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CSE-302-Location Management

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					CSE-302: Location Management

         Dr. R. B. Patel




                               1
         Locating Moving Objects
• Example of moving objects
   – mobile devices (cars, cellular phones, palmtops, etc)
   – mobile users (locate users independently of the device they
     are currently using)
   – mobile software (e.g., mobile agents)

• How to find their location - Two extremes
   – Search everywhere
   – Store their current location everywhere
    Searching vs. Informing

                                                              2
           Locating Moving Objects
• What (granularity), where (availability) and when
  (currency) to store    at all sites



                        Availability
                                          At selective sites (e.g., at
                                          frequent callers)
            the whole
            network
Exact                                     nowhere
location


                                       Never
                                       update
                                                                         Always update (at each
                                                                                           3
                                                                         movement)
    Architectures of Location DBs
• Two-tier Schemes (similar to cellular phones)
   – Home Location Register (HLR): store the location of each
     moving object at a pre-specified location for the object
   – Visitor Location Register (VLR): also store the location of
     each moving object mo at a register at the current region
• Hierarchical Schemes
   – Maintain multiple registries




                                                               4
           Two-tier Location DBs
• Search
  – Check the VLR at your current location
  – If object not in, contact the object’s HLR


• Update
  – Update the old and new VLR
  – Update the HLR

                                                 5
       Hierarchical Location DBs
Maintain a hierarchy of location registers (databases)
A location database at a higher level contains location information
for all objects below it




                                                                      6
       Hierarchical Location DBs
Call




                     caller
                                   7
       Hierarchical Location DBs
Move




                          new location
           old location
                                         8
      Hierarchical vs. Two-tier
(+) No pre-assigned HLR
(+) Support Locality
(-)   Increased number of operations (database
        operations and communication messages)
(-)   Increased load and storage requirements at the
        higher-levels




                                                       9
        Locating Moving Objects
 Partitions




                         P3

P1        P2                  P4            P5




                User x             User x        10
         Locating Moving Objects
• Caching
   – cache the callee’s location at the caller
     (large Call to Mobility Ratio)
• Replication
   – replicate the location of a moving object at its frequent
     callers (large CMR)
• Forwarding Pointers
   – do not update the VLR and the HLR, leave a forwarding
     pointer from the old to the new VLR (small CMR)
   – When and how forwarding pointers are purged?
 Concurrency, coherency and recovery/checkpointing of location
  DBs
                                                            11
     Querying Moving Objects
• Besides locating moving objects, answer more
advanced queries, e.g.,
   • find the nearest service
   • send a message to all mobile objects in a specific
      geographical reafion
• Location queries: spatial, temporal or continuous
   •Issues: representation, evaluation and imprecision
Most current research assumes a centralized location
database
                                                       12
        Querying Moving Objects
 How to model the location of moving objects?

Dynamic attribute (its value change with time without an
explicit update) [e.g., in MOST]
For example, dynamic attribute A with three sub-attributes:
      A.value, A.updatetime and A.function
 (function of a single variable t that has value 0 at time t=0)
   • The value of A at A.updatetime is A.value
   • at time A.updatetime + t0 is A.value + A.function(t0)
                                                            13
       Querying Moving Objects
 How to represent and index moving objects?

 Spatial indexes do not work well with dynamically
     changing values
 Value-time representation
   • An object is mapped to a trajectory [Kollios 99]




                                                        14
      Query Processing
• Location Dependent Query Processing
• QBI (Query by Icons)
• Using Summary Databases




                                        16
Location Dependent Query Processing (LDPQ)

- Queries contain location related and non-location
related attributes.
- Current position of the query issuer is implicitly
involved in the query.
       ex: “Find hospitals within 10 miles”
- This implied location related information will be
added to the query.
- Location service binds the current location to the
query and this binding is known as location binding.


                                                   17
Query By Icons

 QBI include:
 1. An iconic visual language interface that allows
    users to compose queries by using a pointing
    device like a pen to manipulate icons.
 2. A semantic data model that captures most of the
    aspects of database structures.
 3. Metaquery tools that assist on the formulation of
    queries during periods of disconnection.




                                                        18
QBI Architecture


The architecture consists of four modules:
1. Presentation Manager: responsible for all user
   interactions
2. Query Manager: supports the specification of
   queries.
3. GA Evaluator: computes the generalized
   attributes
4. Database Access Manager: is responsible for
   any remote access to the data on a fixed host,
   data updates and metadata management.


                                                    19
QBI Architecture


                   20
Query Processing using Summary Databases
(Madria et al)

  • A condensed form of the main database, which is
  termed as “summary database” is used in replication.
  • Summary databases are small in size and can
  answer the queries approximately during disconnection
  or weak connection.
  • There are three types of connections between host
  and MSS: connected, partially connected, disconnected.
  • The query processor selects main database or
  summary databases depending on the connection
  status.
  • Query processor rephrases the queries to access
  summary database.

                                                       21
Query Processing using Summary Databases
(contd..)
• Summary databases use concept hierarchies.
• Concept hierarchy is a result of mapping from one level
  to another level of data abstractions.
• A concept hierarchy has a tree or lattice structure of
  concepts at different levels of abstraction.

Summarization of the data can take place in two ways:
1. Horizontal summarization (involves attributes of a
   relation)
2. Vertical summarization (involves tuples of a relation)



                                                        22
Mobile Database Environment with Summary
                Databases
                                           23
              Recovery
• Recovery Protocols
• Recovery Guarantees




                         24
Recovery Protocols
1.Timeout protocol: executed by MSS. MSS maintains a timer to measure
   the inactivity period of mobile host and initiates rollback for the
   transaction on timeout.

2.Disconnect protocol: executed by mobile host due to resource problems
   (like battery discharge, weak signal etc.).

3. Hand-off protocol: executed by mobile host, when it switches from one
   MSS to another MSS. Mobile host sends it’s new MSS address to the
   old MSS and conveys old MSS information to the new MSS while
   switching from cell to cell.

4. Migration protocol: In this protocol migration information and new
   settings of mobile host are communicated to the old MSS by the new
   MSS, before timeout or disconnect protocol execution at old MSS



                                                                      25
Mobile Transaction Recovery Protocols



                                        26
                 Recovery Guarantees
Recovery guarantee is a recovery assurance of one subsystem
(eg.MSS) to another subsystem (eg.MH) in case of failures.

Protocol:
          if p succeeds
          then q will also succeed, if invoked
-The guarantee of assurance by a subsystem is up to its
capabilities.
-The guarantee can be given by any subsystem other than the
system where operation p is executed.

-Recovery protocols are prescriptions based on the recovery
guarantees of the system. These protocols satisfy precedence
constraints.



                                                               27

				
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Description: mobile computing