Question: What triggers the measurement for 'Attempted period location update' in the Core Network (CN) Circuit Switched (CS) domain for UMTS and combined UMTS/GSM?
Option 1: Receipt by MSC-S of a 'LOCATION UPDATING REQUEST' service request message from the UE to the VLR with a PERIODIC location update type.
Option 2: Successful completion of a period location update procedure in a location area.
Option 3: Detection of a UE's presence in a new location area during an ongoing call.
Option 4: Scheduled periodic checks by the MSC-S for location updates across all connected UEs.
Explanation: This is correct because the measurement is triggered on receipt by VLR of 'LOCATION UPDATING REQUEST' service request message from the UE to the VLR with a specified location update type of PERIODIC, directly matching Option 1.