Question: Which of the following is NOT a valid reason for cancelling the assigned Serving-CSCF (Call/Session Control Function) in an IP Multimedia Subsystem (IMS)?
Option 1: Timeout of registration at the Serving CSCF itself
Option 2: Explicit de-registration request from the User Equipment (UE)
Option 3: Direct request from the Radio Access Network (RAN) due to mobility issues
Option 4: Request from the Home Subscriber Server (HSS) over the Cx interface due to subscription changes
Explanation: The cancellation of an assigned Serving-CSCF is typically triggered by specific, defined sources within the IMS architecture. These include the Serving CSCF itself (e.g., due to a registration timeout), an explicit de-registration request from the User Equipment (UE), or a request from the Home Subscriber Server (HSS) over the Cx interface (e.g., due to changes in subscription). The Radio Access Network (RAN) is not directly involved in the cancellation of Serving-CSCF assignments, as this process is managed at the core network level. Thus, a direct request from the RAN due to mobility issues is not a valid reason for cancelling an assigned Serving-CSCF.