Softswitches�Part VIII: IP Multimedia Subsystem functions

In our last tutorial, we considered how the IP Multimedia Subsystem, or IMS, integrates a number of technologies and applications, including wireline and wireless carrier systems, voice, data and Internet-centric applications, plus Internet protocols, such as the Session Initiation Protocol, or SIP.

IMS is getting attention from several directions. Carriers are always looking for new value-added services, and the ability to integrate Internet applications with existing wireline services creates some buzz. Along that same line, further integration of wireline and wireless technologies is another recurring theme. On the equipment side of the equation, manufacturers are always eager for new applications, so they can enhance their existing products with support for this new technology. In this tutorial, we will continue our discussion of IMS, and examine the IMS architecture, developed by the 3rd Generation Partnership Project (3GPP).

Before we continue, a roadmap regarding the numbering of the 3GPP specifications might be helpful. The 3GPP divides their specifications into “series,” with the series number as the first part of the specification (see Most of the ISM specifications can be found in Series 21 (Requirements), Series 22 (Service Aspects), Series 23 (Technical Realization) and Series 24 (Signaling Protocols). Of particular interest is a 189-page Series 23 document published in September 2005, and titled IP Multimedia Subsystem (IMS); Stage 2, which describes all of the elements that make up the IMS (see The IMS contains three general functions: call control, media processing, and gateways. Let’s examine those elements individually.

The call control is handled by Call/Session Control Function, or CSCF, which process signaling messages from the Session Initiation Protocol, or SIP. There are three different CSCF functions defined:

  • Proxy CSCF (P-CSCF): is the first point of contact for the IMS terminal, and behaves as a SIP User Agent, forwarding SIP requests and responses.
  • Interrogating CSCF (I-CSCF): is also a SIP proxy, which functions as the contact point for that operator’s network, or for users that are roaming within that operator’s network.
  • Serving CSCF (S-CSCF): is a SIP server that maintains the state of the communication session as needed by the network operator, in order to support the services provided.

The media processing is handled by two associated Media Resource Functions (MRF):

  • Media Resource Function Controller (MRFC): that controls the media stream resources within the MRFP.
  • Media Resource Function Processor (MRFP): that processes that media, such as mixing incoming media streams for multiple parties, any audio transcoding (codec translation) that might be required, generating announcements, and so on.

The gateways interface to other networks:

  • Breakout Gateway Control Function (BWCF): is a SIP server that including routing functions than enable communication sessions between networks. This function would allow a call that originates on the IMS to terminate on the Public Switched Telephone Network (PSTN).
  • Media Gateway Control Function (MGCF): converts the signaling protocols used on one network to those required by another network.
  • Media Gateway (MGW): converts the media stream used on one network (such as the Real Time Protocol, or RTP), to those required by another network (such as Pulse Code Modulation, or PCM).

Developers of softswitches, for both carrier and enterprise applications, are incorporating these IMS functions into their products. Our next tutorial we will begin an examination of those various vendors’ products and architectures.

Copyright Acknowledgement: © 2005 DigiNet ® Corporation, All Rights Reserved

Author’s Biography
Mark A. Miller, P.E. is President of DigiNet ® Corporation, a Denver-based consulting engineering firm. He is the author of many books on networking technologies, including Voice over IP Technologies, and Internet Technologies Handbook, both published by John Wiley & Sons.

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