CSE 535

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CSE 535 Mobile ComputingLecture 7Location
Management

• Sandeep K. S. Gupta
• School of Computing and Informatics
• Arizona State University

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Location Management
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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

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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 cellrequires 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 grouporganize 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

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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

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Handoff

• Handoffs between BSs are required to support
  roaming
• There isnt 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

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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

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Handoff (3)

• Choosing a new BS
• Based on signal strength
• Base on predicted movement of MH
• Based on resources available at BS

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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

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Single LR Scheme Switching ON
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Single LR Scheme Handoff
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Single LR Scheme Search
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Single LR Scheme Search Failure
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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

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Registration Area-Based Location Management
Registration area a group of cells update
only when crossing a registration area boundary
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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 callmobility
  pattern so as to optimize LM cost.

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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

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