r shall be able to end an IP multimedia session at any time during the session. The user or the network can support emergency communications. Support for audio and video conferences that allows users to communicate with all other participants simultaneously. Next Generation Mobile Communications Ecosystem Multicast services allow IMS users and service providers to send multimedia to a group of IMS users simultaneously in a unidirectional way of communication. The underlying network can support mechanisms that optimize the delivery of multimedia to the individual members of that group (e.g., MBMS). 9.3.2 IMS Architecture At a high level there are three layers in the IMS architecture namely transport, session control, and application. The layered IMS model allows each layer to be deployed independently of the other layers, providing flexibility required by the next generation networks. The transport layer provides transport of media streams through the network. The transport layer hosts routers, switches, media gateways, and other user plane functions. The routers and switches provide transport capabilities for both control plane and user plane traffic. The media gateways provide different kinds of interaction on connectivity level, including interaction between different transmission technologies and interaction between different media formats. The session control layer which is the heart of the IMS controls access to services, provides operator control over how subscribers access the services and allow operators to define procedures how to bill the customers for the services. Lastly, the application layer does not contain the real applications, but the servers to create applications. The IMS is an access independent platform and this flexibility is achieved via the separation of transport, control and application mechanisms. A layered IMS model allows each layer to be sourced and deployed independently of other layers as shown in Figure 9.2. Thus, the layers or planes are: The Transport (Media) Layer transports t