ed to (vis- ited) network will support a proxy CSCF. The proxy CSCF will enable the call con- trol to be passed to the home network based serving CSCF, which will provide ser- vice control. When subscribers roam to networks where a serving CSCF exists and the home network decides to use the visited network based serving CSCF solution, the visited network serving CSCF will be used to provide service control to the roamed sub- scriber. While the visited network may support a serving CSCF for inbound roamers, it will usually support proxy CSCF for inbound roamers. Thus, if a visited network decides not to offer serving CSCF capability for inbound roamers, then the home network will pro- vide a serving CSCF to support IM roaming. The home network may provide a serving CSCF for outbound roamers even when a visited network offers the support of a serving CSCF, if so, the visited network provides the proxy CSCF. On the one hand, when users are within their home network, a home network-based serving CSCF provides service control. On the other hand, if the home operator wishes to use home service control for outbound roamers, then a home net- work-based serving CSCF will be used for outbound roamers' service control [7]. 9.7.2 Assignment of Serving CSCF The home network designates the serving CSCF in the home network or with the help of the visited network, requests a serving CSCF in the visited network. This selection occurs on a per subscriber basis at registration time based on consideration of at least the following factors [7]: a. the service capabilities and toolkits supported by the visited network and the home network; b. the subscription profile of the subscriber. SERVICE PLATFORMS Services platforms correspond, for example, to VHE/OSA, SAT, CAMEL, etc. How- ever, we will illustrate mainly VHE and SAT. 9.8.1 VHE/OSA To allow the support of existing CAMEL based services, and to allow the development of new services independent of network domain, the home CSCF will support a CAP The UMTS Network and Radio Access Te