Subject: Mobile Computing

Difficulty: Medium

Marks: 6 Marks

GSM Location Update: The location update procedure allows a mobile device to inform the cellular network, whenever it moves from one location area to the next. Mobiles are responsible for detecting location area codes. When a mobile finds that the location area code is different from its last update, it performs another update by sending to the network, a location update request, together with its previous location, and it’s Temporary Mobile Subscriber Identity (TMSI) In order to make a mobile terminated call, The GSM network should know the location of the MS (Mobile Station), despite of its movement. For this purpose the MS periodically reports its location to the network using the Location Update procedure.

Location Area (LA): A GSM network is divided into cells. A group of cells is considered a location area. A mobile phone in motion keeps the network informed about changes in the location area. If the mobile moves from a cell in one location area to a cell in another location area, the mobile phone should perform a location area update to inform the network about the exact location of the mobile phone.

The Location Update procedure is performed:

• The MS moves from LA1 to LA2, where both LAs are connected to the same MSC

Step 1:

A location update request message is sent from the MS to the MSC through the BTS, include the address of the previously visited LA, MSC, and VLR. In this, the addresses of previous MSC & VLR are same as those for the new MSC & VLR. TMSI is used to avoid sending the IMSI on the radio path. TMSI is temporary mobile subscriber identity of the MS. This temporary identity is allocated to an MS by the VLR at inter VLR registration, and can be changed by VLR after every call setup

Step 2:

The MSC forwards the location update request to the VLR by a TCAP message, MAP_UPDATE_LOCATION_AREA

The message includes:

• Address of the MSC
• TMSI of the MS
• Previous location area identification (LAI)
• Target LAI

Step 3 and Step 4:

MSC updates the LAI field of the VLR record, and replies with an acknowledgment to the MS through the MSC.