Mission Critical Service (MCX) is a communication solution provided for industries with high requirements for communication security, reliability, and real-time performance, such as public safety, emergency rescue, industrial energy, railway transportation, and large-scale events. 'Critical task' can be understood as when the failure/interruption of a task endangers life or puts important assets in danger, the task is called a 'critical task'. Its core goal is to ensure seamless collaboration and efficient information flow among multiple departments and devices in extreme environments or high-risk scenarios, ensuring the safety of life and stable operation of critical infrastructure.
The current mainstream group communication technologies mainly include narrowband digital clustering technology and broadband digital clustering technology. The Land Mobile Radio (LMR) system covers four mainstream narrowband digital cluster technologies, including TETRA, Project 25, DMR, and PDT, and is widely used to build private network systems for digital transmission, supporting group communication needs in public safety and industrial fields
TETRA: Developed by the European Telecommunications Standards Institute (ETSI) in the 1990s for the European PPDR (Public Protection and Disaster Relief) organization. Due to its reliability, high resilience, and security, TETRA technology has gained widespread recognition in critical communication markets and has become a standard for global PPDR organizations and multiple industries.
Project 25: The digital cluster system, abbreviated as P25, developed by the Association of Public Safety Communications Officials (APCO) in the 1990s, is applied in countries such as North America, some Latin America, and Australia and New Zealand to provide services to their public safety agencies.
DMR: A 2-slot TDMA solution developed by ETSI, aimed at providing a smooth transition from traditional analog systems to digital systems for price sensitive critical business customers. Due to its price advantage, DMR dominates global device sales.
PDT: PDT is a digital communication standard developed by China, which combines the advantages of DMR and TETRA technologies and was originally designed to meet the specific needs of the Chinese police. PDT technology has advantages such as low operating costs, wide coverage, and backward compatibility. At present, PDT has become a national standard, which is widely used in public security, fire rescue and emergency management departments in China, and has been applied in some "the Belt and Road" countries. China has currently established the world's largest nationwide PDT interconnection and roaming network.
The broadband digital trunking technology has been officially introduced since 3GPP R12 4G LTE, mainly including China's independently developed B-TrunC (Broadband Trunking Communication), as well as 3GPP's LTE MCX and 5G MCX.
B-TrunC: In 2013, B-TrunC was developed and published by the China Communications Standards Association (CCSA). As a broadband digital cluster communication system, B-TrunC relies on dedicated frequency bands to provide efficient, reliable, and secure communication solutions for industry users. B-TrunC supports seamless integration of multiple services such as voice, video, and data, meeting the diverse needs of modern industry users for multimedia communication. It is widely used in fields such as public safety, transportation, energy, and manufacturing. This technology adopts advanced downlink shared channel technology and supports point to multipoint transmission mechanism. It is the first broadband cluster communication standard in the world to support applications such as point to multipoint voice calls and point to multipoint video calls; Support dynamic resource allocation and priority scheduling, which can ensure the communication quality of critical business in high concurrency scenarios;
Strong network coverage capability, supporting both wide and deep coverage; Having high security, adopting end-to-end encryption technology and strict user authentication mechanism; Support network continuity and stability assurance, support multi-level redundancy design and fault self-healing function. This technology was adopted as a global standard by the International Telecommunication Union (ITU) in 2015, and its open interfaces and protocols also support interconnection with other communication systems, providing users with a wider range of integration and application space.
LTE MCX: 4G LTE has significant advantages in critical task communication compared to previous cluster private network technologies. Firstly, in terms of coverage deployment, 4G LTE has been widely deployed in developed countries and has coverage advantages; Secondly, in terms of data transmission rate and bandwidth, LTE technology can provide higher bandwidth, supporting multimedia critical task applications such as real-time video transmission, high-definition image feedback, and high-speed sensor data acquisition; Thirdly, in terms of ecological construction, equipment manufacturers, solution providers, and application developers can form a stronger ecosystem, which is conducive to end-to-end development and maturity; The fourth aspect is the future scalability towards 5G evolution. LTE, as a natural path for 4G technology to evolve towards 5G networks, can gradually introduce 5G ultra-low latency, massive connectivity, and future 6G capabilities, providing a technological foundation for the intelligent communication of critical tasks in the future.
5G MCX: With the development of 5G technology, the boundaries of mobile broadband networks have been pushed to a new level, and the powerful functions of 5G have brought new vitality to critical task communication. 3GPP has been gradually conducting research and standard development on 5G MCX (hereinafter referred to as MCX) technology since 2020, committed to enhancing technology research, making cooperation between critical task users more efficient, improving emergency rescue capabilities, and enhancing public safety; At the same time, based on cutting-edge technologies such as AI and automation, we innovate performance monitoring and service guarantee capabilities to optimize user experience. The basic functions of MCX include critical task voice MCPTT, video MCVideo, and data MCData, supporting flexible operating modes and adapting to changes in users and environments:
MCPTT: Provides a powerful cluster intercom service based on 5G, which not only provides instant voice communication services, but also supports emergency calls, off network operations, call group management, session maintenance, location sharing, and other functions;
MCVideo: supports various application scenarios such as video point-to-point calls, video group calls, video pull-up, video push, etc., providing users with a high-quality video communication experience;
MCData: Provides data transmission services for critical communication users, supporting multiple transmission methods including point-to-point and point to multipoint transmission, and supporting various data formats such as text messages, images, videos, and file transfers.
Standardization process
The 3GPP standard organization has been researching and developing LTE based MCX technology standards since 2015 to meet the broadband cluster communication needs of industry customers. 3GPP SA6 is mainly responsible for defining the application layer protocols for MC services, and undertaking the standard architecture and interface definition above the MCX application layer. So far, the 3GPP standard organization has completed the standardization work for MCX from Release 12 to Release 18, and Release 19 is currently underway. The commercial version based on LTE is in Release 16, and the commercial version based on 5G is in Release 18. The timeline of technological evolution is shown in Figure 1, and each version is still continuously evolving.
Figure 1 Key Task Communication Technology Evolution Stages
The MCPTT architecture was released in Release 13, and subsequent versions gradually added features such as MCData, MCVideo, MCX system interoperability, high-speed railway, and ship cluster business protocols. The technical standards from Release 12 to Release 19 are described as follows:
Release 12 developed an MC system architecture based on 4G LTE technology, defining QCI values for critical task communication;
Release 13 established the MCPTT standard for cluster voice and LTE based IOPS;
Release 13 established the MCPTT standard for cluster voice and LTE based IOPS;
Release 15 focuses on completing the standards for interconnection and interoperability between MCX and narrowband cluster systems, as well as business migration and interoperability between multiple MC systems; Starting to support the relevant features of the railway cluster field, and introducing standardized support for core functions such as multi speaker and function number for the first time; In terms of broadcast multicast applications, the introduction of end-to-end MBMSAPI middleware has better enabled cluster data to be carried on eMBMS. The basic and key features of MCX are basically covered in R15, and later evolved versions focus more on feature enhancement, problem correction, etc;
In addition to further optimizing features such as enhanced group communication and dynamic QoS scheduling, Release 16 also systematically conducted technical research on legal monitoring and log management for security monitoring and auditing requirements in critical task scenarios, providing compliance guarantees and network behavior traceability capabilities for public safety, railway operations, and other scenarios. Based on this technical report, Release 19 introduces legal monitoring and log management into the standard specifications;
Release 17 focuses on the development of 5G MCX standards. Due to the architecture design of MCX in the first version of Release 13, which adopted the decoupling of application layer services and network bearer transmission, the original application layer standards only need to be connected to the new interfaces of the 5G core network when migrating to the 5G network, including N5 (policy control), Nmb2 (group management), N6 (user plane transmission) Nudm、GC1， The specific migration docking relationship is shown in Table 2. But in Release 17, only the technical report was completed and standard support was not introduced. In addition, an application layer MC IOPS standard based on LTE IOPS has been developed;
The MCPTT architecture was released in Release 13, and subsequent versions gradually added features such as MCData, MCVideo, MCX system interoperability, high-speed railway, and ship cluster business protocols. The technical standards from Release 12 to Release 19 are described as follows:
Release 12 developed an MC system architecture based on 4G LTE technology, defining QCI values for critical task communication;
Release 13 established the MCPTT standard for cluster voice and LTE based IOPS;
Release 14 adds standards for cluster video MCVideo and cluster data MCData;
Release 15 focuses on completing the standards for interconnection and interoperability between MCX and narrowband cluster systems, as well as business migration and interoperability between multiple MC systems;
Starting to support the relevant features of the railway cluster field, and introducing standardized support for core functions such as multi speaker and function number for the first time; In terms of broadcast multicast applications, the introduction of end-to-end MBMSAPI middleware has better enabled cluster data to be carried on eMBMS. The basic and key features of MCX are basically covered in R15, and later evolved versions focus more on feature enhancement, problem correction, etc;
In addition to further optimizing features such as enhanced group communication and dynamic QoS scheduling, Release 16 also systematically conducted technical research on legal monitoring and log management for security monitoring and auditing requirements in critical task scenarios, providing compliance guarantees and network behavior traceability capabilities for public safety, railway operations, and other scenarios.
Based on this technical report, Release 19 introduces legal monitoring and log management into the standard specifications;
Release 17 focuses on the development of 5G MCX standards. Due to the architecture design of MCX in the first version of Release 13, which adopted the decoupling of application layer services and network bearer transmission, the original application layer standards only need to be connected to the new interfaces of the 5G core network when migrating to the 5G network, including N5 (policy control), Nmb2 (group management), N6 (user plane transmission) Nudm、GC1， The specific migration docking relationship is shown in Table 2. But in Release 17, only the technical report was completed and standard support was not introduced. In addition, an application layer MC IOPS standard based on LTE IOPS has been developed;
Release 18 fully completes the 5G MCX standard;
Release 19 continues to refine and enhance the standards for SA1 requirements, and initiates the development of MCX consistency testing specifications and MCX Server testing specifications.
3GPP expects to complete the overall evolution or migration of narrowband cluster communication to broadband cluster communication by 2030 through continuous improvement of technical standards including 5MBS, D2D communication, NTN integration, etc.
technical system
5G brings significant changes to the service mode of operators, which is the key to move from consumer Internet to industrial Internet. To further enrich 5G private network capabilities and expand business scenarios, China Mobile actively builds MCX technology and builds a "1+4+2+N" technology system, helping vertical industry application innovation and development.
Figure 2 China Mobile MCX "1+4+2+N" Technical System
The "1" basic base consists of wireless access, 5G core network, and MCX server, serving as the underlying support for the entire technical system. It provides a unified networking architecture, network connection, and business processing capabilities for multimedia communication services such as MCPTT, MCVideo, MCData, etc. in the upper layer.
The "4" key capabilities include priority assurance, identification system, business continuity, and cross domain integration:
(1) Priority guarantee: Based on QPP (QoS, Priority, Pre exemption) mechanism and 5G network slicing, ensure high priority and reliable stable transmission of mission critical communication services;
(2) Identification system: Adopting a dual layer fusion identification system of control surface and application surface, providing end-to-end bidirectional high security protection for users and networks;
(3) Business continuity: Based on business categories and priorities, achieve dual layer and multi-mode continuity guarantee for the carrying surface and application surface;
(4) Cross domain integration: Through standardized interfaces, it supports the integration and interoperability of multiple network architectures between different MCX systems and between MCX and heterogeneous systems, providing solid support for enterprise cross regional and network operations.
Class 2 communication mode: For both scenarios with network access and without network coverage, stable multimedia cluster communication services can be provided. Through diversified access methods and adaptation to multiple application environments, network call and off network call communication modes are formed, further enhancing the practical value of the network.
N "business characteristics: relying on core features such as point-to-point calls, group calls, dynamic group management, data sharing, emergency scheduling, etc., while meeting the needs of customers for broadband group communication, data information security, cross departmental system collaboration, etc., synchronously providing full process scheduling and management capabilities.
key technology
1 "set of foundation base
In order to meet the core demands of public safety, transportation, energy, and other critical task industries for high quality, high reliability, and high security in scenarios such as voice, video, and data communication, as well as the development needs of interconnection and collaborative operation between various critical task industries, the MCX system provides business support capabilities for upper layer multimedia applications such as MCPTT, MCVideo, MCData, etc. through a unified base.
The MCX networking architecture generally consists of four parts: MCX client, 5G core network, SIP core network, and application servers (MC server, MC user database, MC gateway server),
As shown in Figure 3:
Figure 3 MCX end-to-end network architecture diagram
The modules in the figure are introduced as follows:
MCX Client (MCX UE)
As the access point of the MCX network, the MCX client needs to have core functions such as single call, group call, high priority call, and call preemption. By implementing key task communication priority guarantee, user identity security binding, and one click quick registration upon startup, it fully adapts to the core demands of high reliability, low latency, and fast access in communication scenarios such as public safety, emergency response, and disaster rescue.
5G Core Network (5GC)
The 5G core network, as the "highway" of MCX network, relies on low latency communication, broadcast multicast, QoS guarantee, mobility management, secure communication and other characteristics to provide network resource guarantee and stable, reliable, and low latency data transmission for MCX network.
SIP core network
SIP Core, as the "scheduling center" of MCX network, consists of a standard or simplified IMS core network, including multiple logical units; Provide functions such as proxy routing, SIP encryption, business and application server selection, user registration management, etc.
application server
Composed of MC server, MC user database, and MC gateway server. Through unified user data management (MC user database) and a set of common service functions (CSC), multimedia applications such as voice (MCPTT), video (MCVideo), and data (MCData) are provided to users. The MC gateway server is used to interconnect the MC network with different trust domains, while providing topology hiding to improve network security.
The "4" key abilities
Priority protection
In network congestion or emergency scenarios, normal data transmission is prone to delay and packet loss issues, while critical task cluster data packets generally carry emergency scheduling instructions, real-time business cooperation, and other key information. Transmission obstruction can directly lead to delayed emergency response, core business interruption, and in severe cases, endanger life or put important assets in danger.
In response to the pain points mentioned above, the MCX network needs to have a comprehensive QPP (QCI+Priority+Pre exemption) guarantee mechanism, support for carrying services on dedicated network slices, dynamically adjust end-to-end parameters, configure high preemption capabilities, and establish MCX services on dedicated network slices to ensure priority delivery of cluster packets in network congestion or emergency situations.
Figure 4: Establishment of 5G QoS Flow
End to end parameter dynamic adjustment
To achieve end-to-end parameter dynamic adjustment, the network side needs to be able to dynamically adjust QoS parameters based on business requirements, including QCI and Priority access priority. The 5G network can trigger policy changes, set PCC rules, and create or modify 5G QoS flows through the Policy Control Function (PCF).
Configure high preemption capability
Preemptive capability is to ensure that the MCX network can reject access/resource requests from non privileged users during congestion, while allowing new MC users to preempt network resources from non privileged users, ensuring that MC users can always access cellular network resources and prevent QoS Flow level congestion.
The 5G core network requires local configuration of ARP values or interaction with PCF settings to ensure MCX service preemption capability, with a maximum priority of 1 configurable.
Dedicated network slicing
The 5G network provides dedicated network slicing services for MCX services as needed, ensuring dedicated isolation of network resources; At the same time, it can support second level authentication and authorization that varies depending on the slice, ensuring priority processing and secure isolation of critical task services in the network.
Identification system
Compared with traditional communication networks, MCX networks propose differentiated requirements in user identity management: on the one hand, they need to ensure high security and reliability of MCX user identities, and on the other hand, they need to meet the fast access and network usage needs of ordinary users and MCX users in special scenarios. Taking the public safety field as an example, the MCX network needs to ensure the privacy of MCX user identities and the ability to execute critical tasks, while also safeguarding the basic communication rights of ordinary users.
To meet the requirements of the above scenario, MCX constructs a dual layer fusion identification system of application surface and control surface.
Application surface identification
The MCX network establishes a bidirectional multi authentication mechanism between users and the network by constructing four core identifiers: Critical Task User Identity (MC ID), Critical Task Service Identity (MC service ID), Critical Task Service Group Identity (MC service group ID), and Critical Task System Identity (MC system ID), providing strong support for the high security of the MCX network.
In response to the differentiated security needs of different industries, the MCX network can flexibly configure the same or different identity identification resources for the four types of application surface identifiers mentioned above to adapt to the security requirements of diverse scenarios.
MCX should provide emergency communication capabilities for MCX users in special scenarios or ordinary users. For example, in emergency rescue scenarios, MCX can not only ensure fast access and communication for MCX users, but also provide necessary emergency communication service support for ordinary users. Unlike application plane identification, control plane identification relies on a private user identity and one or more public user identities, and sends registration requests to the SIP core network through SIP signaling. After completing authentication, MCX can provide emergency communication services to users.
Business continuity
When the location of the end user changes, different MCX services have different requirements for business continuity. For example, in the field of railway transportation, compared to train log transmission services, cluster scheduling services such as voice and video have higher requirements for continuity. At the same time, when end users switch between unicast mode and broadcast/multicast mode, the MCX network needs to handle it properly and ensure that business continuity is not affected.
To meet the diverse business continuity requirements in the above scenarios, MCX supports an architecture design that decouples the carrier surface and application surface. On the carrier side, for services such as voice and video that require high business continuity, MCX follows the SSC mode 1 standard and ensures uninterrupted business continuity by maintaining the IP anchor point unchanged during terminal movement; For MCData businesses with lower requirements for business continuity, the SSC Mode 2 standard can be followed to ensure business continuity.
When the terminal switches between unicast mode and broadcast/multicast mode, the continuity of MCX services can be handled and guaranteed by the application side. When the terminal moves to the edge of the broadcast cell, it can report the broadcast monitoring quality to the server, and the server will automatically switch to unicast mode to continue transmitting emergency task cluster communication data for the terminal, avoiding business interruption; After the terminal enters the effective broadcast area and reports the broadcast monitoring quality to the server, the server will maintain unicast transmission for a period of time until the terminal can stably receive broadcast data, and then stop unicast group call data transmission to the terminal to ensure business continuity during the mode switching process.
Cross domain fusion
Cross domain fusion
Inter communication between MCX systems
Many critical task scenarios currently involve the coexistence of multiple MCX systems, such as cross regional emergency rescue, multi departmental linkage tasks, etc. If these independent MCX systems cannot communicate with each other, they will form a "communication island", resulting in the inability to transmit key information such as voice group calls, positioning data, emergency instructions, etc. Different emergency response agencies cannot share information and coordinate actions in real time, directly hindering cooperation efficiency and even delaying key tasks such as rescue and scheduling.
To address the pain points mentioned above, the network side needs to support two modes of interface interoperability and interoperability with MC gateway servers, as shown in Figure 5, in order to achieve cross system communication and data exchange capabilities. When multiple MCX systems are all within the same operator's 5G network, they can communicate with each other through standard interfaces. When the MCX system is distributed across multiple operator 5G networks, interoperability is achieved through the MC gateway server. The MC gateway server is located at the edge of each network, ensuring the security of communication entering each network and implementing topology hiding, so that each network does not know each other's internal details, ensuring secure isolation under interoperability.
Figure 5: Interconnection Architecture between MCX Systems
Heterogeneous system interoperability
Before the advent of MCX technology, LMR had gone through a long development process, including various types of narrowband digital communication systems from early simple analog communication systems. Many countries have deployed narrowband cluster systems on a large scale to ensure public safety, such as P25 systems, TETRA systems, PDT systems, etc. Among them, PDT is a digital wireless communication system specially designed in China to meet the efficient and reliable communication needs of public safety, emergency communication, and industry private networks, and has achieved nationwide coverage. Therefore, in addition to interoperability between systems, MCX also requires the ability to integrate wide and narrow networks.
To meet the above requirements, the MCX communication system should support IWF (Interworking Function) gateway interconnection, thereby achieving interconnection between the MCX system and LMR system, as shown in Figure 6. Among them, the interface between IWF gateway and LMR system needs to adapt to different technical systems of LMR. Interconnection should achieve key services such as voice single call, voice group call, emergency call, and short message. At the same time, the voice encoding and decoding scheme, numbering rules, and security standards should be clearly defined to ensure business consistency, reliability, and security between the two systems.
Figure 6 Interconnection Architecture between MCX System and LMR System
There are certain differences between the MCX system and China's PDT system in terms of cluster business characteristics, protocol standards, and functional implementation. In order to ensure that users can have a consistent and smooth business experience between the two systems, it may be necessary to optimize the architecture and enhance and adapt the interfaces during the actual interoperability process, including coordinating key functions such as voice group calls, single calls, priority processing, emergency calls, etc., to ensure interoperability between different systems. At the same time, adjustments may need to be made to signaling protocols, data transmission, and security mechanisms to achieve seamless integration and business consistency across systems.
Class 2 communication mode
Call online
In critical task scenarios, it is common to face routine work situations with normal network coverage, such as daily cross departmental scheduling, large-scale event security coordination, etc. These scenarios mainly have strong demands for single call, group call, MBS and other functions. Commanders can transmit precise instructions to specific personnel through single call, teams can use group call function to synchronize tactical details, and efficiently issue global deployment information to multiple terminals through MBS, achieving information synchronization and action coordination among all parties involved, significantly improving the efficiency of handling routine critical tasks.
Based on the above scenario requirements, in order to improve the communication reliability of MCX services, when there is 5G network coverage, the MCX communication system should support single call and MBS multicast broadcast functions.
Single call mode
Single call refers to MCPTT point-to-point voice service, where the calling terminal initiates a voice call to the called terminal, and after establishing a link, both parties exchange information. Within the 5G network, PCF needs to support setting up PCC processes, establishing MCX dedicated carriers, and achieving interface interoperability with IMS networks to send call requests and response signaling information, as well as data transmission processes
As shown in Figure 7 below:
Figure 7 MCPTT Single Call Data Transmission Stream
MBS multicast broadcast
MBS is a point to multipoint communication service, which is divided into two session modes: multicast and broadcast. In multicast mode, the source terminal business data is transmitted to the terminal group in the MC group through the 5G downlink, while in broadcast mode, the source terminal business data is transmitted to all terminals in the service area through the 5G downlink. In order to achieve more efficient multicast broadcasting services, 5G networks need to collaborate with MCX systems to support MBS multicast and broadcast session modes:
One is the session management capability, which can dynamically create, adjust, and release MBS sessions, accurately distinguish the transmission objects of multicast to specific MC terminal groups and broadcast to all terminals in the service area, and ensure the directional or global distribution of business data;
The second is resource optimization, efficient allocation of 5G downlink resources, coordination of MBS and unicast service resource occupation, and avoidance of spectrum waste;
The third is differentiated QoS guarantee, providing low latency and high reliability transmission for emergency dispatch, industrial data distribution and other scenarios;
The fourth is comprehensive coverage and stable transmission, ensuring that all terminals within the service area can receive data stably through power regulation and anti-interference technology; At the same time, it is necessary to adapt to 3GPP MBS related protocol standards to ultimately support point to multipoint critical communication requirements in scenarios such as public safety, industry, and transportation. Data transmission architecture
As shown in Figure 8:
Figure 8 MCX Broadcast Multicast Data Transmission Architecture
Off network call
There are also special situations in critical mission scenarios, such as post earthquake base station damage, remote mountainous rescue, underground mining operations, and other extreme scenarios where the network is paralyzed or uncovered. Without off grid communication capabilities, a "communication blind spot" will be formed, where trapped personnel cannot transmit key information such as location and injury to the outside world, and external rescue teams cannot push rescue plans to the scene, resulting in complete loss of contact between the scene and the outside world. This not only hinders the progress of critical tasks such as rescue and operations, but may also delay the best disposal time due to information blockage, endangering the safety of personnel.
Based on the above pain points, the MCX system can provide D2D communication based on Proximity based Services (ProSe) technology, which is served by 5GCs with neighboring UE locations. There are four ways of MCX D2D communication, as shown in Figure 9:
Direct discovery: Support mutual discovery between 5G ProSe UEs based on network authorization and terminal pre configuration information;
Direct communication: Terminals do not require cellular network infrastructure to establish a connection and can exchange data through the PC5 interface;
UE to Network (U2N) relay: Remote UE connects to the 5G network through relay UE, where the remote UE may not be within wireless coverage range. Including two modes of Layer 2 relay and Layer 3 relay, MCX off network calls use Layer 2 relay mode;
UE to UE (U2U) relay: Remote UE exchanges data with target UE through relay UE.
Figure 9 MCX Off Grid Communication Service Mode
N types of business characteristics
MCX provides a variety of business capabilities covering areas such as voice, data, video, location, and scheduling
The rich business features of MCX can be widely applied in key industries such as public safety, rail transit, energy inspection, airports and ports, achieving the modern governance goal of "global perception, precise command, and efficient collaboration".
