Operation Logic of SAS

In Normal state, FTA5120 receives REGISTER request and SIP Invite from UAs in the network and will forward them to the external proxy (i.e., outbound server, proxy server), and send SIP Option message to the cloud PBX to detect its status. Once the proxy replies with a SIP 200 OK, the device records the Contact and address of record (AOR) of the UAs in its internal SAS registration database. FTA5120 will maintain a database of all the registered UAs in the network. Meanwhile FTA5120 will continually send Keep Alive Message to the cloud PBX to check the status of PBX, once the connection is fail, FTA5120 will enter emergency state.

In Emergency state, FTA5120 will server as a proxy for UAs in the network, handle all the Register request and SIP Invite from UAs. Enterprise can carry internal call when the Cloud PBX fails. When FTA5120 receives calls, it will search the internal SAS registration database to locate the destination address (according to AOR or Contact). If the destination address is not found, FTA5120 will forwards the call to itself. FTA5120 has two FXO port, enterprise can preserves its capability for outgoing calls (from UAs to the PSTN network) or convey IP calls through PSTN line (Via SIP Trunk).