by the MU, no location update procedure is performed in the HLR. This strategy is ef cient for users with low mobility and high locality. However, there is always an extra cost associated with the location search. 3. The proposed location management scheme Fig. 2. Updating procedure according to IS-41 standard. to a new LA or receives a call. Reduction of the signaling and database access traf c due to the user mobility indeed constitutes an important research challenge. Many strategies have been recently proposed to reduce both the location update and location search costs [1,6±12,17,18,20]. In Refs. [7,9], a location forwarding strategy is proposed to reduce the signaling cost for location update. In this strategy, a pointer is set up from the old LA whenever the MU moves to another LA. The main disadvantage of this strategy is that a chain of pointers must be traversed during the location search and call setup procedures. Hence penalty must be paid in the form of extra time and cost. In Ref. [9], a per-user location caching strategy is presented to reduce the signaling cost for location search and call delivery. This strategy is ef cient when the frequency of calls coming from a speci c LA to a MU is high. According to this strategy, a cache memory is to be installed in every LA. The LA's cache usually contains the location data of the MUs that are frequently called from that LA. However, the data in a cache may become obsolete and in that case the location search procedure of the per-user 3.1. Basic idea The proposed scheme which can be called the global location management scheme adds two tables to the current network architecture which we identify, respectively, as location data and pointer tables. A location data table is stored at a local signal transfer point LSTP) node to serve all the LAs connected to this LSTP. It contains the location information of some selected MUs, generally the ones that are frequently called from these LAs. This can reduce signi cantly the call delivery procedure cost when the called MU has a pro le in the location data table. Moreover, if this is not the case no extra cost is paid. In general the use of a location data table serving many LAs presents the following advantages: ² reduction of the network traf c by minimizing the number of updating queries sent to the data tables or to the HLR; ² saving in the cost of table installation; ² reducing the redundancy of data storing and the waste of memory space; ² increasing the frequency of locating a MU without accessing the network database. For example, if a MU is often Fig. 3. Location search procedure according to IS-41 standard.

4 1358 H. Safa et al. / Computer Communications ) 1355±1369 Fig. 4. Architecture of the signaling network used in the proposed scheme. called from a location area LA 1 but rarely from a neighboring location area LA 2, since the same location data table may serve the two LAs, the called MU will be locally located even though it is called from LA 2 ). A pointer table is added to each LA. In order to explain the usefulness of the pointer table, we introduce the concept of `anchor LA' of an MU as the LA in which the MU's location information is updated at the HLR. A pointer table of an anchor LA contains a pointer to the current LA of all the MUs having this LA as anchor LA. At this point, we must distinguish between two kinds of MU's moves: an intra-stp move such that the new and the old LAs are connected to the same LSTP, and an inter-stp move such that the new and the old LAs belong to two different LSTPs. Each MU is assumed to have a built-in memory that stores the address of its anchor LA and the addresses of the STP nodes that have the MU location information stored in their location data tables. This built-in memory is updated whenever either the MU location information in the network database HLR is updated or the MU, location information is added to a location data table. When the MU's move is intra-lstp, the pointer table of its anchor LA is queried to update the pointer so as to create a pointer between the anchor LA and the new LA. Hence, no location update operation is performed at the HLR. When the MU's move is inter-lstp, its new LA becomes its anchor LA and all information about the MU is then deleted from the previous anchor LA. The detailed location update and location search procedures of the proposed scheme will be presented later. They operate according to the signaling network architecture shown in Fig. 4. It is implicitly assumed that when a MU's location information is added to a location data table, the MU is informed of this fact when it is called from any LA served by that location data table. Consequently, this operation does not require any additional cost Location update and location search procedures Location update procedure When a MU moves to a new LA, a location update procedure is performed as illustrated in Figs. 5 and 6, respectively, for an intra-lstp and an inter-lstp move. When Fig. 5. Updating procedure for an intra-lstp movement.

5 H. Safa et al. / Computer Communications ) 1355± Fig. 6. Updating procedure for an inter-lstp movement. the MU's move is intra-lstp, its anchor LA is updated instead of the HLR according to the following steps: 1. The MU moves to a new LA and sends a location update message to the MSC/VLR of this new LA. 2. The MSC of the new LA registers the MU with its associated VLR and sends a cancellation message to the old LA. 3. The new LA queries also the MU's anchor LA in order to create a pointer from the anchor LA to the new LA. In other words, no location update operation is performed at the HLR. The anchor LA is the LA which has its address stored in the built-in memory of the MU and in the HLR. 4. and 5. The new LA receives an acknowledgement from both the old and anchor LAs. When the MU's move is inter-lstp, the location update procedure shown in Fig. 6 is performed. The steps of this procedure are described as follows: 1. The MU moves to a new LA and sends a location update message to the MSC/VLR of this new LA. 2. The MSC of the new LA registers the MU in its associated VLR and sends a registration noti cation message to the HLR via the STP. 3. The STP uses the MU's identi cation number and executes the GTT procedure to determine the MU's HLR. The registration message is then forwarded to the HLR. 4. The HLR sends a registration cancellation message to the MU's anchor LA. 5. The MU's anchor LA sends a cancellation message to the previous old) MSC/VLR. 6. The old MSC deletes the MU's pro le in its associated VLR and sends a cancellation acknowledgment message to the MU's anchor LA. Fig. 7. Searching procedure Scenario 1).

6 1360 H. Safa et al. / Computer Communications ) 1355±1369 Fig. 8. Location search procedure Scenario 2). 7. The anchor LA sends an acknowledgment to the HLR and deletes the MU's pro le in its pointer table. 8. The HLR sends a registration con rmation message to the new MSC/VLR and provides the pro le of the MU in this message. The new MSC/VLR becomes the MU's anchor LA. 9. The new MSC/VLR sends an update message to location data tables whenever it is necessary. The MU provides the MSC/VLR with the addresses of those location data tables as stored in its built-in memory.) 10. After updating the MU data, the data location table sends an acknowledgement message to the new LA Location search procedures The location search procedure involves the determination of the current serving LA of a called MU. Figs. 7±10 show the four distinct possible scenarios which must be followed by this procedure according to the proposed location management scheme. Scenario 1. The rst scenario, shown in Fig. 7, addresses the case where the called MU has a record in the location Fig. 9. Location search procedure Scenario 3). data table and is roaming in its anchor LA i.e. the LA address stored in the location data table). The steps of Fig. 7 are described as follows: 1. A call is initiated to a MU and forwarded to the MSC of the calling unit. 2. The MSC sends a location request to its associated location data table which determines the anchor LA of the called MU. 3. The request is then forwarded to the anchor LA of the called MU. 4. The called MU's MSC assigns a TLDN to the call and sends it to the calling MSC. 5. The calling MSC sets up a connection to the called MSC using this TLDN. Scenario 2. The second scenario, shown in Fig. 8, is similar to the rst one. However, it assumes that the called MU has a record in the location data table of the calling MU but is not roaming in its anchor LA. In this case, a pointer should be crossed, at the destination side, to reach the current LA of the called MU. The MU's current LA then assigns a TLDN to the call and sends it to the calling LA that establishes a connection to the called MSC using this TLDN see Appendix A for more details). Scenario 3. The third scenario, shown in Fig. 9, is the IS-41 call delivery scenario. In this case, the called MU has no record in the location data table and the HLR should be queried in order to determine the current LA of the called MU TLDN see Appendix B for more details). Scenario 4. The fourth scenario, shown in Fig. 10, considers the situation where the called MU has no record in the location data table and is not roaming in its anchor LA. In this case, a pointer should be traversed to reach the current LA of the called MU. The MU's current MSC/VLR then assigns a TLDN to the call and returns it to the HLR which forwards it to the calling MSC/VLR before the connection is established TLDN see Appendix C for more details).

7 H. Safa et al. / Computer Communications ) 1355± Fig. 10. Location search procedure Scenario 4). 4. Performance analysis 4.1. Analytical model The performance of location management schemes is highly dependent on users' mobility and incoming calls, characteristics. In this section, we formally investigate classes of users for which the proposed global scheme yields a net reduction in signaling traf c and database loads. All users can be classi ed according to their call-to-mobility ratio CMR) de ned as the ratio between the average number of calls to a user per unit time and the average number of times this user changes LAs per unit time. For each target MU, we de ne the following quantities: l average number of calls to a target MU per unit time. m average number of times the user changes LA per unit time. 1/m average LA residence time for a target MU. p probability that the called MU has a pro le in the location data table. q probability that the new LA VLR/MSC) is served by the same LSTP as the old VLR intra LSTP movement). r probability that the called MU is found in its anchor LA. We furthermore denote the costs of the various operations used in the proposed architecture as follows: U intra cost of a location update operation when the MU's move is intra-lstp. U inter cost of a location update operation when the MU's move is inter-lstp. U global estimated cost of a location update operation. S 1 cost of a location search operation using Scenario 1 i.e. when the called MU has a record in the location data table and is found in its anchor LA). S 2 cost of a location search operation using Scenario 2 when the called MU has a record in the location data table and is not found in its anchor LA). S 3 cost of location search operation using Scenario 3 when the MU has no record in the location data table and is found in its anchor LA). S 4 cost of location search operation using Scenario 4 when the MU has no pro le in the location data table and is not found in its anchor LA). S global estimated cost of a location search operation. C global total cost for location search and location update operations. Clearly the following relations hold between these quantities The estimated cost for location update procedure is given by U global ˆ qu intra q U inter : 1 The estimated cost for location search procedure is given by S global ˆ p rs r S p rs r S 4 : 2 The total cost per unit time for location search and location update is given by C global ˆ mu global 1 ls global : 3 In order to compute the cost of the location update procedure based on the global scheme using the reference network architecture Figs. 1 and 4) and the two procedures presented in Figs. 5 and 6, we de ne the following costs for traversing various network elements: A l cost of transmitting a message on A-link between service switching point SSP) and LSTP. D cost of transmitting a message on D-link between LSTP and RSTP.

8 1362 H. Safa et al. / Computer Communications ) 1355±1369 A r cost of transmitting a message on A-link between RSTP and service control point SCP). L cost of processing and routing a message by LSTP. R cost of processing and routing a message by RSTP. C H cost of a database update or query at the HLR. cost of a database update or query at the VLR. C V Based on the location update procedure shown in Fig. 5, the location update cost for an intra-lstp move is given by U intra ˆ 8A l 1 4L 1 2C V : For an inter-lstp move, the cost of a location update operation is the cost of the location update according to IS-41 standard plus the cost of updating the location data tables that store the location data of the moving MU and the cost of updating the MU's previous anchor LA. According to Fig. 6, this cost is given by U inter ˆ 4 A l 1 L 1 A r 1 D 1 R 1 4A l 1 2L 1 2C V 1 C H 4 1 X k[e U k ; 5) where E is the set of location data tables to be updated after an inter-lstp move and U k is the cost of updating a location data table U k ˆ 2 A l 1 L 1 R 1 2D : The estimated cost of the location update procedure can be derived using Eqs. 1), 4) and 5) as U global ˆ q 8A l 1 4L 1 2C V q 4 A l 1 L 1 A r 1 D 1 R 1 4A l 1 2L 1 2C V 1 C H 1 X! U k k[e ˆ 8qA l 1 4qL 1 2qC V 1 8A l 1 6L 1 4A r 1 4D 1 4R 1 2C V 1 C H 1 X k[e U k 2 8qA l 2 6qL 2 4qA r 2 4qD 2 4qR 2 2qC V 2 qc H 2 q X k[e U k ; which simpli es as U global ˆ 8A l 1 4L 1 2C V q 2L 1 4A r 1 4D 1 4R 1 C H 1 X U k!: 6 k[e Let t be the probability that the LAs of the called MU and calling unit are connected to the same LSTP. Since updating the location data table and the pointer table involves a simple access to a local memory, we assume the cost of updating or querying this kind of tables is 0. In the previous section, four distinct scenarios have been identi ed for locating a MU. In this section we evaluate their cost using the network architecture of Fig. 4. Scenarios 1 and 2, as shown in Figs. 7 and 8, respectively, are applied when the called MU has a record in the location data table of the calling MU. The cost of Scenario 1, which is used when the called MU is found in its anchor LA is given by S 1 ˆ t 4A l 1 2L 1 C V t 4A l 1 4L 1 2R 1 4D 1 C V ˆ 4tA l 1 2tL 1 tc V 1 4A l 1 4L 1 2R 1 4D 1 C V 2 4tA l 2 4tL 2 2tR 2 4tD 2 tc V ˆ 4A l 1 4L 1 2R 1 4D 1 C V 2 2tL 2 2tR 2 4tD: 7) When the called MU is not found in its anchor LA, Scenario 2 is applied. The cost of this scenario is the cost of Scenario 1 plus the cost of traversing a pointer from the anchor LA of the MU to its current LA. This cost is given by S 2 ˆ S 1 1 2A l 1 L ˆ 6A l 1 5L 1 2R 1 4D 1 C V 2 2tL 2 2tR 2 4tD: 8) When the called MU does not have its location information stored in the location data table, Scenarios 3 and 4 are applied. These scenarios are shown in Figs. 9 and 10. The cost of Scenario 3, which is used when the called MU is found in its anchor LA is equal to the cost of the location search in IS-41 given as S 3 ˆ 4 A l 1 L 1 A r 1 D 1 R 1 C V 1 C H : When the called MU is not found in its anchor LA, Scenario 4 is applied. The cost of this scenario is the cost of Scenario 3 plus the cost of traversing a pointer from the anchor LA of the MU to its current LA. As in Eq. 8) this cost is S 4 ˆ S 3 1 2A l 1 L: 9 10 The total cost per unit time for locating a MU can be expressed as follows: S global ˆ p rs r S p rs r S 4 : 11 Using Eqs. 2), 7)± 10), this cost can be rewritten as follows: S global ˆ 6A l 1 5L 1 4A r 1 4D 1 4R 1 C V 1 C H 2 p 2t L 1 R 1 2D 1 4A r 1 2R 1 C H Š 2 r 2A l 1 L : 12 The total cost per unit time for location update and location search using the global architecture is obtained using Eqs. 3), 6), and 12).

9 " C global ˆ m 8A l 1 4L 1 2C V q 2L 1 4A r 1 4D H. Safa et al. / Computer Communications ) 1355± ) Prob[MU move is inter-lstp] ˆ P 1 P 2 ˆ x 2 1)/ x x). 1 4R 1 C H 1 X! # U k 1 l{6a l 1 5L 1 4A r k[e 1 4D 1 4R 1 C V 1 C H 2 p 2t L 1 R 1 2D 1 4A r 1 2R 1 C H Š 2 r 2A l 1 L }: 13) For comparison purposes, we need the costs of the original IS-41 scheme. The IS-41 location update and location search procedures were presented in Section 2. We denote costs for various operations used in the IS-41 scheme as follows: U IS41 S IS41 C IS41 cost for a location update operation. cost for location search operation. total cost per unit time for location search and location update operations. The total cost per unit time for location update and location search under IS-41 scheme is C IS41 ˆ mu IS41 1 ls IS41 ; 14 where U IS41 ˆ 4 A l 1 L 1 A r 1 D 1 R 1 2C V 1 C H ; S IS41 ˆ 4 A l 1 L 1 A r 1 D 1 R 1 C V 1 C H : De ning the relative cost of the proposed global location management scheme as the ratio of the total cost per unit time for the global scheme to that of IS-41 scheme, C global / C IS41. We get as a function of the user's call to mobility ratio CMR ˆ l/m) C global C IS41 ˆ Uglobal 1 l=m S global U IS41 1 l=m S IS41 : 15 Relation 15) uses the four probability terms: p, q, r, and t. Both p and r, which have been de ned earlier, can be used to classify the users. In order to quantify q and t, we assume that an LSTP consists of x x LAs arranged in a square, and each LA is itself a square. MUs are assumed to be uniformly distributed throughout the LSTP area and each MU exhibits the same arrival call rate at every VLR/MSC. Furthermore, each time a MU leaves a LA, one of the four sides is crossed with equal probability. Then, the probability that the MU's move is inter-lstp is equal to the probability that the MU is in a border LA multiplied by the probability that the MU's next move is to a LA belonging to a different LSTP. De ne: P 1 Prob[MU lies in a border LA of the LSTP] ˆ 4 x 2 1)/ x x) P 2 Prob[MU's next move is to a LA belonging to a different LSTP] ˆ 1/4 Hence, q is the Prob[MU move is intra-lstp] ˆ 1 2 x 2 1)/ x x). Let us assume also that all the SSPs in the network are uniformly distributed among n LSTPs and each SSP corresponds to a single LA. For example, in the case of the public switched telephone network where there are 160 Local Access Transport Area LATA) across the seven Regional Bell Operating Company RBOC) regions [2] and assuming one LSTP per LATA, the average number of LSTPs is 160/7 or 23 per region. Given that there are 1250 SSPs per region, then the number of SSPs per LSTP is 1250/23. Hence s 1250 x ˆ < 7:4 ) q < 0:88: 23 Under the conditions stated above, the probability t that both calling and called users are found in the same LSTP is equal to 1=n ) t ˆ 1=23 ˆ 0:043: 4.2. Numerical results To evaluate the performance of the model, we performed computations using various values for p and k, where k ˆ ueu is the number of location data tables that must be updated after a MU's inter-stp move. Several scenarios have been considered assuming that a set of values dominates in Eq. 15) and setting the remaining terms to zero Scenario 1 We rst consider the case where the costs of the network's `high elements' dominate by setting the costs of the network's `low elements' to 0. To be explicit, we include in the high elements of the network the link D, the RSTP node, the link A r and the HLR see Figs. 1 and 4). Since the cost parameters of these elements are intrinsically heterogeneous we will assume that the delay is the common unit of measure for all cost terms. With this assumption, D and A r represent transmission delays of xed transmission speed signaling links, R is the time delay for processing a query at RSTP, C H is the transaction delay for database queries i.e. HLR). We consider the four sets of cost parameters given in Table 1 which allow us to study the effect of varying the cost parameters D, R, A r, C H on the location management scheme. Fig. 11 a) shows the relative signaling and database access cost for p ˆ 0:8 and k ˆ 4; the value of the CMR varying from 0.01 to 10. This gure shows that the relative cost increases with the CMR. When the CMR is low, the mobility rate is high compared with the call arrival rate, and the cost for location updating procedure dominates. Signi cant cost savings can be obtained by reporting location update to the HLR only after an inter-lstp move. When the CMR is high, the mobility rate is low relative to the call incoming rate and the cost saving from location update

10 1364 H. Safa et al. / Computer Communications ) 1355±1369 Table 1 High elements' cost parameters Set D A r R C H 1 7 3D 1/7)D 1/2)D 2 7 3D D 3D 3 1 4D 7D 20D 4 1 4D D 4D procedures diminishes. However, the location search cost dominates and the saving in call delivery cost becomes signi cant when p is high p ˆ 0:8 : Fig. 11 b) shows the performance of the new scheme when the number of location data tables that should be updated after an inter-lstp move increases k ˆ 12 : This gure shows that the relative cost decreases with CMR. This can be explained as follows. As k increases, the global scheme updating procedure cost increases and the call delivery procedure cost remains constant. It is clear that minimal location update cost is attained when k ˆ 0: However, cost saving can still be obtained when the value of k increases. When the cost of call delivery procedure starts to dominate as the CMR increases), the relative cost decreases and the saving in call delivery becomes signi cant. Fig. 11 c) analyzes the behavior of the new scheme as p decreases. Computations have been carried out when p ˆ 0:3 and k ˆ 4: With small p, the relative cost increases as the CMR increases compared with the relative cost obtained for the same CMR in Fig. 11 a) and b). This is to be expected since the value of p affects only the call delivery procedure cost. In fact, minimal call delivery cost is obtained when p ˆ 1: Moreover, signi cant reduction can still be obtained when p is small. Fig. 11. Relative cost for the network's high elements.

11 H. Safa et al. / Computer Communications ) 1355± Fig. 12. Relative cost for the network's low elements for p ˆ 0.8 and k ˆ 4. Fig. 11 d) shows the relative cost when p is small and k is high. This gure shows that the saving obtained decreases when p decreases and k increases. In this gure, it is seen as the CMR increases that the relative cost increases in contrast with Fig. 11 b). This can be explained as follows. It is clear that a higher call delivery cost is obtained for p ˆ 0:3 as compared to p ˆ 0:8: Also, when p ˆ 0:8 the location update cost in the proposed scheme is lower than the call delivery one and this cost is also lower than the cost of the similar procedure in the IS-41 scheme. Furthermore, when p ˆ 0:3 the call delivery procedure in the proposed scheme is less costly than the location update procedure and approaches the cost of the call delivery procedure of the IS-41 scheme. Combining all these facts explains why the relative cost decreases in Fig. 11 b) and increases in Fig. 11 d). We now consider situations where the cost of the low elements of the network dominates. Fig. 12 obtained by using the data sets of Table 2 shows that, whenever the cost of one or more low elements dominates, the IS-41 location management scheme is less costly than the proposed scheme Scenario 2 We evaluate rst the performance of the proposed location management scheme when the database access costs as Table 2 Low elements cost parameters Set C V A l L 1 L L 3 2 0L 0L 1 3 0L 1 1 L L 0L 0 well as query processing and routing costs at STP nodes dominate by setting the signaling cost parameters to 0. We consider the four sets of costs given in Table 3. Since processing a query at LSTP or RSTP node consists of a simple table lookup procedure, the cost parameters R and L are expected to be the lowest and we will assume that R ˆ L: All cost parameters can be normalized to L which can be set to L ˆ 1: The selected data sets allow us to study the effect of varying the cost parameters C V and C H on the performance of the proposed location management scheme. Fig. 13 a) and b) show, for k ˆ 4 and 12, respectively, and p ˆ 0:8; a relative cost decrease as the value of CMR increases. This can be explained as follows. Since the probability of inter LSTP move is low 1 2 q ˆ 0:12 ; the location update procedure in the proposed scheme has cost given by Eq. 6) as the value of 4L 1 2C V 1 0:12 4L 1 C H 1 4KL ; whereas the IS-41 updating procedure cost is given by 8L 1 2C V 1 C H and involves all the cost parameters related to the database access and STP query processing regardless of the MU's. A simple numeric computation can show that the location update procedure of the proposed scheme is less costly than the location update procedure in IS-41 scheme. Using the same approach, it can be veri ed that the call delivery procedure of the proposed scheme is less costly than the call delivery procedure in the IS-41 scheme. Furthermore, it is seen, since the value of p is high, that the value of U global /U IS41 is larger than S global /S IS41. We then conclude that more saving is obtained when the CMR is high, since the call arrival rate is high compared with the mobility rate, and the cost for call delivery procedure dominates. We can use the same approach to explain why the relative cost increases in Fig. 13 c) and d). Brie y, when the value of p is low, the value of U global /U IS41 is less than the value S global /S IS41. We conclude that more saving is obtained when the CMR is low since the mobility rate is high compared

12 1366 Table 3 Database access cost parameters H. Safa et al. / Computer Communications ) 1355±1369 Table 4 Signaling cost parameters Set C H C V R L 1 6L 3L L L 3L L 1 3 6L 6L L L 6L L 1 Set A l D A r 1 1 3A l 5A l A l 5A l 3 1 3A l 10A l A l 10A l with the call arrival rate, and the cost of the location update procedure dominates. Comparisons between data sets of Table 3 demonstrate that an increase in the VLR access cost or in the HLR access cost results in higher relative cost under all CMR values, and the relative cost increases as the VLR or HLR access cost increases. We consider now the situations where the signaling cost dominates by setting the other cost parameters to 0. The cost parameter A l is expected to be the smallest value since it is the lowest link in the network. We consider the four data sets given in Table 4 and normalize the other cost parameters with respect to A l and set A l ˆ 1. Fig. 14 a) shows that, when p ˆ 0:8 and k ˆ 4; the relative cost increases as a function of the CMR. This can be explained using the same type of arguments as previously. It is seen that the saving obtained by the proposed location management scheme is signi cant. Parameters set 2 data show that less reduction is obtained when the transmission cost of a signaling message over link D is high. Fig. 14 b) shows the effect of increasing the value of k. It turns out that, when k becomes high, the proposed scheme may not perform as well. Whenever k increases, the location update procedure cost in the proposed scheme increases and IS-41 scheme can be higher than in IS-41. As shown earlier at low CMR, the location update cost dominates. As the Fig. 13. Relative database access cost.

13 H. Safa et al. / Computer Communications ) 1355± Fig. 14. Relative signaling cost. CMR increases, the call delivery procedure cost starts to dominate decreasing the relative cost. Parameter sets 2 and 4 show that increasing the signaling cost of link D decreases the cost saving when the proposed scheme is used. Sets 1 and 3 show, at high kand low CMR, more savings as D decreases while set 2 shows that, at low CMR, if the value of D exceeds the value of A r, the performance of the proposed scheme is not as good as the IS-41 scheme. Fig. 14 c) shows that at high CMR less reduction is obtained when p is low. Finally Fig. 14 d) considers the values p ˆ 0:3 and k ˆ 12: At low CMR, better performance is obtained when the value of D is much less than the value of A r. As in Fig. 14 b), whenever the value of D exceeds A r, the relative cost exceeds 1 when the mobility rate is high. Consequently, the proposed scheme is outperformed by the IS-41 scheme Scenario 3 This scenario considers situations where one cost term dominates and consequently we can simplify Eq. 15) by setting the other cost terms to zero. Fig. 15 shows that, whenever any of the following cost terms R, A r or C H dominates, the relative cost is less than 1, regardless of the values of p and k, and independently of the CMR value. It also shows that, when the cost term L or A l dominates, the relative cost is greater than 1 and hence the IS-41 scheme is more ef cient than the proposed scheme. With the cost term C V dominating, both schemes are similar. If the cost term D dominates, a small value of k may result in a relative cost lower than 1 and the proposed scheme results in a considerable cost reduction regardless of the value of p. However, at low CMR and high k values, the IS-41 scheme can be more ef cient although this effect decreases as the CMR increases. 5. Conclusions In this paper a new location management strategy for mobile communications systems is proposed. Under this strategy, two tables are added to the current network architecture. A pointer table is added to each LA and tracks the MUs that moved out of this LA by setting a single pointer from this LA to the current LA. The location data table is located on a LSTP node and contains the data location of the MUs that are frequently called from the LAs connected to

14 1368 H. Safa et al. / Computer Communications ) 1355±1369 Fig. 15. Relative cost when one cost parameter dominates. this LSTP. Computations have shown that a signi cant cost saving can be obtained compared with the IS-41 standard location management scheme depending on the value of the MUs call to mobility ratio. Acknowledgement This work was supported in part by the National Science and Engineering Research Council NSERC) of Canada under grant Appendix A A.1. Location search Ð Scenario 2 Scenario 2, shown in Fig. 8, can be described as follows: 1. A call is initiated to a MU and forwarded to the MSC of the calling unit. 2. The MSC sends a location request to its associated location data table which determines the anchor LA of the called MU. 3. The location request is then forwarded to the called MU's anchor LA. 4. If the MU is not found in this particular LA, the request is then forwarded to the current LA of the called MU by traversing one pointer. 5. The current MU's MSC assigns a TLDN to the call and sends it to the calling MSC. 6. The calling MSC sets up a connection to the called MSC using this TLDN. Appendix B B.1. Location search Ð Scenario 3 Scenario 3, shown in Fig. 9, can be described as follows: 1. A call is initiated to a MU and forwarded to the MSC of the calling unit. 2. The MSC sends a location request to its associated location data table. Since the called MU has no record in this table, a GTT is performed to determine its HLR. 3. The location request is forwarded to the HLR of the called MU.

15 H. Safa et al. / Computer Communications ) 1355± Upon receiving the request, the HLR determines the anchor LA of the called MU and sends a routing request message to this LA. 5. The current MU's MSC/VLR assigns a TLDN to the call and returns it to the HLR. 6. The HLR forwards the TLDN to the calling MSC/VLR. 7. The calling MSC sets up a connection to the called MSC using this TLDN. Appendix C C.1. Location search Ð Scenario 4 Scenario 4, shown in Fig. 10, can be described as follows: 1. A call is initiated to a MU and forwarded to the MSC of the calling unit. 2. The MSC sends a location request to its associated location data table. Since, the called MU has no record in this table, a GTT is performed to determine its HLR. 3. The location request is forwarded to the HLR of the called MU. 4. Upon receiving the request, the HLR determines the anchor LA of the called MU and sends a routing request message to this LA. 5. The called MU is not found in its anchor LA. The request is then forwarded from the anchor LA to the current LA of the called MU by traversing one pointer. 6. The MU's current MSC/VLR assigns a TLDN to the call and returns it to the HLR. 7. The HLR forwards the TLDN to the calling MSC/VLR. 8. The calling MSC sets up a connection to the called MSC using this TLDN. References [1] A. Bar-noy, I. Kesseler, M. Sidi, Mobile users: to update or not to update? Wireless Networks ) 175±186. [2] Bellcore, Switching system requirements for interexchange carrier interconnection using integrated services digital network user part ISDNUP), Technical Reference TR-NWT , Bellcore 1992). [3] EIA/TIA., Cellular radio-telecommunications intersystem operations, Technical Report IS-41 Revision B, EIA/TIA, [4] EIA/TIA., Cellular radio-telecommunications intersystem operations, Technical Report IS-41 Revision C, EIA/TIA, [5] M.D. Gallagher, S.A. Randall, Mobile Telecommunications Networking with IS-41, McGraw-Hill, NY, USA, [6] J.S.M. Ho, I.F. Akyildiz, Dynamic hierarchical database architecture for location management in PCS networks, IEEE/ACMTrans. Networking 5 5) 1997) 646±660. [7] J. Ho, I.F. Akyildiz, Mobile user location update and paging under delay constraint, Wireless Networks 1 4) 1995) 413±425. [8] R. Jain, Y.B. Lin, An auxiliary user location strategy employing forwarding pointers to reduce network impacts of PCS, Wireless Networks 1 2) 1995) 197±210. [9] R. Jain, Y.B. Lin, C. Lo, S. Mohan, A caching strategy to reduce network impacts of PCS, IEEE J. Selected Areas Commun. 12 8) 1994) 1434±1444. [10] P. Khrisna, N.H. Vaidya, D.K. Pradhan, Forwarding pointers for ef cient location management in distributed mobile environments, Computer Science Technical Report Texas A&MUniversity, College Station, [11] Y.B. Lin, Reducing location update cost in a PCS network, IEEE/ ACMTrans. Networking 5 1) 1997) 26±33. [12] Y.B. Lin, Determining the user locations for personal communication service networks, IEEE Trans. Veh. Technol ) 466±473. [13] C.N. Lo, R.S. Wolff, R.C. Bernhardt, Expected network database transaction volume to support personal communications services, in: Proceedings of the First International Conference on Universal Personal Communication, September 1992, pp /1-6. [14] J.G. Markoulidakis, G.L. Lyberopoulos, D.F. Tsirkas, E.D. Sykas, Evaluation of location area planning scenarios in future mobile telecommunication systems, Wireless Networks ) 17±29. [15] J.G. Markoulidakis, G.L. Lyberopoulos, D.F. Tsirkas, E.D. Sykas, Mobility modeling in third-generation mobile telecommunications systems, IEEE Personal Commun ) 41±56. [16] A.R. Modarressi, A.R. Skoog, Signaling system no. 7: a tutorial, IEEE Commun. Mag. July 1990). [17] S. Rajagopalan, B.R. Badrinath, An adaptive location management strategy for mobile IP, in: Proceedings of First ACMMobicom 95, November [18] H. Safa, S. Pierre, J. Conan, A new model for reducing location update cost in a wireless network, in: Proceedings of the Eleventh Annual International Conference on Wireless Communications, Wireless'99, Calgary, Alberta, Canada, 12±14 July 1999, pp. 39±47. [19] S. Tabhane, ReÂseaux mobiles, Editions HERMES, Paris [20] J.Z. Wang, A fully distributed location registration strategy for universal personal communication systems, IEEE J. Selected Areas Commun ) 850±860.