Cross-grouping domain switching method, system and device based on IP multimedia subsystem

The present invention discloses a method of cross-field switching packets based on an IP multimedia subsystem, system and apparatus, the present invention is applied, so that the multi-mode terminal to switch the multimode terminal does not need to complete in the IMS network registration packet domain handover destination again process, effectively increasing the speed of real-time multimedia sessions across packet domain switching. Further, in the handover process, the interaction between the terminal and the network signaling flow does not require an IMS core network and the S-CSCF SIP AS, to enhance the efficiency of switching real-time multimedia sessions.