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CSE 535 – Mobile Computing Location Management

CSE 535 – Mobile Computing Location Management. Sandeep K. S. Gupta School of Computing and Informatics Arizona State University. Location Management. Location Management (Ch2) . Fundamental ideas: Mobile hosts (MH) are served by base stations (BS) (also called access points (AP)

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CSE 535 – Mobile Computing Location Management

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  1. CSE 535 – Mobile ComputingLocation Management Sandeep K. S. Gupta School of Computing and Informatics Arizona State University

  2. Location Management

  3. Location Management (Ch2) • Fundamental ideas: • Mobile hosts (MH) are served by base stations (BS) (also called access points (AP) • Mobile hosts can roam about the network • Mobile hosts (or other parties) must locate mobile hosts to communicate with them • Involves finding the base station currently serving the mobile host • Search operation • When a mobile host moves, must let the system know where it is • Update operation (also called registration) • Must allow mobile hosts to switch between base stations to support roaming • Handoff operation

  4. Location Information • MH will be served by one BS at a time • BS coverage is one cell • Location information can be maintained at various granularities • One cell—requires MH to update location every time it moves from one cell to another • Tradeoff: more accurate location info vs. a large number of updates, which may overwhelm the system • Cell group—organize cells into groups, only update when leaving current group • Tradeoff: less accurate location info, which will require paging every BS in the group, fewer updates, so less load on the system

  5. Tradeoffs • There will be always be tradeoffs between cost of search and update operations • More updates == more accurate location info == less cost for search • Fewer updates == less accurate location info == more cost for search

  6. Handoff • Handoffs between BSs are required to support roaming • There isn’t necessarily a one-to-one correspondence between handoffs and updates • Issues: • When to handoff? • Selecting a new BS • Allocation of resources such as channels • Informing old BS so that packets destined for MH can be forwarded

  7. Handoff (2) • Mobile controlled handoff vs. • Network controlled handoff • Handoff may be necessary because: • Mobile host is moving • Current BS is overloaded • Quality of communication with current BS is poor

  8. Handoff (3) • Choosing a new BS: • Based on signal strength • Base on predicted movement of MH • Based on resources available at BS

  9. Location Registrars • Location Registrars (LR) are databases containing location information for MHs • Can be one or many • To get the idea, consider a system with only one LR, a Home Location Registrar • Location is maintained at single-cell granularity

  10. Single LR Scheme: Switching ON

  11. Single LR Scheme: Handoff

  12. Single LR Scheme: Search

  13. Single LR Scheme: Search Failure

  14. Enhancements to Single LR Scheme • Can add a timestamp and time-to-live to registration information • TTL (not same as residence time of node in cell) • Helps in constraining the search cost • Search diameters = max speed x ttl • Related to concept of soft-state • Hard-state – explicit revocation of “state” • Soft-state – implicit revocation of “state” • Makes the system more fault-tolerant – adaptive to changes in system • If time-to-live expires, then location for mobile host is assumed out of date • Can expand the search to neighboring cells when attempting to locate a MH

  15. Registration Area-Based Location Management Registration area == a group of cells; update only when crossing a registration area boundary

  16. Properties of RA based approach • Advantages • Reduces update cost • Bounds the search cost (number of cells queried) • Search is restricted to an RA • Makes LM more scalable • By reducing nos of regs to be processed by the HLr • Makes LM more manageable (as compared to per-user LM schemes) • Issues: • Granularity of RA (how to configure system into multiple RAs) • Same size (homogeneous) or different size (in terms cells)? • Optimization problem: how to partition the cells into RAs taking into account call+mobility pattern so as to optimize “LM cost”.

  17. Other Optimizations • Movement-based update (non-RA based) • Update location when MH crosses a specified number of cell boundaries • Distance-based update (non-RA based) • Update location when MH moves a specified distance away from the last point of update • Time/Movement/Distance-based schemes are per-mobile user based schemes. In contrast to these, RA based scheme looks at aggregate mobility and call patterns. • Forwarding pointers • When maintaining multiple location registrars, use a chain of forward pointers to track the MH • Replication of location registrars • Flat • Hierarchical

  18. Flat Replication

  19. Hierarchical Replication Non-leaf nodes cache all info in attached subtrees

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