5G NR: Troubleshooting Missing SgNB Reconfiguration Complete Message

Introduction

As you know, 5G User Equipment (UE), or your mobile phone, can operate in two primary modes within a 5G New Radio (NR) network: Non-Standalone (NSA) and Standalone (SA). This article focuses on a common problem encountered in NSA mode.

In NSA mode, the UE leverages both an LTE cell (eNB) and an NR cell (gNB), while still relying on a 4G Core Network (CN). We’ll examine an issue that arises when the UE operates in this dual connectivity setup.

Specifically, in NSA mode, the UE uses the LTE cell for initial control plane signaling and access establishment. Subsequently, the eNB and gNB exchange messages to provide service to the UE in Dual Connectivity (DC) mode. The 4G LTE core network, comprising elements like the MME, SGW, and PGW, is also involved.

Issue: gNB Missing SgNB Reconfiguration Complete Message

The core problem: The 5G gNB cell sends an SgNB Addition Request Acknowledge message in response to an SgNB Addition Request received from the LTE eNB.

Following this acknowledgment, the 5G gNB expects to receive an SgNB Reconfiguration Complete message. However, in certain scenarios, this message never arrives.

As a result, a timer, typically named T DCoverall, expires on the gNB side, triggering a release procedure.

Root Causes

So, what causes the gNB to not receive the critical SgNB Reconfiguration Complete message? Here’s a breakdown of potential culprits:

• eNB Transmission Failure: The eNB might simply not be transmitting the “SgNB Reconfiguration Complete” message towards the gNB.

• UE Decoding Failure: The UE might receive the reconfiguration message, but fail to decode it correctly. This prevents the UE from responding with the necessary reconfiguration complete message back to the eNB.

Debugging Steps

To pinpoint the exact cause, start by capturing X2 signaling logs using tools like Wireshark or proprietary software specific to your 5G NR setup.

Analyze the captured log to confirm whether the “SgNB Reconfiguration Complete” message is present. If it’s missing, dive deeper into the possible causes listed above.

References

• 1. ETSI Document: 3GPP TS 36.423 version 15.4.0 Release 15
• 2. www.eventhelix.com/5G/
• 3. For similar 5G NR network access related issues and possible root causes, visit www.techplayon.com