y RLC control Delivery in sequence Adding RLC header Routing DTCH, DCCH Figure 8.12. AM mode operation NG-RAN and 5G-NR 8.3.2. Protocol structure The RLC protocol defines a header for the UM mode, a header for the AM mode and a control message. 8.3.2.1. UM mode The header structure depends on the size of the Sequence Number (SN) field and the number of concatenated PDCP frames. Figure 8.13 demonstrates the RLC header for the following three cases: - a 6-bit SN field or 12-bit SN field, without offset; - a 6-bit SN field or 12-bit SN field, with offset; - no header added when frames are transmitted to MAC without segmentation. RLC UM Frames no segmentation Octet 1 SI RRRRRR Octet 2 PDCP Frame Octet N RLC UM Frames RLC UM Frames SN on 6 bits SN on 12 bits Octet 1 Octet 1 Octet 2 Octet 2 Octet 3 PDCP Frame Octet 3 PDCP Frame Octet N Octet N RLC UM Frames RLC UM Frames SN on 6 bits with offset SN on 12 bits with offset Octet 1 Octet 1 Octet 2 Octet 2 Octet 3 Octet 3 PDCP Frame Octet N PDCP Frame Octet N Figure 8.13. RLC frame structure - - UM mode SI (Segmentation Information): this field is coded on 2 bits. It indicates whether segmentation is implemented or not: - SI = 00, no segmentation; 5G-NR Radio Interface - Data Link Layer - 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. SO (Segment Offset): this field indicates the position of the RCL SDU segment. The unit of SO is in bytes. SN: this field is coded on 6 or 12 bits. It contains the RLC frame sequence numbers. The numbering of RLC frames is used to re-sequence the frames received from the MAC sublayer. R: reserved bit. 8.3.2.2. AM mode The structure of the RLC header consists of a header and PDCP data. The SN field, in terms of size, is 12 bits or 18 bits. Figure 8.14 demonstrates the RLC header for the two lengths of the SN without segment offset. RLC AM Frames RLC AM Frames SN on 12 bits SN o