16-02-2013, 10:24 AM
Location and Handoff Management
Location and Handoff.ppt (Size: 912.5 KB / Downloads: 119)
INTRODUCTION
The current point of attachment or location of a subscriber (mobile unit) is expressed in terms of the cell or the base station to which it is presently connected.
The mobile units (called and calling subscribers) can continue to talk and move around in their respective cells; but as soon as both or any one of the units moves to a different cell, the location management procedure is invoked to identify the new location.
The location management performs three fundamental tasks:
(a) location update,
(b) location lookup, and
© paging.
In location update, which is initiated by the mobile unit, the current location of the unit is recorded in HLR and VLR databases.
Location lookup is basically a database search to obtain the current location of the mobile unit and through paging the system informs the caller the location of the called unit in terms of its current base station.
These two tasks are initiated by the MSC.
Mobility Management
Location Management
Search: find a mobile user’s current location
Update (Register): update a mobile user’s location
Location info: maintained at various granularities (cell vs. a group of cells called a registration area)
Research Issue: organization of location databases
Global Systems for Mobile (GSM) vs. Mobile IP vs. Wireless Mesh Networks (WMN)
Handoff Management
Ensuring that a mobile user remains connected while moving from one location (e.g., cell) to another
Packets or connection are routed to the new location
Handoff Management
Decide when to handoff to a new access point (AP)
Select a new AP from among several APs
Acquire resources such as bandwidth channels (GSM), or a new IP address (Mobile IP)
Channel allocation is a research issue: goal may be to maximize channel usage, satisfy QoS, or maximize revenue generated
Inform the old AP to reroute packets and also to transfer state information to the new AP
Packets are routed to the new AP
Tradeoff in Location Management
Network may only know approximate location
By location update (or location registration):
Network is informed of the location of a mobile user
By location search or terminal paging:
Network is finding the location of a mobile user
A tradeoff exists between location update and search
When the user is not called often (or if the service arrival rate) is low, resources are wasted with frequent updates
If not done and a call comes, bandwidth or time is wasted in searching
Registration Area (RA) and theBasic HLR-VLR Scheme
Current Personal Communication Service (PCS) networks (i.e., cellular networks such as GSM) use RA-based basic HLR-VLR schemes:
The service coverage area is divided into registration areas (RAs), each with a visitor location register (VLR)
Each RA covers a group of base stations (cells).
A user has a permanent home location register (HLR)
Base stations within the same RA broadcast their IDs
If ID is sensed different by the mobile terminal then a terminal, location update is sent to the VLR of the current RA.
When crossing a RA boundary, an update is sent to the HLR.
A search goes by HLR->VLR->cell->paging (by the base
Dynamic Location Update
Location update algorithms can be static or dynamic
With static, an update is triggered because of crossing of RA boundaries, e.g., the basic HLRVLR scheme
With dynamic, update or not depends on a user’s call and mobility patterns
Dynamic Location update schemes:
Time-Based, Movement-Based, Distance-Based
Dynamic Location Update Schemes
Time-Based: A mobile terminal updates in every T time units
Movement-Based: A mobile terminal counts the number of boundary crossings and performs the update when a threshold is exceeded (e.g. M=6)
Forwarding pointers can be considered as a variation of it.
Distance-Based: A mobile terminal tracks the distance (in terms of RAs) it has moved since the last update.
Update is performed when a distance threshold is exceeded
Mobile terminal needs some knowledge of the network topology
Local Anchor can be considered as a variation of it.
LeZi Update
Based on a compression algorithm by Ziv and Lempel
LeZi is a path-based update algorithm by which the movement history, not just the current location, is sent in an update message from the mobile user to the location database server
The history has a list of IDs of the zone (LA or cell) the mobile terminal has crossed.
The location database keeps the history in compact form by means of a search tree structure (called a LeZi trie)
Can be part of the user’s profile
On a call arrival, prediction of the current cell is given and selective paging is performed