Question: What action does a receiving UM RLC entity take when a received UMD PDU's Sequence Number (SN) falls outside the reassembly window defined by the current next expected sequence and window size?
Option 1: Deliver the UMD PDU to the upper layer after removing the RLC header.
Option 2: Discard the received UMD PDU immediately without buffering.
Option 3: Automatically request a retransmission of the UMD PDU from the peer RLC entity.
Option 4: Place it in the reception buffer for potential future reassembly.
Explanation: When an SN falls outside the defined reassembly window, the receiving UM RLC entity cannot properly place it within the current reassembly context. Discarding the PDU is the most appropriate immediate action to prevent potential misordering or corruption of data.