Location Management for Mobile Cellular Systems MOBILE COMPUTING NIT Agartala, Dept of CSE Jan-May,2012 ALAK ROY. Assistant Professor Dept. of CSE NIT Agartala Email-alakroy.nerist@gmail.com
Cellular System Architecture Each cell is served by a base station (BS) Each BS is connected to a mobile switching center (MSC) through fixed links Each MSC is connected to other MSCs and PSTN MSC MSC HLR VLR To other MSCs HLR VLR PSTN PSTN 2
Cellular System Architecture Each MSC is a local switching exchange that handles Switching of mobile user from one base station to another Locating the current cell of a mobile user Home Location Register (HLR): database recording the current location of each mobile that belongs to the MSC Visitor Location Register (VLR): database recording the cell of visiting mobiles Interfacing with other MSCs Interfacing with PSTN (traditional telephone network) One channel in each cell is set aside for signaling information between BS and mobiles Mobile-to-BS: location update, call setup for outgoing calls, response to incoming calls BS-to-Mobile: cell identity, call setup for incoming calls, location updating 3
Location Databases These two databases communicate with each other to authenticate and update each other about the location of an MS. Home Location Register (HLR) Visitor Location Register (VLR) Home Database Every MS is permanently associated with a home database Keep track of the profile of the MS Mobile ID, authentication keys, location, etc. Visiting Database Keeps track of the MSs in its service area. 4
Hand-Offs Hand-off necessary when mobile moves from area of one BS into another BS initiated: BS monitors the signal level of the mobile Handoff occurs if signal level falls below threshold Increases load on BS Monitor signal level of each mobile Determine target BS for handoff Mobile assisted: Each BS periodically transmits beacon Mobile, on hearing stronger beacon from a new BS, sends it a greeting changes routing tables to make new BS its default gateway sends new BS identity of the old BS New BS acknowledges the greeting and begins to route mobile s call Intersystem: Mobile moves across areas controlled by different MSC s Handled similar to mobile assisted case with additional HLR/VLR effort Local call may become long-distance 5
Outgoing call setup: Call Setup User keys in the number and presses send (no dial tone) Mobile transmits access request on uplink signaling channel If network can process the call, BS sends a channel allocation message Network proceeds to setup the connection Network activity: MSC determines current location of target mobile using HLR, VLR and by communicating with other MSCs Source MSC initiates a call setup message to MSC covering target area Incoming call setup: Target MSC (covering current location of mobile) initiates a paging msg BSs forward the paging message on downlink channel in coverage area If mobile is on (monitoring the signaling channel), it responds to BS BS sends a channel allocation message and informs MSC Network activity: Network completes the two halves of the connection 6
GSM Example: Call Delivery (5) HLR (4) VLR (2) (3) (6) MSC MSC VLR (7) (1) Mobile Switching Center Calling MS Called MS 7
Call Delivery Procedure 1. Calling MS sends a call initiation signal to MSC through BS. 2. MSC sends a location request to HLR of the called MS 3. HLR determines serving VLR of called MS and sends a route request message to it. 4. MSC allocates a temporary ID to MS and sends this ID to HLR 5. HLR forwards the ID to MSC of the calling MS 6. Calling MSC requests a call set up to the called MSC 7. Paging messages are sent to cells within the LA. 8
GSM Example: Location Update (5) HLR (4) VLR (3) (6) MSC MSC VLR (2) (1) Mobile Switching Center 9
Location Update Procedures 1. MS transmits a Location Update (LU) to the new BS 2. The BS forwards the LU to the MSC 3. MSC sends LU to the HLR and updates VLR 4. HLR does the following: authenticate the MS record the ID of the new VLR send an ACK to the new VLR 5. HLR sends a registration cancellation message to the old VLR 6. The old VLR removes the record of the MS and returns an ACK to HLR 10
Location Management in Cellular Networks Location update procedure Call delivery procedure Concept of Location area Other location management strategies Location update Paging 11
Location Management in Cellular Networks has to address the following issues When should a MS (Mobile Station) update its location to the network How should the exact location of a MS be determined within a specified time constraint How should user location info be stored and disseminated throughout the network 12
Cellular Systems Where is 9864180075? Send broadcast messages from every base station? 13
Two Major Components in Location Management Location Update The operation of informing the network about the current location of the mobile terminal is known as location update or location registration. May be static (based on topology of the network) or dynamic (based on mobility pattern and call arrival pattern of an user) Paging The operation of determining the location of the mobile terminal is called terminal paging or searching. The system sends messages (from one or more base stations) to find a particular user. There is a Trade-off between the costs of location update and paging Thus central problem of location management is to devise algorithms that minimize overall cost of LU & Paging 14
Two Extreme Cases (Never-Update and Always-Update) Location never update (no cost). Need to page every cells (high cost). Location updates for every cell crossing (high cost). Need to page only one cell (low cost). Location update 15
LOCATION MANAGEMENT FOR MOBILE CELLULAR SYSTEMS
Location Management Approaches 1.Location Area approach 2.Reporting Cells 3.Time Based 4.Movement Based 5.Distance Based 6.Profile Based 17
1.Location Area Approach Service Coverage area is Partitioned into different location areas (LAs). Each LA consists of several contiguous cells The BS of each cell broadcasts the identification (ID) of LA to which the cell belongs. Therefore, a MS knows which LA it is in A MS will update its location (reports its new LA) whenever it moves into a cell which belongs to a new LA When an incoming call arrives for a MS, the cellular system will page all cells of the LA which was last reported by the MS In principle, service area should be partitioned in such a way to minimize both Location-update cost and Paging Cost 18
Location Area Approach (cont d) Partition the region into different location areas. 19
Location Area Approach (Cont d) Trade-off between location update cost and paging cost 2-extreme cases Always-Update and Never-Update Always-Update: Each cell is a LA, cost of location update is very high, no paging cost (can just route an incoming call to the last reported cell without paging) Never-Update: Whole service area is a LA, no location update cost, paging cost is very high because the cellular system need to page every cell in the service area to find the current cell of the MS In classical LA approach, the most recently visited LA-ID is stored in a MS. whenever the MS receives a new LA-ID, it initiates a location update. 20
Location Area Approach (Cont d) Two Location Algorithm (TLA) allows a MS to store and register the IDs of two most recently visited LAs. When a MS moves into a new LA, it checks if the new location is in the memory. If not found, the oldest of the two LAs is replaced by the new one and location update is performed This results in reduction of ping-pong effect Among other techniques to reduce ping-pong effect is defining Overlapped LA 21
Location Update No location update LA-1 LA-2 Location update is performed when there is a boundary crossing. Location update How to determine the size of a LA? 22
2.Reporting Cells/Centers A subset of cells have been selected from all cells as reporting cells. Other cells are called non-reporting cells. BSs broadcast a signal to indicate whether the cell is a reporting one or not. So a MS knows whether it is in a reporting cell or not For each reporting cell i, its vicinity is defined as the collection of all non-reporting cells that are reachable from cell i without crossing another reporting cell. The reporting cell belongs to its own vicinity. A MS will update its location (i.e. Cell ID), whenever it moves into a new reporting cell. 23
Reporting Cells/Centers(Cont d) When an incoming call arrives for a MS, the cellular system will page all cells within the vicinity of the reporting cells which was last reported by the MS 2-Extremes: Always-update & Never-update The goal is how to select a subset of reporting cell to minimize the total location management cost (sum of location update cost and the paging cost) Reporting Cells D E A B C F G 24
3.Time Based Given a time Threshold T, a MS updates its location every T units of time. The corresponding paging strategy is also simple. Whenever there is an incoming call for a MS, the system will first search the cell the MS last reported to, say i. If it is not found there, the system will search in cells i+j and i-j starting with j=1 and continuing until the MS is found. Here a ring cellular topology is assumed 25
4.Movement Based Each MS keeps a counter which is initialized to zero after each location update. Whenever a MS crosses the boundary between two cells, it increases the counter by one. The boundary crossing can be detected by comparing the Ids of those 2 cells. When the counter reaches a predefined threshold, say M, the MS updates its location (i.e. cell ID) Guarantees that the MS is located in an area (residing area) that is within a distance M from the last reported cell. When an incoming call arrives for a MS, the system will page all the cells in residing area. (Easy to impl due to simplicity) 26
5.Distance Based Each MS keeps track of the distance between the current cell and the last reported cell (distance is defined in terms of cells). When the distance reaches a predefined threshold, say D, the MS updates its location (cell ID). Guarantees that the MS is located in an area (residing area) that is within a distance D from the last reported cell. When an incoming call arrives for a MS, the system will page all the cells within a distance D from the last reported cell. Hard to compute the distance between 2 cells or it requires a lot of storage to maintain the distance info among all cells. Although, if cell IDs can be assigned properly, the distance between 2 cells can be computed very easily. 27
6.Profile Based Cellular System keeps individual subscriber s mobility pattern in his/her profile. This info will be used to save the costs of location update and paging The idea behind this strategy is that the mobility pattern of a majority of subscribers can be foretold. For each time period [t i,t j ], the system maintains a list of LAs, (A 1,p 1 ),(A 2,p 2 ),.., (A k,p k ). Here, A f is an LA and p f is the prob that the subscriber is located in A f. Also p 1 >p 2 > p k If a subscriber moves within the recorded LAs during the corresponding period [t i,t j ], the subscriber does not require to perform location update, otherwise the subscriber reports its current location, and the system will trace the subscriber as in classical LA approach. So, location update is significantly reduced 28
OTHER LOCATION MANAGEMENT STRATEGIES 29
Ping-Pong Effect LA-1 A lot of location updates may be generated if a user crosses LA boundary frequently. 30
Terminal Paging Terminal paging or (paging or Searching) is the process of determining the exact location of a particular MS by the network In each search iteration (or polling cycle), search/polling signals are sent over the downlink control channel to all cells where the MS is likely to be present All MSs listen to the page message, and only the target MS sends a response message back over the uplink control channel. Each polling cycle has a timeout period. If the target MS replies before the timeout, paging process is terminated successfully. Otherwise, another group of cells is chosen in the next 31 polling cycle
Terminal Paging (Cont d) To avoid call dropping, the MS must be located within an allowable time limits. Maximum Paging delay corresponds to max number of polling cycles allowed to locate the MS Paging cost is proportional to the no. of polling cycles and the no. of cells being polled in each cycle Paging cost can be reduced by predicting the current location of the MS 32
Terminal Paging Approaches Blanket/Simultaneous Paging: All the cells within the LA in which the MS is located are polled simultaneously Single polling cycle, deployed on top of LA based location update scheme (e.g., GSM) Drawback: Polling cost is very high (determined by the no. of cells within one LA), Generate Excessive amount of paging traffic Sequential Paging: Network pages the MS sequentially starting from one cell where the MS last updated its location, moving to next cell in a particular order. More polling cycles 33
Blanket Paging Page every cells within the LA. 34
Sequential Paging Page the cells sequentially until the user is found. 1 4 2 3 8 7 6 9 10 5 35
How to determine paging order? Method 1: Shortest Distance First Pages the user starting from the cell where he last updated his location. Move outward in a shortest-distance-first order. Ties are broken arbitrarily. Example: (in a highway) 1 2 3 4 5 Last Location Update 36
How to determine paging order? Method 2: Based on Location Probability Estimate the probability that a user is located in each cell within the current LA. Page the cells in decreasing order of probability. Example: (in a highway) Last Location Update A B C D E Suppose Prob. Distribution is: {0.05, 0.2, 0.4, 0.25, 0.1} Paging order: C, D, B, E, A 37
Paging Delay In our previous example, the expected delay is E[D] = 0.4 1 + 0.25 2 + 0.2 3 + 0.1 4 + 0.05 5 = 2.15 (paging cycles) Worst-case delay is 5 paging cycles. The expected number of cells to be paged is also 2.15. Worst-case: 5 cells. 38
Blanket Paging vs. Sequential Paging Blanket Sequential Paging cost Large Small Paging delay Small Large Sequential group paging may be used if there is a constraint on paging delay 39
Sequential Group Paging Page groups of cells sequentially until the user is found. 1 2 2 2 3 3 3 4 4 Worst case delay is 4 cycles. 3 40
References I. F. Akyildiz et. al., Mobility management in nextgeneration wireless systems, Proceedings of the IEEE, pp. 1347-1384, vol. 87, no. 8, Aug. 1999. V. W.-S. Wong and V. C. M. Leung, Location management for next-generation personal communications networks, IEEE Network, pp. 18-24, Sep/Oct 2000. 41