lane interfaces to the CN (S-GW in the case of EPC and UPF in the case of 5GC). A UE configured with MR-DC has a user-plane connec- tion to the corresponding core network entity, through the MN, the SN, or both. Further- more, there is a network interface (X2 or Xn) between the MN and the SN for transferring data between the two nodes. In particular, in EN-DC, the involved core network entity is the S-GW - S1-U is terminated in the MN and/or the SN, and the MN and the SN are interconnected via X2-U. In the MR-DC options with connection to the 5GC (NGEN-DC, NE-DC, and NR-DC), the involved core network entity is the UPF - NG-U is terminated in the MN and/or the SN, and the MN and the SN are interconnected via Xn-U. All four network interfaces (S1-U, X2-U, NG-U, and Xn-U) involved in MR-DC operation use the same protocol stack, with GTP-U protocol used on top of UDP. Additionally, X2-U and Xn-U also provide flow control functionality and enhancements for lossless packet delivery (in case the transport network is not reliable), similar to the ones provided by the F1-U interface (described in Section 4.2.4). An overview of the user-plane connectivity for the different MR-DC options is shown in Figure 4.3.5. ((q)) ((q)) Figure 4.3.5 User-plane connectivity for EN-DC (left) and MR-DC with 5GC (right). (Source: Reproduced by permission of © 3GPP). 4.3 Multi-Radio Dual Connectivity For further details on network user-plane protocols used on network interfaces S1-U, X2-U, NG-U, and Xn-U refer to 3GPP TS 36.414 for S1-U, 3GPP TS 36.424 and TS 36.425 for X2-U, 3GPP TS 38.414 and TS 38.415 for NG-U, and 3GPP TS 38.424 and TS 38.425 for Xn-U. 4.3.4.2.1 Bearer Types The user-plane connection to the core network is performed via the MN or the SN depend- ing on the bearers that are configured for a UE. In fact, a UE can be configured with multiple bearer types in MR-DC. From a network perspective, a UE can be configured with MN terminated bearers, where the user-plane connection to the core network entity is terminated in the MN,