Description
MCPTT Emergency Private Priority (MEPP) is a sophisticated service-level feature defined within the 3GPP framework for Mission Critical Services, specifically for Push-to-Talk (MCPTT). It operates as a layered priority mechanism on top of the standard MCPTT architecture, which itself is built upon the IP Multimedia Subsystem (IMS) core. The system involves several key components: the MCPTT client on the user equipment (UE), the MCPTT server (which may be part of an application server in the IMS), and the underlying LTE or 5G network providing the connectivity and QoS bearers. MEPP functions by allowing an authorized user to initiate an emergency call with a specific, highest-level priority tag. When invoked, this triggers a series of network actions. First, the MCPTT server recognizes the emergency priority indicator from the session initiation signaling (e.g., using SIP protocols). The server then manages the media plane to ensure the emergency call is granted resources. This often involves preempting or downgrading existing, lower-priority MCPTT group calls or private calls on the same group channel. Crucially, MEPP also ensures the call is established as a 'private' call within the emergency context, meaning it is typically point-to-point or to a specific dispatcher, bypassing the standard group talker arbitration, to guarantee immediate and clear communication for the emergency initiator.
The role of MEPP in the network is to provide deterministic, high-assurance communication for public safety and mission-critical users. Its operation is tightly integrated with the QoS framework of the 3GPP network. The MCPTT server interacts with the Policy and Charging Rules Function (PCRF) or the Policy Control Function (PCF) in 5G to establish dedicated QoS Flows with guaranteed bit rate (GBR) for the emergency media. This ensures low latency and high reliability for the voice packets. Furthermore, the 'private' aspect of MEPP is enforced at the application layer by the MCPTT server, which controls floor control and media distribution, ensuring only the emergency caller and the designated recipient(s) are in the active session. This prevents other group members from interrupting the critical dialogue. The feature is a cornerstone for making LTE and 5G networks suitable replacements for traditional land mobile radio (LMR) systems, as it replicates the critical 'emergency button' functionality with the added benefits of broadband media and network integration.
Purpose & Motivation
MEPP was created to address a fundamental requirement in professional and public safety communications: the guaranteed ability to initiate a high-priority, unimpeded call during a life-threatening or critical incident. Prior to its standardization in 3GPP Release 13, commercial cellular networks lacked standardized mechanisms for preemptive emergency voice services within a group communication context. Traditional LMR systems had dedicated hardware buttons (Emergency Alarms) that would immediately seize the channel. The motivation for MEPP was to translate this imperative functionality into the IP-based, 3GPP ecosystem as part of the broader MCPTT service suite, enabling mission-critical users to transition from legacy LMR to modern broadband networks without sacrificing operational capabilities.
The development was driven by global public safety organizations and standards bodies like 3GPP and the European Telecommunications Standards Institute (ETSI) for critical communications (TCCA). The limitations of previous approaches, including basic VoIP or commercial PTT apps, were the lack of network-assisted priority, susceptibility to congestion, and no ability to preempt existing calls. MEPP solves these by leveraging the standardized priority and QoS mechanisms of 3GPP networks, ensuring the emergency call is recognized and serviced by the core network and application servers with the highest precedence. It provides the technical foundation for reliable emergency response coordination, which is essential for the safety of first responders and the public.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (188 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 MEPP (MCPTT Emergency Private Priority) function was introduced, enabling an MCPTT emergency private call to be established between two users with specific session control procedures. This included new mechanisms for remotely initiated private call client and server procedures, as well as corrections to call forwarding procedures for private answer mode handling in private calls. The release also defined the in-progress emergency private call state and introduced related triggering criteria for MCPTT emergency location.
- Remotely initiated private call client procedures TS 24.379CR0356
- Remotely initiated private call server procedures TS 24.379CR0357
- Managing functional alias – MCPTT server serving the user TS 24.379CR0390
- Managing functional alias – MCPTT server owning the functional alias TS 24.379CR0391
- Enter-Exit emergency alert area TS 24.379CR0411
- Corrections to mcpttinfo.xsd TS 24.379CR0383
+ 4 more changes
In Release 16, enhancements to the MEPP function included the introduction of procedures to restrict incoming private communications for call control and to support the use of functional aliases in both private calls and emergency alerts. Furthermore, new capabilities were added for the MCPTT server to store preconfigured regroup information and for triggering an emergency group call automatically following an emergency alert. These updates also encompassed service authorization procedures to limit the number of authorized clients per MCPTT user and refinements to call state machines for off-network private calls.
- Restricting incoming private communications - call control TS 24.379CR0481
- Using functional alias in private calls – client procedures TS 24.379CR0482
- Using functional alias in private calls – server procedures TS 24.379CR0483
- Triggering an emergency group call after an emergency alert automatically TS 24.379CR0485
- List of MCPTT group members who did not acknowledge the group call request TS 24.379CR0486
- Support of functional aliases in emergency alerts TS 24.379CR0500
+ 13 more changes
In Release 17, the MEPP function was enhanced through the introduction of call transfer and call forwarding capabilities specifically for MCPTT private calls, as defined in the call control part. Furthermore, this release significantly expanded interconnect functionality by adding detailed procedures for MCPTT Pre-arranged groups, Chat groups, Emergency alerts, Private calls, and various regroup operations between systems. It also introduced support for using functional aliases in conference event notifications and in private calls, along with server control over the limit of simultaneous user logins.
- Add altitude, timestamp to MCPTT location XML schema TS 24.379CR0625
- Call transfer for MCPTT private call, call control part TS 24.379CR0673
- Add accuracy to MCPTT location XML schema TS 24.379CR0698
- Call forwarding for MCPTT private call, call control part TS 24.379CR0709
- Interconnect - MCPTT Gateway Server functional entity TS 24.379CR0720
- Interconnect - MCPTT Pre-arranged group originating participating procedures TS 24.379CR0721
+ 56 more changes
In Release 18, the MEPP function was enhanced to support emergency private priority calls for users that have migrated to a partner system, ensuring critical communications continuity during migration. Additionally, the release introduced procedures for emergency alerts targeted to private calls, specifically defining the MCPTT private emergency alert state. These updates expanded the robustness of emergency priority handling in inter-system and private call scenarios.
- MCPTT Chat group join to group regroup TS 24.379CR0855
- Addition of 5G MBS inter-RAT information in MCPTT signalling TS 24.379CR0873
- MC GW UE functions in MCPTT TS 24.379CR0901
- General adhoc group call procedures in single system - Protoc impl for MCPTT TS 24.379CR0902
- Add the description of 5MBS in MCPTT TS 24.379CR0899
- Support MCPTT over 5G ProSe TS 24.379CR0905
+ 65 more changes
In Release 19, the MCPTT Emergency Private Priority (MEPP) function was enhanced with new procedures for emergency remote floor requests and refined criteria for determining participants during ongoing ad hoc group emergency alerts. It also introduced the ability for authorized users to release ad hoc group calls and added mechanisms to prevent multiple participations in a group regroup. Furthermore, corrections and clarifications were made for ad hoc group emergency alert cancellation and MCPTT private call transfer operations.
- Modify list of participants by changing the criteria during an ongoing MCPTT ad hoc group call TS 24.379CR1004
- Adhoc group emergency alert add criteria to the SIP message containing the participant lists sent to the authorised users TS 24.379CR1014
- MCPTT adhoc group call to migrated user TS 24.379CR1015
- Modifying the criteria for determining the participants during an ongoing ad hoc group emergency alert TS 24.379CR1018
- Adhoc group call release by an authorized user (MCPTT) TS 24.379CR1022
- FRMCS_Ph5 Adding reason to leave a session in MCPTT TS 24.379CR1029
+ 20 more changes
Explore further
Broader topics and technologies where MEPP plays a role.
Defining Specifications
3GPP specifications that define or reference MEPP, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 24.379 vj50 | Mission Critical Push To Talk (MCPTT) call control | Rel-19 |
| TS 36.579 | 3GPP TR 36.579 | Rel-13 |
| TS 37.579 vi40 | Mission Critical services conformance testing | Rel-18 |