Split/Combine Function

Description

The Split/Combine Function (S/C) is a functional entity within the 3GPP IP Multimedia Subsystem (IMS) architecture, playing a crucial role in advanced communication services that involve multiple endpoints or access legs for a single user. Its primary operation is to manipulate media streams: it can split a single incoming media flow (e.g., audio, video) into multiple outgoing streams directed to different destinations, or conversely, combine multiple incoming streams into a single outgoing flow. This functionality is central to services specified in TS 23.202 (PSS - IP Multimedia Subsystem (IMS) Service Continuity) and TS 23.910 (Circuit Switched (CS) domain and IP Multimedia Subsystem (IMS) service interaction). The S/C is typically implemented as part of an Application Server (AS) or a Media Resource Function (MRF) within the IMS core network.

Architecturally, the S/C interacts with the Serving-Call Session Control Function (S-CSCF) via the IMS Service Control (ISC) interface. When a service requiring media splitting or combining is invoked (e.g., a user wants to answer a call on multiple devices simultaneously), the S-CSCF forwards the SIP session to the AS hosting the S/C logic. The S/C then acts as a Back-to-Back User Agent (B2BUA), inserting itself into the media path. It uses Session Description Protocol (SDP) offer/answer exchanges to establish separate media legs with each involved endpoint. For a split operation, such as in Simultaneous Ringing, the S/C receives the media from the calling party, replicates it, and sends copies to all of the user's registered devices. For a combine operation, it receives streams from multiple sources, mixes them (in the case of audio), and sends a single composite stream to the intended recipient.

How it works involves precise control of media routing and, often, media processing. In a split scenario for call forwarding or simultaneous ringing, the S/C does not typically modify the media content; it merely replicates and routes it. However, in combine scenarios, such as when a user switches a call from a mobile phone to a hands-free car kit while keeping the mobile phone's microphone active, the S/C may need to perform audio mixing. The function is controlled by service logic that determines when to split or combine based on user preferences, network conditions, or service triggers. Its role is indispensable for maintaining session continuity and enabling rich multi-device experiences without requiring the remote party to re-establish the call, thereby providing a seamless user experience across the IMS ecosystem.

Purpose & Motivation

The S/C function was created to solve the fundamental problem of session and media continuity in an all-IP, multi-access, and multi-device world ushered in by IMS. Prior to IMS, circuit-switched telephony offered simple call forwarding but lacked the flexibility to dynamically split or combine media across different devices and networks within an active session. As users began owning multiple communication devices (phone, tablet, laptop) and connecting via diverse access technologies (LTE, Wi-Fi, fixed broadband), there was a need for network-based control to manage media flows for a single subscriber identity across these endpoints.

The key problems it addresses include enabling services like Voice Call Continuity (VCC) and Single Radio Voice Call Continuity (SRVCC), where a voice call must be transferred seamlessly between circuit-switched and IMS domains without dropping. The S/C function in the IMS domain can combine the media path during the handover. Furthermore, it enables innovative services like Simultaneous Ringing, where an incoming call rings on multiple devices, and the user can answer on any of them. It also supports multi-device scenarios for a single session, such as using one device for audio and another for video. Without the S/C, each device would require a separate SIP dialog with the remote party, complicating billing, signaling, and user experience.

Its development was motivated by the desire to make communication services user-centric rather than device-centric. The S/C abstracts the complexity of multiple endpoints from both the user and the remote party, allowing the network to intelligently manage media distribution. This supports business models for service providers to offer premium continuity and multi-device services. By standardizing this function in 3GPP, interoperability between different vendors' IMS cores and application servers was ensured, which was critical for the widespread deployment of advanced IMS-based telephony services replacing traditional circuit-switched networks.