In this post, I will try to present some potential usage of Public Service Identities (PSIs) for the delivery of IMS services.
Automatic Service Discovery and Configuration
Restricted Access for Management of Public User Groups
Anybody can use this PSI to send an instant message to members of the group, access their presence information, or start a multimedia session with them (public access to the PSI). All these services are accessed through the initiation of an adequate SIP request addressed to the PSI (i.e. MESSAGE, SUBSCRIBE for presence event package, INVITE). Routing to the relevant application server is based on the service profile associated the PSI (i.e. MESSAGEs go to messaging server, SUBSCRIBEs for presence event package go to the resource list server, INVITEs go to the multimedia conferencing server).
On the other hand, only a few users can manage the user group, i.e. add/remove users to it. This implies that SIP requests that permit to access and modify the definition of the user group (SUBSCRIBE, PUBLISH for a group management event package) are restricted to a limited set of people. These people will have their service profile defined in such a way that their management requests to the sip:Friends_of_IMS@operator PSI will be routed to the application server managing the user group. The same management requests issued by a non authorized user will not be routed to any application server and will be rejected by the IMS core network (the service profile associated to the PSI does not define any routing for PUBLISH and SUBSCRIBE requests related to the group management event package).
Differentiated Access to Content (see figure, click to enlarge)
A and B serve the subscribers of the operator who are authorized to access the content. Access to these application servers is based on the service profiles associated to each subscriber. The service profiles associated to subscribers in the partition #1 route the service request to A, while service profiles associated to subscribers in the partition #2 route the service request to B. Service logic on A and B delivers the content.
D serves subscribers of other operators and users from the Internet. The service profile associated to the PSI determines that all requests originating from users whose IMPU's domain is not the one of the operator shall be routed to D. The service logic on D may propose the user to pay for viewing the content.
It would be possible to further discriminate between users trying to access the content. For instance, the service profile associated to the PSI could determine that SIP requests which do not include a P-Asserted-Identity header (i.e. requests originated from a non-IMS network) should be routed to an application server E.