LAC Location Area Code in Telecom Cellular system

LAC Location Area Code

           A mobile subscriber who is roaming moves freely within the network and the network knows where the MS is located. This makes it possible for the mobile subscriber to receive a call wherever he or she is. To keep the network up to date with the subscriber’s location, the MS performs a location updating. The request for location updating can indicate one of three procedures. They are:

• location updating type normal

• IMSI attach

• periodic registration

The MS also informs the network when it switches to detached mode

LOCATION UPDATING, TYPE NORMAL

  A location area is by definition the area, handled by one or more BTSs where the MS can move around without updating the system. A location area is controlled by one or more BSCs, but only by one MSC. When the MS is idle, it listens to the system information on BCCH. This is how the MS gets the location area Identity of the serving cell.

Main steps involved in location updating

1.The MS listens to the system information, compares the Location Area Identity (LAI) to the one stored in the MS on the SIM card 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.

 2a. 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.

 2b. 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.

  2c. The MS is now told to tune to the SDCCH.

 The outcome of the procedure (2a, 2b, and 2c) is that a radio resource connection is dedicated to the MS. The procedure is therefore called RR connection establishment.

  3. 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.

4a. 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. If the MS is not already registered in this MSC/VLR the appropriate HLR or the previously used MSC/VLR must be contacted to retrieve MS subscriber data and authentication parameters .

 4b. MS sends an answer calculated using the received authentication parameter.

  5. If the authentication is successful, the VLR is updated. If needed, the HLR and old VLR are also updated (new registration)

6. The MS receives an acceptance of the location updating.

 7a. The BTS is told to release the SDCCH.

 7b. The MS is told to release the SDCCH and switches to idle mode.

 In cases where the MS is busy when it changes location area, it receives the information about the new LAI on the SACCH. The location updating takes place after the call is released. The MS must set up a new connection and perform the procedures described in the previous case.

 IMSI attach is a complement to the IMSI detach procedure and is used by the MS to notify the system that it was powered on, provided that it is in the same location area as it was when it entered the detached state. The system information informs the MS, if IMSI attach and detach is required or not.

1.         The MS requests an SDCCH.
2.         The system receives the IMSI attach message from the MS.
3.         The MSC sends the IMSI attach message to the VLR. The VLR removes the IMSI detached flag and resumes normal call handling for the MS.
4.         The VLR returns the IMSI attach acknowledge message to the MSC.
5.         The MS also receives an acknowledge message.

   The procedure is to be used only when the IMSI detach flag is set in the VLR, as is the case in Ericsson’s GSM system. If the flag is set in the HLR, switching to active mode requires a normal location updating of the MS.

 If the MS changes location area while detached, it must perform a normal location updating when switched on again. The signaling for location updating, type IMSI attach is identical to normal location updating, except for some bits in the message Location Updating Request that indicate the type of updating. Authentication is optional in this case.

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.

  1. The MS is informed via the system information if periodic registration 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 decide 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. A timer in both the MS and in the MSC controls the procedure.

 3a. When the timer in the MS expires, the MS performs a location updating, type periodic registration. The timers in MS and MSC restart

 3b. 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.

4. The MSC sends an acknowledgment to the MS.