Lecture 09 Location Management 第 9 讲 移动位置管理

1. Lecture 09 Location Management第9讲 移动位置管理 §9.1 LM Basics §9.2 LM in Cellular Networks §9.3 LM in Ad Hoc Networks §9.4 Mobile IP

2. What’s It? • Location is useful • Communication: call forwarding , data forwarding • Location based services • Location is? • Geographical coordinate • Cell ID • IP addr… • Location management. • The maintenance of the binding between the logical identifier and physical location of the user. LBS App. Location Management A special kind of data value in a mobile data management system. A data item whose value changes with respect to time. Localization Module Geographic Information System

3. Key Issues in Location Management • A mobile user creates uncertaintyof his/her exact location. • How to know the current location? • From localization/positioning system • Where to store the location information? • Location database • What to do for new location? • Location update • Who to maintain and provide location information? • Location server, with cooperation from clients

4. Location Services • Location services are provided by a location server. • Location server manages a location database, which holds user location. • It receives user position updates and stores into the location database. • Location database (also called moving object database) contains location information about mobile users. • Mobile users or moving objects have their identity. • Current user locations are stored in multiple databases or replicated databases. • Location information is queried by different agency at different places to achieve their functionality, e.g. location-based services.

5. Locating Moving Objects • Moving objects: • Mobile devices, Mobile users, Mobile software • Two extremes to find their current location: • Storeeverywhere: Cost of location update is very high. • Full replication of location • Allow search to be performed locally (fast). • Search everywhere: No cost of update • No location information is stored anywhere. • Searching is performed on demand everywhere (expensive). • One should balance between search and update costs. • Perform location update to certain key places • Search within some area

6. Locating Moving Objects At all sites Availability At selected sites (e.g., at frequently calling cells) The whole network Nowhere Set of locations Exact location Occurrence Never update Periodic update Precision Always update (at each movement)

7. Locating Moving Objects • Three design issues: • Where: availability • At all sites, at selected (frequently visited) sites or at no site. • When: occurrence • Stored location is always updated, updated periodically, or never updated. • What: precision • Exact location, within a set of possible locations, or any location within network. • Two basic operations: • Paging • Search initiated by system to find the mobile unit. • Server broadcasts a search message and target replies via uplink channel. • Update • Impose an upper bound on the location uncertainty. • Mobile unit sends update message via uplink channel.

8. Cost of Location Management • Cost of Location Management System (LMS) includes • Number of database updates • Number of messages, size of messages and distance the messages need to travel. • Major parameters • Relative frequency of the move. • Call operations of each user. • Call to Mobility Ratio (CMR): • the number of calls made to a user during a period / the number of location updates generated by the user. • High CMR “always update” • Low CMR “paging” is better.

9. Paging • To locate a mobile user by querying/searching • Look up the location database if it is accurate. • Simultaneous paging • To pagesimultaneously in the cells where the user may be located. • Short response time • Sequential paging • Paging cells in the order of descendingprobability • Low paging cost • A hierarchical solution • Group related or neighboring cells within the area served by the Message Service Center (MSC) into location areas. • Simultaneous in one area, sequential among areas

10. Location Update • Observation: • it is not necessary to always update, since an object is likely to be near its last location. • The vicinity of the last update information gives the most probable location of the mobile user. • Static scheme vs. Dynamic scheme • Predefined update occurrence • Dynamic decided update occurrence

11. Location Update • Static update scheme: global information is needed. • Group cells into location areas as before. • All location areas contain non-overlapping groups of cells. • Mobile user sends update when it crosses a location area boundary. • Advantage: low update cost. • Disadvantage: accuracy is only to location area. • Selection of designated reporting cells. • Mobile user must send in updates when they enter some designated cells. • Advantage: optimal set of reporting cells can be computed. • Disadvantage: determining the optimal set is expensive.

12. Location Update • Dynamic update scheme: • update based on local information and initiated by client, i.e., the place where the update takes place is dynamic, as determined by client. • Distance-based • Update when the Euclidean distance (or Manhattan distance) to the previously updated location exceeds a threshold. • Movement-based • Update when the number of cell boundaries crossed since last reporting reaches a threshold. • Time-based • Update when the time to the previous update exceeds a threshold (i.e., periodic updates).

13. §9.2 LM in Cellular Networks • General scheme • Location Management Architecture • Two-tier. • Hierarchical • Centralized • Techniques for Efficiency • Caching • Replication • Forwarding pointer

14. Location Management in GSM • The Mobility Management layer (MM) is built on top of the Radio Resources Management (RR) layer. • MM handles the functions due to mobility of subscriber and authentication and security aspects. • A mobile phone that is powered on is informed of an incoming call by a paging message sent over the PAGCH channel of a cell. • Paging vs. update: • Page each cell in the network for each call • Costly paging, suitable for few call but much movement • Page exactly one cell but requires the mobile phone to send updates each time it changes cell: • Costly update, suitable for many calls but little movement

15. Location Management in GSM • Compromised solution used in GSM is to group cells into location areas. • A location area is a collection of adjacent cells. • Update required only when moving between location areas. • To search, mobile units are paged in the cells of their current location area. • HLR: home location register • VLR: visitor location register AuC- Authentication Center EIR – Equipment Identity Register

16. Location Management in GSM • HLR (Home Location Register) Maintains the current location of a user as part of the user’s profile. • At a network location pre-specifiedfor each user U (the home zone). • When U moves from zone A to zone B, the HLR for U is updated. • To locate a user U, the HLR for U is queried to get U’s current location. • VLR (Visitor Location Register) Holds information about a visiting user who moves out of its home zone. • VLR contains a copy of the profile of a visiting user V. (To reduce the expensive querying cost at remote HLR) • When V moves from zone A to zone B, • Entry for V will be removed from the VLR at zone A and • The corresponding entry will be inserted to the VLR at zone B.

17. Location Update in GSM • When mobile phone moves to a new location area: • Location update msg is sent to new MSC/VLR via base station. • If mobile phone is authorized in the new MSC/VLR, it determines the subscriber’s HLR from the mobile identification number. • HLR sends reg. ack. msg to new MSC/VLR and sends reg. cancellation msg to old MSC/VLR to cancel its VLR entry • Periodic location updating • After the updating time period, if the mobile phone has not registered, it is then deregistered.

18. Call Delivery in GSM • Calling phone sends call initiation signal to MSC via a base station. • MSC determines address of HLR of called phone and sends it a location req. msg. • HLR sends a route req. msg to the MSC/VLR currently serving the phone. • MSC/VLR allocates a temporary local directory number to the called phone and replies HLR with this number. • HLR forwards this info. to the MSC of calling phone. • The MSC of calling phone requests a call setup to the MSC of called phone.

19. USER INFO USER INFO CURRLOC A A MSC2 VLR VLR VLR HLR HLR HLR VLR MSC 3 USER INFO A MSC 1 MSC 2 VLR To other MSCs HLR Fixed Network Change from MSC2 to MSC3 on second movement

20. Location Management in GSM • IMSI: international mobile subscriber identity • TMSI: temporary mobile subscriber identity

21. Location Management in 3G • Besides HLR and VLR, there is GLR • Gateway Location Register: • Between HLR and VLR, like the agent in the CAS model. • GLR contains roamer’s profile and location information. • GLR can interact with multiple VLR in the visitor network. • GLR handles location update from VLR by behaving like HLR. • Local location update: at GLR

22. Location Management Architecture • Two-tier. • Only HLR and VLR. • HLR contains actual location of user. • To search, contact VLR first. If not found, ask HLR. • Hierarchical. • Inserts multiple layers between HLR and VLR. • HLR contains actual location of user (direct or indirect). • To search, contact leaf. If not found, traverse up the tree. • Centralized database. • A single location database containing all location info.

23. Two-Tier Scheme MSC HLR HLR MSC VLR VLR MSC Network 1 MSC Network 2 VLR VLR

24. Two-Tier Scheme • Advantage: • Simple architecture and management. • A maximum of 2 operations for each lookup. • Only 3 operations for each update. • Disadvantage: • Does not support locality. • Searching in nearby locations is impossible. • Always need to register with HLR at a possibly far distance upon a move, even if the move is just to a neighboring cell. • Home Location Register is permanent. • Resettlement is not supported: users who had moved to new region permanently still contact old HLR. • Does not scale well to larger distributed systems since HLR is always contacted.

25. Y Y Y Y X X X X 12 16 16 16 16 Hierarchical Scheme • To avoid contacting remote HLR and to serve neighboring search, multiple registers are used. • A hierarchy of location databases is maintained. • Internal node contains information about user registered in the set of zones in its subtree. • Leaf node contains actual location of objects in its coverage. • Internal node contains information about users registered and their location, which are covered by the children nodes. • Two types of schemes: • Pointers to lower level database (e.g., find X via pointers to 12). • Actual location of each object (find Y directly at 16). 1 location scheme pointer scheme 3 2 7 6 4 5 18 20 19 8 12 15 10 14 17 9 13 16

26. 1 X X X X Y Y Y Y LCA(19,16) LCA(8,12) 3 2 7 6 4 5 18 20 19 8 12 15 10 14 17 9 13 16 16 16 12 16 16 Hierarchical Scheme • Let LCA(i, j) denote the least common ancestor of nodes i and j. • To search for object Y which is currently at node i (i.e., 16) initiated by a user at node j (e.g., 19). • Go up the tree from j until LCA(i, j) is reached, where Y is found, with its actual location at 16. • To lookup for object X at node i (i.e., 12) initiated by a user at node j (e.g., 8). • Go up the tree from j to LCA(i, j) is reached, where X is found. • Follow the pointer for X until X is really found at 12.

27. 17 1 Y Y Y X X X 17 3 2 17 7 6 4 5 18 20 19 8 12 15 10 14 17 9 13 16 X Y 16 16 16 Hierarchical Scheme • With pointer, to updateX from i to j: • Databases along path i, …, LCA(i, j), …, j are to be updated. • Entries for X from i to the node just below LCA(i, j) are deleted. • Entries for X are created from the node just below LCA(i, j) to j, each pointing to the proper child. • Entries for X at LCA(i, j) is updated to point to the proper child. • For example X moves from 12 to 14. • With location, to update Y from i to j: • Databases from i to LCA(i, j) and from root to j are to be updated. • Entries for X from i to the node just below LCA(i, j) are deleted. • Entries for X are created from the node just below LCA(i, j) to point to j. • Entries for X from root to LCA(i, j) are updated to point to j. • For exampleY moves from 16 to 17.

28. Hierarchical Scheme • Advantage • Mobile object is not bound to HLR. • Locality of moves and lookups is possible. • It is fast to search for a user nearby. • Disadvantage • Seemingly increased number of communication messages. • Increased load and storage requirements for intermediate databases. • Intermediate databases store location information (actual location or a pointer) for all objects covered by its children. • Root database stores location information for ALL objects.

29. Centralized Database • To simplify location query and update, a centralized database can be assumed to contain the location of all objects. • Centralized database solution is often adopted for simplicity if the object moves continuously, reporting its location. • Recall in previous schemes, location of object is only accurate up to the cell. • A spatial database is often used to store an object location for fast querying, with additional features for moving objects. • Additional operations supported include nearest neighbor search (find the closest gas station), range query (find restaurants within 5 minutes drive), and k-nearest neighbors. • Recall that in the centralized database, there could be numerous objects. Cost of full scan is prohibitive and update is expensive. • Specially designed databases are needed for that purpose.

30. Centralized Database • To reduce update cost, object movement can be predicted. • Predicting object movement, then an object that does not deviate from its predicted location does not need to send in an update. • Trade off can be made with respect to the frequency of updates for lower accuracy. • Send an update only when distance, movement, or time elapsed exceed certain threshold. • Group-based approach: • A group of objects that stay close to one another can have their aggregated location reported together via the group leader. • Safe-region approach: • As long as an object stays within its safe region, no update is needed, since server knows that object is within that safe region. • Size of safe region may be adjusted dynamically according to actual query need (an object seldom interested should update location less).

31. a b c d h g f e Centralized Database • Time-based, movement-based and combined updating example.

32. a b c d h g f e Centralized Database • Distance-based example. 2 4 5 3 2 7 5 8 3 5 6

33. Centralized Database • Group-based example with 3 groups. • There are algorithms to form and maintain the group membership. Group-based Individual-based

34. Techniques for Efficiency • One may perform caching of location of moving object to reduce need to send in query. • Cache the moving object location (e.g. callee’s location) at the caller site. • This is effective with large CMR. • One may extend caching into replication with multiple copies of the location. • Replicate location of a moving object at its frequent callers. • This is also effective with large CMR. • Forwarding pointers is used to reduce updates to VLR and HLR. • Do not update VLR and HLR. • Just leave a forwarding pointer from old VLR to new VLR. • This is effective with small CMR.

35. Caching • To locate a user U, the cache at the VLR of caller’s zone is queried first, before contacting U ’s HLR. • In two-tier scheme, caching enhances VLR and HLR. • If user U at L1 is called by user A at L2. VLR at L1contains U. • Cache at L2 can then indicate that U is at L1. • Subsequent calls originated from L2 can directly contact U without having to look for U’s HLR. • Two caching schemes: • Eager caching: when a user is moved to a new location, all cache entries for this user’s location are updated. • Lazy caching: a move operation does not automatically translate to a cache update. • With lazy caching, during a lookup, either user is still in indicated location (cache hit) or has moved out (cache miss).

36. Replication • One can create copies about this information at selected sites (e.g., at hubs). • Improve lookup response time. • Reduce network load during lookup. • Generate additional overhead during location update. • Replication should be made judiciously for high CMR. • More precisely, if Local CMR (LCMRi,j), i.e., number of lookups for user X in zone i from zone j during a period / number of location updates of Xis greater than a threshold  ( = cost of update / cost saving with local lookup). • Replication may also be performed to indicate the approximate user location (e.g., current partition).

37. X 5 1 (1)? 6 6 Replication • Working set replication is applicable to two-tier scheme. • For each user X, replicas are kept at frequent callers of X. This is called the working set of X. • Ensure that for all j, LCMRi,j for X in zone i from zone j cost of update / cost saving with local lookup. • When a call to X is made from zone k: • If k is in working set, no update is needed. • If k is not in working set but the above inequality holds for k, k is added to the set. • When X moves, the inequality is evaluated for each member k in the working set of X: • If it does not hold for k, k is removed. • Performance is affected by CMR. X 5 1 4 2 6 8 7 3 9 • (2, 3) are in working set. • Y (at 6) queries X (at 1). • 6 is added to working set if inequality is true. • Now X moves to 5. • The working set (2, 3, 6) needs updating.

38. VLR…|HLR-X VLR…|HLR-X VLRi-X |HLRi… VLRi-X|HLRi… VLRi…|HLRi… VLRk-X |HLRk… VLRk-X|HLRk… Forwarding Pointers • Invalidation to caching and replication can be expensive, with low CMR (e.g., CMR < 0.5). • One can provide forwarding pointers to point to the potential current location (ProbLoc). This reduces communication overhead and query load at remote HLR. • In two-tier scheme, when X moves from i to k, a pointer is added at VLR at i to point to VLR at kwithout informing HLR. • During lookup if no information on X is found at current VLR, HLR of X is queried for proper VLR and follow the forwarding pointers. • Chain of pointers should not exceed a length of K. Typical value of K < 5. • X moves from i to k. • Instead of changing HLR for X, setup forwarding pointer at i to k. • Now X moves to n again, and forwarding pointer is added to k. VLRn-X|HLRn…

39. LCA(12,15) 1 X X X X X 3 2 7 6 4 5 18 20 19 8 12 15 10 14 17 9 13 16 X Forwarding Pointers • Forwarding pointers can be used also in hierarchical scheme. • Recall how a location update is done: entries along path from old cell to LCA then new cell, plus those coming from root need to be updated. • Use similar idea as bypass pointers in caching, create forwarding pointers to point to new location. • X moves from 12 to 15 without forwarding.

40. 1 1 X X X X X 15 X 12 X 15 3 2 3 2 7 6 4 5 X 12 7 6 4 5 18 20 19 8 12 15 10 14 17 9 13 16 X 12 18 20 19 8 12 15 10 14 17 9 13 16 X 12 Forwarding Pointers • Example that X moves from 12 to 15 with pointer and location schemes. 

41. There is a need to purge the forwarding pointers. Save storage. Avoid storing stale information. Improve lookup efficiency by compressing forwarding chain. User X moves from 11 to 18 to 26 then to 14. Pointers in 11, 18, 26 can be purged. Pointers in interior nodes leading to these nodes can be purged. Pointer Purging

42. Other Issues • Concurrency control • Location and pointer updates occur asynchronously. It is possible that a user has moved but a caller follows the old pointer down (and cannot reach the user). • Transactions for update are too expensive (lock too many entries). • Recovery • Nodes may crash, and information should be restored upon node recovery. This demands periodic checkpointing of information at HLR and paging for mobile users by VLR. • Precision, currency and performance tradeoff • Finer location granularity within cell is possible, with more frequent reporting from client. Both translate into higher update cost. • Server can keep a predication of the client location/trajectory, and client detects whether it is within allowable distance from the predication. Update/paging is only sent when deviation is too large. • Service discovery • Services provided by a host are similar to its location, i.e., can be registered with service (location) database for lookup. In mobile computing, host providing services may move. Calls (e.g., remote procedure calls) need to be routed to the moving host.

43. §9.3 LM in Ad Hoc Networks • Location in Mobile Ad Hoc Networks • Used as basic of routing protocol • Location based services (LBS) • Location dependent applications • E.g. data collection • Major Issues • Distributed location data storage • Distributed query • Failure resilience • Scalability

44. Categorization of LM in Ad Hoc Networks • Proactive LM: periodically exchange location information • Some-for-some • Some-for-all • All-for-some • Home region based • Quorum node based • All-for-all • Location dissemination • Reactive LM: discover location on demand

45. Host Region based Approach • Similar to HLR/VLR in cellular networks • Region: a rectangle or a circle area • Nodes  Region  location server • Location initialization • Broadcast or mapping function • Trigger of update • timer-based (i.e., periodic updates), • distance-based (i.e., moved more than a threshold) • predictive distance-based • Threshold of difference between predication and actual location • Location update • Send update to nodes in home region • Via a geographical forwarding protocol Problems?

46. Quorum-based Approach • Quorum? • The minimum number of votes that a distributed transaction has to obtain in order to be allowed to perform an operation – Wikipedia • Quorum for location management • Update: to what nodes to send location update • Query: to what nodes to query location information • There must be intersection between the nodes get the update and the nodes be queried. Please Give a naive example of quorum.

47. Uniform Quorum System (UQS) • Quorum construction • A subset of the network nodes are chosen that best serve as the network's virtual backbone • Quorums are then defined as subsets of the VB nodes, such that any two quorums intersect. • Location update • Sends new location information to the nearest VB node • The VB node forward the update to a quorum of nodes • Location query • Similar to update

48. Column/Row Quorum • A node's column • All the nodes to the north and south of the node's current location • The thickness of the column can be configured • A node’s row • Similar to column, but in west and east direction • Update • To the column nodes • Query • To the row nodes

49. Column/Row Quorum • The void region problem: special mechanism to address it.

50. Grid Location Service (GLS) • The set of location servers is determined by a predefined geographic grid and the ordering of node id. • Key issues • Location server selection, • Location query request, and • Location server update