System and method for event notifications in a multimedia network

1. A communication system comprising:

a multimedia network comprising an internet protocol multimedia subsystem, the multimedia network further comprising a home subscriber server configured to store user information and call state control function entities;

an entity that is external to the multimedia network and configured to subscribe to the multimedia network for notifications regarding events that associate with at least one other entity of the communication system, the configuration being such that

subscription messages from the external entity are routed to at least one serving call state control function entity based on information stored in said home subscriber server,

said at least one serving call state control function entity is provided with a storage configured to store information received in said subscription messages, and

said at least one serving call state control function entity sends a notification in response to an event defined by said information stored in the store of the serving call state control function entity.

2. A communication system as claimed in claim 1 , wherein the subscription messages are routed to the at least one serving call state control function entity via the home subscriber server.

3. A communication system as claimed in claim 1 wherein the information stored at the home subscriber server contains information regarding those serving call state control function entities the external entity is able to subscribe to.

4. A communication system as claimed in claim 1 , wherein the notification is sent to the external entity.

5. A communication system as claimed in claim 1 , wherein the external entity comprises an application server.

6. A communication system as claimed in claim 5 , wherein the application server is a presence server.

7. A communication system as claimed in claim 1 , wherein the external entity comprises a user equipment.

8. A communication system as claimed in claim 1 , wherein the at least one other entity comprises a user equipment.

9. A communication system as claimed in claim 1 , wherein the system operates in accordance with a session initiation protocol, and the events are events of that protocol.

10. A communication system as claimed in claim 9 wherein the subscription message comprises a session initiation protocol subscribe message, and the notification comprises a session initiation protocol notify message.

11. A method comprising:

sending notifications in a communication system;

subscribing by an entity that is external to a multimedia network of the communications system for notifications regarding events associated with at least one other entity, the multimedia network comprising an internet protocol multimedia subsystem;

routing of subscription messages from the external entity to at least one of a plurality of serving call state control function entities of the multimedia network based on information stored in home subscriber server of the multimedia network;

storing in a store associated with said at least one serving call state control function entity information received in said subscription messages; and

sending a notification by the serving call state control function in response to an event defined by said information stored in said store associated with said at least one serving call state control function entity.

12. A method as claimed in claim 11 , wherein the subscription messages are routed to the at least one serving call state control function entity via the home subscriber server.

13. A method as claimed in claim 11 , wherein the information stored at the home subscriber server contains information regarding those serving call state control function entities the external entity is able to subscribe to.

14. A method as claimed in claim 11 , wherein the notification is sent to the external entity.

15. A method as claimed in claim 11 , wherein the external entity comprises an application server.

16. A method as claimed in claim 15 , wherein the application server is a presence server.

17. A method as claimed in claim 11 , wherein the external entity comprises a user equipment.

18. A method as claimed in claim 11 , wherein the at least one other entity comprises a user equipment.

19. A method as claimed in claim 11 , wherein the method operates in accordance with a session initiation protocol, and the events are events of that protocol.

20. A method as claimed in claim 19 wherein the subscription message comprises a session initiation protocol subscribe message, and the notification comprises a session initiation protocol notify message.

21. A communication system comprising:

a multimedia network comprising an internet protocol multimedia subsystem, the multimedia network further comprising a home subscriber server configured to store user information and call state control function entities;

an entity that is external to the multimedia network and comprising a means for subscribing to the multimedia network for notifications regarding events that associate with at least one other entity of the communication system;

means for routing subscription messages from the external entity to at least one serving call state control function entity based on information stored in said home subscriber server;

storage means in said at least one serving call state control function entity for storing information received in said subscription messages, and

sending means in said at least one serving call state control function entity configured to send a notification in response to an event defined by said information stored in said storage means in said at least one serving call state control function.