Multi-Stream MTSI

Description

Multi-Stream MTSI (MS-MTSI) is a service capability standardized by 3GPP for the Multimedia Telephony Service for IMS (MTSI). It fundamentally extends the traditional point-to-point video telephony model by enabling the establishment, synchronization, and management of multiple concurrent media streams within a single IMS session. This is achieved through specific signaling extensions and media handling protocols defined for the IMS core. The architecture involves enhancements to both the MTSI client and the IMS network elements to support the negotiation and delivery of multiple media streams. The client indicates its capability to support MS-MTSI during session establishment using Session Description Protocol (SDP) extensions. The network, including the Proxy-Call Session Control Function (P-CSCF) and the Application Server (AS), must be aware of these capabilities to properly route and policy the multiple streams. A key technical aspect is the synchronization mechanism, which ensures that the multiple video streams (e.g., from different camera angles in a sports event or different parts of a 360-degree capture) are presented to the receiving user with aligned timing, creating a coherent and immersive viewing experience. This is typically managed using RTP Control Protocol (RTCP) extensions and synchronization source (SSRC) groupings defined in the associated RTP sessions. MS-MTSI also defines mechanisms for dynamic adaptation, where individual streams can be added or removed during an active session based on network conditions or user interaction, providing flexibility. Its role in the network is as a value-added service layer on top of the core IMS infrastructure, enabling service providers to offer next-generation visual communication services.

Purpose & Motivation

MS-MTSI was created to address the limitations of traditional single-stream video telephony, which could not support emerging immersive and multi-perspective communication scenarios. The motivation stemmed from the growing consumer and enterprise demand for richer communication experiences, such as virtual reality (VR) calls, remote collaboration with multiple camera feeds, and interactive 360-degree video sharing. Prior to MS-MTSI, implementing such services required proprietary, non-interoperable solutions or complex workarounds like establishing multiple parallel IMS sessions, which were inefficient and difficult to synchronize. The historical context is the evolution of MTSI itself, which in earlier releases provided a robust foundation for voice and video over LTE/5G but was fundamentally designed for a single audio and a single video stream per session. MS-MTSI, introduced in Release 15, directly addressed this architectural limitation by standardizing a framework within the IMS ecosystem. This standardization ensures interoperability between different vendors' clients and network equipment, reduces implementation complexity for service providers, and unlocks a new market for immersive telecommunication services that leverage the high bandwidth and low latency of 4G and 5G networks.

Key Features

• Support for negotiation of multiple concurrent video RTP streams within a single IMS session
• Mechanisms for media synchronization between streams using RTCP and RTP header extensions
• Dynamic session modification to add, remove, or replace individual media streams mid-call
• Backward compatibility with legacy single-stream MTSI clients through capability negotiation
• Support for different stream types (e.g., primary view, auxiliary views, depth maps) for advanced rendering
• Integration with IMS policy control for managing QoS and bandwidth allocation per stream

Evolution Across Releases

Defining Specifications