G-RAN Network - Protocol Architecture The 5G physical layer is described by the NR (New Radio) access technology. 2.1.2. The protocol architecture on the Xn interface An NG-RAN node is either a gNB (i.e. a 5G base station) providing NR control plane services and user plane traffic or an ng-eNB providing LTE services towards Like 4G base stations, an NG-RAN node carries resource management and logic control functions (i.e. Radio Resource Management, radio admission control, etc.) and can communicate directly with another node. NG-RAN nodes are interconnected via the Xn interface in order to: - manage user mobility between NG-RAN nodes; - increase the transmission rate by the dual connectivity mechanism; - manage interference management (ICIC: Inter Cell Interference Coordination); - allow optimization of radio settings (SON: Self Optimization Network). The Xn interface is the reference point between two entities of the NG-RAN node which belong to the same area of tracking. The Xn interface is not mandatory. Signaling is exchanged through the XnAP protocol and user traffic plane through the GTP-U (GPRS Tunnel Protocol User) protocol. The protocol stack of the Xn interface is similar to the protocol stack of the X2 interface. It supports the exchange of traffic and the exchange of signaling between two nodes (Figure 2.4). User Traffic Control message GTP-U GTP-U Figure 2.4. The protocol stack of the Xn interface NG-RAN and 5G-NR The Transport Network Layer (TNL) uses: - GTP-U and UDP (User Datagram Protocol) over IP (Internet Protocol) for data transmission; - the SCTP protocol (Stream Control Transmission Protocol) over IP for the control information. The SCTP protocol provides reliable data transport by retransmitting erroneous data. It detects data loss and data duplication. The tunnel established between two NG-RAN nodes is unidirectional and is temporarily established for the transfer of data during a handover or for the dual connectivity mechanism. An NG-RAN node can be split (Figure 2.5) into an NG-RAN CU (Ce