The IM CN subsystem interworks with the external IP networks through the Mb reference point. This document details the interworking between the IM CN subsystem and external IP networks for IM service support. It addresses the issues of control plane interworking and, user plane interworking for specific interworking use cases. Clause 10 describes the IMS-Ix interface requirements in the form of Use Cases which require H.248 protocol procedures. Subclause 10.4 then details the additional Information Elements required to perform the specific procedures. The IP version Interworking, between IP version 4 (IETF RFC 791) and IP version 6 (IETF RFC 2460) detailed in terms of the processes and protocol mappings required in order to support both IMS originated and terminated calls.

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