This TS specifies the principles of interworking between the 3GPP IM CN subsystem and BICC/ISUP
based legacy CS networks, in order to support IM basic voice, data and multimedia calls.
This TS addresses the areas of control and user plane interworking between the IM CN subsystem and CS
networks through the network functions, which include the MGCF and IM-MGW. For the specification of control plane
interworking, areas such as the interworking between SIP and BICC or ISUP are detailed in terms of the processes and protocol mappings required for the support of both IM originated and terminated voice and multimedia calls.