sem- bly function only involves header removal, but in the case of a segmented SDU, the SDU cannot be delivered to upper layers until the PDUs carrying all segments of the SDU arrive at the receiver. The transmission of PDUs continues such that at time t1, PDUs k + 1 and k + 2 are transmitted but, at the receiving end, only PDU k + 2 has arrived. As soon as a complete SDU is received, it is delivered to the higher layers; thus PDU k + 2 is forwarded to the higher sublayer without waiting for the missing PDU k + 1, which could be undergo- ing retransmission by the HARQ protocol. Therefore, the transmission window remains unchanged, since none of the PDUs with SN > k have been acknowledged by the receiver. This could result in retransmission of these PDUs given that the transmitter is not aware of whether they have been correctly received. The reception window is not updated when PDU + 2 arrives because of the missing PDU k + 1. At this point, the receiver starts the t- Reassembly timer. If the missing PDU k + 1 is not received before the timer expires, a retransmission is requested. If the missing PDU arrives at time t2 before the timer expires, the reception window is advanced, the reassembly timer is stopped, and PDU k + 1 is deliv- ered for reassembly with SDU k + 1. The RLC sublayer is also responsible for duplicate detection using the same sequence number that is used for retransmission management. New Radio Access Layer 2/3 Aspects and System Operation 221 RLC PDUs waiting for initial RLC PDUs transmitted but RLC PDUs transmitted and transmission unackowledged acknowledged Transmitting Receiving Transmitting Receiving Transmitting Receiving entity entity entity entity entity entity Transmitting Receiving Transmitting Receiving Transmitting Receiving entity entity entity entity entity entity Figure 2.13 Example RLC-AM operation [17]. If PDU k + 2 arrives again within the reception window, it will be discarded due to exis- tence of another copy. The transmission continues with PDUs k + 3, k + 4, and k + 5, as 