e; - the user traffic transmitted in the DRB (Data Radio Bearer). Signaling data (RRC messages) is transmitted to the data link layer. The data link layer is divided into three sublayers: - PDCP protocol (Packet Data Convergence Protocol); - RLC protocol (Radio Link Control); - MAC (Medium Access Control) protocol. User traffic (IP packets) is transmitted to the data link layer. The data link layer is divided into four sublayers (Figure 2.2): - SDAP protocol (Service Data Application Protocol); - PDCP protocol; - RLC protocol; - MAC protocol. NG-RAN Network - Protocol Architecture IP Packet SDAP SDU PDCP SDU RLC SDU RLC SDU MAC SDU MAC SDU Transport Block (PHY SDU) Figure 2.2. Processing of IP packet in the DataLink layer The SDAP protocol provides a mapping between the quality of service (QoS) associated with the IP flows and the radio media transporting this flow between the mobile and the NG-RAN node. The PDCP sublayer performs IP header compression and decompression through the use of the RoHC (Robust Header Compression) mechanism. The PDCP sublayer implements the security functions by encryption/decryption, as well as protection of the integrity of the data received (IP packets and signaling). Finally, the PDCP sublayer manages the duplication of the transmitted PDCP frames, the reorganization of the frames (in particular for services of the URLLC - Ultra- Reliable Low-Latency Communication types), the reorganization of the packets and the detection of duplicated frames. In the case of the 5G-NSA option 3X (dual connectivity), the PDCP sublayer of the 5G base station separates the incoming traffic bearer from the core network to the RLC sublayers of the two base stations (split-bearer between the MN: Master Node eNB and SN: Secondary Node en-gNB). The RLC sublayer provides data segmentation services for the PDCP sublayer. There is one RLC channel for each radio bearer configured for the mobile. The RLC sublayer provides three modes of data transmission: acknowledged mode (AM), transparent mode (TM) and unack