Location Management in Cellular Networks

1. Location Management in Cellular Networks Presented by Huiqi Zhang

2. Outline • Introduction to Location Management for Cellular Networks • Location Management Issues for Cellular Networks • Location Management Solutions for Cellular Networks • Conclusion • References

3. Introduction • Location management enables telecommunication networks tolocate roaming mobile terminals (MTs) for call delivery • What is location management doing? • location registration(or location update) • call delivery

4. Introduction • Two standards for location management for cellular networks: • 1. Interim Standard 41 (IS-41) (North America) • 2. Global System for Mobile Communication (GSM ) Mobile Application Part (MAP) (Europe, Asia) • Both standards are two-level database hierarchy

5. Introduction • Architecture: key components • Mobile terminal (MT) • Base station (BS) • Mobile switching center (MSC) • Home location register (HLR) • Visitor location register (VLR)

6. Introduction • The signaling messages in the registration and call delivery procedures are exchanged via the signaling system #7 (SS7) network, through which the HLR, the VLRs, and the MSCs in a cellular network are connected together. • The coverage area of a cellular network is partitioned into a number of cells, each of which is served by a base station (BS) that communicates with the MTs in that cell. • A cluster of cells form a location area (LA), in which an MT can roam freely without triggering any location registrations. • The BSs of all cells in an LA are connected to an MSC. • One or several LAs are controlled by an MSC to form an MSC area. • The MSC is a switch that provides the interfaces between the radio network and the fixed peer networks. It is responsible for maintaining and accessing the VLR, and switching between radio ports. • The MSC connects to the SS7 network via a signaling transfer points (STPs) which are switches that perform message routing functions within the SS7 network .

7. Fig. 1: SS7 Signaling Network

8. Introduction • Location registration tasks (location update) • The MT enters a new LA and transmits a location update message to the new base station. • The base station forwards the location update message to the MSC which launches a registration query to its associated VLR. • The VLR updates its record on the location of the MT. If the new LA belongs to a different VLR, the new VLR determines the address of the HLR of the MT from its Mobile Identification Number (MIN). This is achieved by a table lookup procedure called

9. Introduction Global Title Translation. The new VLR then sends a location registration message to the HLR. Otherwise, location registration is complete. 4. The HLR performs the required procedures to authenticate the MT and records the ID of the new serving VLR of the MT. The HLR then sends a registration acknowledgement message to the new VLR. 5. The HLR sends a registration cancellation message to the old VLR. 6. The old VLR removes the record of the MT and return a cancellation acknowledgement message to the HLR.

11. Introduction • Call delivery • Determining the serving VLR of the called MT • Locating the visiting cell of the called MT (Paging) • Determining the serving VLR of the called MT procedure • The calling MT sends a call initiation signal to the serving MSC of the MT through a nearby base station.

12. Introduction 2. The MSC determines the address of the HLR of the called MT by Global Title Translation and sends a location request message to the HLR. 3. The HLR determines the serving VLR of the called MT and sends a route request message to the VLR. This VLR then forwards the message to the MSC serving the MT. 4. The MSC allocates a temporary identifier called temporary local directory number (TLDN) to the MT and sends a reply to the HLR together with the TLDN. 5. The HLR forwards this information to the MSC of the calling MT.

13. Introduction • Locating the visiting cell of the called MT (Paging) • Polling signals are broadcast to all cells within the residing LA of the called MT. • On receiving the polling signal, the MT sends a reply which allows the MSC to determine its current residing cell.

15. ResearchIssues • Database architecture • Centralized: Extensions to IS-41 location management strategy • Distributed • Location update • Time based (at a constant time interval) • Movement based (movement threshold across cell boundaries) • Distance based (distance threshold): best performance but highest overhead • Terminal paging (a tradeoff between paging cost vs. paging delay) • Paging under delay constraints • Update and paging under delay constraints

17. Location Management for Cellular Networks • Centralized Database Architectures • Dynamic Hierarchical Database Architecture • It is based on IS-41 standard with the addition of a new level of database called directory register (DRs). Each DR covers the service area of a number of MSCs. • Drs periodically compute and store the location pointer configuration for the MTs in its service area. • Each MT has its unique pointer configuration and 3 types of location pointer are available at the DRs. • a local pointer is stored at an MT’s serving DR which indicates the current serving MSC of the MT. • A direct remote pointer is stored at a remote DR which indicates the current serving MSC of the MT. • An indirect remote pointer is stored at remote DR which indicates the current serving DR of the MT. • Advantage: reduce the overhead for both location registration and call delivery

18. Location Management for Cellular Networks(Cont.) 2. Pointer Forwarding: • When an MT changes its VLR area, a forwarding pointer is set up in the old VLR pointing to the new VLR. The HLR is not involved in this process, thus access to the HLR due to location change is reduced. • As the MT keeps moving from VLR to VLR, a forwarding pointer chain is formed among the VLRs the MT has visited. • When an incoming call for the MT arrives, the HLR relays the route request to the first VLR in the MT's forwarding pointer chain, which leads the route query to the current VLR of the MT. • It is effective only when the call arrival rate is low relative to the mobility rate for an MT.

19. Location Management for Cellular Networks 3. Local Anchoring: • A nearby VLR is chosen as an MT's local anchor where the location change of the MT is reported. • The HLR maintains a pointer to the local anchor for call delivery. • As an incoming call is initiated, the HLR relays the route request to the called MT's local anchor, which in turn sends the route request to the serving VLR. • The local anchor can be selected as follows. Whenever an incoming call occurs, the local anchor is changed to the serving VLR of the MT. The local anchor may also be changed to the serving VLR after a movement if a cost reduction can be achieved. • It reduces the location update cost at the cost of longer call delivery delay due to one extra query to the local anchor during call delivery. • It is efficient only when an MT's call arrival rate is lower than its mobility rate.

20. Location Management for Cellular Networks(Cont.) 4. Pre-User Location Caching: • The location of a called MT is cached in the originating switch during a call and is re-used for delivering future calls. • If the MT remains at the same location when another call arrives, the cached location can be used to deliver the call; otherwise, a cache miss occurs and the call delivery procedure in the IS-41 standard has to be followed to route the call, thereby incurring a longer setup time. • It reduces the call delivery cost for an MT only when its call arrival rate is high relative to its mobility rate.

21. Location Management for Cellular Networks(Cont.) 5. User Profile Replication: • An MT's location is replicated at selected local databases to facilitate call delivery. • When a call for the MT originates from the service area of any replicated database, the call can be routed without querying the HLR, thus reducing the call delivery delay. • However, each time the MT changes its location all replicated databases need to be updated with the new location, triggering a high location update cost, especially for highly mobile users. • It favors users with a high call arrival rate relative to their mobility rate.

22. Location Management for Cellular Networks(Cont.) 6. Hybrid: • The local anchoring and replicationtechniques are integrated together to reduce the location tracking cost. • The VLR associated with the MT subscriber's home or office is selected as the local anchor where the MT reports its location change. Meanwhile, the local anchor's location is replicated at selected switches (called replicas) originating relatively high call rates to the MT. • Only when the MT changes its local signaling transfer point (LSTP) (MSC connects to the SS7 network via LSTP) region, its local anchor is changed, which requires the updating of both the HLR and the replicas. • The local anchor and replicas can supplement each other to reduce both the location update cost and the call setup delay. • Compared to the IS-41 strategy, the integrated strategy always results in a smaller tracking cost.

23. Location Management for Cellular Networks(Cont.) • Distributed database architectures • One-root tree structure: • The location databases form a tree with the root at the top. Each database contains the location information of every MT residing in its subtree. All non-leaf databases maintain a pointer to an adjacent lower-level database regarding an MT's location. The leaf databases acting like the VLRs in the centralized structure contain the user service profiles for those MTs within their respective coverage areas. • When an MT's movement or call is local, i.e., the MT roams within the same or nearby LA or receives calls originating from the same or nearby LA, the upper-layer location databases are not accessed for the location update or call delivery. Thus, the access burden on the upper-level databases is alleviated and the end-to-end location update delay or call delivery delay is reduced. • The drawback: the crash of the root may paralyze the entire system and the root database may become overloaded as the number of mobile users increases.

24. Location Management for Cellular Networks(Cont.) 2. Multitree structure: • The whole location database system is organized into a multitree structure. which consists of a number of subsystems that assume the tree structure and connect to each other only through their root databases. • It overcomes the single point of failure issue in the one-root tree structure, since each root only maintains the profiles for those MTs residing in its service area and its failure does not affect the operation of other database subsystems. • It is also scalable in that as the capacity of a root database is saturated, a new subsystem is readily added while keeping the end-to-end delays in location registration and call delivery unchanged. • It is suitable for global mobile systems adopting a nongeographic personal telecommunication plan.

25. Location Management for Cellular Networks(Cont.) • Comparison to the 2 Database Architectures • Centralized structure • Advantages: • Easy adaptation to current PLMN networks. • The number of database updates and queries is relatively small. • Disadvantages: • As the number of MTs increases, the signaling traffic may degrade the performance. • Distributed Structure • Advantages: • reducing the access burden on the centralized HLR . • Disadvantages: • The number of database accesses is increased. • No single scheme that clearly outperforms the others under all system parameters

26. Location Management for Cellular Networks(Cont.) • Location Update • The location update determine when or where an MT should report its location to the network. • Static Schemes: e.g. IS-41. • A location update is performed when a MT changes its LA. • It cannot be adjusted based on the parameters of a MT from time to time. • Dynamic Schemes: • It can be adjusted based on the parameters of a MT from time to time. • Most of the recent research focus on the dynamic schemes.

27. Location Management for Cellular Networks(Cont.) • Profile-based: • The system maintains a list of the most probable LAs where an MT may be located. • As the MT moves between the LAs in the list, no location update is needed. Otherwise, the location update is performed according to the IS-41 scheme. • When an incoming call arrives, the MT is searched from the most to the least likely LA within its LA list. • This scheme requires that the LAs in the list be adjacent.

28. Location Management for Cellular Networks(Cont.) 2. Time-based: • A MT performs location updates periodically at given time intervals. • The time interval could be predefined or dynamically changed based on each user's location probability distribution or the system load. • One drawback of the time-based schemes is the difficulty of considering paging delay constraints while putting an upper bound on the paging cost.

29. Location Management for Cellular Networks(Cont.) 3. Movement-based: • A MT performs a location update whenever it completes a predefined number of movements across cell boundaries. • The MT may move back and forth between cells, thereby triggering un­necessary location updates. • An improved movement-based scheme was proposed to eliminate the unnecessary location updates, where when an MT revisits a cell, the movement counter value is reduced to the smaller counter value associated with the MT's previous visit of this cell. This mechanism not only eliminates repeated counts of movements into the same cell, but also counts out the movements since the last visit to the cell during the same location update period. This makes the movement counter reach the movement threshold more slowly than in the basic movement-based scheme.

30. Location Management for Cellular Networks(Cont.) 4. Distance-based: • A MT reports its location to the network when its distance from the last location update point exceeds a distance threshold. • A mechanism to compute the distance between two cells is required to implement this scheme. • A method was proposed to determine the distance between two arbitrary cells in the hexagonal configuration. • An irregular cell configuration would make the implementation of the distance-based scheme considerably more challenging.

31. Location Management for Cellular Networks(Cont.) 5. Group registration scheme: • Instead of sending individual location update requests to the HLR upon each LA change, the new location of all newly moved-in MTs to an LA since the last location update is reported to the HLR in a single location update request, which is conveyed to the HLR in the route response message of the next incoming call to any MT in the LA. • Upon an LA change, a forwarding pointer is set up from the MT's local anchor (which is pointed to from the HLR) to its current VLR, so that incoming calls to the MT can be delivered before the MT's local anchor is changed to its current VLR via the next group registration.

32. Location Management for Cellular Networks(Cont.) • Terminal Paging • Search for and alert the intended MT of an incoming call. • Sequential paging: • The paging area is partitioned into several paging zones based on the user location probability distribution and the paging delay constraint. • As an incoming call arrives, the MT is searched sequentially from the most to the least likely paging zone. • If the MT is found in a paging zone, the paging process stops. • The user location probability distribution may not be readily available. • An intra-LA location update strategy was proposed to reduce the paging cost. When an MT moves between its anchor cell, where it usually stays for a long period, and the rest of the cells within its current LA, an intra-LA location update is performed at the serving VLR of the current LA. • As an incoming call arrives, either the anchor cell or the rest of the cells in the current LA is paged to locate the MT.

33. Location Management for Cellular Networks(Cont.) 2. Intelligent paging: • The physical information of an MT such as the most recent interaction point and velocity can be exploited to improve paging performance. • Paging related information is used to predict the most likely where about of an MT and the MT is searched in the most probable location first as an incoming call arrives. • Other factors such as traffic conditions and the cellular network topology may affect the accuracy of location prediction, and consequently the effectiveness of the intelligent paging strategy.

34. Location Management for Cellular Networks(Cont.) 3. Optimal paging: • The optimal search theory with discrete efforts was appliedfor paging an MT. • An optimal multi-step search plan is developed in such a way that the probability of detecting the MT on or before each step is maximized. • The execution of the optimal search plan requires that the system know the probability distribution of an MT in each cell within its current LA and the probabilities of the system failing to detect the MT on a single page in each cell given that the MT is in that cell.

35. Location Management for Cellular Networks(Cont.) 4. Concurrent paging: • Multiple MTs can be paged simultaneously at different cells within the same LA where these MTs reside. • An ensemble paging schedule should be designed in such a way that the overall average paging cost is minimized under the paging delay constraint. • Coordination among the pollings of the MTs should be provided to warrant that each MT is searched from the most to the least likely cell if each MT's location probability distribution is known. Otherwise, some heuristic algorithms can be applied to concurrently search the MTs.

36. Location Management for Cellular Networks(Cont.) • Comparison to Static and Dynamic Schemes for Update and Paging • Static (e. g. IS-41): • Advantage: Easy to implement • Disadvantage: It cannot be adjusted according to the parameter of individual user. • Dynamic: • Advantage: allow online adjustments based on the characteristics of each individual MT. • Disadvantage: • Some required information such as the distance between cells is not available to the MTs. • Require significant computing power. Implementation of a computation intensive scheme in an MT may not be feasible.

37. Conclusion • There is no straightforward solution that takes account of the multiplicity of location management requirements • Various approaches from multiple angles have been proposed to achieve advanced location management towards a complete solution. • Each approach has pros and cons and is probably not enough by itself. • Therefore, an accommodative and efficient location management solution is called for by the next-generation wireless system.

38. References [1] I. F. AKYILDIZ, etc. Mobility Management in Next Generation Wireless Systems, Proceedings of the IEEE, Vol. 87, No. 8 August 1999, pp. 1347-84. [2]. R. JAIN AND Y.-B. LIN, An auxiliary user location strategy employing forwarding pointers to reduce network impacts of PCS;ACM-Baltzer Journal of Wireless Networks, vol. 1, no. 2, July 1995, pp. 197-210. [3]. J. S. M. Ho AND I. F. AKY1LDIZ, Local anchor scheme for reducing signaling costs in personal com­munications networks, IEEE/ACM Trans. on Networking, vol. 4, no. 5, Oct. 1996, pp. 709-725. [4]. R..IAIN. Y.-B. L1N. C. LO. AND S. MOHAN, A caching strategy to reduce network impacts of PCS, IEEE Journal on Selected Areas in Communications, vol. 12, no. 8, Oct. 1994, pp. 1434-1444.

39. References [5]. Z. MAO AND C. DOULIGERIS, An integrated strategy for reducing location management cost, IEEE Communications Letters, vol. 8, no. 1, Jan. 2004, pp. 33-35. [6]. N. SHIVAKUMAR, J. JANNINK, AND J. WIDOM. Per-user profile replication in mobile environments: Algorithms, analysis, and simulation results. ACM-Baltzer Journal of Mobile Networks and Applications, vol. 2, no. 2, Oct. 1997, pp. 129-140. [7]. Z. MAO AND C. DOULIGERIS, A distributed database architecture for global roaming in next-generation mobile networks, IEEE/ACM Trans. on Networking, vol. 12, no. 1, Feb. 2004, pp 146-160.

40. References [8]. S. TAI3BANE, An alternative strategy for location tracking, IEEE Journal on Selected Areas in Communications, vol. 13, no. 5, June 1995, pp. 880-892 [9]. C. Rose, Minimizing the average cost of paging and registration: A timer-based method, ACM-Baltzer Journal of Wireless Networks, vol. 2, no. 2, 1996, pp. 109-116. [10]. A. BAR-NOY, I. KESSLER. AND M. SIDI, Mobile users: To update or not to update? ACM-BaltzerJournal of Wireless Networks, vol. 1, no. 2, July 1995, pp. 175-186. [11]. I. F. AKYILDIZ. J. S. M. Ho. AND Y.-B. L1N, Movement-based location update and selective pagings for PCS networks;IEEE/ACM Trans. on Networking, vol. 4, no. 4. Aug. 1996, pp. 629-638. [12]. V. W. S. WONG AND V. C. M. LEUNG, An adaptive distance-based location update algorithm for next-generation PCS networks, IEEE Journal on Selected Areas in Communications, vol. 19, no. 10, Oct. 2001, pp. 1942-1952.

41. References [13]. Z. MAO AND C. DOULIGERIS, Group registration for reducing signaling traffic in location tracking, The 28th IEEE Local Computer Networks Conf. (LCN'2003), Bonn/Konigswinter, Germany, Oct. 20-24, 2003. [14]. S. MISHRA AND O. K. TONGUZ, Analysis of intelligent paging in personal communication systems, Electronics Letters, January 1998, pp. 12-13. [15]. R. REZAIIFAR AND A. M. MAKOWSKI, from optimal search theory to sequential paging in cellular networks, IEEE J. Select. Areas Common., vol. 15, no. 7, Sept. 1997, pp. 1253-1264. [16]. R.H. GAU: AND Z. J. HAAS, Concurrent search of mobile users in cellular networks, IEEE/ACM Trans. on Networking, vol. 12, no. 1, Feb. 2004, pp. 117-130. [17]. Y. XIAO, A parallel shuffled paging strategy under delay bounds in wireless systems, IEEE Commu­nications Letters, vol. 7, no. 8, Aug. 2003, pp. 367-369.