1 Purpose

This document addresses the following questions regarding 5G Non-Standalone (NSA) and Standalone (SA) network architecture:

• How are Option 4 series architectures classified within the NSA/SA framework, and why is this a common source of confusion?
• Why is Option 3x the preferred choice among the Option 3 series architectures?
• What are the key advantages of SA networking over NSA networking?
• Why do operators and vendors recommend a phased NSA-to-SA migration strategy rather than direct SA deployment?

2 Overview

To accelerate the provisioning of 5G network services, 3GPP specifications have defined two 5G networking modes: Standalone (SA) and Non-Standalone (NSA). NSA specifications were frozen approximately six months prior to SA.

Figure 2-1 Evolution in 3GPP specifications

During the early stage of 5G deployment, 5G base stations — referred to as gNodeBs (gNBs) as defined in 3GPP specifications — can be connected to a 4G core network, referred to as the Evolved Packet Core (EPC). As 5G matures, the EPC connected to 5G base stations evolves into a 5G Core network (5GC), also referred to as the Next Generation Core (NGC) in certain protocol contexts. Ultimately, 5G base stations connect directly to the 5GC, enabling support for the full range of 5G use cases.

During network evolution, 4G base stations, referred to as eNodeBs (eNBs) can be upgraded to evolved LTE (eLTE) base stations, known as eLTE eNBs, enabling connectivity to the 5GC, as illustrated in Figure 2-2. This upgrade path extends the operational lifespan of existing eNodeB infrastructure and safeguards prior network investments.

Figure 2-2 5G network evolution

3 How to Distinguish Between NSA Networking and SA Networking

Figure 3-1 shows the mainstream 5G network architectures defined in 3GPP specifications, categorized as either NSA (Non-Standalone) or SA (Standalone) networking. The distinction is based on the number of radio access technologies (RATs) involved: architectures using two RATs are classified as NSA, while those using a single RAT are classified as SA.

NSA architectures differ in their choice of signaling anchor (control plane anchor):

• LTE-anchored NSA: The gNB relies on an eNB or ng-eNB as the signaling anchor to provide 5G NR services. Option 3 series (Option 3/3x/3a) and Option 7 series (Option 7/7x/7a) fall into this category, with Option 3 connecting to EPC and Option 7 connecting to 5GC.
• NR-anchored NSA: The ng-eNB relies on a gNB as the signaling anchor to provide LTE services. Option 4 series (Option 4/4a) belongs to this category.

Option 2 represents the SA NR architecture, where the gNB connects solely to the 5G Core (5GC) with no LTE anchor dependency.

Figure 3-1 Mainstream 5G network architectures

For an architecture with 'x', such as Option 3x, different base stations serve as the signaling anchor and the user plane (data) split point. For an architecture with 'a', such as Option 3a, the secondary node connects directly to the core network for its user plane traffic, bypassing the master node for data.

4 NSA Networking

NSA networking includes Option 3 series, Option 7 series, and Option 4 series architectures defined in 3GPP specifications. They apply to different stages of 5G network development.

4.1 Option 3 Series Architectures with NSA NR

At the early stage of 5G network deployment, the primary objective is to deliver 5G services by leveraging existing 4G infrastructure in a fast and cost-effective manner. To address this requirement, the 3GPP specifications define the Option 3 series architectures. In these architectures, the 4G Evolved Packet Core (EPC) is retained as the core network, while 5G base stations (gNBs) are connected to the existing 4G LTE radio access network. This approach enables operators to deploy 5G capabilities at reduced costs and with accelerated timelines. The Option 3 series comprises three variants ,Option 3, Option 3x, and Option 3a — each differing in the choice of network element (NE) that serves as the data split anchor point, as illustrated in Figure 4-1.

Figure 4-1 Option 3 series architectures

Option 3

In Option 3 (EN-DC), the eNodeB serves as the Master Node (MN), anchoring the control plane for 5G UEs operating in dual connectivity. In the base Option 3 configuration, all user-plane data from the EPC is routed through the eNodeB, which may then forward a portion to the gNodeB (Secondary Node) via the X2 interface for transmission to the UE. However, the Option 3 family includes sub-variants: in Option 3A, the EPC can send certain bearers directly to the gNodeB, bypassing the eNodeB on the user plane; in Option 3X, the gNodeB acts as the PDCP anchor for split bearers, receiving data directly from the EPC and optionally forwarding some traffic to the eNodeB., as shown in Figure 4-2.