This document specifies the principles of interworking between the 3GPP2 IP Multimedia (IM) CN subsystem and ISUP based legacy CS networks, in order to support IM basic voice calls.

This document 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 ISUP are detailed in terms of the processes and protocol mappings required for the support of both IM originated and terminated voice calls. This document concerns itself only with mappings at the upper protocol (i.e., signalling) layer and does not address lower layer (i.e., transport) interworking.

Other areas addressed encompass the transport protocol and signalling issues for negotiation and mapping of bearer capabilities and QoS information.

This document specifies the interworking between 3GPP2 profile of SIP (as detailed according to [9]) and ISUP, as specified in [73].