For providing IP multimedia services over mobile communication networks, the Third Generation Partnership Project (3GPP) defined the Internet Protocol (IP) Multimedia Subsystem (IMS) in 3GPP TS 23.228 V6.8.0, IP Multimedia Subsystem (IMS) Stage 2, 3rd Generation Partnership Project. Multimedia services include a variety of services over both traditional circuit switched media (e.g. voice) and packet switched media (e.g. pictures, video, audio, instant messaging, presence, push-to-talk, chat rooms, etc,). IMS makes it possible for users to use multimedia real-time and non-real-time communication services as well as data oriented multimedia services. IMS supports multimedia services over IP-based networks between end-users as well as between end-users and servers. In addition to mobile radio networks, access over wireless local area networks (WLAN) has been included in the IMS specifications and even fixed IP-based networks can be integrated in IMS making IMS a generic framework for multimedia services over IP. For setting up and controlling calls and sessions between user equipment and/or application servers, IMS includes so-called Serving Call Session Control Function (S-CSCF) units using the transaction-based Session Initiation Protocol (SIP) and Session Delivery Protocol (SDP) defined in 3GPP TS 24.229 V6.5.1, IP Multimedia Call Control Protocol based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP), Stage 3, 3rd Generation Partnership Project. A SIP transaction or dialogue is between two end-points called user agents. Typically, a SIP transaction or dialogue is a multimedia session performed between devices. There are different SIP methods for initiating a SIP transaction or dialogue (e.g. INVITE, SUBSCRIBE or MESSAGE).
In order to control access to a multimedia service, users typically must subscribe to the service prior to using the service. However, a service appears to be less attractive to users, if the subscription process is required to take place a significant amount of time, e.g. hours, days or even weeks, before the service can be used. On the other hand, if a subscription to a service is to be possible at the time the user desires to use the service, immediately preceding usage of the service, generally, the user must know how to access the subscription process for the multimedia service and the user's device must be setup for accessing the subscription process. Moreover, if the multimedia services enable dynamic and spontaneous (“on the fly”) subscriptions, performed immediately preceding the usage of the service, the multimedia services include appropriate mechanisms. Considering the vast number and variety of multimedia services, an enormous overhead is required for the implementation and maintenance of such mechanisms. Particularly, each multimedia service requires an associated data repository for storing user subscriptions as well as an interface for accessing and updating the data repository.
US 2005/0155036 describes a system for providing to a user services of a serving entity, such as an application server, in a communication network, such as IMS. According to US 2005/0155036 a user's service registration message is sent via a control entity, such as an S-CSCF, to a serving entity group having a plurality of serving entities, each of which is configured to provide the service. Upon receiving a message acknowledging the registration message and including an address of a selected one of the serving entities, the control entity stores the address of the selected serving entity in an address table. Accordingly, future respective service requests from the user will be routed by the control unit to the address of the selected serving entity stored in the address table.