value ONE) or not (bit value ZERO). The most significant bit of the first byte represents the sequence number following the value of the FMS field. 8.3. RLC protocol The Radio Link Control (RLC) protocol provides control of the radio link between the mobile and the gNB entity. The RLC entity is configured by the RRC layer. Each PDCP entity is associated with one, two or four RLC entities depending on the characteristics of the radio support: uplink, downlink and split bearer. The mobile can simultaneously activate multiple RLC instances, each instance corresponding to a Packet Data Convergence Protocol (PDCP) instance. The RLC protocol operates in three operation modes: - Acknowledged Mode (AM); 5G-NR Radio Interface - Data Link Layer - Unacknowledged Mode (UM); - Transparent Mode (TM), for which no header is added to the data. The RLC protocol does the following: - retransmission in the case of error via the Automatic Repeat reQuest (ARQ) mechanism, for the acknowledged mode only; - segmentation and reassembly of PDCP frames in both the acknowledged and unacknowledged modes; - possible re-segmentation of PDCP frames, in acknowledgment mode, during a retransmission of the RLC frame; - re-sequencing of received data, in both the acknowledged and unacknowledged modes; - detection of duplicate data in both the acknowledged and unacknowledged modes. Figure 8.9 presents the operating modes of the RLC protocol used by the various flow types. Unicast Unicast Unicast System Common Dedicated Multicast Information Paging Control Control Control Packet Packet SRB1,2 Packet Downlink direction only Downlink direction only Figure 8.9. Operating mode of the RLC protocol. MBMS: Multicast Broadcast 8.3.1. Operations 8.3.1.1. TM mode At transmission, the Radio Resource Control (RRC) messages are stored in a memory, before being transmitted to the Medium Access Control (MAC) layer. 220 NG-RAN and 5G-NR At reception, the data received from the MAC layer are transmitted directly to the RRC layer. Figure 8.10 describes the operations 