Media Plane Control Protocols

Description

Media plane control protocols are a set of protocols operating in the media plane to manage the transmission and control of media streams. They are defined in 3GPP specifications TS 24.380 and TS 24.581, primarily for Mission Critical Push To Talk (MCPTT) and other mission-critical services. These protocols handle floor control (also known as transmission control), pre-established session call control, and MBMS subchannel control. They ensure that media sessions are properly managed, especially in group communication scenarios where multiple users share a common channel.

The most prominent media plane control protocol is the Floor Control protocol, which is used in MCPTT to arbitrate who has the right to speak at any given time. It operates over the media plane and uses messages such as Floor Request, Floor Granted, Floor Taken, and Floor Release. This protocol is essential for half-duplex communication, preventing simultaneous transmissions and managing talker priority. It is defined in TS 24.380 for MCPTT and adapted for other services like MCVideo and MCData.

Pre-established session call control is another function of media plane control protocols. In pre-established sessions, the session is set up in advance, and media can be transmitted quickly without the need for full session establishment. The control protocols manage the activation and deactivation of media streams within these sessions, ensuring fast call setup times critical for mission-critical communications.

MBMS subchannel control is used for controlling the reception of MBMS subchannels. In MBMS, multiple subchannels can be multiplexed, and the control protocol allows the receiver to select and manage the subchannels it wants to receive. This is particularly relevant for group communication over MBMS, where different groups may use different subchannels.

The protocols are designed to be robust and efficient, operating over UDP/IP and using binary encoding to minimize overhead. They are part of the overall mission-critical service architecture and work in conjunction with SIP-based session control in the signalling plane. The media plane control protocols ensure that the media plane is properly managed, providing the necessary control functions without relying on the signalling plane for every action, which reduces latency and improves scalability.

These protocols are specified in TS 24.380 (MCPTT media plane control) and TS 24.581 (MCVideo media plane control). They are essential for enabling push-to-talk and other real-time group services over LTE and 5G networks.

Purpose & Motivation

Media plane control protocols were created to address the unique requirements of mission-critical push-to-talk and group communication services. Traditional SIP-based session control is not optimized for the fast, half-duplex communication patterns of push-to-talk, where users need to request and be granted permission to speak within milliseconds. The media plane control protocols provide a lightweight, low-latency mechanism for floor control and session management directly in the media plane.

Before these protocols, push-to-talk services over cellular used proprietary or non-standard mechanisms, leading to interoperability issues and limited functionality. 3GPP standardized them to ensure that mission-critical users (e.g., public safety, first responders) could rely on consistent behavior across different vendors and networks. The protocols also needed to support pre-established sessions to achieve the very low call setup times required for emergency communications.

Additionally, the need to control MBMS subchannels arose from the desire to efficiently deliver group communications over broadcast/multicast bearers. The media plane control protocols allow receivers to dynamically select subchannels, enabling flexible group management without additional signalling overhead.

Thus, these protocols solve the problems of latency, interoperability, and efficient group communication in mission-critical scenarios, providing a standardized foundation for push-to-talk and related services.