There are couple options how the Dual Connectivity could be configured in 4G and 5G. First of all Dual Connectivity in 4G shall be mentioned. Two eNodeB connected with X2 interface. Both of those connected to EPC.
|Figure 1 LTE DC|
As for the migration towards 5G several more options are available:
EN-DCThe first option is the eNodeB connected to gNodeB with X2 and both of nodes are connected to EPC via S1 interfaces. However the gNodeB in this option is only using S1-U towards SGW. The eNodeB has the MN role and the gNodeB has the SN role. This option is called EN-DC (E-UTRA-NR Dual Connectivity). This and three other options described below are grouped in MR-DC (Multi-Radio Dual Connectivity).
|Figure 2 EN-DC Overview|
In this option called NE-DC (NR-E-UTRA Dual Connectivity) the gNodeB is MN and the eNodeB is SN. Nodes are connected with Xn.
|Figure 3 NE-DC Overview|
Yet another option one is called NGEN-DC (NG-RAN E-UTRA-NR Dual Connectivity). It is the eNodeB that is MN and the gNodeB acts as SN. Similar as in NE-DC the gNodeB and the eNodeB are connected with Xn interface.
|Figure 4 NGEN-DC Overview|
The last option is kind of pure NR option. It is called NR-DC (NR-NR Dual Connectivity). Two gNodeBs are connected with Xn interface. One gNode is MN and another one is SN. Both are connected to the 5GC via NG connection
|Figure 5 NR-DC Overview|