User enters the phone number and presses the “send” button.
To set up the phone call, the MS needs to send information to the MSC. The MS sends “Radio Resource Channel Request” to the associated BSS on the Random Access Channel (RACH) according to ALOHA The phone then waits to hear from the BSS at the Access Grant Channel (AGCH).
The BSS allocates a Traffic Channel (TCH), including the frequency and time slot, and broadcast it in the AGCH. It also contains information about time and frequency corrections.
The MS applies the corrections and tune to the assigned TCH.
MSC checks whether the MS is authenticated.
The BSS enables ciphering with the phone. At this step the connection has been set up between the MS and MSC. The BSS just forwards the message.
The MS sends a connection set up request to the MSC with the called phone number. The MSC connects to the PSTN and allocates the voice communication channel between the BSS.
Make the conversation.
User presses the “end” button. The MSC releases the voice channel with the BSS. The MSC informs the PTSN about the call release and the PTSN will inform the call has been released on its end. MSC informs the MS then releases the TCH.
Public switched telephone network mobile user home network visited network correspondent GSM: indirect routing to mobile home Mobile Switching Center HLR Mobile Switching Center VLR 1 call routed to home network 2 home MSC consults HLR, gets roaming number of mobile in visited network 3 home MSC sets up 2 nd leg of call to MSC in visited network 4 MSC in visited network completes call through base station to mobile