There is a wide variety of services provided through communication systems, particularly mobile communication systems, and new services are planned all the time. More and more of these services are likely to be non-standard services. For example, a large number of value added services, such as call forwarding, are standardized in the GSM system (Global System for Mobile communications), but it seems that these value added services will not be standardized in ‘the third generation mobile communication systems’, such as the UMTS (Universal Mobile Telecommunications System), but they are produced by using a variety of toolkits or protocols utilizing intelligent network-like control. Examples of these include CAMEL (Customised Applications for Mobile network Enhanced Logic), SIP (Session Initiation Protocol), OSA (Open Service Architecture), Parlay API (Application Protocol Interface), MexE (Mobile station Execution Environment) and SAT (SIM Application Toolkit).
The object of the 3GPP AII-IP system, i.e. a UMTS system based on IP technology (Internet Protocol technology) and defined in the 3rd generation partnership project 3GPP, is that services associated with a mobile-terminating call are at least started, i.e. triggered from a call state control function S_CSCF serving the subscriber, whereto an interrogative call state control function I_CSCF routes the call on the basis of the address obtained from a home subscriber server HSS. The interrogative call state control function I_CSCF is the function through which calls terminating at the subscriber are routed. In systems where the signaling and the actual call are routed separately, call routing refers to the routing of signaling.
Sometimes, however, a subscriber is not registered in the system, for example because the user equipment of the subscriber is switched off. Thus the subscriber does not have a serving call state control function S_CSCF, nor the control function that would trigger the services associated with a mobile-terminating call, such as services associated with the forwarding of a terminating call.
One solution to the above problem is to provide these services by means of the interrogative call state control function I_CSCF. In this solution, the home subscriber server HSS would transmit the subscriber's service information, for example CAMEL subscriber information CSI, as a routing instruction to the interrogative call state control function, when the subscriber is not registered in the network. The problem of this solution is, however, that the I_CSCF should have a complete service interface, such as a CAMEL interface, it should be capable of dynamically triggering call state models and maintaining outgoing legs possibly associated with the service. In other words, the I_CSCF should contain a wide variety of functions. Since the I_CSCF very likely becomes the critical element of the system, it would be preferable that it would be as simple a device as possible, which is provided with only a few functions, so that its performance could be optimized without the above mentioned features required for providing the service.