CF in the selected network. When a SIP session request hits the MGCF it performs protocol conversion between SIP protocols and the ISDN User Part (ISUP) and sends a converted request via the Signaling Gateway (SGW) to the CS CN. SGW: A Signaling Gateway Function (SGW) is used to interconnect different signaling networks that is, SCTP 2 /IP based signaling networks and Signaling System No. 7 (SS7) signaling networks. The signaling gateway function may be implemented as a stand alone entity or inside another entity [2]. The SGW does not interpret application level messages. IM-MGW: Media Gateway Function (MGW) is required when voice bearers from the IM CN Sub- system are needed to connect with the voice bearers of other networks. MGW supports transcoding between a codec used by the user in the IM CN subsystem and the codec being used in the network of the other party. MGW also allows inter-working between users of the IM CN subsystem and IP mul- timedia fixed terminals. The IM-MGW can also provide tones and announcements to circuit switched users. MRF: The Media Resource Function (MRF) provides media related functions such as conferencing, playing of tones and announcements. The architecture of MRF is shown in Figure 9.3 and MRF is split into Multimedia Resource Function Controller (MRFC) and Multimedia Resource Function Processor (MRFP) [2]. The MRFC is tasked to handle SIP communications to and from the S-CSCF and to control the media stream resources in the MRFP. The MRFP in turn provides user-plane resources that are requested and instructed by the MRFC. The MRCP performs the following tasks: Mixing of incoming media streams (e.g., for multiple parties). Media stream source (for multimedia announcements). Media stream processing (e.g., audio transcoding, media analysis). 2 Stream Control Transmission Protocol (SCTP) is a transport layer protocol. Next Generation Mobile Communications Ecosystem IMS-ALG: The IMS-ALG (IMS-Application Level Gateway) provides the necessary application function for SIP/SDP³ protocol 