EN-DC (E-UTRA NR Dual Connectivity) connects the UE simultaneously to an LTE eNB (master node) and a 5G NR gNB (secondary node). The LTE eNB maintains the control plane connection to the EPC (MME, SGW). The gNB provides additional user plane capacity via a direct data path (Option 3x) or through the eNB (Option 3/3a). The UE establishes an LTE connection first, then the eNB configures the NR secondary cell group (SCG). Data is split at the PDCP layer: some bearers go through LTE, others through NR, or a single bearer is split across both (split bearer). Peak throughput is the sum of LTE + NR.

What are the limitations of NSA vs. SA?

NSA cannot support: (1) Network slicing (requires 5GC). (2) URLLC with guaranteed latency (control plane goes through LTE/EPC, adding latency). (3) NR-only operation (LTE coverage is always required as anchor). (4) Voice over NR (VoNR, requires 5GC IMS). (5) Full NR mobility (handover requires LTE anchor handover first). NSA adds UE complexity (simultaneous LTE + NR operation, higher power consumption). However, NSA enables operators to offer 5G data speeds immediately while building out the 5G Core infrastructure for eventual SA migration.

What Option 3 variants exist?

Option 3: User plane from gNB routes through eNB to EPC. Simple but bottlenecks at eNB. Option 3a: User plane from gNB goes directly to EPC (SGW), bypassing eNB. Requires gNB-EPC interface (S1-U). Better throughput. Option 3x: Split bearer where PDCP is at gNB, and the eNB provides only SCG (secondary cell group) configuration. Best throughput as NR handles its own user plane. Most operators deploy Option 3x for best performance. All three options keep the control plane anchored at the LTE eNB via S1-MME.

mmWave & 5G

5G NSA (Non-Standalone)

A 5G NR deployment architecture (3GPP Option 3) where the NR radio operates as a secondary node under LTE control. The LTE eNB maintains the control plane connection to the existing EPC, while the 5G gNB provides additional user plane capacity via EN-DC (E-UTRA NR Dual Connectivity). NSA enables operators to launch 5G data services immediately by leveraging existing LTE infrastructure, without deploying a new 5G Core network.

Category: mmWave & 5G

3GPP: Option 3/3a/3x

Dual Connectivity: EN-DC

Understanding 5G NSA

When 5G NR was standardized, operators faced a dilemma: the new 5G Core (5GC) required significant investment and was not yet mature. NSA solved this by allowing NR to operate under the control of the existing LTE network. The UE connects to LTE first (anchor), then the network adds an NR carrier for data boost. Peak throughput is LTE + NR combined.

The UE must support both LTE and NR simultaneously, receiving control signaling from LTE and user data from both. This increases UE complexity and power consumption compared to SA mode. Most early 5G deployments worldwide (2019-2022) used NSA, with operators gradually migrating to 5G SA as 5GC infrastructure matures.

NSA Architecture (Option 3x)

Control plane:
UE ↔ eNB ↔ MME (EPC) via S1-MME
gNB configured by eNB (X2 interface)

User plane:
UE ↔ gNB ↔ SGW/PGW (EPC) via S1-U (direct path)
UE ↔ eNB ↔ SGW/PGW (LTE data path)
Split bearer: PDCP at gNB, data split across both

Throughput:
Peak = LTE CA + NR CA
Example: LTE 4CA (400 Mbps) + NR 100 MHz (1.5 Gbps) = ~1.9 Gbps

Most operators deploy Option 3x for maximum NR throughput.

NSA vs. SA Comparison

Feature	NSA (Option 3)	SA (Option 2)
Core network	LTE EPC	5G Core (5GC)
LTE required?	Yes (anchor)	No
Network slicing	No	Yes
URLLC	Limited	Full support
VoNR	No (VoLTE fallback)	Yes
UE power	Higher (dual active)	Lower (NR only)
Deployment	2019-present	2021-present

Common Questions

Frequently Asked Questions

How does EN-DC work?

UE connects to LTE eNB (master) and NR gNB (secondary) simultaneously. eNB maintains control plane to EPC. Data split at PDCP layer: some bearers on LTE, others on NR, or split across both. Peak = LTE + NR combined throughput.

NSA limitations vs. SA?

No network slicing, no URLLC guarantees, no VoNR, requires LTE coverage everywhere, higher UE power consumption. NSA was the fast path to market; SA is the target architecture for full 5G capabilities.

Option 3 variants?

Option 3: NR data routes through eNB. Option 3a: NR data goes direct to EPC. Option 3x: split bearer with PDCP at gNB (best throughput). Most operators use Option 3x.

Related Terms

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