Specialised Resource Function

Description

The Specialised Resource Function (SRF) is a logical entity within the IMS architecture, often implemented as part of a Media Resource Function (MRF). The MRF is split into a controlling part (MRFC - Media Resource Function Controller) and a processing part (MRFP - Media Resource Function Processor). The SRF typically refers to the capabilities provided by the MRFP. Its primary role is to provide media-related services upon request from an application server (AS) or a Serving-Call Session Control Function (S-CSCF) via the MRFC. The SRF contains the physical resources for processing media streams. This includes hardware and software for functions like audio mixing for multi-party conferences, speech and audio signal processing (e.g., for tone generation and detection), media transcoding between different codecs (e.g., G.711 to AMR), playing recorded announcements, and collecting digits (DTMF). When a service requires such a resource, the AS or S-CSCF sends a control message (using protocols like SIP) to the MRFC. The MRFC then instructs the MRFP/SRF, using a control protocol such as H.248 (Megaco), to allocate the necessary resources and apply specific processing to designated media streams. The media streams themselves are routed to the SRF using the IMS media plane, typically via the IP network. The SRF processes these streams according to its instructions and then forwards them to the appropriate destination(s). This separation of control (MRFC) and media (SRF/MRFP) allows for scalable, flexible service deployment. The SRF is a key enabler for Value-Added Services (VAS), allowing network operators and third-party providers to create features that require manipulation of the media content itself, rather than just signaling.

Purpose & Motivation

The SRF was created to centralize and manage complex media processing resources in a packet-switched, IP-based multimedia network (IMS). Prior to IMS, advanced telephony services like conferencing or interactive voice response (IVR) often required proprietary, siloed equipment attached to the circuit-switched core. This was inefficient, difficult to scale, and hard to integrate with new IP-based services. The SRF concept addresses this by providing a standardized, network-centric pool of media resources that can be dynamically allocated and controlled by any authorized service logic within the IMS. It solves the problem of service fragmentation and enables the creation of blended multimedia services (voice, video, text) that can seamlessly use these resources. The motivation was to break the tight coupling between service intelligence and media processing hardware, following the general IMS principle of separating control plane and user plane. This allows operators to deploy and scale media resources independently of the applications that use them, reducing costs and increasing service agility. It facilitates the offering of consistent advanced services (like audio conferencing, lawful interception recording, or personalized announcements) across both legacy and next-generation access networks.

Key Features

• Provides centralized media processing resources for the IMS network.
• Executes functions like audio mixing, transcoding, tone play/collect, and announcement playout.
• Operates under the control of an MRFC using the H.248/Megaco protocol.
• Interfaces with the IMS media plane to receive, process, and forward RTP/RTCP streams.
• Enables advanced telephony services such as conferencing, IVR, and video processing.
• Designed for scalability and shared use across multiple services and applications.

Evolution Across Releases

Defining Specifications