1. 1 Class Number – CS401 Class Name – Mobile and Distributed Data Access Instructor – Sanjay Madria Lesson Title - Location Mgmt

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3. 3 Personal Communication System (PCS) A system where wired and wireless networks are integrated for establishing communication.

4. 4 Personal Communication System (PCS) Wireless Components

5. 5 Personal Communication System (PCS) Mobile cells The entire coverage area is a group of a number of cells. The size of cell depends upon the power of the base stations.

6. 6 Personal Communication System (PCS) Problems with cellular structure  How to locate of a mobile unit in the entire coverage area? Solution: Location management  How to maintain continuous communication between two parties in the presence of mobility? Solution: Handoff  How to maintain continuous communication between two parties in the presence of mobility? Solution: Roaming

7. 7 Personal Communication System (PCS) Handoff A process, which allows users to remain in touch, even while breaking the connection with one BS and establishing connection with another BS.

8. 8 Personal Communication System (PCS) Handoff To keep the conversation going, the Handoff procedure should be completed while the MS (the bus) is in the overlap region.

9. 9 Personal Communication System (PCS) Roaming Roaming is a facility, which allows a subscriber to enjoy uninterrupted communication from anywhere in the entire coverage space. A mobile network coverage space may be managed by a number of different service providers. They must cooperate with each other to provide roaming facility. Roaming can be provided only if some administrative and technical constraints are met.

10. 10 Personal Communication System (PCS) Roaming Administrative constraints  Billing.  Subscription agreement.  Call transfer charges.  User profile and database sharing.  Any other policy constraints.

11. 11 Personal Communication System (PCS) Roaming Technical constraints  Bandwidth mismatch. For example, European 900MHz band may not be available in other parts of the world.  Integration of a new service provider into the network. A roaming subscriber must be able to detect this new provider.  Service providers must be able to communicate with each other. Needs some standard.  Mobile station constraints.

12. 12 Personal Communication System (PCS) Roaming Two basic operations in roaming management are  Registration (Location update): The process of informing the presence or arrival of a MU to a cell.  Location tracking: the process of locating the desired MU.

13. 13 Personal Communication System (PCS) Roaming Registration (Location update): There are six different types of registration.  Power-down registration. Done by the MU when it intends to switch itself off.  Power-up registration. Opposite to power-down registration. When an MU is switched on, it registers.  Deregistration. A MU decides to acquire control channel service on a different type of network (public, private, or residential).

14. 14 Personal Communication System (PCS) Roaming Registration (Location update): There are six different types of registration.  New system/Location area registration: when the location area of the MU changes, it sends a registration message.  Periodic registration: A MU may be instructed to periodically register with the network.  Forced registration: A network may, under certain circumstances, force all MUs to register.

15. 15 Personal Communication System (PCS) Registration Two-Tier Scheme HLR: Home Location Register A HLR stores user profile and the geographical location of each moving object at a pre- specified location VLR: Visitor Location Register A VLR stores user profile and the current location who is a visitor to a different cell than its home cell.

16. 16 Personal Communication System (PCS) Registration Two-Tier Scheme steps. MU1 moves to cell 2.

17. 17 Personal Communication System (PCS) Registration Steps 1. MU1 moves to cell 2. The MSC of cell 2 launches a registration query to its VLR 2. 2. VLR2 sends a registration message containing MU’s identity (MIN), which can be translated to HLR address. 3. After registration, HLR sends an acknowledgment back to VLR2. 4. HLR sends a deregistration message to VLR1 (of cell 1) to delete the record of MU1 (obsolete). VLR1 acknowledges the cancellation.

18. 18 Personal Communication System (PCS) Location tracking (MU2 wants to comm with MU1) Steps 1. VLR of cell 2 is searched for MU1’s profile. 2. If it is not found, then HLR is searched. 3. Once the location of MU1 is found, then the information is sent to the base station of cell 1. 4. Cell 1 establishes the communication.

19. Locating Objects in Mobile Computing Location Management Lookups and updates

20. 20 Introduction Our target is objects capable of changing their location We are interested in objects with identity We store user locations in multiple databases (DBs) Main questions: How do we update data when user moves? How do we locate user in DBs when it is required?

21. 21 Locating Moving Objects Example of moving objects mobile devices (cars, cellular phones, palmtops, etc) mobile users (locate users independently of the device they are currently using) mobile software (e.g., mobile agents) How to find their location - Two extremes Store their current location everywhere  search locally  Cost of updates Search everywhere  No information is stored anywhere; search is expensive  No cost of updates searching verses update cost

22. 22  Availability – either at all sites or at selective sites (frequently visited sites)  Currency – Stored location is always updated (it may not make sense if user moves very frequently  Precision – Exact location verses set of possible locations

23. 23 Locating Moving Objects What (precision), where (availability) and when (currency) to store Availability nowhere at all sites At selective sites (e.g., at frequent callers) Currency Never update Always update (at each movement) Precision Exact location the whole network Set of locations

24. 24 Overview Database schemas Two-tier Hierarchical Replication: Working set replication Replication in hierarchical schema Forwarding pointers: Two-tier schema Hierarchical schema

25. 25 Two Tier Scheme HLR – Home Location Register is associated with each mobile user, maintains current location of the user as part of the user’s profile HLR is located at a network location pre-specified for each user To locate X, X’s HLR is queried When X is moved to new zone, X’s HLR is contacted and updated Disadvantages : Global move is expensive

26. 26 Architectures of Location DBs Two-tier Schemes (similar to cellular phones) Home Location Register (HLR): store the location of each moving object at a pre- specified location for the object Visitor Location Register (VLR): also store the location of each moving object at a register at the current region Hierarchical Schemes Maintain multiple registries

27. 27 Two-tier Location DBs Search Check the VLR at your current location If object not in, contact the object’s HLR Update Update the old (delete) and new VLR (insert) Update the HLR

28. 28 VLR i -X|HLR i … Two-tier Schema- Enhancement User’s X profile is permanently stored in Home Location Register (HLR) Each site maintains Visitor Location Register (VLR), stores info about users not at their home location During lookup at VLR i : VLR i is queried for X location HLR of X is queried upon failure During move from i to k: X’s HLR is updated X’s profile is deleted from VLR i X’s profile is added to VLR k VLR…|HLR-X VLR k -X|HLR k … VLR i …|HLR i …

29. 29 Two-tier Schema Cont. –Does not support locality search in nearby locations impossible, always need to reg. with HA possible distant HLR is always contacted upon move –Home Location register is permanent resettlement is not supported Does not scale well; Home location is always contacted +Relatively simple max 2 operations for lookup 3 operations for update VLR i -X|HLR i … VLR…|HLR-X VLR k -X|HLR k … VLR i …|HLR i …

30. 30 Standards EIA/TIA (Electronic Indus Asso.) and GSM(Global System for mobile Comm.) – Use HLR and VLR Mobile-IP - Two IP addresses – Home address and Care- of –Address (Current point of attachment) Care-of-address is either the address of FA or IP address acquired by the node in the current network Mobile node registers its care of address with its home address

31. 31 Hierarchical Schema A hierarchy of location databases is maintained Internal node maintains information about user registered in the set of zones in its subtree Leaf node contains actual location of objects in its coverage Intermediate node contains location information for all objects covered by its children in a form of: Pointers to lower level DBs or Actual location of each object 18 19 20 7 15 16 17 6 12 13 14 5 8 9 10 4 2 3 1 X X X X 12 Y 16 Y Y Y

32. 32 Hierarchical Scheme LCA(I,j) – least common ancestor of nodes I and j Parameter that affect the performance of the most location management scheme Relative frequency of the move Call operations of each user Call to mobility ratio (CMR i ) = C i /U i Where C i is the expected number of calls to user P i over a period T and U i the number of moves made by P i over a period T LCMR ij (local call to mobility ratio involves origin of calls) = C ij /U i For hierarchical scheme, LCMR ij =  LCMR ik where k is a child of j; the call to mobility ratio for a user P i and an internal node j is the ratio of the number of calls to P i originated from any zone at J’s subtree to the number of moves made by P i

33. 33 Lookup in Hierarchical Schema ijWhenever lookup for object X at i is initiated at j: jThe tree is traversed from j upwards to LCA(i,j) Then iPointers are traversed downwards from LCA(i,j) to i iLocation of X is found at i (pointer case) OR Location of X is found in LCA(i,j) (actual location case) ijLCA(i,j) – Least Common Ancestor of i and j j,i)Example: (j,i) = {(8,12), (19,16)} 18 19 20 7 15 16 17 6 12 13 14 5 8 9 10 4 2 3 1 X X X X 12 Y 16 Y Y Y LCA(8,12) LCA(19,16)

34. 34 Moving in Hierarchical Schema jiWhenever object X moves from j to i: jiPointers on the path j,…,LCA(j,i),…i are altered OR j iAll databases on paths root,…,j and root,…,i are updated with new location of X 18 19 20 7 15 16 17 6 12 13 14 5 8 9 10 4 2 3 1 X X X Y 16 Y Y X Y 14

35. 35 updates Pointer case - When user x moves from 15 to 18, the entries at 18,7, 6, 3, 15 are updated ; x is deleted from the database at 15 and 6, at 3 it is updated and are added at 18 and 7 Actual location case – entries are updated at 0, 7,6,3,18,15

36. 36 Evaluation of Hierarchical Schema +Mobile object is not bound to HLR +Advantage of locality moves and lookups is taken –Increased number of operations DB operations Communication messages –Increased load and storage requirements for DBs Intermediate DBs store location information for all objects covered by its children Root DB stores location information for ALL objects

37. 37 Partitions Avoid maintaining locations at all levels, reduce search cost Partitions grouping zones for each user among which it moves frequently Partition exploits locality Information whether a user is currently in the partition is maintained at the LCA of all nodes in partition called representative of the partition Representative does not know exact location Reduces overall search cost, increases update cost when a user crosses partition, both representatives must be informed

38. 38 Locating Moving Objects Partitions P1 P2 P3 P4P5 User x

39. 39 Overview Database schemas Two-tier Hierarchical Replication: Working set replication Replication in hierarchical schema Forwarding pointers: Two-tier schema Hierarchical schema Other topics Relation to KDE3 project Evaluation

40. 40 Locating Moving Objects Caching cache the callee’s location at the caller side (large Call to Mobility Ratio) Replication replicate the location of a moving object at its frequent callers (large CMR) Forwarding Pointers do not update the VLR and the HLR, leave a forwarding pointer from the old to the new VLR (small CMR) When and how forwarding pointers are purged?

41. 41 Caching – Two Tier Scheme Current location of the callee may be reused by subsequent calls originated from same region Every time a user x is called, its location is cached at the VLR in the caller’s zone so it can be reused Caching Is useful for those users who frequently receives calls relative to the rate at which they relocate To locate a user, the cache at the VLR of the caller’s zone is queries first, if found then query is launched to the indicated zone without contacting the user’s HLR else HLR is queried

42. 42 Caching – Hierarchical scheme When a call is made from zone i to user x located at zone j, the search traverse from i to LCA (i,j) and then down to j Ack is returned back to i from j Forward and Reverse bypass pointers Forward bypass is an entry at an ancestor of i, say s, that points to ancestor of j, say, t Reverse is from t to s During the next call from zone i to user x, search will travel until s then follow t via LCA (i,j) or via a shorter path In case of simple caching, s and t can be at the leaf while in level caching, s and t belongs to any level and possibly to a different one. Placing a bypass at higher level node s makes this entry available to all calls made in s’s subtree, but will travel long path to reach s Placing high node t will increase lookup cost, cache entry remains valid as long as the user move’s inside t’s subtree

43. 43 Cache Invalidation Eager caching – Every time a user is moved new location, all cache entries for this user’s location are updated Location of the cache entries must be centrally known, failure of central location can cause problem Lazy caching – a move operation does not mean updating cache When look up, either the user is still in the indicated location or it has moved out (cache miss) Cache miss- HLR is contacted and then cache is updated Cache update only on cache miss Overhead – cache location must be visited first Saving of lazy caching over eager caching if hit ratio threshold for a user in a zone must exceed the (cost of lookup when there is a hit) / (the cost of lookup in non- caching scheme). Depends on querying HLR and VLR

44. 44 Replication Location (profile) of selected users is replicated at selected sites: Enhances lookup response time Reduces network load during lookup Creates overhead during updates Replication is judicious if following holds: number of lookups for object i from location j during T number of updates for i during T - savings per lookup; - cost per update Additional parameters: service capacity of DBs, etc. Replication is possible at both caller and receiver locations (1)

45. 45 Working Set Replication Applicable to two-tier schema Replicas are kept at frequent callers of X – working set of X Equation holds for every member j of the set: Every time a call to X is made: From a member of a set – no updates required From nonmember k of a set – if (1) holds for k, k is added to the set Every time X moves: (1) is evaluated for each member of X working set If (1) does not hold for member k, it is removed from the set (1) 1 3 4 5 2 8 7 9 X 1 5 6 (1)? 6 6 X

46. 46 Replication in Hierarchical Schema Takes advantage of locality in movement Local Call to Mobility Ratio is used to determine feasibility and are thresholds for determining replication of nodes If, j is assigned a replica If, j is not used for replication If, database depends on topology of network 18 19 20 7 15 16 17 6 12 13 14 5 8 9 10 4 2 3 1 … ++ + +

47. 47 Overview Database schemas Two-tier Hierarchical Replication: Working set replication Replication in hierarchical schema Forwarding pointers: Two-tier schema Hierarchical schema

48. 48 VLR i -X|HLR i … VLR…|HLR-X VLR k -X|HLR k … VLR i …|HLR i … VLR i -X |HLR i … VLR…|HLR-X VLR k -X |HLR k … Forwarding Pointers (two-tier schema) Pointers could be used to reduce communication overhead and query load at the HLR ij ijWhen X moves from i to j a pointer from VLR at i to VLR at j is added During lookup if no information on X is found at current VLR, HLR of X is queried and pointers are followed Chain of pointers is managed not to exceed length K Useful for users receiving calls infrequently and reallocate often It has been showed that cost savings for K<5, are 20%-60% VLR n -X|HLR n …

49. 49 Forwarding Pointers (hierarchical schema) Updating and lookup in hierarchical schema lack efficiency when LCA(i,j) is at higher levels Consider schema with entries as pointers to lower level DBs In simple forwarding a forwarding pointer connects two leaf nodes Allows cheaper updates In level forwarding a forwarding pointer connects two intermediate nodes Allows cheaper lookups 18 19 20 7 15 16 17 6 12 13 14 5 8 9 10 4 2 3 1 X X X X X X LCA(12,15) 18 19 20 7 15 16 17 6 12 13 14 5 8 9 10 4 2 3 1 X X X X Simple forwarding 18 19 20 7 15 16 17 6 12 13 14 5 8 9 10 4 2 3 1 X X X X Level forwarding