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5G NR: SgNB Addition Request Rejection - Causes and Troubleshooting

5g nr sgnb addition network troubleshooting lte enb dual connectivity

This page discusses a common issue in 5G NR networks: the 5G NR cell rejecting the SgNB Addition Request message, along with its root causes.

Introduction

As we know, 5G User Equipment (UE), or mobile phones, operate in two primary modes within a 5G New Radio (NR) network: Non-Standalone (NSA) and Standalone (SA).

In NSA mode, the UE leverages the LTE cell (eNB), the NR cell (gNB), and the 4G Core Network (CN). This article focuses on an issue encountered with UEs in NSA mode.

In NSA mode, the UE uses the LTE cell for control and initial handshake. To establish access, both the eNB and gNB then exchange messages to provide service to the UE in Dual Connectivity mode. The 4G LTE core network, consisting of MME, SGW, and PGW, is also utilized.

Message flow between UE eNB gNB in dual connectivity mode

Figure: Message flow between UE eNB gNB in dual connectivity mode

The figure above depicts the typical message flow between the eNB and gNB during a successful SgNB addition.

Issue: 5G NR Cell Rejects SgNB Addition Request

After the LTE eNB cell receives a “Measurement Report” from the UE over the air interface, it initiates an “SgNB Addition Request” message over the X2 interface towards the 5G NR gNB cell. Ideally, upon receiving the “SgNB Addition Request,” the 5G NR gNB cell should respond with an “SgNB Addition Request Acknowledge” message to the 4G eNB cell, assuming everything is in order. However, in this scenario, the gNB sends an “SgNB addition request reject” message instead.

5G NR SgNB addition reject

Figure: 5G NR SgNB addition reject

Root Cause for SgNB Addition Request Reject Message

The “SgNB addition reject” message is sent by the gNB along with a cause value. This cause value needs to be analyzed to determine the root cause of the rejection. The following table outlines common cause values and their corresponding reasons for the reject message.

Cause ValueDescription
Transport resource unavailable1. The 5G NR cell does not have an X2-U link established between the eNB and gNB. OR 2. The 5G NR cell does not have an S1-U link established between the gNB and the 4G LTE Core.
No Radio Resources are available in target cellThe target cell (i.e., the gNB cell) does not have sufficient radio resources available to accommodate the new connection.

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