Call Initiation Procedure

How does the network know where the subscribers are?

1. The radio network consists of a large number of BTSs. Each of these is given an identity.
2. The BTSs are grouped in Location Areas which also are given an identity.
3. Each MSC/VLR (Mobile Services Switching Centre/Visitor Location Register) serves the BTSs in an number of Location Areas
4. The GSM phones reports to the network (VLR) when it moves from a BTS in one Location Area to a BTS in another Location Area.

How does the network know where these subscribers are?

1. VLR always knows in which Location Area the GSM subscriber is located in at the moment
2. HLR always knows in which MSC/VLR the GSM subscriber is just now
3. The GSM subscriber’s telephone number tells the network to which HLR the actual GSM subscriber belongs.

1) Mobile A is staying in Location Area 1. MSC/VLR1 has reported this to Mobile A’s HLR.

2), 3) Mobile A moves to Location Area 2, and identifies a new Location Area info (LA2). It reports its arrival to MSC/VLR 2.

4) MSC/VLR 2 informs HLRA, and receives sets of Authentication Data for Mobile A

1) Via the radio path and the base station network a call request for GSM subscriber B 9212345 is sent from Mobile A to MSC/VLR A.

2-3) MSC/VLR A collects authentication data from HLR A (if such data has not been collected earlier) .Normally a bulk of such data is transferred, thus avoiding repeating authentication requests to HLR A.

4) MSC/VLR A requests HLR B of the actual location of GSM subscriber B.

5) Gateway MSC checks with HLR B”Where is the GSM subscriber?

6) The call is established to the actual MSC/VLR (Visiting MSC) either directly or through the fixed or international telephone network

7) The request for mobile 9212345 is transmitted over all BTSs in the actual Location Area of the called GSM subscriber. Mobile B recognises its own identity, and ringing is generated.

Call Termination Procedure

When a station calls a mobile station, we can term it as a Mobile Terminated Call (MTC). For e.g. when a call is made from a MTNL landline to an Airtel mobile phone we call it as an MTC. The mobile terminated call set-up is described below.

1. Initially the user dials the mobile number. It reaches the PSTN/ISDN/Switching office where it is identified as a GSM call (by the destination code mentioned) and forwards it to the gateway MSC i.e. GMSC.

2. The GMSC identifies the HLR for the subscriber (which is coded in the phone number) and signals the call set-up to the HLR.

3. The HLR then checks if the number is a valid number and whether that user has subscribed to this particular service. If so then an MSRN (Mobile Subscriber Roaming Number) is requested from the subscriber’s current VLR.

4. After receiving the MSRN, the HLR determines the MSC responsible for the mobile station and sends this information to the GMSC.

5. The GMSC then forwards the call setup request to the concerned MSC.