# An Approach to Designing Critical Railway Voice Communication
## 3 Reusable Functionality Related to Security Services  
### Management of FRMCS User and Application Data  
According to [3], FRMCS applications require an enhanced level of security. FRMCS security covers functionality to protect services and security functions, including identification and authentication, authorization, key management, and secure data transmission. The implementation aspects of the encryption mechanisms based on identifiers, credentials, cryptographic keys, and encryption algorithms have been standardized to a very limited extent in 5G. The logic of FRMCS applications, including security functions, is based on the functional identity, which defines the user's functional role on a specific train, engine, station, or control center. An FRMCS user may be a human or a device. The user's functional identity describes the user's role in communications as a calling or called party. It is used to route the call based on function or identity, rather than user subscription. In addition to humans, some FRMCS devices such as public announcement machines also have functional identities that reference where the device is used. The following functional roles are defined [1]. The network operator has the responsibility for the proper operation of the FRMCS network. A controller is a person who coordinates all train-related operations, such as supervising, monitoring, analyzing, as well as controlling train movements. Different functional identities are defined for the controllers, e.g., infrastructure manager controller, railway undertaking controller, power \begin{table}
\begin{tabular}{c c c} \hline \hline
**Subjects** & **Aspects** & **References** \\ \hline \multirow{6}{*}{FRMCS} & Specifications & [1; 2; 3] \\  & Operational rules & [24; 25; 26; 27] \\  & 5G New Radio & [4; 5; 6; 7; 8; 9; 10] \\  & Cloudification & [20; 21; 22; 23] \\  & Cybersecurity & [16; 17; 18; 19] \\  & 5G Mission critical services & [11; 12; 13; 14; 15] \\ \hline \multirow{2}{*}{Services and applications} & Service components & [28; 29; 30; 31; 32; 33] \\  & Mathematical formalisms & [34; 35; 36; 37] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of related work. supply controller, and signaler, among others. An entitled controller is a controller responsible for safe train movements within a defined geographic area. A train driver is a person who is authorized to drive different types of trains, including route locomotives, shunting locomotives, work trains, passenger trains, freight trains, and others, in a safe, responsible, and autonomous manner. Train staff are onboard the train and they are not train drivers, e.g., security staff, conductors, catering staff, and so on. A group of people who safely move the trains between platforms and yards is called a shunting team. Trackside staff include workers who are shunting and/or maintenance workers. A ground user is not on the train but can access the network via a stationary or wireless connection. A public emergency operator is responsible for answering emergency calls from the public. Personal staff other than train drivers, train staff, trackside staff, or controllers who are employed by the railways are regarded as railway staff. In order to use FRMCS applications, the user needs to be registered in the system with one or more functional identities. User credentials are stored as a part of the user profile. The FRMCS role management and presence application allows for user login/logout, functional identity registration/deregistration, and the submission of identities. The use of FRMCS applications, as well as access to voice and data communications, are configurable. Access to applications and communications is based on the user's identity. The network operator defines FRMCS applications that could be used by a given functional identity. Only authorized applications are presented to the FRMCS user depending on his/her user profile, registered identity(ies), and the context.