They are mentioned here as an explanation as to how services are
reached through the IMS. Otherwise, it would be difficult to identify specific services
that a subscription pays for as part of the subscriber profile.
A service identity can be static or dynamic and is assigned by the application server.
The service identity is used within the IMS for routing to the proper application server.
When an application server registers in the network, it will advertise its capabilities to
the HSS and S-CSCF, along with its service identity. The service identity is also sent in
the charging parameters sent through SIP in the IMS domain.
The service identity then becomes a part of the subscriber profile within the HSS.
Notice that a service profile could be in multiple public user identities. It simply identifies
what service each identity is to be able to access, so even partner networks will be
able to access and provide the same service even when the subscriber is outside their
home network.
This is yet another important reason for the use of IMS identities and service identities.
As subscribers roam from network to network, the visited network needs the ability
to access the same application servers and deliver the same services the subscriber
uses at home. This is not always possible in today??™s networks.
In the IMS model, however, partner networks would be able to identify what services
the subscriber is allowed to use (through the subscriber profile stored on the HSS) and
deliver that same service no matter where the application server resides.
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