M&MA

Mobile Terminated Call         • The calling party dials the MSISDN for the mobile subscriber. • The PSTN identifies the network (PLMN) that the dialed MSISDN belongs to and will locate a GMSC for that network.         • The PSTN sends an Initial Address message to the GMSC. •T he GMSC forwards the MSISDN to the HLR and requests routing information for it. • The HLR looks up the MSISDN and determines the IMSI and the SS7 address for the MSC/VLR that is servicing the MS. •T he HLR then contacts the servicing MSC/VLR and asks it to assign a Mobile Station Routing Number (MSRN) to the call. (a MSRN is assigned to the call not to the MS itself. ) • The MSC/VLR allocates the MSRN and forwards it to the HLR.

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