Subject: Mobile Computing

Difficulty: Medium

Marks: 4 Marks

Location Tracking:

• 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 HLR maintains a database for the mobile subscribers. At any point of time, the HLR knows the address of the MSC VLR that controls the current location area of the mobile.
• The HLR is informed about a location area update only if the location area change has resulted in a change of the MSC VLR. Visitor Location Register (MSC VLR) is responsible to switching voice calls and it also keeps track of the exact location area where the mobile user is present.

Call Setup in GSM:

1. Channel Request: The MS requests for the allocation of a dedicated signaling channel to perform the call setup.
2. After allocation of a signaling channel the request for MOC call setup, included the TMSI (IMSI) and the last LA1, is forwarded to the VLR
3. The VLR requests the AC via HLR for Triples (if necessary).
4. The VLR initiates Authentication, Cipher start, IMEI check (optional) and TMSI Re-allocation (optional).
5. If all this procedures have been successful, MS sends the Setup information (number of requested subscriber and detailed service description) to the MSC.
6. The MSC requests the VLR to check from the subscriber data whether the requested service an number can be handled (or if there are restrictions which do not allow further proceeding of the call setup)
7. If the VLR indicates that the call should be preceded, the MSC commands the BSC to assign a Traffic Channel (i.e. resources for speech data transmission) to the MS
8. The BSC assigns a Traffic Channel TCH to the MS
9. The MSC sets up the connection to requested number (called party)