Question: Under what circumstance must the Secondary eNodeB (SeNB) request the Master eNodeB (MeNB) to update the Secondary KeNB (S-KeNB) over the X2-C interface?
Option 1: When the SeNB's battery level falls below a certain threshold, indicating a need for security context refresh.
Option 2: As a periodic maintenance task, performed at set intervals (e.g., daily) to ensure ongoing security.
Option 3: When uplink or downlink PDCP COUNTs are about to wrap around for any of the Data Radio Bearers (DRBs).
Option 4: Immediately after each successful X2-handover to a new SeNB, as a precautionary security measure.
Explanation: Updating the S-KeNB when PDCP COUNTs are about to wrap around is crucial for maintaining data integrity and preventing potential security vulnerabilities that could arise from count rollover. This ensures continuous secure communication across the network, particularly important in scenarios involving multiple hops (e.g., X2-handovers), where end-to-end security must be preserved.