Non-Standalone

Description

The Non-Standalone (NSA) mode of operation, formally specified in 3GPP Release 15, is a network architecture where the new 5G New Radio (NR) radio access is deployed alongside and is tightly integrated with an existing 4G LTE network. In NSA, the device (UE) maintains a dual-connection: it remains anchored to a 4G LTE cell (acting as the Master Node or MeNB) for all control plane signaling (such as connection establishment, mobility management, and paging), while simultaneously connecting to a 5G NR cell (acting as a Secondary Node or SgNB) to provide additional user plane bandwidth. This architecture is most commonly associated with Option 3 (specifically variants 3, 3a, and 3x), where the LTE eNB is connected to the 4G Evolved Packet Core (EPC), and the 5G gNB provides a secondary data path also anchored at the EPC.

From a technical perspective, NSA operation is enabled by the Dual Connectivity (DC) framework originally developed in LTE-Advanced. The UE establishes a primary connection with the LTE eNB via the LTE-Uu interface. When conditions allow and the network decides to add 5G capacity, the LTE eNB, acting as the master, coordinates with the 5G gNB to establish a secondary cell group for the UE. This involves signaling over the X2 interface (specifically the X2-C for control and X2-U for user plane in Option 3 variants). The user plane data flow can be split at different points: in Option 3, all data goes via the LTE eNB; in Option 3a, some data bearers are routed directly from the EPC to the gNB; and in Option 3x, the LTE eNB handles the signaling anchor and some data, while the gNB can handle the majority of the user plane traffic, with data splitting occurring at the eNB. The core network remains the EPC, meaning the UE uses 4G NAS protocols to communicate with the MME, and services like IMS voice continue to rely on VoLTE.

The role of NSA was pivotal as a transition technology. It allowed Mobile Network Operators (MNOs) to introduce 5G NR radios in selected high-traffic areas to boost data rates and capacity without the immediate need to invest in and deploy a completely new 5G Core (5GC) network. This significantly accelerated the time-to-market for 5G services. For the UE, it simplified early 5G device design, as the complex 5G core network signaling (5G NAS over N1) was not required. However, NSA does not enable the full suite of 5G capabilities, such as network slicing based on a Service-Based Architecture (SBA), ultra-reliable low-latency communication (URLLC) with edge computing, or advanced session management features inherent to the 5GC. These are enabled by the Standalone (SA) mode, where both the radio (NR) and the core (5GC) are new.

Purpose & Motivation

The creation of the NSA mode was driven by a clear market and technical imperative: to enable a faster and more cost-effective introduction of 5G services. In the late 2010s, operators had heavily invested in ubiquitous and stable 4G LTE networks. Deploying a completely new 5G system with a new radio and a new core network (SA) represented a massive, simultaneous capital expenditure and operational challenge. NSA provided a pragmatic stepping stone. It solved the problem of how to deliver the most immediately marketable 5G benefit—dramatically higher data speeds—by simply adding 5G NR carriers to the existing network fabric.

This approach addressed several key limitations. First, it leveraged the mature coverage and reliability of the LTE network for control functions, ensuring service continuity and mobility support from day one. Second, it allowed for a phased investment, where the expensive new core network could be deployed later, once its more advanced features were needed and the technology was more mature. Third, it created a market for 5G devices and services earlier, fostering ecosystem development. Without NSA, the commercial launch of 5G would have been delayed by several years. Thus, NSA's purpose was fundamentally about migration and risk reduction, providing a clear path from 4G to the full 5G vision of SA, while delivering tangible user benefits in the interim.