Method and apparatus for distributing application server addresses in an IMS

1. A method of distributing application server addresses in an IP multimedia subsystem, the method comprising the steps of:

receiving a Session Initiation Protocol (SIP) request associated with a subscriber at a Serving Call Session Control Function (S-CSCF), the SIP request including a subscriber identity;

in response to receiving the SIP request at the S-CSCF, selecting an application server by the S-CSCF and forwarding the SIP request to the selected application server;

in response to receiving the SIP request at the selected application server, dynamically allocating the subscriber to the selected application server and sending the subscriber identity and at least one application server address from the selected application server to a Home Subscriber Server (HSS) associated with the subscriber thereby registering as the subscriber's serving application server;

in response to receiving the at least one application server address at the HSS, storing the at least one address at the HSS in association with the subscriber identity for responding to future subscriber-related messages;

sending the at least one application server address from the HSS to the S-CSCF, wherein if an explicit address has been allocated, the allocated explicit address is sent to the S-CSCF by the HSS, and if no explicit address has been allocated, the HSS sends to the S-CSCF a generic application server identity and the S-CSCF resolves the received generic identity to one of a plurality of explicit addresses; and

caching the at least one address at the S-CSCF in association with the subscriber identity, wherein the S-CSCF utilizes the at least one address to send subscriber-related messages to the allocated application server.

2. The method according to claim 1 , wherein said SIP request is one of:

a) a SIP register request from said subscriber; and

b) a call termination request for said subscriber and where said subscriber is unregistered.

3. The method according to claim 1 , wherein the at least one application server address is sent from the application server to the HSS via an Sh interface.

4. The method according to claim 1 , wherein said step of sending the at least one address from the HSS to an S-CSCF allocated to the subscriber includes updating Initial Filter Criteria at the S-CSCF over a Cx interface.

5. An application server for distributing application server addresses in an IP multimedia subsystem, the application server comprising:

communication means for receiving an indication from a Serving Call Session Control Function (S-CSCF) that the S-CSCF has selected the application server to serve a subscriber having a subscriber identity;

means responsive to receiving the indication, for dynamically allocating the subscriber to the application server; and

means for registering as the subscriber's serving application server by sending the subscriber identity and at least one application server address to a Home Subscriber Server (HSS) associated with the subscriber upon allocation of the application server to the subscriber;

wherein if an explicit address has been allocated, the allocated explicit address is sent to the S-CSCF by the HSS, and if no explicit address has been allocated, the HSS sends to the S-CSCF a generic application server identity and the S-CSCF resolves the received generic identity to one of a plurality of explicit addresses.

6. A method of distributing application server addresses in an IP multimedia subsystem, the method comprising the steps of:

receiving at an application server, a request from a subscriber, the request including a subscriber identity;

in response to receiving the request, allocating the application server or another application server to said subscriber;

sending at least one application server address from the allocated application server or other application server to a Home Subscriber Server (HSS) associated with the subscriber;

in response to receiving the at least one application server address at the HSS, storing the at least one application server address at the HSS in association with the subscriber identity for responding to future subscriber-related messages;

sending the at least one address from the HSS to a Serving Call Session Control Function (S-CSCF) allocated to the subscriber, wherein if an explicit address has been allocated, the allocated explicit address is sent to the S-CSCF by the HSS, and if no explicit address has been allocated, the HSS sends to the S-CSCF a generic application server identity and the S-CSCF resolves the received generic identity to one of a plurality of explicit addresses; and

caching the at least one address at the S-CSCF in association with the subscriber's identity, wherein the S-CSCF utilizes the at least one address to send subscriber-related messages to the allocated application server.

7. The method according to claim 6 , wherein receipt of a request at an application server causes that application server to send a query to the HSS to determine whether or not an application server has already been allocated to the subscriber.

8. The method according to claim 7 , wherein, if an application server has been allocated, the receiving application server forwards the request to the allocated application server after having received the address of the allocated server from the HSS if the allocated server is not the receiving application server.

9. The method according to claim 7 , wherein, if no application server has been allocated, the method further comprises sending the address of the receiving application server from the receiving application server to the HSS.

10. The method according to claim 7 , wherein, if no application server has been allocated, the method further comprises forwarding the request from the receiving application server to another application server, the request causing the other application server to send its address to the HSS.