ent operators' net- works. Protocols for the Mw, Mm, Mg to apply SIP4 for the single call control protocol applied to the interfaces between MGCF and CSCF, between CSCFs within one operator's network and between CSCFs in different operator's net- works. UNI versus NNI call control We may assume that the SIP based signalling inter- actions between CN elements may be different than SIP based signalling between the UE and the CSCF in some cases if not all. 9.7.1 Support of Roaming Subscribers Here we assume that R4 and R5 architectures will be based on the principle that the service control for a roaming subscriber is designated by the home network. The serving CSCF can then be located either in the home network or in the visited network as illus- trated in Figure 9.15. This assignment of the serving CSCF takes place in the home network during the registration of the UE at the visited network. Application Server Serving Application Network Server Network Serving Visited Visited Network Network Figure 9.15 Support of the UE via serving CSCF in the home and visited networks. The R4 and R5 standards will thus support roaming for IP multimedia (IM) users be- tween operators. As noted earlier, to achieve roaming the required serving-CSCF can be located in the visited network or in the home network. The decision as to whether the UE is served by a serving CSCF in the home network or a serving CSCF in the visited network occurs at the home network. According to [7] the following takes place: 4 As defined by RFC 2543, other relevant RFCs, and additional enhancements required to support 3GPP's needs. Towards IP Based Networks When subscribers roam to networks where a serving CSCF does not exist, the roamed to (visited) network will support a proxy CSCF. The proxy CSCF will en- able the call control to be passed to the home network based serving CSCF, which will provide service control. When subscribers roam to networks where a serving CSCF exists but the home network decides to use a home network-based serving CSCF, the roam