5G Standalone architecture and it can be com- pared with the use of the NR CA feature. CA already allows the combination of different NR carriers (also belonging to different bands) to increase the maximum data rate for UEs supporting such features. However, CA requires that the different carriers are tightly synchronized and controlled by the same MAC entity. In practice this means that, on the network side, the different carriers need to be controlled by the same gNB, or possibly by different access nodes connected with an ideal backhaul (e.g. in a proprietary manner, as opposed to using a standardized Xn network interface). Also, in cases where a split architecture is used, with a single gNB-CU connected to multiple gNB-DUs (e.g. each one operating in a different frequency band) inter-band CA is not possible. NR-DC can then be used to combine the radio resources controlled by different gNBs connected via a non-ideal backhaul with standardized Xn network interface (inter-gNB NR-DC) or by different gNB-DUs connected to the same gNB-CU via a F1 network interface (intra-gNB NR-DC). 4.3.4 Functional Description 4.3.4.1 Control Plane From the UE point of view, each UE configured with MR-DC has one single control-plane connection to the corresponding core network entity (Mobility Management Element [MME] or AMF), through the MN. From the network point of view, there is an interface (X2 or Xn, depending on the MR-DC option) between the MN and the SN for coordination messages between them and for exchanging RRC control messages intended for the UE. The interface is used for example for SN node addition, modification, and release messages used to establish an MR-DC operation for a UE, to modify it, or to release it, respectively. The details of these message exchanges are described in more detail below.6 In particular, in EN-DC, the MN and the SN are interconnected via X2-C, and the involved core network entity is the MME, to which MN is connected via S1-MME. In the MR-DC options with connection to the 5GC (NGEN-DC, NE-