An improved movement-based location management ... - IEEE Xplore

An Improved Movement-Based Location Management Scheme for PCS Network Lei Li Dept. of Computer Information System Georgia State University Atlanta, GA 30303

Yi Pan Dept. of Computer Science Georgia State University Atlanta, GA 30303

authors present a new dynamic location management scheme and introduce the concept of master HLR and current HLR. With this scheme, the MT is always served by the nearest HLR, so the cross-HLR communication cost can be dramatically reduced. However, Ref. [3] doesn’t address the excessive updates occurring at the boundaries of LAs.

Abstract-Location management is a key issue in the operation of wireless networks. A lot of location management schemes have been proposed. Movement-based location management strategy is the one that not only efficient but also easy to implement. However, when the number of roaming users increases, the access to the Home Location Register (HLR) and the associated signaling cost may increase explosively under the movementbased strategy. In this paper, we introduce the concept of Master HLR and Current HLR. In our presented scheme, the mobile terminal (MT)’s HLR is changed dynamically and the MT will always be served by its nearest (current) HLR. A follow up simulation study shows the proposed scheme gives better performance than current movement-based scheme in term of location management cost.

In this paper, we proposed an improved movement-based location management scheme. We combine the concept of Master HLR and Current HLR [3] with movement-based scheme. At the SA level, we apply the dynamic location management. Inside the SA, we use movement-based strategy. With our improved scheme, both the cross-SA and crossboundary location update cost can be effectively reduced. At the same time, the current network topology doesn’t need to be changed which makes the proposed scheme easy to implement. To evaluate the effect of presented scheme, a follow-up simulation study is conducted.

1. INTRODUCTION Personal Communication Service (PCS) networks have grown very rapidly over the past few years. PCS subscribers are also increasing at exponential rate. Unlike static network (e.g. telephone network), the network access points of the mobile terminals (MT’s) change as the users travel to different locations in PCS network. In order to successfully deliver an incoming call, the wireless network must keep track of the location of each mobile terminal continuously [2]. The process of tracking and locating MT’s so that calls arrive for them can be directed to their current location is called location management [3]. Two activities are involved in location management: location update and paging. Location update is the process that MT reports its location information to the network. Paging is the search procedure for certain MT to deliver a call.

The remains of the paper is organized as follows: section two presents system description for our proposed scheme; section three describe the simulation model; experiments and results are showed in section four; finally, section five draws conclusion. 2. SYSTEM DESCRITION A PCS network can be divided into several service areas (SA’s). Each SA has a Home Location Register (HLR). HLR stores permanent information of the MT whose primary subscription is in this area. A HLR contains a group of mobile switching centers (MSC). Each MSC is associated with a Visitor Location Register (VLR) that stores the temporary record for MT current active in its area. The area served by MSC is called location area (LA). Each LA is composed of a set of cells. In each cell, there is a base station that communicates with MT’s. The topology of PCS network is illustrated in figure 1.

As significant of wireless bandwidth and processing power being consumed for Location update and paging, location management becomes a key issue in the operation of PCS networks. Various location cost-efficient schemes [2] [3] [7] [8] [9] [10] [11] [12] [13] have proposed. Movement-based scheme is one of those. Under current movement based scheme [2], a MT will update its location when the number of cells it crosses equals to a threshold value. This effectively eliminates the excessive location updates occurred at the boundaries of cells. Current network topology isn’t changed in the movement-based scheme, so the scheme is easy to implement. However, when the number of roaming users increases, the access time to the Home Location Register (HLR) and the associated signaling cost may increase explosively under the movement-based strategy. In [3], the

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Jie Li Institute of Info. Sc. & Electronics University of Tsukuba Ibaraki, Japan

Current PCS network uses LA-based location update and blanket paging. A MT updates its location only if it moves out a location area. MT first updates its information in VLR, then VLR forwards the record to the MT’s home HLR. When there is incoming call for a MT, the network will first search the HLR for which VLR the MT locates, then page the whole LA. This scheme is simple and easy to implement, but inefficient.

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Inner-SA Level Scheme Inside the SA, we adapted the movement-based scheme [2]. A movement-threshold is predefined at the network configuration. A MT will update its location whenever the distance it moved exceeds the threshold value. The location area for a MT isn’t fixed. The LA is a circle centered with the cell that MT last updated, has a diameter of threshold value. The movement-based scheme has following advantage: 1) effectively reduce the excessive updates at the boundary of LA when MT moves back and forth in LA-based update strategy; 2) easy to implement, doesn’t change current network structure; 3) theoretically the movement-base threshold value could be dynamically adjusted to achieve better performance.

SA1

HLR

LA1 MSC

…

BS

M T

MSC

…….

…….

BS

Paging Scheme If paging is needed, we apply Shortest Distance First (SDF) paging strategy [2]. The LA can be divided into different layer of rings (see Figure 3). The network starts searching at MT last reported cell, and then goes to outer layer until the MT is found.

M T

3 SIMULATION MODEL Figure 1. Topology of PCS Network

In order to study the performance of proposed scheme, a simulation study is conducted. We designed an object-oriented discrete-event simulator. The simulator is implemented using Java programming language. The object-oriented style makes the model not only reflect the nature behavior of the objects, but also easy to extend and for modification.

In our proposed scheme, we combine two existing location management strategies: movement-based scheme [2] and a dynamic scheme presented by [3]. Correspondingly, we divide the location management into two parts: SA level and innerSA level. SA Level Scheme A PCS network may contain several SAs. The inter-SA communication cost is much higher than the inner-SA communication cost. When MT is roaming outside its home SA, the cost of update and paging will increase dramatically. We adapt the concept of the concepts of master HLR and current HLR from [3]. The HLR that contains the permanent data information of the MT the master HLR for the MT. The Service Area (SA) serviced by the master HLR is called the master SA for the MT. When an MT moves from an old SA to a new SA, the new SA that the MT resides is called the current SA. The associate HLR that services the MT is called the current HLR for the MT. Once an MT moves out its master HLR or current HLR, the master HLR will send a copy of MT’s profile to the new current HLR that the MT just moves in. The MT will always served by current (nearest) HLR. The cross-SA location management cost can be effectively reduced. This is showed in Figure 2. Old current

Network Configuration We assume that the PCS network is partitioned into cells of same size. We use hexagon as the cell configuration. The target network has following components: two HLRs and two corresponding SAs; each HLR has two VLRs; Each VLR has sixty-one BSs; Each BS serves four MTs. TeleTraffic Modeling Based on [1] [5] [14], We choose following teletraffic models for our simulation: 1) Residence time model The residence time represents the amount of time the MT stays in that location before moving somewhere else. We assume a geometric (exponential) cell residence time distribution in our study. The distribution is assumed to be independent and identically distributed for all cells.

New current

Layer 2

Send record about the MT Layer 1

Layer 0 The MT moves to another SA SA

SA

Figure 3 Shortest Distance First Paging Scheme

Figure 2 Location Management Scheme for SA level

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2) Mobility model We assume each MT resides in a cell for a time period then moves to one of its neighbors with equal probability. 3) Call arrive model The Poisson process and exponential process are used to describe the incoming call arrivals and the service time of a phone call, respectively. Discrete-event Simulator Discrete-event simulator contains ten classes. The class diagram is showed in Figure 4.

TABLE 1 EXPERIEMENTAL RESULTS FOR THREE SCHEMES

Simulation controls flow When simulation begins, the simulation engine first initializes the target network such as HLR, VLR, BS, MT, etc. The running time is loaded. Then a migrate event and a call arrive event will be assigned to each MT initially and then added to the event list. A random number generator is used to ensure each event has its own independent random number. The event that has the smallest time will be executed first. Based on the event type, different action will be taken. After an event is executed, a new event will be scheduled and added into the event list. The program will not stop until the event time is larger than the predefined running time.

Item

Data

Target CMR

0.1

0.5

1.0

2.0

4.0

Actual CMR

0.103

0.501

0.9996

1.998

3.993

Total cost of static scheme

333277

1061322

1973315

3803881

7451849

Total cost of movementbased scheme Total cost of improved movementbased scheme

365096

950263

1682968

3154474

6085784

344716

927827

1661796

3130706

6060464

Total cost verse CMR 8000000

4. EXPERIMENT and RESULTS

static scheme

7000000

An experiment is conducted using the discrete-event simulator. Three location management schemes are compared in the experiment: static scheme (LA-based location update and blanket paging), movement-based scheme, and our improved movement based scheme. The threshold value for movement-based schemes is pre-tested and set up as three. The experimental result is showed in Table 1 and Figure 5.

movement-based scheme

Total Cost

6000000

Improved movement-based scheme

5000000 4000000 3000000 2000000 1000000 0 0.1

0.5

1

2

4

CMR

Figure 5 Experimental Results for Three Schemes CMR is the abbreviation of Call Migration Rate. CMR is defined as call arrive rate over migrate rate. Target CMR is initial parameter for the system. Actual CMR is the ratio of number of calls and number of migrations. From table 1, we can see actual CMR is very close to target CMR, this shows that the system reach steady state at the end of the simulation. From the Table 1, we can conclude that under our target network, when CMR is very small, location cost for movement-based and improved movement-based is higher than static scheme. This is because the threshold value for movement-based schemes is smaller than LA diameter in the static scheme. There are more updates in movement-based schemes. The reduced paging cost can’t compensate the increased update cost. However, when CMR get a little bigger,

Figure 4 Class Diagram for Discrete-event Simulator

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the movement-based schemes show the advantage. Clearly, our proposed scheme has the best performance in those three schemes.

[7] J. Ho and I. F. Akyildiz, “Mobile User Location Update and Paging under Delay Constraints,” ACM/Baltzer J. Wireless Networks, vol. 1, no. 4, Dec. 1995, pp. 413–25. [8] C. Rose, “Minimizing the Average Cost of Paging and Registration: A Timer-Based Method,” ACM/Baltzer J. Wireless Networks, vol. 2, no. 2, June 1996, pp. 109–16. [9] C. Rose, “State-Based Paging/Registration: A Greedy Technique,” IEEE Trans. Vehic. Tech., vol. 48, no. 1, Jan. 1999, pp. 166–73. [10] Y-B. Lin, “Reducing Location Update Cost in a PCS Network”, IEEE/ACM Transactions on Networking, Vol. 5, No. 1, February 1997, pages 25-33. [11] Ben Liang and Zygmunt J. Haas, “Predictive DistanceBased Mobility Management for PCS Networks” IEEE INFOCOM’99, New York, NY, March 21-25, 1999 [12] G. P. Pollini and C.-L. I, “A Profile-Based Location Strategy and Its Performance,” IEEE JSAC, vol. 15, no, 8, Oct. 1997, pp. 1415–24. [13] G. Wan and E. Lin, “A Dynamic Paging Scheme for Wireless Communication Systems,” Proc. ACM/IEEE MobiCom ’97, Budapest, Hungary, 1997, pp. 195–203. [14] J.G. Markoulidakis, G.L. Lyberopoulos and M.E. Anagnostou, “Traffic model for third generation cellular mobile telecommunication systems”, Wireless Networks No. 4, 1998, 389–400

5. CONCLUSION In this paper, we present an improved movement-based location management scheme. We also design and implement a discrete-event simulator. A follow-up experiment shows that our proposed scheme performs better than the current movement-based scheme. The main contribution of this paper is the improved movement-based location management scheme for the PCS network. The proposed scheme combines two existing location management schemes. It inherits the advantages of movement-based scheme: 1) avoid excessive updates from the MT that locates at the boundary of location boundary; 2) easy to implement under current network topology. The proposed scheme can also greatly reduce the sky rising cross-SA update cost caused by increasing number of roaming users that may be a serious problem for future wireless network. Another possible contribution is the simulation model. The design of the model is object-oriented which is easy to modify and extend. In our future study, we want to study the effect of our proposed scheme on real world application such as mobile commerce. We also plan to develop a more sophisticated simulation model for the wireless network. The location management issue in emerging 3G, 4G networks is another area we can work on. 6. REFERENCE [1] Vincent W. S. Wong and Victor C. M. Leung, “Location Management for Next-Generation Personal Communications Networks”, IEEE Network, September/October 2000. [2] Ian. F. Akyildiz, J. S. M. Ho, and Y-B. Lin, “MovementBased Location Update and Selective Paging for PCS Networks”, IEEE/ACM Transactions on Networking, Vol. 4, No. 4, August 1996, pages 629-638. [3] Jie Li and Yi Pan, ``Dynamic Database Management for PCS Networks,'' Proc. IEEE 21st Intl Conf. on Distributed Computing Systems (IEEE ICDCS-21), Phoenix, Arizona, USA, pp. 683-686, April 16-19, 2001. [4] Jie Li, Yi Pan, and X. Jia, “Analysis of Dynamic Location Management for PCS Networks”, IEEE Transactions on Vehicular Technology, Vol. 51, No. 5, September 2002, pp. 1109-1119. [5] S. K. Sen, A. Bhattacharya, and S. K. Das, “A Selective Location Update Strategy for PCS Users”, ACM/Baltzer Journal on Wireless Networks, No. 5, 1999, pages 313326. [6] Padmini Ramakrishnan, “A Simulation Study of Basic Location Management Schemes”, Thesis for Degree of Master of Science, Computer Science department, Georgia State University, 2000.

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