shown in Fig. 2.13. At time t3, PDUs with SN < k + 5 have been transmitted, but only PDU k + 5 has arrived and PDUs k + 3 and k + 4 are missing. Similar to the previous case, this causes the t-Reassembly timer to start. However, in this example no PDUs arrive prior to the expiration of the timer. The expiration of the timer at time t4 triggers the receiver to send a control PDU containing a status report indicating the missing PDUs to its peer entity. The control PDUs have higher priority than data PDUs to avoid delayed transmission of the status reports, which would adversely impact the retransmission delay. Upon receipt of the status report at time t5, the transmitter is informed that PDUs up to k + 2 have been correctly 222 Chapter 2 received; thus the transmission window is advanced. The missing PDUs k + 3 and k + 4 are retransmitted and are later correctly received. At time t6, all PDUs, including the retransmis- sions, have been transmitted and successfully received. Since k + 5 was the last PDU in the transmission buffer the transmitter requests a status report from the receiver by setting a flag in the header of the last RLC data PDU. Upon reception of the PDU with the flag set, the receiver will respond by transmitting the requested status report, acknowledging all PDUs with SN k + 5. The reception of the status report allows the transmitter to declare all PDUs as correctly received and to advance the transmission window. The status reports can be triggered for many reasons. However, to avoid transmission of excessive number of status reports, a t-StatusProhibit timer is used where status reports cannot be transmitted more than once within a time interval determined by the timer. In the above example, we assumed that each PDU carries an unsegmented SDU. Segmented SDUs are handled in the same way, but an SDU cannot be delivered to the higher layers until all segments have been received. The status reports and retransmissions operate on individual segments, and only the missing segments of a PDU are retran