## 4 Modeling the Identity Registration Status
An HTTP GET method retrieves information about the individual FRMCS system. The UEs resource represents the list of all of the UEs in the railway operator networks (both registered and unregistered). The HTTP GET method retrieves the list of UEs. The UEID resource represents an individual UE. An HTTP GET method retrieves information about given UE, including information on the FRMCS system in which it is registered and its serving profile. An HTTP PATCH method is used to change the FRMCS system in which the UE is registered. The UEFunctionalNumbers resource represents all of the functional numbers related to the UE, and an HTTP GET method is used to retrieve the list of UE functional numbers. The UEFunctionalNumberID resource is for an individual UE functional number. Information about it is retrieved by an HTTP GET, and its serving FRMCS system is updated by an HTTP PATCH method. The Users resource represents the list of all of the users in the railway network. An HTTP GET method retrieves the user list. The UserID resource represents an individual user. Information about an individual user can be retrieved using an HTTP GET method. The FRMCS system in which the user is registered is updated by an HTTP PATCH method. The UserFunctionalIdentities resource represents all of the functional identities of the user, and an HTTP GET method is used to retrieve a list of all of the user functional identities. The UserFunctionalIdentityID resource is for an individual user functional identity. Information about the resource is retrieved by an HTTP GET method, while the serving FRMCS is updated using an HTTP PATCH method. The subscriptions resource represents all of the subscriptions related to identity registration management. The FRMCSareaRegistrations resource represents the subscriptions for notifications about registration changes in FRMCS-serving areas. An HTTP GET method returns all of the subscriptions of this type, and an HTTP POST method creates a new subscription. The FRMCSareaSubscriptionID resource is for given subscription for registration change notifications and it allows an HTTP GET method to retrieve information about the subscription, an HTTP PUT method to update the subscription, and an HTTP DELETE method to terminate the subscription. The UERegistrations resource, UEFNRegistrations resource, UserRegistrations resource, and UserFIDRegistrations resource represent all of the subscriptions to notifications about registration/deregistration events related to UE, UE functional numbers, users, and user functional identities, respectively. An HTTP GET method retrieves information about all the subscriptions, and an HTTP POST method creates a new subscription for each subscription type. The UERegistrationSubscriptionID resource, UEFNRegistrationSubscriptionID resource, UserRegistrationSubscriptionID resource, and UserFIDRegistrationSubscriptionID resource represent individual subscriptions to notifications about registration/deregistration events related to UE, UE functional numbers, and users and user identities, respectively. The supported HTTP methods are GET, PUT, and DELETE, used to retrieve, update, and delete information about the subscription. A UE and a user only need to register its and her/his identity and the related functional identities once. Events related to UE and user movements across the serving areas of different FRMCS systems can be reported by the FRMCS location service. When a UE enters a serving area of a new FRMCS system, the registration procedure for the UE and user identities in the new FRMCS system is triggered without the participation of the UE or the user. Figure 11 shows the flow for the initial registration of a piece of UE and its functional numbers in FRMCS system 1 and subsequent registration in FRMCS system 2. As a precondition, each FRMCS system knows which UE can enter and leave its serving area in advance.