g SDU can significantly delay the delivery of the subsequent 220 Chapter 2 SDUs. The RLC-AM is bidirectional which means the data may flow in both directions between the two peer entities (see Fig. 2.11), enabling the acknowledgement of the received RLC PDUs to the transmitting entity. The information about missing PDUs is provided by the receiving entity to the transmitting entity in the form of status reports. The status reports can either be transmitted autonomously by the receiver or requested by the transmitter. The PDUs in transit are tracked by the sequence number in the header. The transmitting and receiving RLC entities maintain two windows in RLC-AM, that is, the transmission and reception windows. The PDUs in the transmission window are only eligible for transmis- sion, thus PDUs with sequence numbers less than the start of the window have already been acknowledged by the receiving RLC entity. In the same way, the receiving entity only accepts PDUs with sequence numbers within the reception window. The receiver discards any duplicate PDUs and delivers only one copy of each SDU to higher layers. The concept of RLC retransmissions is exemplified in Fig. 2.13, where transmitting and receiving RLC entities are illustrated. When operating in RLC-AM mode, each RLC entity has transmitting and receiving functionality; nevertheless, in this example we only show one direction as the other direction is identical [17]. In this example, the PDUs numbered k to k + 4 are awaiting transmission in the transmis- sion buffer. At time to, it is assumed that the PDUs with sequence number SN < have been transmitted and correctly received; however, only PDUs with sequence number SN < k - 1 have been acknowledged by the receiver. The transmission window starts at k, that is, the first unacknowledged PDU, while the reception window starts at k + 1, that is, the next PDU expected to be received. Upon reception of kth PDU, the SDU is reassembled and delivered to the higher layers. For a PDU containing an unsegmented SDU the reas