Mobility Management in GPRS

Transcription

1 Rheinisch-Westfälische Technische Hochschule Aachen Lehrstuhl für Informatik IV Prof. Dr. rer. nat. Otto Spaniol Mobility Management in GPRS Seminar: Data Communication and Distributed System Winter 2003/04 Wichan Threamthrakanpon Matrikelnummer: Betreuung: Roger Kalden Ericsson Research, Corporate Unit Ericsson Eurolab Deutschland GmbH

2 Table of Contents ABSTRACT 3 1. INTRODUCTION 3 2. CONCEPT OF MOBILITY IN MOBILE COMMUNICATION Mobility Management Framework GPRS in Mobility Management Framework 5 3. GPRS MOBILITY MANAGEMENT GPRS System Architecture GPRS Session Management Attach, Detach Procedure PDP Context Activation, Deactivation Procedure GPRS Mobility Management State Model Location Update RA Update Cell Reselection Paging of GPRS Mobile Station USER MOBILITY INVESTIGATION Investigation by Mathematical Formulation and Analytical Method Investigation by Measuring Geographic Position Investigation by Exploring Trace Data in Cellular Network CONCLUSION REFERENCES 21 2

3 Abstract Mobility management plays a significant role in the new cellular wireless network standard GPRS. GPRS provides a number of functions, such as location update, cell reselection, paging mobile station, etc., to support subscriber mobility. The characterization of different forms of user mobility and their effects on communication traffic are also important in planning, design, and operation of mobile communication network. This paper describes concepts of mobility in mobile communication, the GPRS mobility management and various user mobility investigation techniques. 1. INTRODUCTION The General Packet Radio Service (GPRS) is a step between GSM and 3G cellular networks. GPRS offers faster data transmission via a GSM network up to the maximum speed kbps. This new technology makes it possible for users to make telephone calls and transmit data at the same time. (For example, if you have a mobile phone using GPRS, you will be able to simultaneously make calls and receive messages.) The main benefits of GPRS are that it reserves radio resources only when there is data to send and it reduces reliance on traditional circuit-switched network elements. GPRS can be thought of as an overlay network onto the GSM network because GPRS uses most of existing GSM network elements, plus new network elements, interfaces, and protocols for building a packet-based mobile cellular network. GPRS is the technology that provides packet data access to an existing GSM Networks. It is designed to support an efficient mobile packet network. One of the greatest advantages of GPRS is that it allows a subscriber to access data services at a higher data rate while on the move. Therefore, reliable seamless mobility handling is a significant key requirement for wireless data networks and in order to achieve the quality of service (QoS), the efficient mobility handling algorithm is needed. Consequently the user mobility model, used to predict the movement of subscriber, is used in planning and implementing the mobility handling algorithm. This causes an emerging of several researches in user mobility investigation techniques. The rest of this paper explores the issues of concepts of mobility in mobile communication, the GPRS mobility management and various user mobility investigation techniques. To describe the concept of mobility, a mobility framework will be first introduced and then we analyze how the GPRS mobility is fixed in our framework in section 2. In addition, in section 3 the GPRS mobility management is presented in detail. In section 4, we present various user mobility investigation approaches, such as an investigation by mathematical formulation, by measuring geographic position, and by exploring trace data, including examples and results from the existing papers [4]-[8], and compare the advantage of each approach. 3

4 2. CONCEPT OF MOBILITY IN MOBILE COMMUNICATION In order to define a complete mobility management solution, we need to define the framework of mobility management. In the Mobility Management in Mobile Internet, S. Uskela [2], the mobility framework has been examined in three different aspects of mobility: use cases, realization, and functionality. They refer to end user s perception of the provided mobility, and basic procedures that each mobility management technology supports. In this section, mobility framework will be discussed. 2.1 Mobility Management Framework Use Cases S. Uskela [2] has defined the mobility use case from the end-users point of view as three different use cases: static, nomadic and continuous mobility. Static mobility is the case that there is no movement at all. Nomadic mobility is the ability to retain access at the intermediate stop and at the destination, for instance, the users use laptops to connect to the internet from different locations. Continuous mobility refers to the model that the users are always reachable and capable to access the network during the movement. The example for the last use case is the use of cellular network which, the users are always connected to the network and reachable while on the move. In the paper On Fundamental Concept of Mobility for Mobile Communication, Jun-Zhao Sun and Jaakko Sauvola [3], the mobility use case is defined further to pervasive mobility. This scenario is known as mobile ad hoc network communication where the mobile hosts without using any pre-existing network infrastructure are free to move randomly. Realization Based on the use cases discussed above, there are two main different realization approaches to realize mobility: personal and device mobility. The former approach focuses on movement of the user while the latter approach focuses on movement of the user s device [2]. Personal Mobility focuses on providing communication services and ubiquitous network access for users independent of the device they are currently using, for example, the use of SIM card in GSM system. Another interesting aspect of personal mobility is personalizing the operating environment and maintaining the personalization when changing the device. Device Mobility focuses on movement of device and can be realized in different protocol layers: link, network, session and application layer. While using link layer mobility, device s point of attachment to IP network remains the same, for instance, IEEE and GPRS networks can provide link layer mobility. The network layer mobility has to be provided by the IP routing, there fore Mobile IP (MIP), solution for IPv4 and IPv6, supports the network layer mobility. For session layer mobility, the application can be running on top of session without awareness of the movement. Function In general there are a numbers of functions used in the mobility management schemes and since there is some variation in different technologies, these functions can be variant. As a result, only the key functions are discussed here. 4

5 Registration. Usually this function is used when the mobile device is switch on. The mobile device informs the network that it is ready to start sending and receiving communication data. The registration function also include the process of checking the authentication of the user to use the network. Paging. Normally the network knows the accurate location of the mobile device only when the device is in the state that performs cell updates frequently. If the device is in power saving mode, the paging function is essential in order to retrieve the current cell, the device is located. Location Update. Location update is used to inform the current location of the device to the network. This location update function can be triggered by both movement or timer, for instance, the mobile device updates the current location to the network when it moves into the new location or the timer is expired. Handover. When the device is in dedicated mode, in order to maintain the connection session, handover must be performed when the device move from one coverage area of one access point to another. The length of time which is used to perform handover is critical to maintain the quality of the communication especially in an interactive session. Rerouting. After a handover procedure, the traffic path in the network often is sub optimal. Then rerouting may be done after the handover to optimize the traffic path. Since the handover is done, the rerouting process is no more time-critical. 2.2 GPRS in Mobility Management Framework The General Packet Radio Service (GPRS) is a new non-voice value added service that allows information to be sent and received across a mobile telephone network. GPRS is an enhancement over the GSM by adding some nodes into the network to provide the packet switch services. In our framework, GPRS is mapped to the continuous mobility use case because it uses the same infrastructure as the GSM cellular network and the GPRS system provides link layer mobility. The GPRS provides also personal mobility through the use of the Subscriber Identity Module (SIM), in which the user identity is stored, and SIM card can be inserted into any GPRS device. Consequently the GPRS provides both device mobility in the link layer and personal mobility through the use of SIM card. The key mobility management functions in our framework are also implemented in GPRS. When the GPRS device is switched on, it performs registration or attach procedure to the network. Location update is done to inform its current location to the network, when the device moves into the new location. If there is any data packet sent to the device while the device is in standby mode, paging procedure is required to retrieve the current cell onto which the device has camped. During active communication, handovers are performed whenever needed to maintain the session and if the traffic routing in the core network after the handover procedures is sub optimal, rerouting can be done. 3. GPRS MOBILITY MANAGEMENT The General Packet Radio Service (GPRS) is a new nonvoice value added service that allows information to be sent and received across a mobile telephone network [11]. It supplements today's Circuit Switched Data and Short Message Service. GPRS has several unique features which can be summarized as: 5

6 SPEED Theoretical maximum speeds of up to kilobits per second (kbps) are achievable with GPRS using all eight timeslots at the same time. This is about three times as fast as the data transmission speeds possible over today's fixed telecommunications networks and ten times as fast as current Circuit Switched Data services on GSM networks. By allowing information to be transmitted more quickly, immediately and efficiently across the mobile network, GPRS may well be a relatively less costly mobile data service compared to SMS and Circuit Switched Data. IMMEDIACY GPRS facilitates instant connections whereby information can be sent or received immediately as the need arises, subject to radio coverage. No dial-up modem connection is necessary. This is why GPRS users are sometimes referred to be as being "always connected". Immediacy is one of the advantages of GPRS (and SMS) when compared to Circuit Switched Data. High immediacy is a very important feature for time critical applications such as remote credit card authorization where it would be unacceptable to keep the customer waiting for even thirty extra seconds. PACKET SWITCHING GPRS involves overlaying a packet based air interface on the existing circuit switched GSM network. This gives the user an option to use a packet-based data service. To supplement a circuit switched network architecture with packet switching is quite a major upgrade. However, as we shall see later, the GPRS standard is delivered in a very elegant manner- with network operators needing only to add a couple of new infrastructure nodes and making a software upgrade to some existing network elements. With GPRS, the information is split into separate but related "packets" before being transmitted and reassembled at the receiving end. The Internet itself is another example of a packet data network, the most famous of many such network types. SPECTRUM EFFICIENCY Packet switching means that GPRS radio resources are used only when users are actually sending or receiving data. Rather than dedicating a radio channel to a mobile data user for a fixed period of time, the available radio resource can be concurrently shared between several users. This efficient use of scarce radio resources means that large numbers of GPRS users can potentially share the same bandwidth and be served from a single cell. The actual number of users supported depends on the application being used and how much data is being transferred. Because of the spectrum efficiency of GPRS, there is less need to build in idle capacity that is only used in peak hours. GPRS therefore lets network operators maximize the use of their network resources in a dynamic and flexible way, along with user access to resources and revenues. 3.1 GPRS System Architecture In order to integrate GPRS into the existing GSM architecture, two new GPRS Support Nodes, a Serving GPRS Support Node (SGSN) and a Gateway GPRS Support node (GGSN) have been introduced (See Figure 1). 6

7 Figure 1 GPRS System Architecture. [9] The SGSNs can be viewed as a "packet-switched MSC;" it delivers packets to mobile stations (MSs) within its service area. SGSNs send queries to home location registers (HLRs) to obtain profile data of GPRS subscribers. SGSNs detect new GPRS MSs in a given service area, process registration of new mobile subscribers, and keep a record of their location inside a given area. Therefore, the SGSN performs mobility management functions such as mobile subscriber attach/detach and location management. The SGSN is connected to the base-station subsystem via a Frame Relay connection to the PCU in the BSC. The GGSNs are used as interfaces to external IP networks such as the public Internet, other mobile service providers' GPRS services, or enterprise intranets. GGSNs maintain routing information that is necessary to tunnel the protocol data units (PDUs) to the SGSNs that service particular MSs. Other functions include network and subscriber screening and address mapping. One (or more) GGSNs may be provided to support multiple SGSNs. In general, the relationship between the SGSNs and the GGSNs is many-to-many because a SGSN may route the data packets over the several GGSNs in order to reach the different packet data network and a GGSN can be an interface to external data packet network for several SGSNs. The European Telecommunications Standards Institute (ETSI) has defined the interfaces between the new GPRS network nodes and the GSM network nodes in GSM Network Architecture (See Figure 1). All GSNs are connected via interfaces and by using GPRS Tunneling Protocol (GTP) GSNs exchange the data by transmit the encapsulated PDN packets through the IP-based GPRS backbone Network. There are two kinds of GPRS backbones; Intra-PLMN backbone network which connects GSNs of the same PLMN and Inter-PLMN backbone network which connect GSNs of the different PLMNs. 3.2 GPRS Session Management The function of the session management is to support PDP context handling of the user terminal. After a successfully attach of the MS, a PDP context is created describing the characteristics of the session. To exchange packet with external Packet Data Networks (PDNs) a mobile station must apply for one or more addresses used in the PDN. These addresses are called PDP addresses. Each PDP address is described by one or more PDP contexts in the MS or the Network. 7

8 3.2.1 Attach, Detach Procedure Before the mobile station can use the GPRS Service, first the mobile station must register with the SGSN of the GPRS Network. The network checks for authorization of the user, copies the user profile from HLR database to the SGSN and assign the packet temporary mobile subscriber identity (P-TMSI) to the user. This procedure is called GPRS attach. The disconnection from the GPRS network is called GPRS detach PDP Context Activation, Deactivation Procedure After a successful GPRS attach process, data packets still cannot be routed in GPRS because no address is assigned to the mobile station. This address is called PDP address (Packet Data Protocol Address), e.g., IP address in case the PDN is an IP network. The procedure to create PDP context is shown in Figure 2. For each session, a so-called PDP context is created which contains PDP type, PDP address assigned to the mobile station, the requested QoS, and the address of GGSN that serves as the access point to the PDN. This context is distributed and stored in MS, SGSN, and GGSN. In the other way, the PDP context deactivation procedure is called to deactivate an existing PDP context between the MS and the network. After deactivation, no data transfer is anymore possible. Figure 2 PDU Context Activation. [9] 3.3 GPRS Mobility Management The main task of the mobility management is to keep track of the user s current location. The MS sends the location update message to the SGSN so that the network can be always aware of the current location of the MS. There are three states exist in the GPRS mobility management (see figure 3) and the different location information is available in each state. As a result, the different mobility management strategies are applied in the different states. 8

9 3.3.1 State Model By Performing a GPRS attach, the MS gets into READY state and if the MS does not transmit any packet for a long period of time until the READY timer is expired, the MS will get into STANDBY state. It is possible to transmit data only if the MS is in READY state, thus the MS in STANDBY state can switch back to the READY state, if a PDU transmission occurs and in the same way, at READY state if the GPRS detach is performed, the MS will be back into IDLE state and all PDP context will be deleted. The GPRS state model is shown in Figure 3. Figure 3 State Model of GPRS Mobile Station. [9] In the STANDBY state, the MS sends the location update message seldom, so its location is not known exactly and the paging is necessary for every downlink packet, resulting in a significant delivery delay. In the READY state, the MS updates its location frequently. Consequently the MS s location is known precisely and no paging delay during delivery downlink packet. However this consumes much more the uplink radio capacity and battery of the MS Location Update The State Model of GPRS Mobile Station deploys an appropriate location update strategy in order to maintain the optimum network capacity as well as the MS battery drain. Figure 4 shows the fundamental concept of network cell-structure. Cell is the coverage area of the radio transmission of base station (BS). Location Area (LA) and Routing Area (RA) consist of one or several cells and RA is always in one LA. When MS crosses LA border, a location update and RA update shall be done. In case MS moves within the same LA but crosses different RA, the RA update is needed. When the MS moves within the same LA and RA, cell update may be needed. It depends on the current state of the MS. The first case, that the MS updates the location every cell change, is used in READY state. This strategy ensures that the accurate location of the MS is always known and packet data can be delivered faster as no paging procedure is necessary. However the MS battery is drained more and uplink radio capacity is wasted for cell updates. The second case, used in STANDBY state, is that the MS updates the location only when the MS moves to a new routing area (RA). In this strategy, when data packet is sent to the MS, 9

10 paging is required in order to find out the current location of the MS. Thus, uplink capacity will be wasted for paging response and every downlink packet requires paging of the mobile delay. Figure 4 Cell, Routing Area and Location Area RA Update Whenever the MS moves to a new RA, it sends a routing area update request including the routing area identity (RAI) of the old RA to its assigned SGSN. When the message arrives at the base station subsystem (BSS), the BSS adds the cell identifier (CI) of the new cell. Based on the RAI and CI data, the SGSN can derived the new RAI. Two different cases are possible; Intra-SGSN and Inter-SGSN routing area update. Intra-SGSN routing area update: The MS has moved to an RA, assigned to the same SGSN as the old RA. In this case, the SGSN knows already all necessary user profile, and can assign a new packet temporary mobile subscriber identity (P-TMSI) to the user without the need to inform other network elements. Figure 5 shows the message exchange diagram of the Intra- SGSN routing area update. Figure 5 Intra-SGSN routing are update. [9] 10

11 Inter-SGSN routing area update: In this case, the MS has moved to an RA, assigned to a different SGSN, thus, the new SGSN does not have the user profile of the MS. The SGSN contacts the old SGSN and requests the PDP context of the user. After receiving the PDP context of the user, the new SGSN informs the involved network elements, such as the GGSN about the new PDP context of the user, and the HLR about the user s new SGSN, etc. Figure 6 shows the message exchange diagram of the Inter-SGSN routing area update. Figure 6 Inter-SGSN routing area update Cell Reselection When Mobile Station is in IDLE state, if the MS initiates attach procedure and the currently camped-on cell already supports GPRS then no cell reselection is required. On the other hand, if the currently camped-on cell does not support GPRS then a reselection procedure is required before execution of GPRS attach procedure. When MS is in STANDBY and READY state, it continuously monitors the surrounding cells. If the more suitable cell is found, a cell reselection procedure is performed. The cell reselection procedure in this case can be helpful to minimize the cell changes. Besides, when the MS moves to a new location, the cell reselection is needed to select a new cell most appropriate to the new location. While MS is in dedicated mode, then the changes from one cell to another is performed according to the network-controlled handover procedures Paging of GPRS Mobile Station When the MS is in STANDBY state, the network does not know the precise location of MS, thus paging procedure is required to retrieve the accurate cell on which the MS has camped. The MS in STANDBY state is paged by the SGSN before a downlink transfer to that MS. The paging procedure cause the MS to move to READY state to allow the SGSN to forward downlink data to the radio resource. The SGSN supervises the paging procedure with a timer. If the SGSN receives no response from the MS to the Paging Request message, the SGSN will repeat the paging. Figure 7 demonstrates the message exchange in the Paging procedure. 11

12 Figure 7 GPRS Paging Procedure. 4. USER MOBILITY INVESTIGATION Understanding the traffic characteristic in mobile communication is highly useful in planning, designing and operating cellular networks. In a mobile cellular network, a handover is performed every time when the user crosses from one cell to another while communication, thus the network characteristics depend on the user mobility. The mobility pattern has been investigated in different ways. In this section we explain the following user mobility investigation methods; the investigation by analytical method with mathematical formulation, by measuring geographic position, and by exploring trace data in cellular network. 4.1 Investigation by Mathematical Formulation and Analytical Method The paper of M. M. Zonoozi and P. Dassanayake [4], the mobility patterns is analytically investigated by mathematical formulation. First a system tracking of the random movement of a mobile station in a cellular environment is formulated mathematically. Based on this formulation, a model is developed to obtained different mobility-related parameters under generalized case. The proposed model is used to characterize different mobility related traffic parameters in cellular mobile communication systems including cell resident time, channel holding time, and average number of handovers. The results show that the generalized gamma distribution is adequate to describe the cell residence time distribution and the negative exponential distribution is a good approximation for the channel holding time distribution in cellular mobile systems. The increase and decrease in speed of the mobile in the cell can be treated as the decrease and increase in cell size respectively. The investigation approach and the results above show that mathematical formulation can be used to create the model to describe the mobility pattern in mobile cellular network under the given assumptions. However, it is questionable if the given assumptions are always correct. As a results, the models from this investigation method are probably unrealistic. The better model can be found by measurement. 12

13 4.2 Investigation by Measuring Geographic Position In [5] the teletraffic characteristics are investigated based on measuring geographic position. In the paper, speed and cell dwell time distributions and transition probability for taxis in large- and small-city regions are analyzed. The simulation are used to evaluate differences in the traffic characteristics of handover rate and failed communications probability between these two regions by computing the mapping of collected movement patterns on hypothetical cell structures. The measured data consists of collected position information of taxi movement in large and small cities. This data was gathered by using Global Positioning System (GPS) receiver on the taxi. The measurement is done in the large city, Yokohama and the small-city, Yokosuka. Figure 8 represents the census and other data characterizing the difference between Yokohama, and Yokosuka. With GPS receiver, the movement position of the taxi has been gathered and the vehicle loci (shown in Figure 9) have been generated. Figure 8 Census and other data characterizing the difference between Yokohama (large city) and Yokosuka (small city) as of [5] Figure 9 The measured loci of the taxis. (a) Yokohama (large city) and (b) Yokosuka (small city). [5] 13

14 Figure 10 Overlay of hypothetical cells on the vehicle loci. [6] The cell dwell time, the period from the time a vehicle enters a certain cell to the time it leaves that cell, is computed by overlaying hypothetical cells (see figure 10) on the vehicle loci. Figure 11,12 show the cumulative distribution of vehicle speed cell dwell times of vehicles computed based on the measured locations and the hypothetical cell structure. It was found out that the difference in the cell dwell time distribution for the large- and small-city is small due to small a difference in the speed distribution. Figure 11 Cumulative distribution of vehicle speed.[5] In addition, handover rate and failed communication probability are calculated on the basis of simulations [5] with the cell dwell time distribution. Again it was found out that the difference in the handover rate and the failed communication probability between large and small cities is small. 14

15 Figure 12 Cumulative distribution of cell dwell time.[5] Another similar example of mobility investigation by measuring geographic position has been done in teletraffic characteristics of cellular communication for different types of vehicle [6]. In this paper, the geographic position has been measured from four types of vehicle (intercity buses, recreational vehicle, freight trucks, and taxi) with mounted GPS receiver. By overlaying hypothetical cells, the cell cross-over rate, which is the number of times a vehicle crosses cell boundaries per hour, and the cell-dwell-time are determined. The results are shown in Figure 13 and 14. The cell cross-over rate varies according to the vehicle type, but is inversely proportional to cell size. The cell dwell time is approximated well by a log-normal distribution, with a mean and standard deviation that depend on the characteristics of speed and direction in the motion of vehicle type [6]. To obtain a handover rate and cell dwell time for the communication terminals, the communication holding time is taken into account and the same method as in [10] is applied. Figure 13 Cell cross-over rate for the four types of vehicles. [6] 15

16 Figure 14 Cell dwell time for the four types of vehicles plotted on log-normal-distribution paper. [6] Figure 15 shows the result of taking holding time into consideration and evaluating handover rate. The handover rate of four types of vehicles is decreased in this order: inter-city buses, RVs, freight trucks, and taxis, which is the same order as in the cell cross-over rate of the vehicle, when communication were not taken into consideration (Figure 13). Based on Figure 16 and 17, it can be concluded that the cell dwell time during communications mainly depends on the motion of the vehicle when the cells are small. When the cells are large, the cell dwell time depends on the holding time distribution. Figure 15- Handover rate for the four types of vehicles. [6] In conclusion, measuring geographic position is a method to investigate user mobility. The investigation is done by using GPS receiver to gather the measured data and by simulating cell dwell time and handover based on hypothetical cell layout. However, the realization level of the model obtained from this method depends on the simulation. Besides, it might be difficult to mount GPS devices on many vehicles. 16

17 Figure 16 Cell dwell time distribution for communicating terminals when the cell size is 100m. [6] Figure 17 Cell dwell time distribution for communicating terminals when the cell size is 10,000m. [6] 17

18 4.3 Investigation by Exploring Trace data in Cellular Network The investigation by exploring trace data in cellular network is used for teletraffic characterization in paper [7] and [8]. In the paper of S. Thajchayapong and J. M. Peha [8], the data examined in this paper was measured from the Wireless Andrew network, the enterprisewide broadband micro-cellular wireless network that blankets the CMU campus. In the network, there were approximately 90 access points covering 6 buildings and approximately 100 users. The data packets associated with every sign-on or handover were captured along with the timestamp and they focused on only the sign-on and handover data of mobile device. The sign-on interval time, the time between a sign-on and the previous sign-on, and the dwell time, the time between a handover and the previous handover or sign-on, are examined from the trace data. The raw trace data is analyzed and the descriptive statistic dwell data and the cumulative distribution function (CFD) of dwell time are shown in Figure 18. In Figure 19, Pareto and Exponential distribution methods are applied on the distribution of dwell time to find closedform expressions of the distribution dwell time, the estimation of the distribution of dwell time (in seconds and minutes) and the predictions of dwell time. Based on figure 19, the Pareto distribution yields a lower error than the exponential. As a result, the Pareto is the suitable distribution. Similar to analysis of dwell time, Pareto distribution and Exponential distribution are also used in sign-on interval and handover rate analysis. Figure 18 Descriptive Statistics of Dwell data and CDF of Dwell time.[8] 18

19 Figure 19 the estimation of the distribution of dwell time (in seconds and minutes) and the predictions of dwell time. [8] In summary, user mobility investigation can be done by exploring trace data of the network. Trace data exploring is separated into 2 processes; data collection and data analysis. In data collection process, trace data source and the preparation of the trace data are taken into consideration. For example, in [8] the data was gathered from the access points in Andrew Network. The data packets associated with every sign-on or handoff were captured along with a timestamp. In data analysis process, mathematical methods and data analysis techniques are utilized in order to recognize the estimation and prediction of the network characteristics. The Pareto and Exponential distribution are used in [8] and the results show that Pareto is an appropriate distribution because it yields less error than Exponential distribution. The results of user mobility investigation by using this method are more realistic because the real measured data has been examined. The results of this investigation method are, however, themself particular to only specific network, for instance a network provider that provides the network services to business customers and a network provider that provides services to general customers. The results from the data obtained from these two network providers might be different. Therefore, the problem for this approach is that the result of the data obtained from one network may be particular to that specific network. Another possible problem is that the trace data is unobtainable in some cases. 19

20 5. CONCLUSION Mobility management plays a significant role in GPRS, the new cellular wireless network standard. In this paper, we explore how the GPRS mobility is settled on the concept of mobility in mobile communication. To understand the mobility of GPRS, we define the mobile communication mobility framework. In our framework, we have examined three different angles of the mobility: use cases, realization, and functionality. Three different mobility use cases can be identified: static, nomadic, and continuous mobility. The users of cellular networks (e.g., GSM, GPRS) experience continuous mobility model today because they are always connected to the network and reachable while on the move. In relation to the framework, GPRS provides personal mobility through the use of Subscriber Identity Module (SIM), and device mobility in the link layer. We have given the definitions of the common key functions in mobility management, such as registration, paging, location update, handover and rerouting. The GPRS standard also supports the mobility management functionality defined in the framework. Understanding the traffic characteristic in mobile communication is highly useful in planning, designing and operating cellular networks. In mobile cellular network, handover is performed every time when user crosses cell boundary to another cell while communication. As a result, the network characteristics depend on user mobility. We have presented three different approaches to investigate user mobility. Applying analytical investigation with mathematical formulation is the first possible method, however if a wrong assumption is given, the results may be unrealistic. The second method mentioned in this paper is the investigation by measuring geographic position. The measured data in the simulation can be obtained by using GPS receiver mounted to the vehicle. The traffic characteristic parameters can be computed by overlaying hypothetical cells on the vehicle loci. The problem in this approach remains in the simulation to get the result close to the realistic user mobility. The last mobility investigation technique, addressed in this paper, is the investigation by exploring cellular movement data. The data packets associated to the traffic characteristics like sign-in and handover can be obtained from the network access point. This is different from the previous approaches, because the measured data is collected from the actual movement of users. The advantage of this method is that the results are based on realistic measured data, however the results are particular to the specific network. In addition, since the trace data is a private information of the network providers, it is impossible in some cases to obtain the data. 20

21 6. REFERENCES [1] 3GPP TS , General Packet Radio Service (GPRS); Service Description; Stage2 [2] S. Uskela, Mobility Management in Mobile Internet [3] Jun-Zhao Sun and Jaakko Sauvola, On fundamental Concept of Mobility for Mobile Communications [4] Mahmood M. Zonoozi and Prem Dassanayake, User Mobility Modeling and Characterization of Mobility Patterns [5] K. Saitoh, H. Hidaka, N. Shinagawa, T. Kobayashi, Vehicle motion in Large and Small Cities and Teletraffic Characterization in Cellular Communication Systems, IEICE- Transactions-on-Communication. April 2001; E84-B(4): [6] K. Saitoh, H. Hidaka, N. Shinagawa, T. Kobayashi, Teletraffic Characteristics of Cellular Communication for Different Types of Vehicle Motion, IEICE-Transactions-on- Communications. March 2001; E84-B(3): [7] D. Tang, M. Baker, Analysis of a Metropolitan-Area Wireless Network, Wireless Networks, 8(2-3), pp , March-May 2002 [8] S. Thajchayapong, J. M. Peha, Mobility Patterns in Microcellular Wireless Networks, Proceedings of IEEE Wireless Communications and Networking Conference (WCNC), March 2003 [9] GPRS: Architecture and Protocols. [10] T. Kobayashi, N. Shinagawa, and Y. Watanabe, Vehicle mobility characterization based on measurements and its application to cellular communication systems, IEICE Trans. Communication., vol.e82-b, no.12, pp , Dec [11] GSM-The Wireless Evolution: GPRS Platform 21