29.4213GPPEndorsement of 3GPP TS 29.162 Interworking between IM CN Sub-system and IP networksNGN Release 1Release 8TISPANTS
4.1 General interconnection overview
The IM Subsystem interworks with SIP RFC 3261  based IP Multimedia networks. These IP Multimedia networks include:
- SIP User Agents (UAs);
- SIP Servers.
The IM Subsystem also interworks with non SIP-based IP multimedia networks (e.g. network controlled using H.323).
The general interworking model is shown in figure 1. The IMS network and the SIP based Multimedia networks may use IP version 4 RFC 791  or IP version 6 RFC 2460 .
Figure 1: Interworking Model for IMS to IP Multimedia Network
The IMS uses the IBCF/I-BGF and possibly the IWF entity in order to communicate with external IP multimedia entities.
The IBCF is inserted on the SIP signalling path between the CSCF/BGCF and external IP Multimedia networks.
The I-BGF is inserted along the media path at IP interconnect between the IMS network and an external IP multimedia network.
The IBCF and the I-BGF may be implemented as a part of other physical entities in the IMS.
The IWF performs the interworking between the TISPAN SIP profile (ES 283 003 – see bibliography) and other IP multimedia protocols when required (e.g.H.323 or SIP-I as defined for profile C ITU‑T Recommendation
4.2 Interworking scenarios
4.2.1 UE with TISPAN SIP profile capability connecting to an external SIP device
ES 283 003 (see bibliography) specifies the procedures used by a UE compliant to the TISPAN SIP profile to communicate with an external SIP device possibly lacking TISPAN SIP profile capabilities.