Location Management in PCS Networks

1. Location Management in PCS Networks D. Manivannan

2. Overview • Background • The location management problem • Implemented solutions • Proposed solutions

3. Background • Cellular communication • Was tried in the late 70’s, and was commercially available in mid 80’s • Now, a widely used service • An affordable alternative to wired telephone service • Led to the development of a new generation of mobile communication network, called personal communication services (PCS) network • PCS can provide various types of services that are unavailable in traditional cellular systems

4. Background… • Some issues in PCS networks • Location management schemes • Channel allocation schemes • Development of new architectures • Security issues

5. The Problem • In wireline networks • There is a fixed relationship between a terminal (i.e. its ID) and its location • Changing location of a terminal generally involves network administration • Incoming calls for a particular terminal are always routed to its associated location • In contrast, PCS networks support Mobile Terminals (MTs). So, the ID of an MT does not provide the location information of that MT.

6. The Problem • In PCS networks • Support for mobility of terminals (MTs) should be provided • MTs change network access point as they move. • The ID of an MT does not provide the location information of that MT. • So, a location tracking mechanism is needed for the effective delivery of incoming calls

7. Solution to the location management problem • The MTs report their location to the network periodically • The network stores the location information for each MT in location databases • During call delivery, this information is retrieved from the database to locate the MT. • Current methods employ a centralized database architecture

8. The Cellular Model

9. Location Management

10. Current PCS Architecture

11. Current PCS Architecture • BSC manages the radio resources of its base stations • MSC provides typical switching functions and coordinates location registry and call delivery • Location management scheme is based on two level hierarchy VLR, HLR • A user is permanently associated with an HLR. Information about each user such as services subscribed, billing information and location information, … are stored here. • VLR stores information about the MTs visiting its associated area • In general, one HLR for each PCS network

12. SS7 Network • Network management functions of PCS such as, call processing and location registration are achieved by the exchange of signaling messages through the signaling network SS7 • In this figure, each VLR is collocated with one MSC

13. Location management • Two standards for location management • Electronic/Telecommunication Industry Associations (EIA/TIA) Interim Standard 41 (IS-41) - used in North America for Advanced Mobile Phone Systems (AMPS) • Global System for Mobile Communications (GSM) Mobile Application Part (MAP) - used in Europe Both are based on the two level hierarchy mentioned earlier

14. Location Management in IS-41 • Location registration procedures • Are procedures for updating the location databases HLR and VLR • Authenticating the MTs • Call delivery procedures • These procedures locate the MT based on the information available at the HLR and VLR when a call for an MT is initiated

15. Location registration • Network coverage area is partitioned into registration areas (RAs) • All BSs belonging to the same registration area belong to the same MSC

16. Location registration… • When an MT moves to a new Registration Area (RA). • MT enters a new RA and transmits a location update message to its new base station. • The base station forwards the location update message to the MSC through a wired link, which launches a registration query to its associated VLR • The VLR updates its record on the location of the MT. If the new RA belongs to a different VLR, the new VLR determines the address of the HLR of the MT through a lookup table and sends a location registration message to the HLR

17. Location registration… • When an MT moves to a new RA… 4. The HLR records the ID of the new serving VLR of the MT and sends a registration acknowledgement to the new VLR 5. The HLR then sends a registration cancellation message to the old VLR 6. The old VLR removes the record of the MT and returns an ack. to the HLR

18. Call Delivery • The calling MT sends call initiation signal to the serving MSC through the BS • MSC determines the address of the HLR of the called MT and sends a location request to the HLR • The HLR determines the serving VLR of the called MT and sends a route request message to the VLR. The VLR forwards it to the MSC • The MSC allocates a temporary identifier called TLDN (temporary local directory number) to the MT and sends a reply to the HLR together with the TLDN. • The HLR forwards this to the MSC of the calling MT. • The calling MSC requests call set up to the MSC of the called MT using the TLDN.

19. Disadvantages of this registration and call delivery approach • Location registration may generate significant traffic load to the SS7 network since depending on the distance between the current and home locations of the MT, signaling messages may have to go through several signal transferring points (STPs) • This is a centralized method and hence will not scale well as the number of MTs increases • When a call originates from an MT in a nearby registration area, the MSC of the calling MT still has to query the HLR that is far away.

20. Some proposed solutions – centralized database architectures (Ravi Jain et al) • Uses per-user location caching strategy • Cache of location information is maintained at nearby STP • Whenever the MT is accessed through the STP, an entry containing the mapping of the ID of the MT and the VLR is added to cache

21. Some proposed solutions – centralized database architectures (Ravi Jain et al) • Uses a pointer-forwarding strategy • Every time an MT moves to an RA belonging to a different VLR, a pointer from the old to new VLR is set up • When a call is initiated, the network locates the MT by following the pointer chain • For efficiency, the length of the pointer chain can be limited to a predefined maximum value.

22. Some proposed solutions – centralized database architectures (J.S.M.Ho. et al) • A local anchoring scheme • A VLR close to the MT is selected as a local anchor • Location registration messages are reported to the local anchor instead of the HLR • When an incoming call arrives, the HLR queries the local anchor of the called MT which in turn queries the serving VLR to obtain a routable address to the called MT • Local anchors can be selected dynamically or statically

23. Distributed database architectures – J.Z. Wang • The two level HLR/VLR architecture is replaced by large number of location databases • The location databases are organized as a tree with the root at the top and the leaves at the bottom • The MTs are associated with the leaf • This approach increases the number of updates and queries and thus increases the delay in location registration and call delivery.

24. Distributed database architectures – B.R. Badrinath et al. • Presents a partitioning scheme for the fully distributed database hierarchy • Partition the location servers by grouping location servers among which the MT moves frequently • Location registration is performed only when the MT enters a partition • No registration is performed when the MT moves to another location server in the same partition

25. Distributed database architectures – V. Anantharaman et al. • Similar to the one seen earlier. • The root of the tree contains the database, but it is not necessary for other nodes to have the database installed • These other databases store pointers for MTs. If an entry for an MT is residing at a subtree of a database, a pointer is set up in this database pointing to the next database along the path to the MT

26. Some drawbacks of the location update and paging schemes • Excessive location updates may be performed if an MT located at the boundaries of RAs moves back and forth between two RAs • Mobility and call arrival patterns of MTs vary. If an MT will not receive calls frequently, then no need to perform location updates frequently. The following schemes take this into consideration.

27. Other mechanisms – Time based mechanisms • An MT performs location updates at constant time intervals • This approach is suitable if an MT moves constantly; at times, location information will not be correct.

28. Other mechanisms – Movement based mechanisms • An MT performs a location update whenever it completes a predefined number of movements across cell boundaries • Figure shows updates done for movement threshold 3

29. Other mechanisms – Distance based mechanisms • An MT performs a location update whenever its distance from the cell where it performed the last update exceeds a predefined value. • Figure shows updates done for movement threshold 3

30. Source • Ian F. Akyildiz and Joseph S. M. Ho Georgiatech “On location management for personal communication networks” appeared in IEEE communications magazine September 1996.