 different bearer types (MN and SN terminated, MGG/SCG/split bearer), QoS aspects, and bearer type selection. 4.3.2 MR-DC Options Several different MR-DC variants have been specified by 3GPP. The first option, providing connectivity to the EPC, is EN-DC, in which a UE is connected to one eNB (providing E-UTRA access) that acts as an MN and one gNB (providing NR access) that acts as an SN. In this architecture, the gNB may also be referred to as en-gNB, to discriminate this from a gNB that connects to the 5GC. Similarly, in some architectures, when an eNB is connected to a 5GC it is referred to as ng-eNB. That is, the prefix "en-" indicates connectivity to EPC and the usage of X2 and S1 network interfaces, whereas the prefix "ng-" indicates connec- tivity to 5GC and the usage of Xn and NG network interfaces. From a 3GPP specifications point of view, strictly speaking EN-DC is not an NG-RAN architecture, as it uses "old" net- work interfaces (X2 and S1) and EPC. Nevertheless, it is described in the book as this is an important 5G deployment option. Figure 4.3.1 illustrates the EN-DC architecture, in which the eNB is connected to the EPC via the S1 interface and to the gNB via the X2 interface. Optionally, the gNB might also be connected to the EPC via the S1-U interface. All the other MR-DC options provide connectivity to the 5GC. In NG-RAN, E-UTRA-NR dual connectivity (NGEN-DC) illustrated in Figure 4.3.2, a UE is connected to one ng-eNB that acts as an MN (connecting the UE to the 5GC) and one gNB that acts as an SN connected to the ng-eNB via the Xn interface. In NR-E-UTRA dual 4 NG-RAN Architecture MME/S-GW Figure 4.3.1 EN-DC architecture. (Source: Reproduced by permission of © 3GPP). ((q)) ((p)) AMF/UPF AMF/UPF ((q)) ((p)) ((q)) ((Q)) ng-eNB ng-eNB Figure 4.3.2 NGEN-DC (left) and NE-DC (right) architectures. (Source: Reproduced by permission of © 3GPP). connectivity (NE-DC) also shown in Figure 4.3.2, a UE is connected to one gNB that acts as an MN (again connecting the UE to the 5GC) and one ng-eNB that acts 