quired. In other words, when there is no segmentation, the RLC header is minimized. Second, because RLC SDUs are transmitted in sequence and segmentation only occurs when needed, what fits into the payload indicated from lower layers can be described as follows: {0;1} last segment of SDU + [0;n] complete SDUs + {0,1} first segment of SDUitn+1; 1 segment of SDU. 7.5 RLC Real-Time Operations Non-Real-Time Operations Header Header with Offset RLC PDU Payload Segmentation Figure 7.10 Segmentation. That is, the first segment of an SDU always comes last and is always followed by the next segment. Third, segmentation is a real-time operation, which can only be performed once the size of the grant is known. If the offset were added to the first segment, the RLC PDU header size would be fixed only after knowing whether segmentation has occurred. Not having an offset in the first segment makes the RLC PDU header size stable from the viewpoint of real-time operations (see Note 1). When segmentation occurs, only the one-bit flag in the header of the RLC PDU needs to be changed and the PDU cut to produce a first segment. The second segment can then be built with the offset, but in non-real time. This is depicted in Figure 7.10. 7.5.3 Error Correction ARQ in 5G is based on the same principles as in LTE: the retransmissions of RLC PDUs or RLC PDU segments in the transmitter are based on status reports from the receiver; status reports can be polled by the transmitter or triggered by the receiver; reaching the maximum number of retransmissions triggers radio link failure (RLF) (see Section 7.5.5 for one exception). The main difference with LTE lies in the details of the RLC status report. Without con- catenation in RLC, there can be a large number of RLC PDUs sent in a single transport block, and then lost. The status report in 5G was therefore enhanced to handle multiple SNs: a range of lost PDUs can be signaled to keep overhead under control. 7.5.4 Transmissions Modes In TM, an RLC entity does not do anything and remains trans