GSM Functionalities – How does GSM work

1. to initiate a call, the MS sends a request for radio resource allocation to the BSS, which mediates further connection to the Mobile Switching Center (MSC). The BSS assigns the MS a channel with a given frequency and time slot, which constitutes the communication route between the MS and the BSS.
2. once the MS confirms the established channel, the BSS can initiate the connection to the MSC.
3. once the MS has connected to the network, the subscriber needs to be authenticated. This can be done using the IMSI number stored in the SIM card, which allows the Authentication Center in the core network to verify the subscriber’s identity. After this step, the MS and the MSC can start communicating.

To make sure the data sent over the radio network between the BSS and MS/MSC is secure, the MSC initiates a ciphering procedure which is transmitted to the BTS, which in turn forwards the message to the MS. The MS enables ciphering towards the BTS; as the BTS starts receiving ciphered data, it will start the ciphered transmission of information, finalizing the encryption procedure.

4. in order to initiate the call setup, the MSC verifies that the requested service is allowed for the subscriber. This information is available in the Virtual Location Register, which maintains temporary subscriber data (location, preferences, allowed services). Once the VLR confirms the service requested by the originating MS, the MSC starts the call setup.

5. for the call to take place, the MSC allocates a voice channel between the MSC and the BSS. The BSS notifies the MS about the change to voice mode, and the MS returns a confirmation message. The MSC routes the call to the dialled number. When the call is received in the PSTN, the MSC is notified that the called subscriber is being alerted, at which point the originating MS receives a ring notification.

To disconnect the call by either party, a disconnect message is sent to the MSC, which releases the communication channels created with the PSTN and the BSS.