n 18 bits Octet 1 Octet 1 Octet 2 Octet 2 Octet 3 Octet 3 PDCP Frames PDCP Frames Octet N Octet N Figure 8.14. RLC structure of frame - AM mode Data/Control (D/C): this bit indicates whether the frame contains PDCP frames (bit value ONE) or control messages (bit value ZERO). P (Polling): this bit indicates whether the transmission of an acknowledgment report for received frames is required (bit value ONE) or not (bit value ZERO). SI (Segmentation Information): this indicates whether retransmission is performed with segmentation or without: - SI = 00, no segmentation; NG-RAN and 5G-NR - SI = 01, start of segmentation of the last PDCP frame; - SI = 10, end of segmentation of the first PDCP frame; - SI = 11, end of segmentation of the first PDCP frame and start of segmentation of the last PDCP frame. In the case of retransmission of an RLC frame for which the original resource is not available, the originally transmitted PDCP frame must be able to be segmented. Figure 8.15 demonstrates the RLC header in the following two cases with a segment offset, for an SN on 12 bits or 18 bits: - retransmission of a PDCP frame as a segment; - retransmission of three PDCP frames, with the first PDCP frame as a segment. RLC AM Frame with segmentation RLC AM Frame with segmentation SN on 12 bits SN on 18 bits Octet 1 Octet 1 Octet 2 Octet 2 Octet 3 Octet 3 Octet N Octet N PDCP Frame PDCP Frame Figure 8.15. Structure of an RLC frame - AM mode SO (Segment Offset): this field indicates the position of the initial PDCP segment. The unit of SO is in bytes. 8.3.2.3. Control message The control message concerns the RLC frame acknowledgement and the indication of lost frames and frame segments. Figure 8.16 demonstrates the RLC protocol control message either with a sequence numbering of 12 bits (left) or 18 bits (right). 5G-NR Radio Interface - Data Link Layer Octet 1 ACK SN Octet 1 Octet 2 Octet 2 ACK_SN ACK_SN Octet 3 E1RRRRRRR Octet 3 ACK_SN Octet 4 NACK_SN Octet 4 NACK SN Octet 5 Octet 5 NACK SN NACK SN E1E2E3 Octet 6 Octet 6 NACK SN E