Outgoing - Media Gateway Control Function

Description

The O-MGCF is not a separate network element, but a specific operational role or perspective of the Media Gateway Control Function (MGCF) within the 3GPP IMS architecture. The MGCF is the central controller responsible for sessions that require interworking between the IP-based IMS domain and external circuit-switched (CS) networks like the Public Switched Telephone Network (PSTN) or legacy GSM/UMTS CS core. The 'Outgoing' designation refers to the MGCF's function when it is handling the call leg that originates from within the IMS and is destined outward to the CS network. In this role, the O-MGCF acts as the SIP User Agent Client (UAC) for the outgoing leg.

When an IMS user initiates a call to a PSTN number, the S-CSCF routes the SIP INVITE to an MGCF based on destination analysis. The MGCF, assuming the O-MGCF role, performs several key functions. First, it performs protocol mapping and translation between the SIP signaling from the IMS and the ISUP (ISDN User Part) or BICC (Bearer Independent Call Control) signaling used in the CS network. This involves translating SIP methods, headers, and SDP (Session Description Protocol) offers/answers into equivalent ISUP/BICC messages and bearer information. Second, the O-MGCF controls one or more Media Gateways (MGWs) using the H.248 (Megaco) protocol. It instructs the MGW to establish the necessary bearer connections, including codec transcoding between the IMS codecs (e.g., AMR, EVS) and CS codecs (e.g., G.711), and to connect the IP bearer from the IMS to the TDM or IP bearer leading to the CS network.

Architecturally, the O-MGCF interfaces with the IMS core via the Mg interface (SIP), with the CS network via the Nb interface (using ISUP/BICC over MTP3/SCTP/IP), and with the MGW via the Mn interface (H.248). Its role is critical for seamless service continuity, allowing IMS subscribers to communicate with users on any legacy telephone network. The complementary role is the T-MGCF (Terminating MGCF), which handles calls entering the IMS from a CS network. The O-MGCF ensures that call control, billing information, and supplementary services are properly translated and maintained across the network boundary.

Purpose & Motivation

The O-MGCF role was formally defined to clarify the call control functions of the MGCF in a decomposed, next-generation network architecture, particularly as IMS matured through 3GPP Releases 7 and beyond. Earlier releases introduced the MGCF, but the distinction between its outgoing and terminating behaviors became more important with the full decomposition of the MSC into MSC Server and MGW, and the need for precise interworking scenarios. The purpose of defining the O-MGCF is to specify the exact procedures, protocol mappings, and state management required when an IMS-originated session breaks out to the legacy telephony world.

It addresses the critical problem of protocol and service interworking between the session-oriented, text-based SIP protocol of IMS and the connection-oriented, message-based signaling (ISUP/BICC) of circuit-switched networks. Without the O-MGCF's translation functions, IMS networks would be isolated islands. The O-MGCF enables operators to deploy IMS as a core network while maintaining full connectivity to the global PSTN, protecting existing investments and ensuring universal service. Its creation was motivated by the practical necessity of a phased network migration, where IMS and legacy networks must coexist for many years, and by the need for a standardized, vendor-interoperable way to handle the complex signaling translation for outbound calls, including support for essential telephony features like caller ID, call forwarding, and speech codec negotiation.

Key Features

• Defines the MGCF's role for IMS-to-CS network call legs
• Performs protocol interworking between SIP and ISUP/BICC
• Controls Media Gateways using H.248 (Megaco) protocol
• Manages bearer resource establishment and codec transcoding
• Acts as a SIP User Agent Client (UAC) for the outgoing leg
• Translates IMS session parameters into CS network bearer information

Evolution Across Releases

Defining Specifications