IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 15, NO. 8, OCTOBER 1997 1455 One-Step Pointer Forwarding Strategy for Location Tracking in Distributed HLR Environment Kuen-Liang Sue and Chien-Chao Tseng Abstract— One of the main challenges in personal communi- cation service (PCS) is to locate many mobiles that may move frequently from place to place. Such a system operation is called location tracking. Many network signals flow, and database queries are required to achieve such a task. In addition to the two-level hierarchical strategy in IS-41 and GSM, several strategies have been proposed to improve the efficiency of location tracking. Pointer forwarding was used to reduce the expensive home location register (HLR) accesses. Previously, the distributed HLR scheme was proposed to prevent HLR from becoming a bottleneck in the signaling network. However, the length of a for- warding pointer chain may be lengthened in a distributed HLR environment. In this paper, we propose a more efficient strategy to overcome this potential problem. This strategy attempts to migrate the locating chains in a distributed HLR system when a mobile issues a registration operation. As a consequence, the length of any forwarding pointer chain does not exceed one in our strategy. Simulation results indicate that our strategy significantly decreases the locating cost. In fact, this strategy provides an upper bound of location tracking time owing to the fact that the length of any locating path does not exceed one. Furthermore, obsolete entries in local databases visiter location registers can be reclaimed in this strategy. I. INTRODUCTION T HE personal communication service (PCS) is a system that aims to allow for communication anywhere in the world. Such a system attempts to locate many mobiles that may move frequently from place to place [1]. Location tracking operation in a PCS network is expensive because that many signal flow and database queries are needed to achieve such a task. Moreover, locating a mobile is also a time-consuming process. In fact, the time to deliver an incoming call to a mobile is dominated by the locating time. If a location tracking strategy is efficient, the call delivery time is shortened significantly. Therefore, devising a good location tracking strategy is necessary in PCS systems. Common location tracking strategies use a two-level hierar- chical mechanism to maintain two classes of databases of user location information [2]. One class of the databases is called the home location register (HLR), and the other is the visitor location register (VLR). Manuscript received September 12, 1996; revised April 16, 1997. This work was supported in part by the National Science Council, R.O.C., under Grant NSC86-2213-E-009-076. K.-L. Sue is with the Department of Information Management, Ling Tung College, Taichung, Taiwan, R.O.C. C.-C. Tseng is with the Department of Computer Science and Information Engineering, National Chiao Tung University, Hsinchu, Taiwan, R.O.C. Publisher Item Identifier: S 0733-8716(97)06087-3. In addition to a two-level hierarchical strategy in IS-41 and GSM [2], several methods have been proposed to improve the efficiency of location tracking [3]–[8], [10]. The pointer forwarding strategy has been proposed to avoid the expensive HLR access each time a mobile moves to a new registra- tion area (RA) [7]. Lin suggested that distributing HLR’s in several areas can prevent HLR from becoming a signal bottleneck in the signaling network [8]. However, in the IS- 41 and GSM protocols, distributed HLR requires multiple HLR updates to maintain all distributed HLR’s containing the valid locating information. In that study, the forwarding pointer strategy and the concept of distributed HLR’s are combined to provide a new locating tracking strategy. It uses the characteristic of forwarding pointers to make the concept of distributed HLR [3] more attractive and efficient. However, a potential limitation of the strategy is that the length of the forwarding pointer chains may become too long. Therefore, this paper provides a more efficient strategy to overcome such a limitation, and makes the notion of distributed HLR’s more feasible. II. RELATED LOCATION TRACKING STRATEGIES A. Location Tracking in IS-41 In the IS-41 protocol, all service areas are divided into many registration areas (RA’s) [12]. To record every mobile’s loca- tion information, there are two kinds of databases, HLR/VLR. When a user subscribes to the service, a record associated with this user is created in the system database, HLR. As a mobile roams and arrives at a new RA, a record for this mobile is created in this RA’s database, i.e., VLR. In fact, some RA’s may share a VLR. To simplify research, we assume that every RA has its own VLR. Notably, the mobile switching center (MSC) is near the associated VLR [2]. “VLR” is used to represent “MSC/VLR” later. Signaling flow between MSC and VLR will be ignored in this paper. In IS-41, if a mobile moves from some RA to another one, it must be registered at the VLR of the new RA by sending a registration-request message [2]. This VLR creates a temporary record for the mobile, and sends a message to inform the HLR about the mobile’s new location. To erase the obsolete record in the VLR of the old RA, HLR sends a registration-cancel message to the old VLR [2], [9], [11]. The composition of the above actions is referred to as registration operation, and is shown in Fig. 1. 0733–8716/97$10.00  1997 IEEE