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

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

Sandeep K. S. Gupta

School of Computing and Informatics

Arizona State University

agenda
Agenda
  • Location management
    • Managing the mobile phone connectivity to the cell tower during user mobility
location management ch2
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
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
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
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
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
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
  • 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
enhancements to single lr scheme
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-Based Location Management

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

area boundary

properties of ra based approach
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
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
hierarchical replication
Hierarchical Replication

Non-leaf nodes cache all info in attached subtrees

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