Access transfer for a DRVCC mobile terminal

#The present invention relates to the Packet Switched to Circuit Switched transfer of a voice or video call involving a Dual Radio Voice Call Continuity mobile terminal. More particularly, the invention concerns the handover of a Dual Radio Voice Call Continuity mobile terminal on the terminating call side during a call alerting phase.

#IP Multimedia Subsystem (IMS) is a standardised and established architecture for delivering IP multimedia services to end users. IMS is to a large extent agnostic concerning the access network used by the end users: access networks may be wireless or fixed line. In the context of IMS, it is important to allow end users to seamlessly move between access networks and access technologies, e.g. to allow voice and video call continuity during such movements.

#3GPP TS 23.237 v11.4.0 specifies Single Radio Voice Call Continuity (SRVCC) as a functionality defined for the Long Term Evolution (LTE) architecture to allow voice and video calls to be transferred or “handed over” from the LTE packet switched (PS) domain to a legacy circuit switched (CS) domain, in the case where a mobile terminal has only a single radio interface available to it. Such a transfer might be required, for example, when a call is established by a user terminal over a packet switched (PS) LTE connection and it becomes necessary, due to a deterioration in the LTE radio quality, to transfer the call to a 3G UMTS circuit switched (CS) connection. FIG. 1 illustrates schematically the reference architecture for SRVCC (using the Access Transfer Control Function, ATCF, enhancements).

#FIG. 1 illustrates schematically the reference architecture for SRVCC (also applicable to DRVCC);

#FIG. 2 illustrates a flow associated with a successful DRVCC transfer from PS to CS for an IMS session in alerting phase, where the UE being transferred is on an originating side;

#FIG. 3 illustrates a flow associated with an unsuccessful DRVCC transfer from PS to CS for an IMS session in alerting phase, where the UE being transferred is on an terminating side;

#A procedure is proposed here for handling the transfer of a call in an alerting state for DRVCC, assuming a PS to CS transfer on the terminating call side. The procedure comprises, upon a determination at a terminating UE that a DRVCC transfer is required, placing the MSC within the CS access into a state in which it expects an answer from the terminating UE. As such, when the MSC receives an answer from the UE, no error state arises at the MSC. The procedure is such that no changes are required at the MSC.

#The terminating UE may send to the SCC AS a message indicating that a DRVCC transfer should be started, whereupon the SCC AS sends an INVITE towards the MSC (this may be converted to a CS trunk signalling message on its path towards the MSC). The MSC sends a setup request to the terminating UE. This process causes the MSC to be placed in the correct state.

#A user terminal is proposed that is capable of sending an appropriate notification to the SCC AS to cause the SCC AS to place the MSC into the correct state.