DUs and SDUs are bit strings that are octet-aligned. The TM data (TMD PDU) consists only of a data field and does not include any RLC headers. In RLC-AM and RLC- UM modes, a sequence number is generated and attached to the incoming SDUs using 6 or 12 bits for the RLC-UM and 12 or 18 bits for the RLC-AM. The sequence number is included in the RLC PDU header as shown in Fig. 2.12. If the SDU is not segmented, the RLC PDU consists of the RLC SDU and a header, which allows the RLC PDUs to be gen- erated in advance as the header does not depend on the transport block size. However, depending on the transport block size after multiplexing at MAC sublayer, the size of the last RLC PDU in a transport block may not match the RLC SDU size, thus requiring divid- ing the SDU into multiple segments. If no segmentation is done, padding need to be used which would adversely impact the spectral efficiency. As a result, dynamically varying the number of RLC PDUs in a transport block along with segmentation to adjust the size of the last RLC PDU, ensures that the transport block is efficiently utilized. Segmentation is done by dividing the last preprocessed RLC SDU into two segments, the header of the first seg- ment is updated, and a new header is added to the second segment as shown in Fig. 2.3. Each RLC SDU segment carries the same sequence number as the original SDU and the 218 Chapter 2 RLC-AM channel RLC-AM channel [RLC-UM channel] [RLC-UM channel] Generate RLC header and store in RLC control SDU reassembly transmission buffer Remove RLC header Segmentation Retransmission modify RLC header buffer Reception buffer Add RLC header Routing RLC-AM: DTCH/DCCH RLC-AM: DTCH/DCCH RLC-UM: DTCH RLC-UM: DTCH RLC-AM mode only Figure 2.11 RLC-AM and RLC-UM entity models [10]. sequence number is part of the RLC header. To distinguish an unsegmented RLC PDU from a segmented one, a segmentation information field is included in the RLC header, indicating whether the PDU is a complete SDU, the first segment of the SDU, the last seg- ment of th