tive state; 2. UE data forwarding for lossless mobility; 3. Resource coordination, including coordination between NR and LTE; 4. Network energy saving, i.e. cell activation and deactivation; 5. Dual and multiconnectivity (described in Section 4.3). 3.3.4.1 Xn Control Plane (Xn-C) Interface The Xn-C protocol stack (3GPP TS 38.423) is similar to that of NG-C (3GPP TS 38.413). Therefore, in the present section we focus on the functions of the Xn Application Protocol (Xn-AP), used on the Xn-C interface. Xn-C primary functions are: Xn interface management UE mobility in connected and inactive states Dual and multiconnectivity. 3.3 NG Radio Access Network As the last one is described in Section 4.3, here we focus on the two first points. Similar to the NG interface, Xn also supports multiple SCTP connection functionality to facilitate virtualized deployments. In the present chapter we describe only the subset of the most commonly used proce- dures. For detailed description of all the procedures, refer to 3GPP TS 38.423. 3.3.4.1.1 Interface Management The purpose and the general structure of the Xn interface management procedures are the same as those of the NG. The procedures are: Xn Setup Request/Response/Failure NG-RAN Node Configuration Update/Update Acknowledge/Update Failure Cell Activation Request/Response/Failure Reset Request/Response and Error Indication Xn Removal Request/Response/Failure. One point worth mentioning regarding the interface management (and some other) Xn pro- cedures is that because the interface supports two radio accesses (NR and LTE), depending on whether the interface is between two gNBs or between a gNB and ng-eNB, they support carrying the information about either NR or E-UTRA. The Xn Setup Request/Response messages are used to establish the interface and to exchange the information about the served NR and E-UTRA cells, such as Physical Cell ID (PCI), Tracking Area Code (TAC), and frequency information. The information about served cells exchanged during the setup can be updated using th