performed for the transparent mode. Memory BCCH, PCCH, CCCH Figure 8.10. TM mode operations 8.3.1.2. UM mode At transmission, the PDCP frames are stored in a memory. When transmitting to the MAC layer, the PDCP frames may be segmented, and are then encapsulated by an RLC header. At reception, the data received from the MAC layer are stored in a memory. The received data are ordered, following desequencing which can be introduced by the Hybrid Automatic Repeat reQuest (HARQ) mechanism, and the RLC header is then removed. The last operation is to reassemble the segments to reconstruct the original PDCP frame. Figure 8.11 describes the operations for the unacknowledged mode. Memory Reassembly Segmentation Removing RLC header Memory Adding RLC header Delivery in sequence DTCH, MCCH, MTCH Figure 8.11. UM mode operations 5G-NR Radio Interface - Data Link Layer 8.3.1.3. AM mode At transmission, the PDCP frames are stored in a memory. When transmitting to the MAC layer, the PDCP frames may be segmented, and are then encapsulated by an RLC header. The RLC frames transmitted are stored in a retransmission memory. The data received from the MAC layer are stored in a memory and are placed in sequence, and the RLC header is then removed. The last operation is to reassemble the segments to reconstruct the original PDCP frame. A request may be sent from the source to the destination to receive the status report of an RLC frame reception. An RLC control message is sent from the destination to the source for the positive or negative acknowledgment of RLC frames. If the RLC frame is not received correctly, the source will perform a retransmission. If the Modulation and Coding Scheme (MCS) of the physical layer has been modified, it is possible that the size of the RLC frame can be reduced for the retransmission, and in this case, it is necessary to perform a re-segmentation. Figure 8.12 demonstrates the operations for the acknowledged mode (AM). Memory Reassembly Memory Segmentation Removing RLC header Retransmission Control Memor