Interconnection

Description

Interconnection is a key concept in 3GPP Mission Critical (MC) services, initially defined for Mission Critical Push To Talk (MCPTT) in Rel-14 and later generalized to all MC services in Rel-16. It enables communication between different MC systems, allowing MC service users obtaining service from one MC system to communicate with MC service users who are obtaining MC service from one or more other MC systems. This is crucial for interoperability in scenarios such as public safety, where different agencies or regions may operate their own MC systems but need to collaborate during emergencies. The architecture for interconnection is defined in 3GPP TS 23.280 (Common functional architecture to support mission critical services) and TS 23.781 (Interconnection between MC services). The interconnection can be implemented at various levels, including the service layer, where MC service providers interconnect their systems to exchange signaling and media. Key components involved include the MC Service Server (MCSS) in each system, which handles call control, group management, and media distribution. The interconnection interface allows for the exchange of session control messages, group affiliation information, and media streams. It supports both direct and indirect interconnection models. In direct interconnection, MC systems communicate directly via an inter-system interface. In indirect interconnection, they may use a broker or gateway to facilitate communication. The interconnection must ensure security, including authentication and authorization of users across systems, and confidentiality of communication. It also must handle quality of service (QoS) and priority to maintain mission-critical performance. The concept has evolved from MCPTT-specific to all MC services (MCVideo, MCData) in Rel-16, reflecting the growing need for integrated mission-critical communications. Interconnection enables features such as cross-system group calls, where users from different systems can join the same group communication, and individual calls between users on different systems. It also supports location sharing and other data services across systems. The implementation requires standardized protocols and interfaces to ensure multi-vendor interoperability. 3GPP specifies the use of SIP (Session Initiation Protocol) for session control and RTP (Real-time Transport Protocol) for media transport, with appropriate extensions for MC services. The interconnection also involves the exchange of policy and charging information to manage resource usage and billing. As MC services continue to evolve towards 5G, interconnection will play a vital role in enabling nationwide and cross-border mission-critical communications.

Purpose & Motivation

Interconnection exists to enable seamless communication between different Mission Critical (MC) systems, which is essential for public safety and other mission-critical users who may be served by different network operators or agencies. Without interconnection, users of one MC system would be isolated and unable to communicate with users of another system, severely limiting the effectiveness of mission-critical communications during emergencies that require multi-agency or cross-border cooperation. The concept was introduced in Rel-14 for MCPTT to address the need for interoperability between separate MCPTT deployments, such as those operated by different public safety organizations. Prior to this, each system was standalone, and there was no standardized way to interconnect them. The 3GPP specifications provide a common framework for interconnection, ensuring that different vendors' systems can interoperate. In Rel-16, the scope was expanded to all MC services (MCVideo, MCData), reflecting the integrated nature of modern mission-critical communications. Interconnection also facilitates resource sharing and fallback scenarios, enhancing overall resilience.