he 5GC. The NG-RAN comprises a radio network layer (RNL) and a TNL. The NG-RAN architecture, that is, the NG-RAN logical nodes and the corresponding interfaces, is defined as part of the RNL, whereas the NG-RAN interfaces (NG, Xn, F1) are specified as part of TNL protocols and functionalities. The TNL provides services for user-plane and signaling transport. The protocols over Uu (i.e., the radio air-interface) and NG interfaces are divided into two classes: user-plane protocols, which are the protocols implementing the actual PDU carrying user data through the AS; and control-plane protocols, which are the proto- cols for controlling the PDU sessions and the connection between the UE and the network from different aspects including requesting the service, controlling different transmission resources, handover, etc. as well as a mechanism for transparent transfer of NAS messages 114 Chapter 1 Central control-plane entity Central user-plane entity -Xn-C Xn-U- CU-CP CU-UP Distributed entity Figure 1.52 Disaggregated gNB model [15]. via encapsulation in RRC messages. The NG interface, comprising a user-plane interface (NG-U) and a control-plane interface (NG-C), connects the 5GC and the NG-RAN. In this architecture, the NG-RAN termination is an NG-RAN node which can be either an ng-eNB or a gNB and the 5GC termination is either the control-plane AMF logical node or the user- plane UPF logical node. There may be multiple NG-C logical interfaces toward 5GC from any NG-RAN nodes which are selected by the NAS node selection function. Likewise, there may be multiple NG-U logical interfaces toward 5GC from any NG-RAN node which can be selected within the 5GC and signaled to the NG-RAN node by the AMF [15,17] (see Fig. 1.52). The NG interface supports procedures to establish, maintain, and release NG-RAN part of PDU sessions; to perform intra-RAT handover and inter-RAT handover; the separation of each UE on the protocol level for user-specific signaling management; the transfer of NAS signaling messages between UE and AMF;