Cse 598 494 mobile computing systems and applications class 13 location management
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CSE 598/494 – Mobile Computing Systems and Applications Class 13 : Location Management. Sandeep K. S. Gupta School of Computing and Informatics Arizona State University. Agenda. Location management Managing the mobile phone connectivity to the cell tower during user mobility.

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CSE 598/494 – Mobile Computing Systems and Applications Class 13 : Location Management

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CSE 598/494 – Mobile Computing Systems and ApplicationsClass 13:Location Management

Sandeep K. S. Gupta

School of Computing and Informatics

Arizona State University


Agenda

  • Location management

    • Managing the mobile phone connectivity to the cell tower during user mobility


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


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


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


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


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


Handoff (3)

  • Choosing a new BS:

    • Based on signal strength

    • Base on predicted movement of MH

    • Based on resources available at BS


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


Single LR Scheme: Switching ON


Single LR Scheme: Handoff


Single LR Scheme: Search


Single LR Scheme: Search Failure


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


Registration Area-Based Location Management

Registration area == a group of cells; update only when crossing a registration

area boundary


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”.


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


Flat Replication


Hierarchical Replication

Non-leaf nodes cache all info in attached subtrees


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