E-UTRAN EN-DC (New Radio - Dual Connectivity) is a technology that enables 5G services and data speeds to be introduced into a largely 4G network. EN-DC UEs could connect to both the LTE Master Node eNB (MN-eNB) and the 5G-NR Secondary Node gNB at the very same time (SN-gNB). This technique allows cellular providers to launch 5G services without having to invest in a full-scale 5G Core Network. In places with heavy traffic congestion, 5G gNBs can be deployed early. 

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Overall Network Architecture EN-DC:

On the radio side, the UE communicates with both LTE eNB (eNodeB) and NR gNB (gNodeB), but all communications (signaling and data) are routed through the LTE core network.

As you see here, in case of data plane both Master Node (LTE) and Secondary Node (gNB) has direct interface with LTE core network(S-GW), but in case of control plane only Master Node(LTE) has direct interface with LTE core network(MME).

This figure simply shows how 4G / 5G RANs are connected to one another and to the core network (EPC). It is based on the overall notion of Option 3, although it does not depict any specific variant of the option (e.g, plain option 3 , 3a or 3x).

Demonstration

In the case of C-Plane

• The MN (Master Node: MeNB in this csae) and the SN have a connection (Secondary Node : En-gNB in this case). X2-C is the name of this interface.

• MN and CN have a same interface (Core Network : MME in this case). S1-MME is the name of this interface.

• Between SN and CN, there is no direct interaction (connection).

In the case of U-Plane

• The MN (Master Node: MeNB in this csae) and the SN have a connection (Secondary Node : En-gNB in this case). X2-U is the name of this interface.

• MN and CN have a same interface (Core Network : MME in this case). S1-U is the name of this interface.

• Between SN and CN, there is a link (interface). S1-U is the name of this interface.

Ending remarks

The majority of operators opt for non-standalone networking (NSA) mode during the early stages of 5G rollout to save money and provide services rapidly. Because 5G base stations in NSA mode are given precedence to access the 4G core network (EPC) due to the expense of the 5G core network and its maturity, the Option 3x series (Option 3x) is recommended as the initial 5G launch of eMBB services.