ce) is chosen as IMS main protocol to facilitate development and interconnectivity between Cellular networks and Fixed IP networks. From the mobile point of view, IMS is a signaling application that can be used to provide various services. IMS only implements signaling procedures and application-common functions but does not offer services themselves. Thus, IMS is an open signaling system, based on standard Internet technology, which supports the migration of Internet applications (like VoIP, video conferencing, messaging, etc.) to the mobile environment and offers enhanced service control capabilities. This is done by having a horizontal control layer that isolates the access network from the service layer. From a logical architecture perspective, services need not to have their own control functions, as the control layer is a common horizontal layer. However, in implementation this does not necessarily map into greater reduced cost and complexity. IMS, Services and Applications 3GPP IMS 3GPP has specified IMS network as an access-independent and standard-based IP connectivity and service control architecture, which enables various types of multimedia services to end-users using common Internet-based protocols. 3GPP started the IMS standardization process in 1999/2000 with the vision of evolving beyond GSM and delivering multimedia services. The first IMS specifications were frozen in 2002 by 3GPP in its Release 5. The objective of this feature is to support applications involving multiple media components like audio and video, and tools like shared online whiteboards. However, Release 5 IMS specifications contain some GPRS access-specific features which were contrary to its access-independent concept. Thus, in 3GPP Release 6 the GPRS access-specific features were removed from the core IMS description and thus IMS architecture was returned to its inception state. The other IMS related developments in Release 6 were the addition of IP IMS messaging, voice over IMS, location services support for IMS public identit