DC, and NR-DC), the involved core 6 General principles of the Xn interface operation are described in Section 3.3. In the present section we focus on Xn functions defined for MR-DC operation. The X2 interface in general and MR-DC messages and procedures in particular are very similar to those defined for Xn. 4.3 Multi-Radio Dual Connectivity ((p)) ((o)) Figure 4.3.4 Control-plane connectivity for EN-DC (left) and MR-DC with 5GC (right). (Source: Reproduced by permission of © 3GPP). network entity is the AMF, NG-C interface is used between the AMF and the MN, and the MN and the SN are interconnected via Xn-C. The control-plane connectivity for the different MR-DC options is shown in Figure 4.3.4. Both radio nodes have their own RRC entity that can generate RRC messages to be sent to the UE. Each node, that is the MN or the SN, is responsible for the handling of its group of cells (called MCG and SCG, respectively) and then for the generation of the correspond- ing MCG/SCG RRC Reconfiguration messages. RRC messages generated by the SN can be conveyed from the SN to the MN via the X2/Xn interface and then encapsulated in an RRC message generated by the MN that may also carry an MCG reconfiguration decided by the MN. The combined configuration can be jointly processed by the UE and the UE uses a joint success/failure procedure for RRC messages transmitted by the MN and encapsulat- ing an RRC message generated by the SN. Each RRC Reconfiguration message has its own RRC response message even when the RRC message is encapsulated in another RRC mes- sage. The RRC response message for the SN can then be encapsulated in the RRC response message for the MN and then forwarded over X2/Xn to the SN. If the SN is a gNB (i.e. for EN-DC, NGEN-DC, and NR-DC), the UE can also be config- ured to establish an SRB with the SN, called SRB3. SRB3 is different from the other SRBs (SRBO, SRB1, and SRB2) established between the UE and the network, as it is defined exclu- sively for MR-DC options where the SN is a gNB, to enable SN RRC mess