Description
The IMS Media Gateway Function (IMS-MGW) is a transport plane entity responsible for the interworking of user media (e.g., voice, video) between the packet-switched IP domain of the IMS and legacy circuit-switched networks like the Public Switched Telephone Network (PSTN) or legacy GSM/UMTS core networks. It performs the actual conversion of media streams between different formats and transport protocols. The IMS-MGW is a critical component for enabling seamless voice and multimedia calls between IMS subscribers and users on traditional telephony networks.
Architecturally, the IMS-MGW is controlled by a Media Gateway Control Function (MGCF), which is the signaling plane entity that handles protocol conversion between SIP (used in IMS) and ISUP/BICC (used in circuit-switched networks). The MGCF controls the IMS-MGW using the H.248 protocol (also known as MEGACO) as defined in the 3GPP Mn reference point. The MGCF instructs the IMS-MGW to create, modify, and delete terminations and contexts, which represent logical media endpoints and connections. For example, the MGCF may command the IMS-MGW to establish a connection between an IP termination (receiving RTP audio stream on a specific IP/port) and a TDM termination (on a specific E1 timeslot).
The IMS-MGW's key operation involves real-time media processing. On the IMS side, it typically handles Real-time Transport Protocol (RTP) and Real-time Transport Control Protocol (RTCP) streams carrying encoded voice (e.g., using AMR, G.711 codecs). On the circuit-switched side, it interfaces with TDM (E1/T1) trunks carrying Pulse Code Modulation (PCM) voice channels or with other legacy interfaces. Its core functions include media transcoding (converting between different voice codecs), transport processing (packetization/depacketization, jitter buffering), and echo cancellation. It may also support tone detection and generation (e.g., for DTMF tones) and play announcements.
Within the IMS network, the IMS-MGW does not participate in SIP signaling; it is purely a media plane resource under the command of the MGCF. This separation of control and media (as per the gateway decomposition model) allows for scalable, flexible, and cost-effective network design. Different vendors can provide the MGCF and IMS-MGW. The IMS-MGW is essential for the phased migration from circuit-switched to all-IP networks, allowing operators to protect investments in legacy networks while introducing IMS-based services. It ensures that media quality and bearer path connectivity are maintained across the technological boundary.
Purpose & Motivation
The IMS-MGW was created to solve the fundamental problem of media interworking between the new, all-IP IMS domain and the vast installed base of legacy circuit-switched telephony networks (PSTN, ISDN, GSM/UMTS CS core). When IMS was introduced as the architecture for delivering multimedia services over IP, a migration strategy was necessary. It was not feasible to instantly replace all existing telephony infrastructure and subscriber devices. There was a critical need for a bridge that could connect IP-based voice/video calls to traditional telephone lines.
The purpose of the IMS-MGW is to provide this bridge at the media bearer level. It addresses the incompatibility between the packet-switched, RTP-based media transport in IMS and the circuit-switched, TDM-based transport in legacy networks. Without such a gateway, IMS subscribers would be isolated in an "IP island," unable to communicate with the majority of telephone users worldwide. The IMS-MGW, under the control of the MGCF, performs the necessary real-time conversion of media streams, enabling transparent end-to-end calls.
Its design follows the media gateway control protocol paradigm (H.248/MEGACO), which was a well-established standard at the time. This allowed reuse of existing media gateway technology and expertise while adapting it for the 3GPP IMS context. The IMS-MGW enables operators to deploy IMS incrementally, using it initially as a modern overlay for trunking and interconnection with legacy networks. It solves the technical challenge of maintaining high voice quality and low delay while performing complex codec and transport conversions, which is essential for customer acceptance of new network technology.
Key Features
- Media Transcoding Between IP Codecs (e.g., AMR) and Circuit-Switched Codecs (e.g., G.711 PCM)
- Transport Processing Between RTP/UDP/IP and TDM (E1/T1) Bearers
- H.248 (MEGACO) Protocol Control Interface (Mn Reference Point) with MGCF
- Echo Cancellation and Voice Quality Enhancement
- Support for Tone Detection/Generation (DTMF) and Announcements
- Resource Management for Multiple Concurrent Media Connections
Evolution Across Releases
Introduced as the IMS-Media Gateway Function (IMS-MGW) for interworking between the IMS and circuit-switched networks. The initial architecture defined its role as a H.248-controlled media gateway, specifying the Mn reference point to the MGCF. It detailed support for media conversion between RTP/IP and TDM formats, establishing its foundational function in the IMS transit and interconnection architecture.
Defining Specifications
| Specification | Title |
|---|---|
| TS 28.705 | 3GPP TS 28.705 |
| TS 29.292 | 3GPP TS 29.292 |