Multimedia Streaming and Messaging Telephony Service Interworking Client Implemented by Media Resource Function

Description

The MSMTSI MRF (Multimedia Streaming and Messaging Telephony Service Interworking Client Implemented by Media Resource Function) is a functional entity defined in 3GPP specifications 26.114 and 26.980. It represents a specific implementation of an MSMTSI client where the client functionality is realized within the Media Resource Function (MRF) of the IP Multimedia Subsystem (IMS). The MRF itself is split into two components: the Media Resource Function Controller (MRFC), which handles signaling and control, and the Media Resource Function Processor (MRFP), which performs media processing tasks such as transcoding, mixing, and streaming. By embedding the MSMTSI client into the MRF, the network can provide interworking between IMS-based multimedia telephony (e.g., MMTel) and legacy messaging or streaming services without requiring changes to user equipment or other network nodes.

The MSMTSI MRF operates as a back-to-back user agent (B2BUA) or as a media resource that terminates and re-originates media streams. When a user initiates a session that requires interworking—for example, sending a multimedia message from an IMS client to a legacy messaging system—the MRFC receives the SIP signaling and instructs the MRFP to allocate media resources. The MRFP then acts as an MSMTSI client, converting between the protocols and media formats used in IMS and those used in the legacy domain. This may involve transcoding audio/video codecs, adapting messaging protocols (e.g., from SIP MESSAGE to SMS or MMS), and handling streaming protocols such as RTSP or HTTP-based streaming. The MRF can also provide announcements, tones, and conferencing functions as part of the interworking service.

Architecturally, the MSMTSI MRF sits between the IMS core and external networks or application servers. It communicates with the IMS core via the ISC interface (using SIP) and with legacy networks via gateways or interworking functions. The MRFC controls the MRFP using the Mp interface, which is based on H.248/Megaco. The MRFP may also interact with other media resources, such as media servers or content delivery networks, to retrieve or store streaming content. The MSMTSI MRF can be deployed in a centralized or distributed manner, depending on network requirements and scalability needs.

Key components of the MSMTSI MRF include the MSMTSI client logic, which handles the interworking procedures; the media processing functions, which perform transcoding, mixing, and streaming; and the control interface, which allows the MRFC to manage the MRFP. The MSMTSI client logic is responsible for session establishment, modification, and termination, as well as for handling mid-session events such as DTMF detection or media stream changes. The media processing functions ensure that media is adapted to the capabilities of the receiving terminal or network, which may involve codec negotiation, rate adaptation, and error resilience.

In the context of 3GPP releases, the MSMTSI MRF was introduced in Release 13 as part of the work on multimedia telephony service interworking. Its definition has remained stable across subsequent releases (Rel-14 through Rel-19), indicating that the fundamental concept and architecture have not changed. The MSMTSI MRF is particularly relevant for operators who need to support interworking between IMS-based services and legacy circuit-switched or messaging services, as it allows them to leverage their existing MRF infrastructure to provide these capabilities without deploying separate interworking gateways. This can reduce capital and operational expenditures while ensuring a consistent user experience across different access technologies and service domains.

Purpose & Motivation

The MSMTSI MRF exists to enable interworking between IMS-based multimedia telephony services and legacy messaging or streaming services. In the evolution of mobile networks, operators often have a mix of IMS-capable and legacy terminals, as well as external networks that may not support IMS protocols. Without interworking, users of IMS services would not be able to communicate with users on legacy networks or access legacy services. The MSMTSI MRF solves this problem by providing a media resource function that can act as an MSMTSI client, translating between protocols and media formats. This allows operators to offer seamless services across heterogeneous networks and devices.

Historically, interworking between IMS and legacy domains was handled by standalone gateways or interworking functions, which could be complex and costly to deploy and manage. The MSMTSI MRF leverages the existing MRF infrastructure, which is already deployed for conferencing, announcements, and other media services, to provide interworking capabilities. This approach reduces the need for additional network elements and simplifies the architecture. It also allows operators to reuse their MRF investments and to scale interworking services in line with other media services.

The motivation for introducing the MSMTSI MRF in Release 13 was to support the growing demand for multimedia services that span different domains. As operators migrate to all-IP networks, they need to ensure that users can continue to communicate with others who may still be on legacy networks. The MSMTSI MRF provides a standardized way to achieve this interworking, ensuring interoperability between different vendors' equipment and simplifying network planning and deployment.