Question: In an IP Multimedia Subsystem (IMS), when a User Equipment (UE) acquires IP connectivity via an IP Connectivity Access Network (IP-CAN), what triggers the UE's registration with the IMS if it's not already registered?
Option 1: Receipt of a dedicated registration request from the Serving Call Session Control Function (S-CSCF)
Option 2: The UE's acquisition of IP connectivity itself, as per IMS registration procedures
Option 3: Completion of a successful CS (Circuit Switched) call setup
Option 4: Update of the UE's capabilities via Open Mobile Alliance Device Management (OMA DM)
Explanation: According to IMS principles, whenever a UE gains IP connectivity through an IP-CAN and is not already registered with the IMS, it automatically initiates registration. This process enables IMS services and is independent of the UE's CS state. The correct trigger is thus the acquisition of IP connectivity itself, aligning with how IMS manages network access for multimedia services.