e SDU, or a segment between the first and last segments of the SDU. Furthermore, in the case of a segmented SDU, a 16 bit segmentation offset (SO) is included in all segments except the first one to indicate which byte of the SDU is represented by the segment. The RLC header may further include a poll (P) bit, which is used to request status New Radio Access Layer 2/3 Aspects and System Operation 219 Octet 1 Octet 1 Octet 2 Octet N Octet 3 TMD PDU Octet 4 Octet 5 Octet 1 Octet 2 Octet N Octet 3 UMD PDU with 12-bit SN and so Octet 4 Octet 5 ACK_SN Octet 1 ACK_SN Octet 2 Octet N E1RRRRRRR Octet 3 AMD PDU with 12-bit SN and so NACK_SN Octet 4 NACK_SN Octet 5 Octet 1 NACK_SN Octet 6 Octet 2 NACK_SN Octet 7 Octet 3 SOstart Octet 8 Octet 4 SOstart Octet 9 Octet 5 SOend Octet 10 Octet 6 SOend Octet 11 NACK range Octet 12 Octet N NACK_SN Octet 13 AMD PDU with 18-bit SN with so NACK_SN Octet 14 STATUS RLC PDU with 12-bit SN Figure 2.12 Example RLC PDU formats 10]. report for RLC-AM, and a data/control (D/C) indicator, indicating whether the RLC PDU contains data to/from a logical channel or control information required for RLC operation [10,17]. The retransmission of missing RLC PDUs is one of the main services of the RLC-AM entity. While MAC sublayer HARQ protocol provides some level of error correction, when combined with the ARQ retransmission mechanism, they can provide a higher level of reli- ability. The missing RLC PDUs can be detected by inspecting the sequence numbers of the received PDUs, and a retransmission requested from the transmitting side. The RLC-AM mode in NR is similar to LTE except the in-sequence delivery service is not supported in NR. As we mentioned earlier, eliminating the in-sequence delivery from the RLC helped reduce the overall latency. It further reduces the buffering requirements at the RLC sub- layer. In LTE, which supports the in-sequence delivery service at RLC sublayer, an RLC SDU cannot be forwarded to higher layers unless all previous SDUs have been correctly received. A single missin