5G NR Architecture Option 4 and 4a Explained: Dual Connectivity with 5GC

The launch of 5G networks follows several architectural paths set by 3GPP, designed to facilitate a smooth transition from LTE (4G). The initial deployments of 5G were based on Option 3 variants, which relied on the LTE EPC, but later stages brought in Option 4 and 4a, where the setup is anchored on the 5G Core (5GC).

This change is a vital move towards full 5G Standalone (SA) networks since Option 4 variants start to weave in 5GC, while still utilizing LTE evolved nodes.

The diagram included shows the NR architecture for Option 4 and 4a, highlighting how the gNB (next-gen 5G base station), ng-eNB (enhanced LTE eNodeB with next-gen interfaces), and 5GC work together for both the control plane and user plane.

In this article, we'll break down the tech flow, compare Option 4 and 4a, and clarify their parts in the 5G migration journey.

What is Option 4 in 5G NSA?

Option 4 is a Non-Standalone (NSA) dual connectivity architecture where:

The gNB (NR base station) serves as the Master Node (MN).

The ng-eNB (LTE base station upgraded for NG interfaces) acts as the Secondary Node (SN).

The core network is the 5G Core (5GC).

Control Plane (NG-C, Xn-C):

The EPC is no longer in use; now, 5GC manages signaling through the gNB (MN).

The gNB handles control-plane signaling for the UE and coordinates the ng-eNB using the Xn-C interface.

User Plane (NG-U, Xn-U):

User data can go directly from 5GC to gNB (NG-U).

If extra LTE connectivity is needed, the gNB forwards traffic to the ng-eNB using Xn-U.

The UE collects data from both nodes.

✅ Key Point: In Option 4, NR (gNB) is the main player, so 5G is the anchor technology, contrasting with Option 3 where LTE was the anchor.

Option 4a Architecture

Option 4a tweaks the user-plane path for better efficiency.

Control Plane: * Same as Option 4: gNB (MN) controls signaling with 5GC through NG-C. * gNB still manages ng-eNB via Xn-C.

User Plane: * Both gNB and ng-eNB connect directly to 5GC through NG-U. * User data paths don’t need to go through the gNB, which cuts down on latency.

✅ Key Point: Option 4a enables direct EPC-to-ng-eNB user-plane connections, speeding up data transfer and allowing gNB to avoid unnecessary traffic forwarding.

Comparing Option 4 and Option 4a

Here’s a comparison:

Feature Option 4Option 4aMaster Node (MN)gNB (5G NR base station)gNB (5G NR base station)Secondary Node (SN)ng-eNB (LTE base station)ng-eNB (LTE base station)Control Plane5GC → gNB (NG-C) → ng-eNB (Xn-C)5GC → gNB (NG-C) → ng-eNB (Xn-C)User Plane5GC → gNB (NG-U) → ng-eNB (Xn-U) → UE5GC → gNB (NG-U) + 5GC → ng-eNB (NG-U)Latency Higher (traffic forwarded via gNB)Lower (direct ng-eNB to 5GC link)Throughput Moderate Higher (parallel connections)Complexity Simpler More complex (multiple NG-U paths)

Why Option 4 Variants Matter

The shift from LTE EPC (Option 3 variants) to anchoring on 5GC (Option 4 variants) is crucial for operators because:

5GC Adoption: Option 4 marks the first time 5GC enters the architecture while still depending on LTE for coverage.

NR as Master Node: This changes the hierarchy from Option 3 — now 5G NR takes the lead, with LTE just providing supplementary coverage.

Performance Improvements: Particularly, Option 4a reduces latency and boosts throughput with direct 5GC-to-ng-eNB user plane paths.

Future-Ready: This setup eases the transition toward Option 2 (Standalone 5G), where only gNB and 5GC are used.

Real-World Adoption and Challenges

Even though Option 4 is quite powerful in theory, it's been less widely adopted compared to Option 3 due to some practical hurdles:

Infrastructure Readiness: Operators needed to deploy 5GC early, which many delayed to cut costs.

Device Compatibility: Early smartphones were made for LTE-anchored NSA (Option 3), so they weren't as ready for NR-anchored NSA (Option 4).

Coverage Gaps: In the early days, NR coverage was patchy, making LTE a more dependable anchor.

Interoperability: There were complex Xn interfaces between gNB and ng-eNB that needed vendor cooperation, which wasn’t fully developed yet.

Operators’ Strategies

Option 3 First: Most global operators (like Verizon, Vodafone, SK Telecom) opted for Option 3 first because it was easier with EPC.

Option 4 Later: Option 4 variants became more appealing for operators aggressively deploying 5GC early on, including those in China and South Korea.

Migration Path: From Option 4 to Standalone (Option 2)

Option 4 is really just a stepping stone:

Step 1: Roll out Option 3 NSA (LTE as anchor, EPC).

Step 2: Bring in 5GC and switch to Option 4 NSA (NR as anchor, 5GC).

Step 3: Gradually transition to Option 2 SA (gNB directly tied to 5GC without LTE).

This approach guarantees:

Backward compatibility for LTE users.

A gradual shift to 5GC without a sudden overhaul.

A smooth path towards true 5G services, like ultra-reliable low-latency communication (URLLC), massive IoT, and network slicing.

Benefits of Option 4 and 4a

Better Alignment with 5G Vision: Anchoring on NR and 5GC brings networks closer to a Standalone structure.

Enhanced User Experience: Especially, Option 4a speeds up latency and throughput.

Future-Proofing: Early adoption of 5GC sets operators up for advanced 5G applications.

Flexible Transition: Dual connectivity lets operators move traffic between LTE and NR smoothly.

Conclusion

The NR architecture of Option 4 and 4a represents a pivotal phase in the deployment of 5G NSA, shifting the anchor point from LTE EPC to NR with 5GC.

Option 4 handles user-plane traffic via the gNB, with ng-eNB linked through the gNB.

Option 4a takes it a step further by allowing direct connections from ng-eNB to 5GC for user-plane traffic.

In a nutshell, Options 4 and 4a act as bridges: connecting LTE to NR while introducing the 5G Core, ensuring that the transition from NSA to SA happens without a hitch.