Secondary Node

Description

In 3GPP multi-connectivity architectures, notably MR-DC (Multi-RAT Dual Connectivity), the Secondary Node (SN) is a radio access node that supplies additional radio resources to a UE whose connection is anchored and controlled by a Master Node (MN). The MN maintains the control-plane connection to the core network (e.g., via NG or S1 interface), while the SN adds user-plane capacity. Common deployments include EN-DC (E-UTRA-NR Dual Connectivity) where the MN is an LTE eNB and the SN is a 5G NR gNB, and NE-DC where the roles are reversed.

The SN is added, modified, or released via procedures coordinated by the MN. The MN decides to initiate SN addition based on measurement reports from the UE and radio resource management policies. The MN and SN communicate over the Xn (in 5G) or X2 (in LTE) interface to exchange context information, manage bearer (or QoS flow) mapping, and coordinate mobility. User data can be split at the MN (MCG Split Bearer), at the SN (SCG Split Bearer), or use separate bearers (SCG Bearer), depending on the architecture.

Key functions of the SN include transmitting and receiving user data on its allocated cells (the Secondary Cell Group, SCG), performing its own radio resource management for those cells, and executing intra-SN mobility (e.g., handovers within its cells). It receives control commands (for bearer setup, etc.) from the MN via inter-node RRC messages and may also have a direct user-plane connection to the core network (in 5G standalone architectures with SN-terminated bearers). The SN concept is fundamental to leveraging multiple frequency bands or radio access technologies simultaneously.

Purpose & Motivation

The Secondary Node concept was introduced to address the growing demand for higher data rates and more reliable connections beyond what a single radio node could provide. Early LTE networks used carrier aggregation within a single eNB, but dual connectivity (introduced in Release 12) extended this by aggregating resources from two distinct nodes, often on different frequency layers or even different RATs.

It solves the problem of limited peak data rates and coverage holes by allowing a UE to connect to two nodes simultaneously. This is particularly crucial for the phased rollout of 5G NR, where EN-DC allows operators to use existing LTE coverage as an anchor while adding 5G NR capacity. The SN architecture also enhances mobility robustness by providing a more seamless handover process and enables load balancing between network layers. It represents a key evolution towards truly integrated multi-RAT networks.