Question: What action does a receiving Unacknowledged Mode (UM) Radio Link Control (RLC) entity take with received Unacknowledged Mode Data (UMD) Protocol Data Units (PDUs) that cannot be reassembled into an RLC Service Data Unit (SDU) due to lower layer losses?
Option 1: Buffer them for potential future reassembly
Option 2: Send a negative acknowledgement to the transmitter
Option 3: Discard them to maintain link efficiency
Option 4: Convert them to Acknowledged Mode (AM) for retransmission
Explanation: In Unacknowledged Mode (UM), reliability is not guaranteed, and the RLC layer does not request retransmissions. If a UMD PDU is lost at a lower layer and cannot be reassembled into an RLC SDU, the receiving UM RLC entity must discard it to maintain link efficiency and prevent buffer overflow, as there's no mechanism for recovery in UM mode.