| 摘要 |
A method of Dual Radio Access Transfer of an IP Multimedia Subsystem, IMS, controlled voice or video call in an alerting phase for the Packet Switched, PS, to Circuit Switched, CS, direction. The method comprises initiating a SIP session towards a terminating mobile station, UE and, during a voice or video call alerting phase, making a determination at the UE that a DRVCC transfer is required from a PS to a CS access. The method comprises sending a SIP message in the initiated SIP session from the UE to a Service Centralization and Continuity Application Server, SCC-AS, the SIP message including a request to initiate session transfer from PS to CS, and responding to receipt of the SIP message at the SCC-AS by sending a SIP INVITE from the SCC-AS towards the UE, the INVITE including the Session Transfer Number, STN, as the calling party identity, and receiving the INVITE at a Mobile Switching Centre, MSC, serving the UE in the CS access. The method further comprises responding to receipt of said INVITE at the MSC, or to receipt of associated CS signalling, by sending from the MSC to the UE a CS Setup message, whereupon the MSC is placed in a state in which it expects a CS Answer from the UE. |
