erface protocol stacks for the control and user planes are shown in Figure 5.9. The control plane part is based on the use of the Stream Control Transmis- sion Protocol (SCTP) on top of an IP connection. While the user plane part transports the user data, the key functions of the control plane part of the NG-interface are as follows: NG interface management UE context and mobility management Transport of NAS messages Paging PDU session management 5.4.2 Xn-Interface The Xn-interface connects gNBs to another gNB (or to eNB). The term Xn-interface is always used when operating together with the NG-core (5G-core). When the architec- ture option 3 is used with the 4G EPC core, then the interface is called the X2-interface. The Xn-interface protocol stack is shown in Figure 5.10. As with the X2-interface in LTE networks, the Xn-interface is defined to enable operation as a multivendor inter- face. Figure 5.9 shows the Xn-protocol split. From the dimensioning point of view, the Xn-interface differs from the LTE-based X2 interface as the user data is routed via Xn/X2 more continuously than in LTE. In LTE, it was used only for temporary packet forward- ing. Dimensioning is addressed in Chapter 9. For the user plane side, the key functionality is data forwarding and flow control, while on the control plane side of the Xn-interface key functionalities are as follows: Xn-interface management UE mobility management, including context transfer and RAN paging Dual connectivity 5 5G Architecture GTP-U GTP-U Xn Control Plane Xn User Plane Figure 5.10 Xn control and user plane protocols. Figure 5.11 E1-interface protocol stack. 5.4.3 E1-Interface The E1 interface connects the user plane and control plane parts of the CU functionality. The E1 interface was added in the June 2018 version of the specifications, together with the standalone 5G option and the 5G core. The E1 interface protocol stack is shown in Figure 5.11. 5.4.4 F1-Interface The F1-interface connects the CU and DU. In Release 15, the supported functional split, as pre