Interworking between telecommunications networks

A method of interworking between first and second mobile telecommunications networks, for use with a mobile user terminal for a subscriber to the first network which can roam to the second network, the first network including a home location register for maintaining a record of the network with which the subscriber is currently registered, the second network including at least one visitor location register with which the user terminal becomes registered when roaming to the second network, and the second network offering a service to the subscriber not available through the first network;

• the method comprising:
• maintaining in an interworking home location register a record of when the subscriber's user terminal becomes registered with a visitor location register of the second network and those of the services of the second network to which the subscriber is permitted access, whereby when the user terminal has roamed to the second network and registered with a visitor location register of the second network, the visitor location register is provided with data corresponding to said services to which access is permitted, and
• maintaining an interworking visitor location register coupled to the interworking home location register, the interworking visitor location register being operable to communicate to the home location register of the first network that the subscriber's user terminal has become registered with the visitor location register of the second network whereby when the user terminal has roamed to the second network and registered with the visitor location register of the second network, a call may be routed to the user terminal through the second network by being directed to the home location register of the first network such that the home location register obtains routing information through the interworking function unit for routing the call to the user terminal through the second network.

An interworking function unit for providing interworking between first and second mobile telecommunications networks, for use with a mobile user terminal for a subscriber to the first network which can roam to the second network, the first network including a home location register for maintaining a record of the network with which the subscriber is currently registered, the second network including at least one visitor location register with which the user terminal becomes registered when roaming to the second network, and the second network offering a service to the subscriber not available through the first network;

• the interworking function unit comprising:
• an interworking home location register to maintain a record of when the subscriber's user terminal becomes registered with a visitor location register of the second network and those of the services of the second network to which the subscriber is permitted access, whereby when the user terminal has roamed to the second network and registered with a visitor location register of the second network, the visitor location register is provided from the interworking function unit with data corresponding to said services to which access is permitted, and
• an interworking visitor location register coupled to the interworking home location register, the interworking visitor location register being operable to communicate to the home location register of the first network that the subscriber's user terminal has become registered with the visitor location register of the second network whereby when the user terminal has roamed to the second network and registered with the visitor location register of the second network, a call may be routed to the user terminal through the second network by being directed to the home location register of the first network such that the home location register obtains routing information through the interworking function unit for routing the call to the user terminal through the second network.