Question: During an intra-eNB handover, what determines whether the eNB uses a vertical key derivation (NH) or a horizontal key derivation (current KeNB) to derive KeNB*?
Option 1: The type of UE (User Equipment) connected to the network.
Option 2: The availability of an unused {NH, NCC} pair in the eNB (Evolved Node B).
Option 3: The frequency EARFCN-DL (E-UTRA Absolute Radio Frequency Channel Number for Downlink) of the target cell.
Option 4: The PCI (Physical Cell Identity) of the source cell.
Explanation: The eNB's decision to use either a vertical key derivation (with NH, Next Hop) or a horizontal key derivation (from the current KeNB, eNB Key) hinges on whether an unused {NH, NCC} (Next Hop, Network Colour Code) pair is available within the eNB itself. This ensures secure and efficient key management during handovers without exhausting available key derivation resources.