ACCESS TRANSFER FOR A DRVCC MOBILE TERMINAL

摘要

A method of Dual Radio Access Transfer of an IP Multimedia session IMS in an alerting phase for the Packet Switched to Circuit Switched direction. The method comprises initiating an IMS media session towards a terminating mobile station UE and during a session alerting phase making a determination at the UE that a DRVCC transfer is required from a Packet Switched PS to a Circuit Switched CS access. The method comprises sending a SIP message from the UE to a Service Centralization and Continuity Application Server SCC AS over the PS access informing the SCC AS of the DRVCC transfer and responding to receipt of the SIP message at the SCC AS by sending a SIP INVITE from the SCC AS towards a Mobile Switching Centre MSC serving the UE in the CS access the INVITE including an identity of the UE as a called party identity and the STN as the calling party identity. 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.