bearer bearer NR PDCP NR PDCP NR PDCP NR PDCP LTE RLC LTE RLC LTE RLC NR RLC NR RLC NR RLC LTE MAC NR MAC Figure 1.76 Radio protocol stack for MCG, MCG split, SCG, and SCG split bearers in MR-DC with EPC (EN-DC) [13]. MCG split bearer bearer split bearer bearer NR PDCP NR PDCP NR PDCP NR PDCP MN RLC MN RLC MN RLC SN RLC SN RLC SN RLC MN MAC SN MAC Figure 1.77 Radio protocol stack for MGC, MCG Split, SCG, and SCG split bearers in MR-DC with 5GC (NGEN-DC, NE-DC) [13]. As we mentioned earlier, there are four bearer types identified as MCG bearer, MCG split bearer, SCG bearer, and SCG split bearer in MR-DC scenarios. These four bearer types are depicted in Fig. 1.76 for MR-DC with EPC (EN-DC) and in Fig. 1.77 for MR-DC with 5GC (NGEN-DC, NE-DC). For EN-DC, the network can configure either LTE PDCP or NR PDCP for MCG bearers while NR PDCP is always used for SCG bearers. For split bearers, NR PDCP is always used and from the UE perspective there is no difference between MCG and SCG split bearers. In MR-DC with 5GC, NR PDCP is always used for all bearer types. From system architecture point of view, in MR-DC, there is an interface between the MN and the SN entities to facilitate control-plane signaling and coordination. For each MR-DC 5G Network Architecture S1-MME ((H)I) ((III) )) (III)) Figure 1.78 Control-plane and user-plane architecture for EN-DC and MR-DC with 5GC [13]. capable UE, there is also one control-plane connection between the MN and a correspond- ing core network entity. The MN and the SN entities involved in MR-DC operation for a UE control their own radio resources and are primarily responsible for allocating radio resources of their cells. Fig. 1.78 shows control-plane connectivity of an MN and SN involved in MR-DC with a UE. In MR-DC with EPC (EN-DC) scenario, the involved core network entity is the MME. S1-MME is terminated in MeNB and the MeNB and the SgNB are interconnected via X2-C. In MR-DC with 5GC (NGEN-DC, NE-DC) scenario, the ter- minating core network entity is the AMF. The NG-C interface i