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5G NSA SgNB Addition: Key Timers Impacting the Success Rate

Mohamed ELAdawi

07 Sep 2026 — 2 min read

During SgNB Addition, there are three important timers that can directly affect the SgNB Addition Success Rate. The figure below shows where each timer starts and stops within the call flow.

After the B1 Measurement Report is received, the MeNB starts the SgNB Addition procedure:

MeNB → SgNB: SgNB Addition Request
SgNB → MeNB: SgNB Addition Request Acknowledgement
MeNB → UE: RRC Reconfiguration
UE → MeNB: RRC Reconfiguration Complete
MeNB → SgNB: SgNB Reconfiguration Complete
The UE can then proceed with the Random Access procedure toward the SgNB.

The three timers supervise different parts of this procedure:

A timer configured too short may expire before the expected response is received and therefore increase SgNB Addition failures. Increasing the timer may help in some cases, but unnecessarily large values can also increase the time required before declaring a real failure.

Important Timer Relationship

The most important configuration relationship is:

TDCoverall > RRC Reconfiguration Guard Timer

This is required because TDCoverall covers the complete execution period that includes the RRC Reconfiguration procedure. If TDCoverall is shorter, it may expire while the MeNB is still waiting for the UE to complete the RRC Reconfiguration.

So, when troubleshooting a degraded SgNB Addition Success Rate, it is worth auditing not only each timer value individually, but also the relationship between these timers.

Note: Timer naming is not fully standardized across vendors, so the equivalent parameter names may be different in each network implementation.

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YouTube Video for the same:

Reference

3GPP TS 36.423 — X2 Application Protocol (X2AP)

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SgNB Addition Call Flow and Related Timers