ssion streams and the radio bearers through the SDAP layer. The gNB-CU provides a radio QoS profile to the gNB-DU (DRB QoS Profile). The Carrier Aggregation (CA) packet duplication function is handled by the gNB-CU. The gNB-CU controls the establishment of two GTP-U tunnels with the gNB-DU and instructs the gNB-DU to reconfigure the special secondary radio cell (SPcell). 2.1.3.2. User plane functions F1-U User plane functions F1-U support: - transfer of user data; - flow control. This function controls the transmission of user data in the donwlink direction to the gNB-DU; - inter-gNB-DU mobility when the user moves between two gNB-DUs controlled by the same gNB-CU; - intra-gNB-DU inter-cell user mobility when the user moves between two cells of the same gNB-DU. The gNB-CU sends the UE context modification procedure to the mobile; - NE-DC mobility: the user moves from an ng-eNB-DU to a gNB-DU controlled by the same gNB-CU. 2.1.4. Protocol stack on the NG interface The N1 interface is the interface between the AMF function and the mobile. It supports signaling carried by the NAS (Non-Access Stratum) protocol. The N2 interface is the interface between the NG-RAN radio node and the AMF function. NAS messages, exchanged between the mobile and the AMF function, are transported by RRC messages on the Uu interface and NG-AP (Next Generation Application Protocol) messages on the N2 interface. NG-RAN Network - Protocol Architecture N1-NAS N1-NAS N1-NAS N1-NAS HTTP2 HTTP2 NG-AP NG-AP Figure 2.10. Protocol stack of the control plane between the mobile and SMF function The NG-RAN access node establishes an association with the AMF function. Several associations are created in order to connect the access node to several AMF functions. The functions of the N2 interface are established by NG-AP messages and are divided into two groups: - generic functions intended to manage the association between the NG-RAN access node and the AMF function. These functions are independent of the mobiles; - the functions associated with the mobi