December 02, 2005
IMS functions
IMS enables service carriers to combine several Internet applications with already present wireless services and has hence generated a fair amount of interest with the vendors. The IMS architecture is being developed by the 3rd Generation Partnership Project (3GPP). 3GPP has laid down several specifications that deal with various issues such as requirements, service aspects, technical realization, etc.
Call control, media processing, and gateways are three functions of IMS. The Call/Session Control Function (CSCF) handles the call control function. Signaling messages from SIP are processed by the CSCF. The three CSCF functions include Proxy CSCF which performs like an SIP User Agent and handles the forwarding of SIP requests and responses; the Interrogating CSCF is also an SIP proxy and functions as a contact point for the operator’s network and the users that are roaming that network; the serving CSCF is an SIP server that helps to maintain the state of a session as desired by the network provider.
The Media Resource Functions (MRF) manage media processing. The Media Resource Function Processor (MRFP) handles functions such as the processing of mixed incoming media streams, audio transcoding, announcements, etc; the Media Resource Function Controller (MRFC) performs a controlling function for the media stream resources within the MRFP.
The gateway interface consists of the Breakout Gateway Control Function (BWCF) which is an SIP server that performs routing functions to allow calls between networks. This enables calls to begin on the IMS and finish on the PSTN; the Media Gateway Control Function (MGCF) transforms the signaling protocols such that they can be used by a given network; the Media Gateway (MGW) is used for changing the media streams that are used on a network such as RTP to a format that can be used by another network such as the PCM.
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