22-11-2012, 04:13 PM
GSM Location Update
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LOCATION UPDATE, TYPE NORMAL:
The MS listens to the system information, compares the Location Area Identity (LAI) to the one
stored in the MS on the SIM card (on BCCH channel if idle or SACCH channel if actice) and
detects whether it has entered a new location area or is still in the same location area. If the
broadcast LAI differs from the one stored on the SIM card, the MS must perform a location
update, type normal:
1) The MS sends a channel request message including the reason for the access. Reasons
other than location updating can be for example, answering a page or emergency call.
2) The message received by the BTS is forwarded to the BSC. The BSC allocates an SDCCH, if
there is one idle, and tells the BTS to activate it.
3) The MS is now told to tune to the SDCCH.
4) The MS sends a location updating request message that contains the identity of the MS, the
identity of the old location area and the type of updating.
5) The authentication parameter is sent to the MS. In this case the MS is already registered in
this MSC/VLR and the authentication parameter used is stored in the VLR.
LOCATION UPDATING, TYPE IMSI ATTACH:
IMSI attach is a procedure and is used by the MS to notify the system that it was powered on.
The procedure is described below:
1) The MS requests an SDCCH.
2) The system receives the IMSI attach message from the MS.
LOCATION UPDATING, TYPE PERIODIC REGISTRATION:
Periodic registration is a type of location updating procedure that is used to avoid unnecessary
paging of the mobile in cases where the MSC never receives the IMSI detach message and
also to prevent damage in case of database failure. The procedure is described below:
1) MS listens on the BCCH to specify if Periodic Registration Location Update is used in the
cell. If periodic registration is used, the MS is told how often it must register. The time is
set by the operator and can have values from 0 to 255 deci-hours (a unit of six minutes). If the
parameter is equal to zero, periodic registration is not used in this cell. If the parameter is set to
ten, for example, the MS must register every hour.
2) Both the MS and the MSC have the timer which controls the procedure. When the timer in
the MS expires, the MS performs a location updating, type periodic registration. After that, the
timers in MS and MSC restart.
In the MSC there is a time scanning function for the MSs. If the MS does not register within the
determined interval plus a guard time, then the scanning function in MSC detects this
and the MS is flagged as detached.