T301 RRC Timer in 5G NR

The T301 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 Connection Re-establishment. Understanding the exact behaviour of this timer is essential for optimizing network performance, mobility, and radio link recovery.

Purpose and Overview

Purpose: RRC Connection Re-establishment Request timeout
Description: Time to wait for RRCReestablishment message after sending RRCReestablishmentRequest

Timer Specifications

Typical Value: 1000 ms
Allowed Values (ms): 100, 200, 300, 400, 600, 1000, 1500, 2000
Value Range: 100 ms to 2000 ms
3GPP Specification Reference: 3GPP TS 38.331 Section 5.3.7

Operational Triggers

Started When

UE transmits RRCReestablishmentRequest message

From TS 38.331: Upon transmission of RRCReestabilshmentRequest.

Stopped When

RRCReestablishment or RRCReestablishmentReject is received, or T301 expires

From TS 38.331: Upon reception of RRCReestablishment or RRCSetup message as well as when the selected cell becomes unsuitable.

At Expiry

RRCReestablishment not received within timer duration Actions Taken: UE goes to RRC_IDLE, releases all radio resources, informs upper layers (reestablishment failure)

From TS 38.331: Go to RRC_IDLE.

Related Timers: T300, T310, T311
Related Counters: N311
Associated Procedures: RRC Connection Re-establishment, Radio Link Failure Recovery, Handover Failure Recovery, Beam Failure Recovery

Troubleshooting Tips

If you are observing anomalies or frequent expirations related to the T301 timer, consider checking the following:

Check radio link failure (RLF) causes
Verify cell reselection performance
Analyze handover failure rates
Check for inconsistent RRC configuration
Review UE context availability at gNB
Verify beam failure recovery procedures