# An Approach to Designing Critical Railway Voice Communication
## 2 Related Work
The accurate positioning of trains is also required for passenger information systems, proactive maintenance, and resource management. A discussion on the capabilities of 5G mechanisms for accessibility differentiation to maintain mission-critical services is provided in [14]. The prioritization of mission-critical communications is vital for railway applications, whose disruption or failure could cause collisions, train derailments, and human casualties. Prioritization is based on the context of communications, the type of application, and the user's functional identities [2]. It requires the FRMCS to determine the behavior of the device in the case of multiple communications competing for the user's attention. The prioritization rules should handle incoming and outgoing communications by enabling, disabling, or holding them. The prioritization function allows for concurrent communications and the user's choice to accept, reject or hold communications. The prioritization of 5G traffic determines the quality of service (QoS) of the services in use based on the QoSClass Identifier (QCI). Using 5G Access Class Baring (ACB), emergency calls are treated with high-priority access and low-priority calls may be rejected temporarily. The Allocation and Retention Priority (ARP) is used in congestion scenarios. Emergency railway communication scenarios include public train emergency services with a limited/local impact (e.g., on a train or platform) and they are used to warn about potentially hazardous situations. These services provide emergency help points, where calls can be placed to report an emergency. An emergency situation requires an immediate response and 5G low-latency and highly reliable communications can expand the precision of response teams. The 5G mission-critical services provide a location-based mechanism for triggering/canceling an emergency alert upon entering/leaving a predefined area [11]. Strong security is one of the essential requirements for critical communication applications. Railway digitalization introduces threats to security, reliability, and costs. Connected users, train control systems, signaling systems, and trackside infrastructure are all potentially at risk. The control systems are built on open platforms, where train control can be accessed remotely via public networks and, consequently, they are vulnerable to cyber-attacks. MC railway communications can be the object of attacks such as denial-of-service or man-in-the middle attacks or snuffing. An advanced cybersecurity solution for railway systems should include constant monitoring, provisioning of full visibility, and prompt addressing of potential threats. A systematic review of cybersecurity challenges and approaches to railway infrastructure is provided in [15]. Key operational metrics to evaluate cybersecurity in critical railway infrastructure are identified and evaluated through simulations in [16]. In [17], the authors propose an approach for combined security and safety risk assessment in railway systems. Based on specific security requirements, a security framework tailored to the needs of railways is proposed in [18]. In [19], a multilayered approach to encrypting audio signals in secure voice communications is presented. According to [1], critical communication applications in railways require authorization for both the application and the communication. The aim of communication authorization is to avoid communication disruptions, maintain communication privacy, and reduce the network load. It is based on the communication context, user's functional identity, time frame, location, network identifier, and other context-based criteria. Application authorization requires the railway operator to configure the communication rules to control access to the application. It is envisaged that FRMCS applications will embrace 5G capabilities by using a radio access network, core network functionality, MC services, and railway-specific support services.