 and SN termi- nated bearers, where the user-plane connection to the core network entity is terminated in the SN. Regardless of the network termination point, bearers can be further categorized into MCG bearers, SCG bearers, or split bearers, depending on whether the transport of user-plane data over the Uu interface involves MCG, SCG radio resources, or both. For MCG bearers, only MCG radio resources are used; for SCG bearers, only SCG radio resources are used; while for split bearers, both MCG and SCG radio resources are used. For split bearers, MN terminated SCG bearers, and SN terminated MCG bearers, user-plane data are trans- ferred between the MN and the SN via the X2-U/Xn-U interface. A summary of all the possible MR-DC bearer types, from a network perspective, for the EN-DC case is shown in Figures 4.3.6 and 4.3.7. It is important to note that even if only SCG bearers are configured for a UE (i.e. no MCG resources are used to exchange user-plane data with the network), SRB1 and SRB2 always use at least MCG resources. In other words this is still an MR-DC configuration. Also, if only MCG bearers are configured for a UE (i.e. no SCG resources are configured), this is still considered as MR-DC as long as at least one of the bearers is terminated in the SN. In terms of protocol stack, at the network side, for each radio bearer the PDCP entity (for EN-DC) or the SDAP/PDCP entities (for NGEN-DC, NE-DC, and NR-DC) is always hosted by the node that terminates the radio bearer: in the MN for MN terminated bearers and in the SN for SN terminated bearers. Furthermore, a radio bearer may have an RLC and MAC ((p)) ((a)) ((a)) X2-U- X2-U- Figure 4.3.6 MN terminated bearers: MCG bearer (left), SCG bearer (center), split bearer (right). 4 NG-RAN Architecture ((q)) ((@)) ((q)) ((a)) ((q)) ((p)) Figure 4.3.7 SN terminated bearers: SCG bearer (left), MCG bearer (center), split bearer (right). logical channel configuration (i.e. an RLC bearer) in one cell group (for MCG bearers and SCG bearers) or two cell groups (for split 