nality in the CPS. The main procedures that require information transfer through the Cx interface are: procedures related to registration and the serving CSCF (S-CSCF) assignment; procedures related to routing information retrieval from UMS to CSCF, as in a mobile terminating session; procedures related to updating the user's data in the S-CSCF. There is also a proprietary interface between the service execution environment (SEE) of the CPS and the service and subscription repository (SSR). This interface is called Xc and is also based on the Diameter Protocol with application extensions. The Xc interface is used to deliver service profiles, service logic and service data from the data repository in order to service execution. IP Multimedia Service What is a common feature of the successful mobile services of today, namely mobile voice and short message service (SMS)? They are person-to-person services and satisfy a basic human need: the need to communicate. Operators worldwide are currently deploying networks with IP capabilities (GPRS, EDGE and WCDMA). These capabilities can today be used to fill some human needs, such as the need for information (content downloading and browsing), the need for basic entertainment (content downloading, streaming, downloadable games), etc. However, in order to increase revenue further, operators need to find other human needs and to fulfil existing needs better. One such need is the need to share experiences: share things that are seen, done together and emotions. IP has been designed to carry multimedia traffic. Today's networks offer IP capabilities for person-to- content services, but lack a mechanism to establish IP connections between terminals. The IP multimedia core enables IP based person-to-person services over mobile networks as well as to/from alternative access networks. IP connections between people enable a range of new multimedia communication services as well as the ability to create interactive applications like interactive games. Communication services no longe