n between two IMS users, between an IMS user and a user on the Internet, or between two users on the Internet are all established using the same protocol. Moreover, the Fundamentals of EMS, NMS and OSS/BSS IMS network Application layer Session control layer Access layer Figure 9.6 Three layered IMS network. interfaces for service developers are also based on IP protocols. IMS merges the Internet with mobile, using cellular technologies to provide ubiquitous access and Internet technologies to provide appealing services. The rapid spread of IP-based access technologies and the move toward core network convergence with IMS has led to an explosion in multimedia content delivery across packet networks. This transition has led to a much wider and richer service experience. The IP multimedia subsystem can be thought of as composed of three layers (see Figure 9.6): The service/application layer: The end services reside in the application layer. It includes a host of application servers that execute services and communicates with a session control layer using SIP. It can be a part of the service provider home network or can reside in a third-party network. With open standards defined on interactions with an application server, it is easier to build appli- cations on application servers. The power of IMS lies in easily rolling in/out services on the fly in minimal time using application servers. The IMS core/session control layer: Session control and interactions between transport and application layer happen through the session layer. The main components of the core or session control layer are: Call session control function (CSCF): It can establish, monitor, maintain, and release sessions. It also manages the user service interactions, policy of QoS and access to external domains. Based on the role performed, a CSCF can be serving (S-CSCF), proxy (P-CSCF), or interrogating (I-CSCF) call session control function. Communication Networks Home subscriber server (HSS): Database for storing subscriber and service information.