as separation of Xn interface radio net- work and transport network functionalities in order to allow future extensions. The Xn inter- face supports intra-NG-RAN mobility and DC procedures. The control-plane protocol stack of Xn interface is also illustrated in Fig. 1.54. The TNL uses stream control transmission protocol (SCTP)57 57 over IP. The application layer signaling protocol is referred to as Xn application protocol (Xn-AP). The SCTP layer provides guaranteed delivery of application layer messages. In the transport IP layer, point-to-point transmission is used to deliver the signaling PDUs. The Xn-C interface supports Xn inter- face management and error handling (the functionality to manage the Xn-C interface), mobility support for UE in CM-CONNECTED (the functionality to manage the UE mobility for connected mode between nodes in the NG-RAN), context transfer from the current serv- ing NG-RAN node to the new serving NG-RAN node, and control of user-plane tunnels between the existing serving NG-RAN node and the new serving NG-RAN node. It further supports DC (the functionality to enable usage of additional resources in a secondary node in the NG-RAN) as well as support of paging (sending of paging messages via the last serv- ing NG-RAN node toward other nodes in the RAN-based notification area to a UE in RRC_INACTIVE state). Stream control transmission protocol is a transport layer protocol for transmitting multiple streams of data at the same time between two end points that have established a connection in a network. It is message- oriented protocol like user data protocol and ensures reliable, in-sequence transport of messages with conges- tion control like transmission control protocol. Stream control transmission protocol supports multihoming and redundant paths to increase resilience and reliability. 5G Network Architecture 119 1.2.1.1.2 F1 Control-Plane/User-Plane Functions and Procedures As we described earlier, the NG-RAN gNB functions may be implemented in one gNB-CU and one or more gNB-DUs. The g