# An Approach to Designing Critical Railway Voice Communication
## 5 Use Cases of Critical Voice Communication Applications  
The train driver switches on his/her handheld device. The device attaches to the FRMCS network at the telecommunication level and is reachable based on its subscriber identity. All the applications included in the subscriber profile initialize and are ready to be used. The train driver's handheld device logs into the FRMCS on the application level. The use of handheld devices requires the registration of the user's login and functional identity(ies). The train driver initiates a login dialogue that confirms their credentials. After the successful registration of the train driver, they are identified by the FRMCS and their device is reachable through the user identity and device subscriber address. The train driver registers one or more of their functional identities and, thereafter, their device is accessible through their functional identity(ies). The identity registration is carried out by applying  
Figure 10: Structure of resource URIs related to multiuser talker control. Figure 9: UML communication diagram illustrating the communications in three use cases. Use case 1: Change in the number of simultaneous talkers (the number of simultaneous talkers is higher than the allowed number). Use case 2: The MUTC service is aware of the (initial) permissions to talk and automatically makes the decision about the requested permission to talk. Use case 3: The MUTC service delegates the permission to talk to the entitled call participant. the HTTP PUT method to the respective resource related to the role management and presence function. Assuming that it is a normal situation, e.g., the network is not congested, all applications associated with the registered functional identity(ies) are authorized to be used. The train is on its journey. The train driver's device periodically reports its location and its location information can also be provided to external systems. Figure 11 illustrates the services interaction flow for the initial registration of the device and user. A fire suddenly breaks out on a train. The train driver stops the train and uses a critical voice application to initiate a call to the controller responsible for the traffic in the respective geographical area. The communication authorization application retrieves the call party locations and requests an authorization decision from the policy service, which provides the calling party's functional identities and locations. The call is established and the QoS resources are assigned according to the configured application requirements. The connection between the train driver and the train controller is established. Due to the critical nature of the established connection, the assured voice communication application is activated. The application requests connectivity monitoring from the assured voice communication service and subscribes to notifications related to link-quality disruptions. During the communication, the assured voice communication service periodically sends notifications if the link quality is intact or notifies the application about link-quality degradation using the address provided by the application in the subscription request. Figure 12 shows the procedures for communication authorization and assurance. Figure 11: Services interaction flow for the initial registration of the device, user, and user’s functional identity. A passenger evacuation begins and the fire brigade arrives in order to extinguish the fire. Calls are established between some of the firemen and the controller responsible for the train on fire and the establishment of each call is accompanied by an application and communication authorization. The MUTC application is activated and a controller in the role of an entitled call participant makes a decision about permission to talk. To use the multiuser talker control function, the MUTC application submits a request to the multiuser talker control and subscribes to MUTC service notifications about the request to talk and the need to select users to talk.