S 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. Multimedia resource function plane (MRFP): It implements all the media- related functions like play media, mix media, and provide announcements. Multimedia resource function controller (MRFC): It handles communi- cation with the S-CSCF and controls the resources in the MRFP. Breakout gateway control function (BGCF): It mainly handles interaction with PSTN. Media gateway con