(MCG, SCG, split bearer) can end on the MN and/or on the SN. The split bearer is transparent for the core network entities. Several deployment scenarios (Figure 1.2) have been defined for the 5G-NSA: - option 3: the E-UTRAN access network is connected to the 4G core network. The master node is the 4G radio node (eNB - evolved Node B). The secondary node is the 5G radio node (en-gNB). The MR-DC architecture is called EN-DC (E-UTRAN NR Dual Connectivity); - option 4: the NG-RAN access network is connected to the 5G core network. The master node is a 5G radio node (gNB - next generation Node Base Station). The secondary node is a 4G radio node (ng-eNB). The MR-DC architecture is called NE-DC (NR - E-UTRAN Dual Connectivity); - option 7: the NG-RAN access network is connected to the 5G core network. The master node is a 4G radio node (ng-eNB - next generation eNB). The secondary node is a 5G radio node. The MR-DC architecture is called NGEN-DC (NG-RAN E-UTRAN NR Dual Connectivity). NSA Configuration Option 3 Option 4 Option 7 Control plane Master Node Secondary Node User plan Figure 1.2. NSA configuration options NG-RAN and 5G-NR 1.1.1. Option 3 Option 3 is the non-standalone EN-DC configuration. Option 3 uses the MN (Master Node) terminated MCG (Master Cell Group) bearer for signaling: the eNB is the master node, and the gNB (gNodeB) acts as the secondary node. The radio access network is connected to EPC. The 4G base station (eNB) controls the 5G base station (en-gNB) through the X2 interface. The eNB supports signaling with the MME (Mobile Management Entity) through the S1-MME interface and supports the user plane traffic (MCG bearer) with the SGW (Serving Gateway) entity through the S1-U interface. The en-gNB base station supports signaling with the eNB. The 5G-NR interface is activated by the eNB over the X2 interface. Once activated, en-gNB controls its own radio resource allocation. The user traffic is either transmitted from the eNB to en-gNB or transmitted from the 4G core network (SGW) over the S1-U interfa