twork (N2 handover). From a system point of view, the functions of the NG-C interface allow us to: - update the AMF configuration and NG-RAN radio nodes to ensure the interoperability of procedures on the NG-C interface; - report errors which are not defined in specific messages; - manage a 5G core network congestion state: the AMF function indicates to the NG-RAN the start or end of the core network congestion state; - ensure bi-directional transfer of radio access configuration information to the AMF function (e.g. for a SON functionality); - broadcast ETWS (Earthquake and Tsunami Warning System) type safety messages; - re-initialize the radio access node. NG-RAN Network - Protocol Architecture 2.1.4.2. NG-U interface The NG-U user plane interface is the interface between the network node and the The NG-U interface provides unreliable PDU data transfer services between the radio access node and the UPF. 2.2. Procedures on the radio network access 2.2.1. XnAP procedures XnAP procedures consist of elementary procedures (EP) which can be of two types: - class 1: request with response (failure or success, see Table 2.1); - class 2: request without response (Table 2.2). Elementary procedure - class 1 Request Handover Preparation HANDOVER REQUEST Retrieve UE Context RETRIEVE UE CONTEXT REQUEST S-NG-RAN node Addition Preparation S-NODE ADDITION REQUEST M-NG-RAN node initiated S-NG-RAN node S-NODE MODIFICATION REQUEST Modification Preparation S-NG-RAN node initiated S-NG-RAN node S-NODE MODIFICATION REQUIRED Modification S-NG-RAN node initiated S-NG-RAN node S-NODE CHANGE REQUIRED CHANGE M-NG-RAN node initiated S-NG-RAN node S-NODE RELEASE REQUEST Release S-NG-RAN node initiated S-NG-RAN node S-NODE RELEASE REQUIRED Release Xn Setup XN SETUP REQUEST NG-RAN node Configuration Update NG-RAN NODE CONFIGURATION UPDATE Cell Activation CELL ACTIVATION REQUEST Reset RESET REQUEST Xn Removal Xn REMOVAL REQUEST Table 2.1. XnAP elementary procedures - Class 1 NG-RAN and 5G-NR Elementary procedure - class 2 Handover Cancel SN Sta