Question: In a 5G network, when is the 'Number of failed legacy handover preparations' measurement incremented in the source NR (New Radio) cell CU (Central Unit)?
Option 1: Upon successful receipt of a HANDOVER REQUEST message by the target NR cell CU.
Option 2: Upon receipt of a HANDOVER PREPARATION FAILURE message by the source NR cell CU from either the AMF (Access and Mobility Management Function) or the target NR cell CU.
Option 3: When a UE (User Equipment) initiates a handover from a legacy network to a 5G network.
Option 4: After a successful handover completion, for statistical purposes.
Explanation: This is correct because the measurement is specifically incremented when the source NR cell CU receives a HANDOVER PREPARATION FAILURE message, indicating that the preparation of resources at the target NR cell CU has failed.