Question: In IMS (IP Multimedia Subsystem), when there is a change in the user's subscription data utilized by the Serving-Call Session Control Function (S-CSCF), how does the Home Subscriber Server (HSS) update the S-CSCF with the new information?
Option 1: HSS uses the Pull model, where the S-CSCF periodically requests updates from the HSS.
Option 2: HSS and S-CSCF synchronize through a third-party server that manages all subscription data updates.
Option 3: HSS employs the Push model, sending the complete updated subscription data set to the S-CSCF upon any modification.
Option 4: The update is facilitated by the User Equipment (UE) relaying the changed subscription data to both the HSS and S-CSCF.
Explanation: This correct answer reflects the specified procedure in IMS for handling updates to a user's subscription data. By using the Push model, the HSS proactively sends the updated information to the S-CSCF as soon as a change occurs, ensuring the S-CSCF always has the most current data without needing to request it. This approach enhances the efficiency and accuracy of subscription management within the IMS infrastructure.