as an SN. Finally, in NR-NR dual connectivity (NR-DC), a UE is connected to one gNB that acts an MN and another gNB that acts as an SN (see Figure 4.3.3). The master gNB is connected to the 5GC via the NG interface and to the secondary gNB via the Xn interface. The sec- ondary gNB might also be connected to the 5GC via the NG-U interface. NR-DC can also be used to provide a UE with NR access via two separate gNB-DUs, serving different cells, connected to the same gNB-CU, acting both as an MN and as an SN. 4.3.3 Market Drivers The full benefits of 5G can only be exploited by a 5G "Standalone" architecture, where gNBs providing NR access are directly connected to the 5G core network. This architecture is the 4.3 Multi-Radio Dual Connectivity AMF/UPF AMF/UPF ((p)) ((a)) ((p)) gNB-CU ((q)) gNB-DU gNB-DU Figure 4.3.3 NR-DC inter-gNB (left) and intra-gNB (right) architectures. (Source: Reproduced by permission of © 3GPP). final destination of the technological migration in many operators' networks, but it is also the 5G architecture used from day one in several deployments in different parts of the world. Whenever the Standalone architecture is not adopted from the very beginning, EN-DC is useful to realize early 5G deployments because it allows the benefits of NR technology to be experienced, with its increased data rates and the possibility to exploit new frequency bands, while fully reusing the existing 4G infrastructure. In fact, EN-DC is the architectural option that allows the deployment of NR, that is a 5G radio access technology, without the need to deploy a 5G core network at the same time, but simply reusing the EPC, that is the 4G core network. It also does not require ubiquitous NR coverage from the very beginning, as it is always possible to fall back to the connectivity provided by the LTE network and maintain service without interruptions due to NR coverage holes. Thanks to this attribute of allowing NR access while relying on an existing LTE infrastructure to be able to connect to the operator's network