Question: In deriving KeNB* for handover purposes in a 3GPP SAE security architecture, what determines the length (L1) of the EARFCN-DL input parameter to the Key Derivation Function (KDF)?
Option 1: The type of UE's wireless communication standard (e.g., LTE, 5G)
Option 2: The value range of EARFCN-DL (e.g., 0-65535 or 65536-262143)
Option 3: The network's security protocol version (e.g., Rel-8, Rel-15)
Option 4: The target physical cell ID's format (e.g., macro cell, small cell)
Explanation: The length of the EARFCN-DL (L1) is determined by its value range to ensure backward compatibility. For EARFCN-DL values between 0 and 65535, L1 is set to 2 bytes, and for values between 65536 and 262143, L1 is set to 3 bytes. This dynamic adjustment prevents discrepancies in derived keys between entities of different releases.