Question: What action does a receiving Unacknowledged Mode (UM) RLC entity take when it receives an UMD PDU with a Sequence Number (SN) = x from the lower layer, where x falls outside the range of VR(UR) to VR(UH) but within the UM Window Size from VR(UH)?
Option 1: Send an acknowledgement for the received UMD PDU to the transmitting entity.
Option 2: Place the received UMD PDU in the reception buffer for reassembly.
Option 3: Request retransmission of the UMD PDU with SN = x from the peer RLC entity.
Option 4: Discard the received UMD PDU to prevent potential duplication or misordering.
Explanation: According to the specified conditions for handling UMD PDUs in Unacknowledged Mode, if an incoming UMD PDU's Sequence Number (SN) falls within a range that indicates it has been received before (outside the expected window but within the UM Window Size from VR(UH)), or is deemed not immediately usable due to sequence numbering, the receiving RLC entity should discard such PDUs. This prevents potential duplication errors or misordering in the data stream, ensuring integrity of the received data.