ntralized Unit) and one or more NG-RAN DU (Distributed Unit). The Xn-C interface is the logical interface between the gNB entity and the gNB-CU. The E1 interface is the logical interface between two gNB-CUs. gNB-CU gNB-CU Higher Layer gNB-DU gNB-DU Split (HLS) gNB-DU gNB-DU Lower Layer NG L1 Split (LLS) Figure 2.5. Interfaces between gNB entities and gNB-CU/gNB-DUs The F1 interface is the logical interface between the gNB-CU and the gNB-DU. It supports signaling via F1-C functions and traffic via F1-U functions. The F2 interface is the logical interface between the gNB-DU and the RRH (Remote Radio Head). The data is transported by a serial bus respecting the eCPRI (evolved Common Public Radio Interface) protocol on the optical fiber, also called RoF (Radio Over Fiber). NG-RAN Network - Protocol Architecture 2.1.2.1. Control plane functions The XnAP control plane on the Xn-C interface supports: - functions used for management of signaling associations between NG-RAN nodes, surveying the Xn interface and recovering from errors (i.e. reset function after abnormal failure); - functions to allow initial setup, modification or release of the Xn interface between two NG-RAN nodes; - functions to allow two NG-RAN nodes to update application level data at any time, including activation or deactivation of the interface within the framework of an energy management procedure; - function to allow exchange of information between the source and target NG-RAN nodes to initiate a handover or function informing an existing target NG-RAN node that a handover will ultimately not take place; - function to exchange the context of a mobile between two NG-RAN nodes; - function to manage paging procedure when the mobile is in the RRC INACTIVE state; - functions to establish or release a tunnel and data transfer between two NG-RAN nodes. 2.1.2.2. User plane functions User plane functions (UPF) on the Xn-U interface support: - data transfer in the case of dual connectivity or the handover mechanism; - user plane flow control allowing an NG