MCG, SCG, and split bearers from a UE Flows perspective in MR-DC with 5GC (NGEN-DC, NE-DC, and NR-DC). (Source: Reproduced by permission of © 3GPP). Split Bearer Bearer Bearer NR PDCP NR PDCP NR PDCP MN RLC MN RLC SN RLC SN RLC MN MAC SN MAC 4 NG-RAN Architecture 4.3.4.2.2 QoS Aspects and Bearer Type Selection EN-DC The QoS aspects for EN-DC follow the QoS framework that applies in E-UTRAN and defined in 3GPP TS 36.300. The MN decides whether a given bearer should be termi- nated at the MN or at the SN. For an MN terminated bearer, the corresponding S1-U bearer is established between the EPC and the MN. For an SN terminated bearer, the corresponding S1-U bearer is established between the EPC and the SN. Besides deciding which PDCP entity (in the MN or in the SN) terminates each radio bearer, the MN also decides in which cell group(s) radio resources are to be configured, that is whether a bearer is an MCG, an SCG or a split bearer. However, once an SN terminated split bearer is established, through a Secondary Node Addition procedure (as described in Section 4.3.4.3.1) or a Secondary Node Modification procedure (as described in Section 4.3.4.3.2), the SN may later on remove SCG resources, as long as QoS for the respective E-UTRAN Radio Access Bearer (E-RAB) is guaranteed. In EN-DC, all the possible bearer type change options are supported: MCG bearer to/from split bearer, MCG bearer to/from SCG bearer, and SCG bearer to/from split bearer. Also bearer termination point change (MN terminated bearer to/from SN terminated bearer) is supported for all bearer types, and can be performed with or without a simultaneous bearer type change. For E-RABs for which a bearer termination point changes from/to MN termi- nated bearer to/from SN terminated bearer, user data forwarding may be performed. In this case, the behavior of data forwarding follows that of handover, where the node from which data are forwarded behaves as a "source eNB" and the node to which data are forwarded behaves as a "target eNB" during handover. MR-DC 