Mobility and Location Management I

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Mobility and Location Management I

• Mobility Management Problems
• Handoff Detection and Operations
• Location-Dependent Queries
• Location Update Methods and Data Modeling

After getting your position data, you have to
maintain them in location databases to support
different types of location-dependent services
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• Mobility Management Problems
• Roaming Vs. Handoff
• Monitoring the movement of an object
• for call connection (requests from the mobile
  object) and
• execution of location-dependent queries
• (requests on the mobile object)

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Mobility Management Problems

• Handoff (in connection) Vs. Roaming (not in
  connection)
• Handoff
• Providing continuous connection to a moving
  mobile station
• from a cell into another cell (horizontal
  handoff)
• from a network into another network (vertical
  handoff)
• When a mobile user moves into the coverage area
  of another network (cell), the radio link to the
  old network is eventually disconnected (becoming
  weak)
• A new link to the new network should be
  established before the disconnection to maintain
  the link quality

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Handoff Operations

• Handoff operations (MT -gtBSs-gt MSC)

How about the situation in other networks?
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Mobility Management Problems

• Roaming
• Moving but not in connection
• location update and paging (called)
• Keeping the current location of an object while
  it is moving
• Positioning techniques (I.e., GPS and cell ID
  updates)
• Location update (registration), when? under which
  conditions?
• Efficient link connection to a mobile object
  (calling a mobile station)
• For example, in a PCS, if a mobile station wants
  to connect to another mobile station, how??
  First, identify the current location
• Search for the called mobile station by paging
• What are the cells (areas) to be paged?
• A database maintained the previous reported
  locations of the mobile stations (location
  database)
• Location update cost proportional to the number
  of update generated
• Paging cost depends on the number of areas to be
  searched
• Location update cost Vs. paging cost
• Searching delay (paging delay)

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Detection of Handoff

• Handoff operations are expensive and need to be
  minimized
• Select and then establish the new link with the
  new base station
• A mobile station may be at a location that it may
  connect to several networks. Which network should
  it choose to connect to?
• Removal of the old connection after obtaining a
  new connection
• What will be the problem if a handoff cannot be
  completed?
• I.e., the new base station does not have any free
  channel for connection (forced termination)
• To initiate a handoff, two issues must be
  considered
• Who initiates the handoff operation?
• How is the need for handoff detected?
• Handoff detection is based on link quality
  measured
• It determines the needs for handoff
• Link quality affected by many factors and always
  not stable diffusion, reflection and multi-path
  propagation (fading problems)
• Techniques such as frequency hopping are employed
  to make the link quality stable (changing to
  another frequency when it is not stable)

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Detection of Handoff

• Mobile-controlled handoff (MCHO)
• The mobile station continuously monitors the
  strength of the signals from the surrounding base
  stations and initiates the handoff process when
  some handoff criteria (threshold) are met
• Network-controlled handoff (NCHO)
• The surrounding base stations (BSs) measure the
  signal from a mobile station, and the network
  initiates the handoff process when some handoff
  criteria are met
• Mobile-assisted handoff (MAHO)
• The network asks the mobile station to measure
  the signals from the surrounding BSs. The network
  makes the handoff decision based on the reports
  from the mobile station (MS).
• It requires a higher level controller (MSC) on
  top of the base stations

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Handoff Failure and Channel Assignment Schemes

• Handoff failure
• No channel is available on the selected base
  station
• Normally, the number of channels in a cell or the
  total bandwidth in a network is fixed (Vs.
  dynamic channel allocation scheme)
• Channel Assignment Strategies
• Minimize the probability of handoff failure due
  to no free channel (forced termination)
• Channel requests new call and handoff operations
• Reservation Vs. no reservation (same as new call)
• Some of the channels are reserved for handoff
  operations
• Channel reservation may increase the number of
  call blocking (from new call connection)
• What is the number of channels to be reserved for
  handoff users?
• A balance between call blocking probability and
  forced termination probability

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Handoff Failure and Channel Assignment Schemes

• Queuing priority scheme handoff operation may be
  delayed due to overlapping service areas
  (degradation interval and handoff area)
• FIFO
• Measured-based priority scheme (MBPS)
• Priority defined based on power level received
  from the links
• Higher priority to the mobile station with a
  weaker link quality
• Channel Assignment in shared networks
• A connection may require more than one channel,
  i.e., in multimedia transmission
• How many number of channels to be assigned?
• If insufficient number of free channels, what to
  do?
• Look at the one with smaller channel requirement
• A guarantee in quality of services (QoS)
• The number of channels required for a connection
  may be included in the initial call for
  connection
• If it cannot meet the requirement, it may reject
  the request

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BS_A
BS_B
May choose between two base stations for
connection Currently is connected to BS_A but
moving towards BS_B
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• Location Update and
• management of location databases for execution of
  location-dependent queries

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Applications of Mobility Management

• Location-dependent services
• Different services are provided at different
  areas to different users
• Different areas have different application
  characteristics and the users will have different
  types of requests (spatial properties)
• Sales news broadcasting in shopping malls
• Traffic and weather information at different
  areas (Central, Kowloon Tong, )
• Active detection of location information of
  clients (proactive and context-aware services).
• How? i.e., obtained from the network or by radio
  detectors
• Enter a shopping mall -gt your location is updated
  -gt rule-evaluation -gt location-dependent services
  (i.e., big sales information)

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Location-Dependent Applications

• Example applications
• Automobile navigation, traffic information,
  etc..
• Tracking
• Trucking companies use LD services to minimize
  the cost and maximize productivity (courier
  services)
• Helpful for dispatching taxis
• Advertising and marketing
• Discount and promotional coupons
• Agriculture and environmental protection
• Identity the living habit of animals and the
  health conditions of the plant at different
  regions
• GPS map and development of harvest strategies
• Animal tracking
• Security and theft control
• Track stolen items
• Accessibility
• Lost people (old man)
• Many more

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Making Uses of Mobility Information

• Location updates show the movement behavior of
  users
• What are the hot areas? Then
• Traffic conditions of the roads real-time
  navigation
• Which paths/roads people are preferred to choose
  when going from areas A to B
• Based on the location and movement behavior of
  the users, we can predict how they will move in
  the future, a temporal (time) and spatial (area)
  relation
• The movement of mobile users are not totally
  random
• To reduce the number of location updates

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Movement is affected by the connection of the
roads
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Location-Dependent Queries

• Queries processing for supporting various types
  of location dependent services LDQ and LDCQ
• Location-dependent queries (LDQ)
• The location information of a mobile client is
  submitted together with the query
• The result to be returned depends on the location
  of the client
• I.e. What is the nearest Italian restaurant from
  my current position (in Kowloon Tong Vs.
  Tsimshatsui)
• Location-dependent continuous queries (LDCQ)
• The LDQ is submitted with a begin time and an end
  time (i.e., from now to 10 min later)
• Continuously monitor the database throughout the
  activation period (i.e. re-evaluation after each
  update)
• What is the nearest Italian restaurant from my
  current position from now on until 10 min later
  (re-evaluate after each movement)
• I.e. Navigation for the shortest path to the
  nearest hospital

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Location-Dependent Queries

• LDCQ
• From a moving object
• Searching for a car park
• From a stationary object on moving objects (or
  other dynamic information)
• EQ1 A police officer may ask How many patrol
  cars are within 2 km of the airport?
• From a moving object on moving objects
• A patrol car is looking for other patrol cars

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Centralized Vs. Distributed Management

• Centralized approach use a powerful server to
  manage the space status and connection
  information
• Distributed approach multiple devices (service
  providers) manage the information
• Comparisons
• Problems in distributed computing
• Perform operations at device level because of
  limited bandwidth
• Due to the dynamic properties of the smart space
  and objects, a lot of updates are needed to be
  generated
• A distributed approach can make the management of
  objects to be localized and adaptive to the
  changing systems status (in networking
  processing). But, the communication overhead
  could be very heavy
• A hierarchical approach multiple levels with
  different types of coordinators may be used

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Location Management Architecture
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Location Data and Representation

• Location databases
• A database maintains location information of all
  moving objects (i.e., coordinates, identify, ..)
• The objects periodically/conditionally (how?)
  generate location update to refresh their
  locations maintained in the location database (to
  minimize the number of updates)
• Location information
• Coordinates gt X 23.111, 104.202
• Vs Location area ID gt X in school
• Accuracy Vs cost in processing
• In reporting the query results, we usually not
  simply present the coordinates
• The accuracy is highly affected by update
  frequency

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Location Update and Evaluation Problems

• Query result problem
• If the location of any objects accessed by a
  query changes, the result of the query may change
  too
• For example, EQ1 asking the number of cars within
  2km of the airport
• Ten cars satisfy EQ1 at time t1. At time t2
  (where t2 t1d), a car has moved and it is no
  longer within 2km of the airport
• Thus the result of EQ1 at time t2 has to be
  re-evaluated
• But, the location database may still keep its old
  location and no re-evaluation will be performed
• The client may still believe that the old results
  are still valid
• Evaluation problem
• Re-evaluate the query whenever an update has
  arrived? Heavy evaluation cost if the update rate
  is high but most of them are similar
• Periodic re-evaluation? What is the best period?
  Long Vs short
• Data transmission volume
• Proportional to the number of updates

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• What are the location update methods?
• A tradeoff between certainty and update cost

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Basic Location Update Methods

• Time-based
• A periodic update scheme
• Generate an update every fixed time threshold T
• How to define T? Based on the number of updates
  that the system can support and the movement
  behavior of the objects
• Distance-based
• If the difference between the current distance
  and the last update location is larger than D, an
  update is generated
• Hybrid (time-based distance-based)
• Either condition from TB and DB
• Speed-dead-reckoning
• An update is generated if the deviation of its
  current location is greater than the predicted
  location by a pre-defined distance threshold. How
  to make the prediction?
• Location area
• The system is divided into inter-connected
  location areas
• An update is generated if a moving object enters
  into a new LA
• How to divide the areas? (Location area planning
  problem)
• How to organize the location areas to minimize
  the searching cost

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Time-based Location Update Method
Uncertainty in location Vs. no. of updates
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Distance-based Location Update Methods
Uncertainty in location Vs. no. of updates
No update because of disconnection?
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Hybrid Method (time-based distance-based)
Reset the threshold whenever an update is received
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Speed-dead-reckoning (SDR)
Number of updates depends on the accuracy in
prediction
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Location Area (LA)
What will be the problem if an object is moving
in the boundary?
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Modeling of Moving Objects

• A predictive approach
• Moving Object Spatio-Temporal (MOST)
• For location management and for location
  prediction
• To minimize the update cost (frequency)
• Attributes static dynamic
• Dynamic attribute A of a moving object
• A.value A.updatetime A.function
• At time A.updatetime, the value of A is A.value
• A.function is a function time (t) which has value
  0 at t 0
• The value of A at time A.time t0 is given by
  A.value A.function(t0)

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MOST Model

• Using previous velocity to predict its current
  location as a function of time
• If the prediction is correct, the number of
  location update will be smaller
• If the prediction is incorrect??
• Another way for prediction is to use the movement
  of other objects for that area
• What is the assumption?

O
Time
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Example MOST Model

• An object O has a location attribute L
• If it is stationary, the location attribute has
  two sub-attributes L.x and L.y
• If it is moving, its location attributes are
• L.route, L.startlocation, L.starttime,
  L.direction, L.speed and L.uncertainty
• L.route we assume that the object is moving
  along a pre-defined route
• L.startlocation a point on the route and is the
  location of the object at L.starttime
• With L.speed and L.direction, we can estimate the
  current distance of object at last update time
  t from L.startlocation on the route
• L.uncertainty its value depends on the update
  threshold and the update scheme adopted

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Location Update with Prediction

• Under the MOST model, the results of an
  evaluation of a LDCQ is a set of tuples with each
  tuple consisting of ltobject, begin time, end
  timegt, Why?
• The begin time and end time of a tuple indicate
  the duration when the object satisfies the
  conditions of the query
• The result is determined based on its last
  recorded velocity
• The result is a prediction assuming that the
  object is moving following the information
  maintained in the location database
• The next update time of an object can be defined
  based on the begin time of the object, i.e., a
  shorter update interval is assigned if begin time
  is closer to the current time (for LDCQ)

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Location Update with Prediction
Distance / Speed
Based on current movement, the typhoon will hit
HK at 4pm and leave at 10pm
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References

• Y.B. Lin and I. Chlamtac, Wireless and Mobile
  Network Architecture, John Wiley and Sons
  (chapter 3)
• Fundamentals of Mobile and Pervasive Computing,
  Chapter 2