Rheinisch-Westfälische Technische Hochschule AachenLehrstuhl für Informatik IV

Prof. Dr. rer. nat. Otto Spaniol

Betreuung: Roger KaldenEricsson Research, Corporate UnitEricsson Eurolab Deutschland GmbH

Mobility Management in GPRS

Seminar: Data Communication and Distributed SystemWinter 2003/04

Wichan ThreamthrakanponMatrikelnummer: 244535

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Table of Contents

ABSTRACT ____________________________________________________ 3

1. INTRODUCTION _____________________________________________ 3

2. CONCEPT OF MOBILITY IN MOBILE COMMUNICATION ______ 4

2.1 Mobility Management Framework ______________________________ 42.2 GPRS in Mobility Management Framework ________________________________ 5

3. GPRS MOBILITY MANAGEMENT _____________________________ 5

3.1 GPRS System Architecture ____________________________________ 63.2 GPRS Session Management ___________________________________ 7

3.2.1 Attach, Detach Procedure ________________________________ 83.2.2 PDP Context Activation, Deactivation Procedure _____________ 8

3.3 GPRS Mobility Management __________________________________ 83.3.1 State Model ___________________________________________ 93.3.2 Location Update _______________________________________ 93.3.3 RA Update____________________________________________ 103.3.4 Cell Reselection _______________________________________ 113.3.5 Paging of GPRS Mobile Station ___________________________ 11

4. USER MOBILITY INVESTIGATION ___________________________ 12

4.1 Investigation by Mathematical Formulation and Analytical Method ____124.2 Investigation by Measuring Geographic Position ___________________134.3 Investigation by Exploring Trace Data in Cellular Network __________ 18

5. CONCLUSION _______________________________________________ 20

6. REFERENCES _______________________________________________ 21

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Abstract

Mobility management plays a significant role in the new cellular wireless networkstandard GPRS. GPRS provides a number of functions, such as location update, cellreselection, paging mobile station, etc., to support subscriber mobility. Thecharacterization of different forms of user mobility and their effects on communicationtraffic are also important in planning, design, and operation of mobile communicationnetwork. This paper describes concepts of mobility in mobile communication, the GPRSmobility 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 171.2kbps. This new technology makes it possible for users to make telephone calls and transmitdata at the same time. (For example, if you have a mobile phone using GPRS, you will be ableto simultaneously make calls and receive e-mail messages.) The main benefits of GPRS arethat it reserves radio resources only when there is data to send and it reduces reliance ontraditional circuit-switched network elements.GPRS can be thought of as an overlay network onto the GSM network because GPRS usesmost of existing GSM network elements, plus new network elements, interfaces, andprotocols for building a packet-based mobile cellular network. GPRS is the technology thatprovides packet data access to an existing GSM Networks. It is designed to support anefficient mobile packet network. One of the greatest advantages of GPRS is that it allows asubscriber to access data services at a higher data rate while on the move. Therefore, reliableseamless mobility handling is a significant key requirement for wireless data networks and inorder to achieve the quality of service (QoS), the efficient mobility handling algorithm isneeded. Consequently the user mobility model, used to predict the movement of subscriber, isused in planning and implementing the mobility handling algorithm. This causes an emergingof 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 thenwe analyze how the GPRS mobility is fixed in our framework in section 2. In addition, insection 3 the GPRS mobility management is presented in detail. In section 4, we presentvarious user mobility investigation approaches, such as an investigation by mathematicalformulation, by measuring geographic position, and by exploring trace data, includingexamples and results from the existing papers [4]-[8], and compare the advantage of eachapproach.

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2. CONCEPT OF MOBILITY IN MOBILE COMMUNICATION

In order to define a complete mobility management solution, we need to define the frameworkof mobility management. In the Mobility Management in Mobile Internet, S. Uskela [2], themobility 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 CasesS. Uskela [2] has defined the mobility use case from the end-users’ point of view as threedifferent use cases: static, nomadic and continuous mobility. Static mobility is the case thatthere is no movement at all. Nomadic mobility is the ability to retain access at theintermediate stop and at the destination, for instance, the users use laptops to connect to theinternet from different locations. Continuous mobility refers to the model that the users arealways reachable and capable to access the network during the movement. The example forthe last use case is the use of cellular network which, the users are always connected to thenetwork and reachable while on the move.In the paper On Fundamental Concept of Mobility for Mobile Communication, Jun-Zhao Sunand Jaakko Sauvola [3], the mobility use case is defined further to pervasive mobility. Thisscenario is known as mobile ad hoc network communication where the mobile hosts withoutusing any pre-existing network infrastructure are free to move randomly.

RealizationBased on the use cases discussed above, there are two main different realization approaches torealize mobility: personal and device mobility. The former approach focuses on movement ofthe user while the latter approach focuses on movement of the user’s device [2].Personal Mobility focuses on providing communication services and ubiquitous networkaccess for users independent of the device they are currently using, for example, the use ofSIM card in GSM system. Another interesting aspect of personal mobility is personalizing theoperating environment and maintaining the personalization when changing the device.Device Mobility focuses on movement of device and can be realized in different protocollayers: link, network, session and application layer. While using link layer mobility, device’spoint of attachment to IP network remains the same, for instance, IEEE 802.11 and GPRSnetworks can provide link layer mobility. The network layer mobility has to be provided bythe IP routing, there fore Mobile IP (MIP), solution for IPv4 and IPv6, supports the networklayer mobility. For session layer mobility, the application can be running on top of sessionwithout awareness of the movement.

FunctionIn general there are a numbers of functions used in the mobility management schemes andsince there is some variation in different technologies, these functions can be variant. As aresult, only the key functions are discussed here.

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Registration. Usually this function is used when the mobile device is switch on. The mobiledevice 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 touse the network.Paging. Normally the network knows the accurate location of the mobile device only whenthe device is in the state that performs cell updates frequently. If the device is in power savingmode, the paging function is essential in order to retrieve the current cell, the device islocated.Location Update. Location update is used to inform the current location of the device to thenetwork. This location update function can be triggered by both movement or timer, forinstance, the mobile device updates the current location to the network when it moves into thenew location or the timer is expired.Handover. When the device is in dedicated mode, in order to maintain the connectionsession, handover must be performed when the device move from one coverage area of oneaccess point to another. The length of time which is used to perform handover is critical tomaintain 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 thehandover 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 allowsinformation to be sent and received across a mobile telephone network. GPRS is anenhancement over the GSM by adding some nodes into the network to provide the packetswitch services. In our framework, GPRS is mapped to the continuous mobility use casebecause it uses the same infrastructure as the GSM cellular network and the GPRS systemprovides link layer mobility. The GPRS provides also personal mobility through the use of theSubscriber Identity Module (SIM), in which the user identity is stored, and SIM card can beinserted into any GPRS device. Consequently the GPRS provides both device mobility in thelink 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 thenetwork. Location update is done to inform its current location to the network, when thedevice moves into the new location. If there is any data packet sent to the device while thedevice is in standby mode, paging procedure is required to retrieve the current cell onto whichthe device has camped. During active communication, handovers are performed wheneverneeded to maintain the session and if the traffic routing in the core network after the handoverprocedures 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 allowsinformation to be sent and received across a mobile telephone network [11]. It supplementstoday's Circuit Switched Data and Short Message Service. GPRS has several unique featureswhich can be summarized as:

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SPEEDTheoretical maximum speeds of up to 171.2 kilobits per second (kbps) are achievable withGPRS using all eight timeslots at the same time. This is about three times as fast as the datatransmission speeds possible over today's fixed telecommunications networks and ten times asfast as current Circuit Switched Data services on GSM networks. By allowing information tobe transmitted more quickly, immediately and efficiently across the mobile network, GPRSmay well be a relatively less costly mobile data service compared to SMS and CircuitSwitched Data.

IMMEDIACYGPRS facilitates instant connections whereby information can be sent or receivedimmediately as the need arises, subject to radio coverage. No dial-up modem connection isnecessary. 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 SwitchedData. High immediacy is a very important feature for time critical applications such as remotecredit card authorization where it would be unacceptable to keep the customer waiting foreven thirty extra seconds.

PACKET SWITCHINGGPRS involves overlaying a packet based air interface on the existing circuit switched GSMnetwork. This gives the user an option to use a packet-based data service. To supplement acircuit 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- withnetwork operators needing only to add a couple of new infrastructure nodes and making asoftware upgrade to some existing network elements.With GPRS, the information is split into separate but related "packets" before beingtransmitted and reassembled at the receiving end. The Internet itself is another example of apacket data network, the most famous of many such network types.

SPECTRUM EFFICIENCYPacket switching means that GPRS radio resources are used only when users are actuallysending or receiving data. Rather than dedicating a radio channel to a mobile data user for afixed period of time, the available radio resource can be concurrently shared between severalusers. This efficient use of scarce radio resources means that large numbers of GPRS userscan potentially share the same bandwidth and be served from a single cell. The actual numberof users supported depends on the application being used and how much data is beingtransferred. Because of the spectrum efficiency of GPRS, there is less need to build in idlecapacity that is only used in peak hours. GPRS therefore lets network operators maximize theuse of their network resources in a dynamic and flexible way, along with user access toresources and revenues.

3.1 GPRS System Architecture

In order to integrate GPRS into the existing GSM architecture, two new GPRS SupportNodes, a Serving GPRS Support Node (SGSN) and a Gateway GPRS Support node (GGSN)have been introduced (See Figure 1).

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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) toobtain profile data of GPRS subscribers. SGSNs detect new GPRS MSs in a given servicearea, process registration of new mobile subscribers, and keep a record of their location insidea given area. Therefore, the SGSN performs mobility management functions such as mobilesubscriber attach/detach and location management. The SGSN is connected to the base-stationsubsystem 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, othermobile service providers' GPRS services, or enterprise intranets. GGSNs maintain routinginformation that is necessary to tunnel the protocol data units (PDUs) to the SGSNs thatservice particular MSs. Other functions include network and subscriber screening and addressmapping. 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 aSGSN may route the data packets over the several GGSNs in order to reach the differentpacket data network and a GGSN can be an interface to external data packet network forseveral SGSNs. The European Telecommunications Standards Institute (ETSI) has definedthe interfaces between the new GPRS network nodes and the GSM network nodes in GSM03.02 Network Architecture (See Figure 1).All GSNs are connected via interfaces and by using GPRS Tunneling Protocol (GTP) GSNsexchange the data by transmit the encapsulated PDN packets through the IP-based GPRSbackbone Network. There are two kinds of GPRS backbones; Intra-PLMN backbone networkwhich connects GSNs of the same PLMN and Inter-PLMN backbone network which connectGSNs of the different PLMNs.

3.2 GPRS Session Management

The function of the session management is to support PDP context handling of the userterminal. After a successfully attach of the MS, a PDP context is created describing thecharacteristics 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. Theseaddresses are called PDP addresses. Each PDP address is described by one or more PDPcontexts in the MS or the Network.

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3.2.1 Attach, Detach ProcedureBefore the mobile station can use the GPRS Service, first the mobile station must registerwith 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 temporarymobile subscriber identity (P-TMSI) to the user. This procedure is called GPRS attach. Thedisconnection from the GPRS network is called GPRS detach.

3.2.2 PDP Context Activation, Deactivation ProcedureAfter a successful GPRS attach process, data packets still cannot be routed in GPRS becauseno address is assigned to the mobile station. This address is called PDP address (Packet DataProtocol Address), e.g., IP address in case the PDN is an IP network. The procedure to createPDP context is shown in Figure 2. For each session, a so-called PDP context is created whichcontains PDP type, PDP address assigned to the mobile station, the requested QoS, and theaddress of GGSN that serves as the access point to the PDN. This context is distributed andstored in MS, SGSN, and GGSN. In the other way, the PDP context deactivation procedure iscalled to deactivate an existing PDP context between the MS and the network. Afterdeactivation, 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. TheMS sends the location update message to the SGSN so that the network can be always awareof the current location of the MS. There are three states exist in the GPRS mobilitymanagement (see figure 3) and the different location information is available in each state. Asa result, the different mobility management strategies are applied in the different states.

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3.3.1 State ModelBy Performing a GPRS attach, the MS gets into READY state and if the MS does not transmitany packet for a long period of time until the READY timer is expired, the MS will get intoSTANDBY state. It is possible to transmit data only if the MS is in READY state, thus theMS in STANDBY state can switch back to the READY state, if a PDU transmission occursand in the same way, at READY state if the GPRS detach is performed, the MS will be backinto IDLE state and all PDP context will be deleted. The GPRS state model is shown inFigure 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 isnot known exactly and the paging is necessary for every downlink packet, resulting in asignificant 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 deliverydownlink packet. However this consumes much more the uplink radio capacity and battery ofthe MS.

3.3.2 Location UpdateThe State Model of GPRS Mobile Station deploys an appropriate location update strategy inorder to maintain the optimum network capacity as well as the MS battery drain. Figure 4shows the fundamental concept of network cell-structure. Cell is the coverage area of theradio transmission of base station (BS). Location Area (LA) and Routing Area (RA) consist ofone or several cells and RA is always in one LA. When MS crosses LA border, a locationupdate and RA update shall be done. In case MS moves within the same LA but crossesdifferent RA, the RA update is needed. When the MS moves within the same LA and RA, cellupdate 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 datacan be delivered faster as no paging procedure is necessary. However the MS battery isdrained 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 theMS moves to a new routing area (RA). In this strategy, when data packet is sent to the MS,

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paging is required in order to find out the current location of the MS. Thus, uplink capacitywill be wasted for paging response and every downlink packet requires paging of the mobiledelay.

Figure 4 – Cell, Routing Area and Location Area.

3.3.3 RA UpdateWhenever the MS moves to a new RA, it sends a routing area update request including therouting area identity (RAI) of the old RA to its assigned SGSN. When the message arrives atthe base station subsystem (BSS), the BSS adds the cell identifier (CI) of the new cell. Basedon 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 asthe old RA. In this case, the SGSN knows already all necessary user profile, and can assign anew packet temporary mobile subscriber identity (P-TMSI) to the user without the need toinform 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]

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Inter-SGSN routing area update: In this case, the MS has moved to an RA, assigned to adifferent SGSN, thus, the new SGSN does not have the user profile of the MS. The SGSNcontacts the old SGSN and requests the PDP context of the user. After receiving the PDPcontext of the user, the new SGSN informs the involved network elements, such as the GGSNabout the new PDP context of the user, and the HLR about the user’s new SGSN, etc. Figure6 shows the message exchange diagram of the Inter-SGSN routing area update.

Figure 6 – Inter-SGSN routing area update.

3.3.4 Cell ReselectionWhen Mobile Station is in IDLE state, if the MS initiates attach procedure and the currentlycamped-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 isrequired 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 cellreselection procedure in this case can be helpful to minimize the cell changes. Besides, whenthe MS moves to a new location, the cell reselection is needed to select a new cell mostappropriate to the new location.While MS is in dedicated mode, then the changes from one cell to another is performedaccording to the network-controlled handover procedures.

3.3.5 Paging of GPRS Mobile StationWhen 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. Thepaging procedure cause the MS to move to READY state to allow the SGSN to forwarddownlink 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 SGSNwill repeat the paging. Figure 7 demonstrates the message exchange in the Paging procedure.

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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 isperformed 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 beeninvestigated in different ways. In this section we explain the following user mobilityinvestigation 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 analyticallyinvestigated by mathematical formulation. First a system tracking of the random movement ofa mobile station in a cellular environment is formulated mathematically. Based on thisformulation, a model is developed to obtained different mobility-related parameters undergeneralized case. The proposed model is used to characterize different mobility related trafficparameters in cellular mobile communication systems including cell resident time, channelholding time, and average number of handovers. The results show that the generalized gammadistribution is adequate to describe the cell residence time distribution and the negativeexponential distribution is a good approximation for the channel holding time distribution incellular mobile systems. The increase and decrease in speed of the mobile in the cell can betreated as the decrease and increase in cell size respectively.The investigation approach and the results above show that mathematical formulation can beused to create the model to describe the mobility pattern in mobile cellular network under thegiven 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 bettermodel can be found by measurement.

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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 inlarge- and small-city regions are analyzed. The simulation are used to evaluate differences inthe traffic characteristics of handover rate and failed communications probability betweenthese two regions by computing the mapping of collected movement patterns on hypotheticalcell structures.The measured data consists of collected position information of taxi movement in large andsmall cities. This data was gathered by using Global Positioning System (GPS) receiver on thetaxi. 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 betweenYokohama, and Yokosuka. With GPS receiver, the movement position of the taxi has beengathered and the vehicle loci (shown in Figure 9) have been generated.

Figure 8 – Census and other data characterizing the difference between Yokohama (largecity) and Yokosuka (small city) as of 1995. [5]

Figure 9 – The measured loci of the taxis. (a) Yokohama (large city) and (b) Yokosuka(small city). [5]

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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 itleaves that cell, is computed by overlaying hypothetical cells (see figure 10) on the vehicleloci. Figure 11,12 show the cumulative distribution of vehicle speed cell dwell times ofvehicles computed based on the measured locations and the hypothetical cell structure. It wasfound out that the difference in the cell dwell time distribution for the large- and small-city issmall 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 ofsimulations [5] with the cell dwell time distribution. Again it was found out that the difference inthe handover rate and the failed communication probability between large and small cities issmall.

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Figure 12 – Cumulative distribution of cell dwell time.[5]

Another similar example of mobility investigation by measuring geographic position has beendone 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 (inter-city buses, recreational vehicle, freight trucks, and taxi) with mounted GPS receiver. Byoverlaying hypothetical cells, the cell cross-over rate, which is the number of times a vehiclecrosses cell boundaries per hour, and the cell-dwell-time are determined. The results areshown in Figure 13 and 14. The cell cross-over rate varies according to the vehicle type, but isinversely proportional to cell size. The cell dwell time is approximated well by a log-normaldistribution, with a mean and standard deviation that depend on the characteristics of speedand direction in the motion of vehicle type [6]. To obtain a handover rate and cell dwell timefor the communication terminals, the communication holding time is taken into account andthe same method as in [10] is applied.

Figure 13 Cell cross-over rate for the four types ofvehicles. [6]

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Figure 14 Cell dwell time for the four types of vehiclesplotted on log-normal-distribution paper. [6]

Figure 15 shows the result of taking holding time into consideration and evaluating handoverrate. 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 thevehicle, when communication were not taken into consideration (Figure 13). Based on Figure16 and 17, it can be concluded that the cell dwell time during communications mainlydepends on the motion of the vehicle when the cells are small. When the cells are large, thecell 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. Theinvestigation is done by using GPS receiver to gather the measured data and by simulatingcell dwell time and handover based on hypothetical cell layout. However, the realization levelof the model obtained from this method depends on the simulation. Besides, it might bedifficult to mount GPS devices on many vehicles.

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Figure 16 – Cell dwell time distribution for communicating terminalswhen the cell size is 100m. [6]

Figure 17 – Cell dwell time distribution for communicating terminalswhen the cell size is 10,000m. [6]

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4.3 Investigation by Exploring Trace data in Cellular Network

The investigation by exploring trace data in cellular network is used for teletrafficcharacterization in paper [7] and [8]. In the paper of S. Thajchayapong and J. M. Peha [8], thedata examined in this paper was measured from the Wireless Andrew network, the enterprise-wide broadband micro-cellular wireless network that blankets the CMU campus. In thenetwork, there were approximately 90 access points covering 6 buildings and approximately100 users. The data packets associated with every sign-on or handover were captured alongwith 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 dwelltime, the time between a handover and the previous handover or sign-on, are examined fromthe trace data.The raw trace data is analyzed and the descriptive statistic dwell data and the cumulativedistribution function (CFD) of dwell time are shown in Figure 18. In Figure 19, Pareto andExponential distribution methods are applied on the distribution of dwell time to find closed-form expressions of the distribution dwell time, the estimation of the distribution of dwelltime (in seconds and minutes) and the predictions of dwell time. Based on figure 19, thePareto distribution yields a lower error than the exponential. As a result, the Pareto is thesuitable distribution.Similar to analysis of dwell time, Pareto distribution and Exponential distribution are alsoused in sign-on interval and handover rate analysis.

Figure 18 Descriptive Statistics of Dwell data and CDF of Dwell time.[8]

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Figure 19 the estimation of the distribution of dwell time (in seconds and minutes) and the predictions of dwelltime. [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 datacollection process, trace data source and the preparation of the trace data are taken intoconsideration. For example, in [8] the data was gathered from the access points in AndrewNetwork. The data packets associated with every sign-on or handoff were captured along with atimestamp. In data analysis process, mathematical methods and data analysis techniques areutilized in order to recognize the estimation and prediction of the network characteristics. ThePareto and Exponential distribution are used in [8] and the results show that Pareto is anappropriate distribution because it yields less error than Exponential distribution. The results ofuser mobility investigation by using this method are more realistic because the real measureddata has been examined. The results of this investigation method are, however, themselfparticular to only specific network, for instance a network provider that provides the networkservices to business customers and a network provider that provides services to generalcustomers. The results from the data obtained from these two network providers might bedifferent. Therefore, the problem for this approach is that the result of the data obtained from onenetwork may be particular to that specific network. Another possible problem is that the tracedata is unobtainable in some cases.

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5. CONCLUSION

Mobility management plays a significant role in GPRS, the new cellular wireless networkstandard. In this paper, we explore how the GPRS mobility is settled on the concept of mobilityin mobile communication. To understand the mobility of GPRS, we define the mobilecommunication mobility framework. In our framework, we have examined three differentangles of the mobility: use cases, realization, and functionality. Three different mobility usecases can be identified: static, nomadic, and continuous mobility. The users of cellularnetworks (e.g., GSM, GPRS) experience continuous mobility model today because they arealways connected to the network and reachable while on the move. In relation to theframework, 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 keyfunctions in mobility management, such as registration, paging, location update, handover andrerouting. The GPRS standard also supports the mobility management functionality defined inthe framework.Understanding the traffic characteristic in mobile communication is highly useful in planning,designing and operating cellular networks. In mobile cellular network, handover is performedevery 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 differentapproaches to investigate user mobility. Applying analytical investigation with mathematicalformulation is the first possible method, however if a wrong assumption is given, the resultsmay be unrealistic. The second method mentioned in this paper is the investigation bymeasuring geographic position. The measured data in the simulation can be obtained by usingGPS receiver mounted to the vehicle. The traffic characteristic parameters can be computed byoverlaying hypothetical cells on the vehicle loci. The problem in this approach remains in thesimulation to get the result close to the realistic user mobility. The last mobility investigationtechnique, 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 beobtained 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 ofthis method is that the results are based on realistic measured data, however the results areparticular to the specific network. In addition, since the trace data is a private information ofthe network providers, it is impossible in some cases to obtain the data.

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6. REFERENCES

[1] 3GPP TS 23.060, 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 MobileCommunications

[4] Mahmood M. Zonoozi and Prem Dassanayake, User Mobility Modeling andCharacterization of Mobility Patterns

[5] K. Saitoh, H. Hidaka, N. Shinagawa, T. Kobayashi, Vehicle motion in Large and SmallCities and Teletraffic Characterization in Cellular Communication Systems, IEICE-Transactions-on-Communication. April 2001; E84-B(4): 805-13

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[7] D. Tang, M. Baker, Analysis of a Metropolitan-Area Wireless Network, WirelessNetworks, 8(2-3), pp 107-120, March-May 2002

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