T312 RRC Timer in 5G NR

The T312 is a critical Radio Resource Control (RRC) timer in 5G New Radio (NR) networks, defined in 3GPP TS 38.331. It falls under the category of Radio Link Failure. Understanding the exact behaviour of this timer is essential for optimizing network performance, mobility, and radio link recovery.

Purpose and Overview

Purpose: SCG (Secondary Cell Group) failure timer
Description: Time for MCG to continue data transmission while SCG is being reconfigured after failure

Timer Specifications

Typical Value: 1000 ms
Allowed Values (ms): 0, 50, 100, 200, 500, 1000, 2000, 4000, 8000, 10000
Value Range: 0 ms to 10000 ms
3GPP Specification Reference: 3GPP TS 38.331 Section 5.3.10.4

Operational Triggers

Started When

SCG failure detected in EN-DC or NR-DC scenarios

From TS 38.331: Upon triggering a measurement report for a measurement identity for which T312 is configured, while T310 is running.

Stopped When

SCG successfully reconfigured or added, or T312 expires

From TS 38.331: Upon receiving N311 consecutive in-sync indications, reconfigurationWithSync, MobilityFromNRCommand, or T310 expiry.

At Expiry

SCG not recovered within timer duration Actions Taken: UE releases SCG configuration, continues on MCG only

From TS 38.331: Initiate connection re-establishment or failure information procedure.

Related Timers: T310, T313
Related Counters: None
Associated Procedures: EN-DC (E-UTRA-NR Dual Connectivity), NR-DC (NR Dual Connectivity), SCG Failure Recovery, Split Bearer Management

Troubleshooting Tips

If you are observing anomalies or frequent expirations related to the T312 timer, consider checking the following:

Check SCG radio link quality
Verify EN-DC/NR-DC configuration
Analyze X2/Xn interface performance
Review SCG addition/modification success rate
Check for SCG beam failure
Verify split bearer configuration
Analyze MCG and SCG load distribution