## 2 Identities and Registration Management in FRMCS  
According to [10], a functional identity describes a function performed by a user who is involved in a communication. It is used to address a user/system by identity or function instead of by user subscription or FRMCS equipment identity. FRMCS user equipment (UE) is a device that can use the FRMCS. There are different types of user devices that can be used in rail systems. The following are types of UE [10]:  
* UE without a man-machine interface that does not require a functional identity, such as a \"Sensor\",
* UE without a man-machine interface that requires a functional entity, such as \"Public Address System (PA system)\".
* UE with a man-machine interface that is used in a train by a train driver, such as a \"Cab radio\",
* UE with man-machine interface for general purposes such as a \"Handheld\".  
When UE is switched on, it attaches to the FRMCS network at the telecommunication level. The UE is reachable via its subscriber identity address. The associated FRMCS applications start up and are ready for use.  
The UE registers itself in the FRMCS system at the application level, and the FRMCS identifies the type of UE. This is the first step of registration for UEs of all types. \"Sensor\"-type UE is reachable via its UE identity and subscriber identity address, and no other actions are required, as shown in Figure 1.  
The second step of registration is for FRMCS UEs of \"PA system\" and \"Cab radio\" types. This step includes registration of UE functional number(s). The UE functional number is related to the equipment and points where the device is used, e.g., a PA system on a train.  
The role management process of \"PA system\"-UE is shown in Figure 2. A UE may have more than one functional identity. The UE can be addressed via its functional identity and via the identity address of its subscriber.  
Figure 1: Flow of the registration for a “Sensor”-type FRMCS UE.  
Figure 2: Flow of the registration for a “PA system”-type FRMCS UE.  
Next steps of registration are for UE of \"Handheld\" and \"Cab radio\" types. Third step of registration includes user login to the FRMCS system. The user logs into the FRMCS system using a man-machine interface. The user logs via his/her credentials, which can be functional or personal. The user login procedure may require a username and password, smart card, fingerprint, or eye scan depending on the UE's capabilities. The fourth step of registration includes registering of user functional number(s). This functional number is related to the user and identifies his/her functional role (e.g., controller, train driver, train staff, etc.). A user may have more than one functional identity.  
Figure 3 shows the registration of \"Handheld\"-UE and Figure 4 shows the registration of \"Cab radio\"-UE. \"Cab radio\"-UE is reachable via its UE identity, its functional number(s), user identity, user functional identity(ies), and the identity address of its subscriber.  
In the FRMCS system, a user or equipment can deregister one or multiple functional identities. The deregistration procedure must be performed to change a functional identity and in this case the user deregistration can be driven by the user.  
A user or UE can be identified by others who are involved in a communication. The available identities of the communication initiator are sent to the UE(s) of the so-called party(ies), and the receiving UE presents the identities based on a context (e.g., destination, time, status, location, etc.). A user or UE can be identified by others within a specific context (e.g., train, all drivers at a station, railway emergency communications, etc.).  
Figure 3: Flow of registration for a “Handheld” FRMCS UE.