Question: For RLC UM (Unacknowledged Mode) in a cellular network, what part of the UMD PDU is excluded when forming the plaintext block for encryption?
Option 1: The entire UMD PDU header and the first payload octet
Option 2: Only the first octet, which is the RLC UM PDU header
Option 3: The last two octets of the UMD PDU for integrity checks
Option 4: No part of the UMD PDU is excluded for RLC UM mode
Explanation: In RLC Unacknowledged Mode (UM), reliability is not guaranteed, and thus, only the header that provides sequence information (among other controls) is excluded from the encryption process to maintain efficiency. Excluding just the first octet, which contains the RLC UM PDU header, ensures that user traffic or signaling data (payload) is encrypted while still allowing for basic protocol functionality.