1. Call Request from a MS

Whenever there is call from or for a GSM user, different logical channels are transmitted for different intervals of time. The following diagram shows the sequence of processes whenever there is call request from Mobile station.

As shown in Figure 1, when the mobile is in ON condition, it is locked or tuned to the strongest BTS signal. From the BCH, it gets all synchronization data such as time, frequency, Base Station Identity code (BSIC) etc. of the Base station.

2. Call Destined to a GSM MS

As shown in figure 2, A MS is always tuned to a BCH. The serving BTS transmits the IMSI of the target mobile as a PACH message. The target mobile acknowledges this with an RACH data burst. The flow diagram explains the remaining process diagrammatically.