acity; however, the technology is designed in such a way that other deployments are also possible. One specific flavor of MR-DC, referred to as E-UTRA-NR dual connectivity (EN-DC), is particularly important as it is expected to be used in the first phase of 5G deployments. In EN-DC, a 5G "capacity layer" is added to an existing 4G network in the form of 5G SN nodes, while using the 4G Evolved Packet Core (EPC) network, as opposed to 5GC. This allows 5G deployment with relatively low capital expenditure (CAPEX) (as opposed to upgrading the whole network) and provides an evolution path toward "full" 5G deployment. 4.3 Multi-Radio Dual Connectivity 4.3.1 Key Ideas From the UE point of view, MR-DC allows simultaneous connection to two different radio access nodes, one providing NR access and the other one providing either LTE or NR access. From the network point of view, one network node (typically deployed for cover- age) serves as the MN and the other network node (typically deployed for capacity) serves as the SN. Different MR-DC network architecture options are supported in 5G (EN-DC, NGEN-DC, NE-DC, and NR-DC), which differ in connectivity to the core network (EPC or 5GC) and in which nodes serve as MN or SN. One particular MR-DC flavor (EN-DC) is especially important for early 5G deployment, as it allows gradual 5G roll out while still using the legacy EPC CN. EN-DC has been selected as the first 5G deployment option by many operators. EN-DC uses the legacy network interfaces X2 and S1, while MR-DC connected to 5GC relies on new interfaces called Xn and NG corresponding to X2 and S1, respectively. In order to support spectrum sharing between LTE and NR, network interfaces used for MR-DC allow for resource coordination between MN and SN network nodes. Control-plane MR-DC aspects covered in this section include: dual RRC architecture, split SRB, SRB3, UE capability coordination, radio resource coordination, measurement framework, and security. User-plane aspects of MR-DC operation covered in this section include: