Mobile call origination in GSM:

Diagram:

Description :

1. The MS sends the dialed number indicating service requested to the MSC(via BSS)

2. The MSC checks from the VLR if the MS is allowed the requested service. If so, MSC asks BSS to allocate necessary resources for the call.

3. If the call is allowed, the MSC routes the call to GMSC.

4. The GMSC routes the call to the local exchange of called user.

5. The LE alerts (applies ringing) the called terminal.

6. Answer back (ring back tone) from the called terminal to LE.

7. Answer back signal is routed back to the MS through the serving MSC which also completes the speech path to the MS.

OR

Mobile call termination:

1. The PSTN user dials the MSISDN of the called user in GSM.

2. The LE routes the call to the GMSC of the called GSM user.

3. The GMSC uses the dialed MSISDN to determine the serving HLR for the GSM user and interrogates it to obtain the required routing number.

4. The HLR requests the current serving VLR for the called MS for a MSRN(MS roaming number) so that the call can be routed to the correct MSC.

5. The VLR passes the MSRN to the HLR.

6. The HLR passes the MSRN to the GMSC.

7. Using the MSRN, the GMSC routes the call to the serving MSC.

8. The MSC interrogates the VLR for the current location area identity (LAI) for the MS.

9. The VLR provides the current location for the MS.

10. The MSC pages MS via the appropriate BSS. The MS responds to the page and sets up the necessary signaling links.

11. When the BSS has established the necessary radio links, the MSC is informed an the call is delivered to the MS.

12. When the MS answers the call, the connection is completed to the calling PSTN user.