s terminated at the MN and the MN and the SN are interconnected via Xn-C interface [13]. There are different user-plane connectivity options for the MN and SN involved in MR-DC operation with a certain UE, as shown in Fig. 1.78. The user-plane connectivity depends on the configured bearer type. For MCG bearers, the user-plane connection to the core network entity is terminated at the MN. The SN is not involved in the transport of user-plane data for this type of bearer over the Uu interface (i.e., the radio interface). For MCG split bearers, the user-plane connection to the core network entity is terminated in the MN. 158 Chapter 1 PDCP data is transferred between the MN and the SN via MN-SN user-plane interface. The SN and MN participate in transmitting data of this bearer type over Uu interface. For SCG bearers, the SN is directly connected to the core network entity via a user-plane interface. The MN is not involved in the transport of user-plane data for this type of bearer over Uu interface. For SCG split bearers, the user-plane connection to the core network entity is ter- minated at the SN. The PDCP packets are transferred between the SN and the MN via MN/ SN user-plane interface. The SN and MN transmit data of this bearer type over the Uu inter- face. For MR-DC with EPC (EN-DC), X2-U interface is the user-plane interface between MeNB and SgNB, and S1-U is the user-plane interface between MeNB and SGW. For MR- DC with 5GC (NGEN-DC, NE-DC), Xn-U interface is the user-plane interface between MN and SN, and NG-U is the user-plane interface between MN and UPF [13]. 1.4 LTE-NR Interworking and Deployment Scenarios 1.4.1 RAN-Level and CN-Level Interworking To provide full 5G services to the users, 5G cells will have to be deployed with full cover- age and all UEs will have to be able to connect to 5G network everywhere. However, in the early phases of 5G deployments, the 5G cells will be partially deployed and there will be 5G coverage gaps. Large-scale 5G commercial services and deployments are expected in 2020