MC service Off-Network Protocol

Description

The MC service Off-Network Protocol (MONP) is a comprehensive protocol suite standardized by 3GPP for enabling Mission Critical (MC) services in off-network conditions. It is specified across multiple technical specifications, including TS 24.281 (MCPTT off-network), TS 24.282 (MCData off-network), TS 24.379 (MCVideo off-network), and the associated conformance test specs TS 36.579 and 37.579. MONP allows Mission Critical User Equipment (MC UEs) to establish direct device-to-device (D2D) communication links using Proximity Services (ProSe) when they are not served by a 3GPP network (e.g., in remote areas, during network failures, or inside buildings with no coverage). The protocol stack operates over the PC5 reference point, the direct interface between UEs defined for ProSe. MONP encompasses several layers and functions: it defines discovery mechanisms for UEs to find each other and form groups off-network, session management for setting up and tearing down group calls (e.g., MCPTT calls), floor control for managing talker permission, security protocols for authentication and encryption of the direct link, and media plane protocols for transporting voice, data, or video. A key architectural concept is the Off-Network UE, which can autonomously function as a client, and in some configurations, certain UEs can take on limited network-like roles (e.g., for relaying or managing discovery) to extend the communication range or organize the group. MONP is tightly integrated with the on-network MC service architecture; a UE can seamlessly transition between on-network and off-network modes based on availability. The protocols ensure that critical features like group management, emergency calls, and security are maintained even without infrastructure.

Purpose & Motivation

MONP was created to fulfill a critical requirement for public safety and mission-critical professional users: reliable communication must be available even when the cellular network is damaged, overloaded, or simply not present. Traditional cellular services are entirely dependent on network infrastructure (base stations, core network). In disaster scenarios like earthquakes, fires, or terrorist attacks, this infrastructure can fail. MONP solves this problem by standardizing a direct mode of operation using D2D technology. It addresses the limitations of previous proprietary solutions (like traditional walkie-talkies) by providing a standardized, interoperable, and feature-rich off-network protocol that integrates seamlessly with the broader 3GPP Mission Critical services ecosystem (MCPTT, MCData, MCVideo). This allows first responders from different agencies or using equipment from different vendors to communicate directly. The motivation stems from lessons learned from major incidents where communication breakdowns occurred. By providing a standardized off-network protocol, 3GPP enables the development of ruggedized UE that can operate in both infrastructure-based and direct modes, ensuring communication resilience, which is a non-negotiable requirement for public safety organizations worldwide.