Description
The Mission Critical MBMS subchannel Control Protocol (MCMC) is a control-plane protocol specified in 3GPP TS 24.380. It operates within the framework of Mission Critical Services (MCS), specifically for Mission Critical Push-to-Talk (MCPTT) over MBMS (Multimedia Broadcast Multicast Service). Its primary function is to manage the establishment, modification, and release of MBMS subchannels that are allocated for MCPTT group communication sessions. An MBMS subchannel is a logical channel within an MBMS bearer service, used to carry media and control information for a specific MCPTT group. The protocol enables the MCPTT server to request the BM-SC (Broadcast Multicast Service Centre) to activate or deactivate these dedicated subchannels based on the dynamic needs of group calls.
Architecturally, MCMC resides between the MCPTT application server and the BM-SC in the core network. It defines specific procedures and messages for subchannel control. For instance, when an MCPTT group call is initiated, the MCPTT server uses MCMC to send a subchannel activation request to the BM-SC. This request includes parameters such as the TMGI (Temporary Mobile Group Identity), session description, and QoS requirements for the subchannel. The BM-SC, upon receiving this request, coordinates with the LTE or 5G NR network to establish the necessary MBMS radio bearers and allocate resources for broadcasting the media to all subscribed users within the service area.
The protocol ensures that MBMS resources are used efficiently for mission-critical group communications. Instead of establishing multiple unicast connections, a single MBMS broadcast stream serves all members of an MCPTT group, conserving radio and core network resources, especially in scenarios with many listeners. MCMC handles the lifecycle of these subchannels, including modifications during a session (e.g., adding a new talker, changing QoS) and timely release when the group call ends. This management is crucial for maintaining service continuity, prioritizing critical traffic, and ensuring that network capacity is optimally utilized for public safety operations where reliable, low-latency group communication is paramount.
Purpose & Motivation
MCMC was introduced to address the specific requirements of Mission Critical Services (MCS) for efficient, one-to-many group communications. Traditional MCPTT over unicast paths can strain network resources when serving large groups, as it requires individual data streams to each user. This is inefficient for public safety scenarios where many first responders need to receive the same voice transmission simultaneously. The purpose of MCMC is to leverage the broadcast/multicast capabilities of MBMS to offload this traffic, but a dedicated control mechanism was needed to integrate MBMS with the MCPTT application layer.
Prior to MCMC, MBMS control was more generalized and not optimized for the dynamic, session-based nature of mission-critical group calls. The creation of MCMC solved the problem of how an MCPTT application server could directly and dynamically control MBMS subchannels. It provides the necessary signaling interface to request broadcast resources on-demand, align them with specific MCPTT group sessions, and manage their QoS parameters. This enables network operators to support large-scale mission-critical communications with predictable latency and high reliability while making efficient use of limited radio spectrum, a critical consideration during major incidents or disasters.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (31 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-13, normative work from Rel-15.
In Release 15, the MCMC function was enhanced to support multi-talker floor control, including procedures for floor control messages to support functional alias and updates to the non-controlling MCPTT function. It also introduced specific MBMS procedures for group dynamic data and the capability for MBMS bearer announcement over an MBMS bearer. Furthermore, the release defined the use of a UDP port as the parameter for differentiating media and control packets for different groups over the same MBMS bearer.
- Multi-Talker floor control server TS 24.380CR0178
- Mutli-talker – floor control towards the participant TS 24.380CR0179
- Updates to Non-controlling MCPTT function for Multi Talker TS 24.380CR0183
- Floor Release Multi Talker message over MBMS subchannel TS 24.380CR0185
- Floor Control Server towards participant TS 24.380CR0187
- Coding of floor control messages to support functional alias TS 24.380CR0192
+ 5 more changes
In Release 16, the MCMC function was enhanced with specific corrections to its floor control procedures for both operational modes. These updates included corrections to the Off-Network Floor Control procedures and to the timers and events of the On-Network Floor Control procedures.
In Release 17, the MCMC function saw specific corrections and enhancements to improve the reliability of mission-critical broadcast calls. These included fixes for floor control message handling during the upgrade or downgrade of a broadcast call and corrections to the floor control procedures when groups are regrouped. Additionally, updates were made to the state transition diagram for basic floor control operation towards the floor participant to ensure proper server behavior.
- Corrections to floor indicator of On-Network Floor Control procedures TS 24.380CR0274
- Corrections to floor control messages handling for upgrade/downgrade of broadcast call TS 24.380CR0289
- MCPTT client and Participating MCPTT function alignments TS 24.380CR0296
- Updates to clause 6.3.5 Floor control server state transition diagram for basic floor control operation towards the floor participant and related editorials TS 24.380CR0298
- Corrections in call setup control over pre-established session state machine TS 24.380CR0307
- Error in floor control when groups are regrouped. TS 24.380CR0316
+ 2 more changes
In Release 18, the MCMC protocol was enhanced to integrate 5G Multicast/Broadcast Service (5G MBS) into the MCPTT media plane, enabling media delivery over MBMS bearers. New timers and counters were added within the participating MCPTT function for improved control of MBS channels. Furthermore, support for multiplexing was introduced, specifying the use of separate SSRCs for RTP audio and RTCP floor control signaling when transported over 5MBS.
- Addition of 5G MBS in MCPTT media plane TS 24.380CR0332
- Add timers and counters in the participating MCPTT function for MBS channel control TS 24.380CR0347
- MCPTT Adding user ID in Floor Request message from NCF to CF TS 24.380CR0348
- MCPTT support of multiplexing - SSRC used in RTCP signalling over 5MBS TS 24.380CR0363
- MCPTT support of multiplexing - SSRCs used for RTP audio and RTCP floor control TS 24.380CR0356
- Addition of Reason cause value in Pre-Established Session Call Control Disconnect Message to indicate media bearer failure TS 24.380CR0349
+ 2 more changes
In Release 19, the MCMC protocol was enhanced with an addition to the floor control release message and received miscellaneous corrections to floor control procedures. These updates specifically refined the protocol used for managing MBMS subchannels over MBMS bearers, which are logical channels for group communication. The changes provided more precise control actions for when a user finishes talking and releases the floor.
Explore further
Broader topics and technologies where MCMC plays a role.
Defining Specifications
3GPP specifications that define or reference MCMC, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 24.380 vj10 | MCPTT Media Plane Control Protocol | Rel-19 |