GSM Call Flow (GSM Originating Call) Call Setup GSM Flow Call Setup GSM Flow: • The MSC sends an Initial Address Message (IAM) to the GMSC. The IAM contains the MSISDN of the called party as the MS dialed it. • The MSC will also send a Call Proceeding (CALL_PROC) message down to the BSS and this is when the BSS would assign a TCH to the MS, as described earlier • Based on the dialed number, the GMSC decides where to route the IAM within the PSTN. • The PSTN will continue to route the IAM until it reaches the correct Switching Center and the call routing is complete. The PSTN will then establish the call circuit and send an Address Complete Message (ACM) back to the GMSC. • The GMSC then forwards the ACM back to the responsible MSC indicating that the call circuit has been established. Call Establishment • Once the MSC receives the ACM, it sends an ALERT message