# An Approach to Designing Critical Railway Voice Communication
## 1 Introduction  
Railways are expected to provide highly reliable, safe, secure, robust, and predictable services. The digitalization of the rail industry can meet this unique set of demands by enabling innovative solutions that optimize operational efficiency and enhance the customer experience. The digitalization of railways relies on stable, reliable, and critical connectivity. Communication solutions for the complex needs of rail operations and infrastructure include connected trains and trackside connectivity, enabling improved onboard monitoring systems and more efficient maintenance. The International Union of Railways (UIC) established a framework called the Future Railway Mobile Communications System (FRMCS), which was intended to replace the railways' legacy communication system known as GSM-R. The transition from the GSM-R to the FRMCS, which is based on fifth-generation (5G) mobile technology, will modernize railway services; improve performance, reliability, and security; maximize capacity using critical broadband, and provide a seamless passenger experience. Technologies that will drive the transition to the FRMCS include the Internet of Things, sensor technologies for environmental perception, automated interruption detection, Artificial Intelligence-based automated train control that allows for driverless train movements at an optimal distance, and intelligent solutions for proactive maintenance. The FRMCS standardization process has been initiated by the UIC. Basic FRMCS standardization documents are the User Requirement Specifications [1], FRMCS Use Cases [2],and the Study of System Architecture [3], which was developed by the European Telecommunications Standard Institute (ETSI). In [1], a set of technology-independent FRMCS user requirements were defined in the form of individual voice, video, and data applications with railway-specific requirements for latency, bandwidth, and reliability. FRMCS applications should provide intuitive human-machine interfaces and support machine-to-machine communications for the exchange of information and performance of tasks. There are also requirements for the mitigation of the risk of miscommunications and the prevention of unauthorized access. The applications are classified as communication types and support types. Critical applications are essential for train control and signaling or trackside surveying and maintenance; performance applications are used for the improvement of railway operations such as telemetry, monitoring, and control of non-critical infrastructure; business applications support railway business operations, e.g., Internet access for passengers, video streaming, and other infotainment use cases. Critical communication applications are required for automatic train control, automatic train protection, emergency calls, shunting, and banking operations. Critical communications use 3 GPP mission-critical voice, video, and data services and rely on reliable and resilient network connectivity. The authors of [2], discuss use cases related to the applications listed in [1]. The use cases capture the users' (devices or humans) particular goals and interactions with the system, without considering the internal system implementation details. Each use case specifies a different way to interact with the system and defines the expected behavior. In [3], based on an analysis of the FRMCS user requirements, a description of the system's logical architecture is provided and key deployment and border-crossing scenarios are derived. The document also focuses on the possible technical realization of the FRMCS system. The logical architecture describes the system in the form of functional blocks and the reference points between them, whereas the technical realization describes alternatives for building the system using 3 GPP or other body blocks.