subscriber availability, QoS and more, that are needed by all applications created by the application developers in the IMS environment. The Application Servers are entities that provide value-added multimedia services in the IMS, such as presence and Push to talk over Cellular. It could reside either in the user's home network or in a third-party location (network or standalone AS). It can process the SIP session received from the IMS; originate SIP requests and send accounting information to the charging functions. Beside SIP servers, OSA (Open Services Architecture) application servers and legacy IN (Intelligent Network) servers are also part of the application plane. The interaction between these application servers and IMS elements such as the S-CSCF is through IMS Service Control (ISC) messages which are essentially SIP messages. OSA Service Capability Server (OSA-SCS) mediates between ISC messages and OSA services. The IN services are supported through the CAMEL application server. CAMEL (Customized Applications 1 OSA allows operators to share network services such as location, presence, etc with application partners. OSA based application servers allow greater variety of services to be developed in IMS networks. IMS, Services and Applications S-CSCF Figure 9.3 Architecture of MRF. Reproduced with permission from C 2004 3GPP for Mobile networks Enhanced Logic) allows an operator to define services over and above standard GSM/UMTS services. The IM-Serving Switching Function (IM-SSF) supports mediation between SIP based ISC messages and legacy CAMEL messages. BGCF: The Breakout Gateway control function (BGCF) selects the network in which PSTN domain breakout is to occur. If the BGCF determines that the breakout is to occur in the same network in which the BGCF is located within, then the BGCF selects a MGCF (Media Gateway Control Function) which will be responsible for the inter-working with the PSTN/CS domain. If the break out is in another network, the BGCF will forward this session signaling to another BG