Description
The IMS Access Media Gateway (IMS-AGW) is a media processing entity defined in the IMS media plane architecture. It is not a signaling element but a bearer plane function responsible for the manipulation and forwarding of user media streams (voice, video). The IMS-AGW operates under the control of a Media Gateway Control Function (MGCF) or, more specifically in later architectures, an IMS Media Gateway Control Function (IMS-MGW-CF) or a Media Resource Function Controller (MRFC). Its operation is governed by protocols like H.248 (Megaco) or its IMS profile, which allows the controller to instruct the IMS-AGW to create, modify, and delete media processing contexts and terminations.
Functionally, the IMS-AGW performs several key roles. First, it acts as a media anchor point. This is crucial for services like Voice Call Continuity (VCC) or Single Radio Voice Call Continuity (SRVCC), where a call needs to be seamlessly transferred between different access networks (e.g., from LTE/VoLTE to 3G circuit-switched). By anchoring the media in the IMS-AGW, the remote party's media path remains unchanged, and only the leg to the moving UE is switched, minimizing interruption. Second, it provides media interworking functions, such as transcoding between different voice codecs (e.g., AMR-NB used in legacy networks to EVS used in VoLTE) and packetization protocol conversion (e.g., RTP to circuit-switched TDM bearers).
Third, the IMS-AGW enables media-based services like conferencing (mixing multiple audio/video streams), playing tones and announcements, and facilitating lawful interception by providing a controlled point for duplicating media streams. Architecturally, it sits at the boundary between the all-IP IMS domain and legacy circuit-switched networks or other IP networks with different capabilities. It contains resources like Digital Signal Processors (DSPs) for codec operations and packet processing engines. The separation of control (MGCF/MRFC) and media (IMS-AGW) follows the gateway decomposition model, promoting scalability, flexibility, and the use of standard interfaces.
Purpose & Motivation
The IMS-AGW was introduced to address specific challenges in the evolution towards a pure, end-to-end IP multimedia network. Early IMS visions imagined direct IP-based media paths between endpoints. However, practical deployment revealed several scenarios where intelligent media plane intervention was necessary. Legacy networks (2G/3G circuit-switched) would coexist with IMS for decades, requiring seamless interworking for voice calls. Furthermore, not all user equipment or access networks support the same set of advanced codecs, necessitating transcoding to ensure interoperability.
A key driver was the need for service continuity during mobility events, particularly SRVCC. Without a media anchor, handing over a VoLTE call from LTE to a 3G circuit-switched network would require re-establishing the entire media path with the remote party, causing unacceptable audio gaps. The IMS-AGW provides a stable anchor point in the home IMS network, allowing for a swift update of only one leg of the call. This solved a major obstacle for the commercial launch of VoLTE, as it guaranteed voice service continuity even when users moved out of LTE coverage.
Additionally, regulatory requirements for lawful interception mandated a network-controlled point where media could be copied. A distributed, peer-to-peer media model made this difficult. Concentrating media processing in a managed network function like the IMS-AGW provided a natural point for such intercepts. The decomposition into a separate control function (MGCF) and media gateway (IMS-AGW) also aligned with broader industry trends towards softswitch architectures, allowing operators to source control and media hardware/software independently and scale them according to different traffic patterns.
Key Features
- Media anchoring for service continuity during inter-access handovers (e.g., SRVCC)
- Transcoding between different audio and video codecs (e.g., AMR to EVS, G.711)
- Protocol interworking between RTP/IP and legacy circuit-switched TDM bearers
- Media mixing for conferencing services under MRFC control
- Support for playing tones, announcements, and voice prompts
- Provides a controlled access point for lawful interception of media streams
Evolution Across Releases
Introduced the IMS-AGW as a key component of the IMS Media Plane architecture. Defined its functional split from the Media Gateway Control Function (MGCF) and its control via the H.248 protocol. Primary focus was on supporting SRVCC for voice call continuity from LTE to 3G/2G networks, where the IMS-AGW acts as the essential media anchor point.
Enhanced IMS-AGW capabilities to support the optimized eSRVCC procedure, reducing handover interruption time. Improved H.248 package definitions for more efficient control of media resources and context management during handovers.
Further refinements to the IMS Media Plane architecture. Enhanced support for IMS-AGW in scenarios involving access transfer for multiple media streams (e.g., video calls) and better integration with the Policy and Charging Control (PCC) architecture for resource management.
Strengthened the role of IMS-AGW in Voice over WiFi (VoWiFi) and fixed-mobile convergence scenarios, where it provides media interworking between different IP access networks with varying QoS characteristics.
Maintenance and optimization of IMS-AGW procedures. Consideration for its role in network function virtualization (NFV) environments, defining requirements for virtualized media gateway implementations.
Integrated IMS-AGW into the 5G system architecture for voice and media services. Defined its role in supporting Voice over NR (VoNR) and interworking with the 5G core network, particularly for fallback to EPS or legacy systems.
Continued evolution for cloud-native implementations, focusing on scalability, automation, and support for advanced media services like immersive communications which may require complex media processing.
Specifications maintained for backward compatibility and support in hybrid network environments. Focus on ensuring IMS-AGW remains a viable component for legacy interworking and specific media anchoring use cases.
Defining Specifications
| Specification | Title |
|---|---|
| TS 23.334 | 3GPP TS 23.334 |
| TS 24.229 | 3GPP TS 24.229 |
| TS 29.334 | 3GPP TS 29.334 |
| TS 29.866 | 3GPP TS 29.866 |
| TS 32.260 | 3GPP TR 32.260 |
| TS 32.298 | 3GPP TR 32.298 |
| TS 32.299 | 3GPP TR 32.299 |
| TS 33.107 | 3GPP TR 33.107 |
| TS 33.108 | 3GPP TR 33.108 |
| TS 33.127 | 3GPP TR 33.127 |