The operation of the GSM system can be understood by studying the sequence of events that takes place when a call is initiated from the Mobile Station.
Call from Mobile Phone to PSTN:
When a mobile subscriber makes a call to a PSTN telephone subscriber, the following sequence of events takes place:
- The MSC/VLR receives the message of a call request.
- The MSC/VLR checks if the mobile station is authorized to access the network. If so, the mobile station is activated. If the mobile station is not authorized, service will be denied.
- MSC/VLR analyzes the number and initiates a call setup with the PSTN.
- MSC/VLR asks the corresponding BSC to allocate a traffic channel (a radio channel and a time slot).
- The BSC allocates the traffic channel and passes the information to the mobile station.
- The called party answers the call and the conversation takes place.
- The mobile station keeps on taking measurements of the radio channels in the present cell and neighboring cells and passes the information to the BSC. The BSC decides if handover is required, if so, a new traffic channel is allocated to the mobile station and the handover is performed. If handover is not required, the mobile station continues to transmit in the same frequency.
Call from PSTN to Mobile Phone:
When a PSTN subscriber calls a mobile station, the sequence of events is as follows:
- The Gateway MSC receives the call and queries the HLR for the information needed to route the call to the serving MSC/VLR.
- The GMSC routes the call to the MSC/VLR.
- The MSC checks the VLR for the location area of the MS.
- The MSC contacts the MS via the BSC through a broadcast message, that is, through a paging request.
- The MS responds to the page request.
- The BSC allocates a traffic channel and sends a message to the MS to tune to the channel. The MS generates a ringing signal and, after the subscriber answers, the speech connection is established.
- Handover, if required, takes place, as discussed in the earlier case.
