Question: In deployments without IMS-level roaming interfaces, how does the home network determine the serving PLMN of a UE during IMS Initial Registration, and what triggers this determination?
Option 1: The P-CSCF requests the PCRF/PCF to report the PLMN identifier upon receiving a SIP REGISTER request from the UE, as part of the initial IMS registration procedure.
Option 2: The UE directly informs the P-CSCF about its current PLMN during the initial attachment process, which then triggers an update in the IMS registration.
Option 3: The I-CSCF queries the PCRF/PCF for the UE's PLMN ID based on periodic location updates from the UE.
Option 4: This determination is not necessary for IMS Initial Registration in deployments without IMS-level roaming interfaces.
Explanation: According to the specified procedure for deployments without IMS-level roaming interfaces, when a UE initiates an IMS registration (sending a SIP REGISTER request), the P-CSCF (Proxy-Call Session Control Function) responds by subscribing to the PCRF/PCF (Policy and Charging Rules Function/Policy Control Function) to obtain the PLMN (Public Land Mobile Network) identifier where the UE is currently located. This step is crucial for updating the UE's location information within the IMS (IP Multimedia Subsystem) context, ensuring accurate routing and policy application. The correct sequence involves the P-CSCF's proactive query upon initial registration, making Option 1 the accurate description of this process.