The IP Multimedia Subsystem (IMS) is a 3GPP/3GPP2 standards-defined intelligent network architecture which has also been embraced by the international wireline standards bodies including TISPAN, ITU-T and ATIS. Adoption in the fixed line space positions IMS as the key enabler for Fixed Mobile Convergence (FMC).
IMS is the next step in the evolution of the core network for mobile operators - merging the Internet with the cellular world. Based on Session Initiation Protocol (SIP), access to the 3GPP IMS is agnostic. The IMS architecture defines a flexible service domain which is independent from the access layer, providing support for GSM, W-CDMA, cdma2000, WLAN, WiMAX, and wireline broadband access.
Figure 5 is a representation of the 3GPP IMS architecture which is a layered model divided into an IP-based Transport plane, a Control plane, and a Services and Application plane.
Standards-based interfaces enable integration between each of the planes. The Control Plane provides session control and management, and is responsible for setting up and taking down packet sessions. It also contains the information on subscriber authentication, service authorization, and location. Policy decisions are made in the Control Plane. The IMS AAA Server resides in the Control Plane of the 3GPP IMS architecture and in conjunction with the HSS, performs the authentication, authorization, and accounting functions.
By itself, IMS does not specify any new services; rather, it provides a framework for network operators to build and launch their access-agnostic services. New applications can be integrated quickly onto IMS because of the nature of the architecture.
Some of the initial mobile applications in IMS included "presence" services and Push-to-Talk over Cellular (PoC). Future applications will feature richer multimedia services such as video telephony messaging and conferencing, and entertainment functions such as mobile IPTV. IMS also provides the functionality to bill and account for these services, and to roll them out quickly and cost-effectively.