T300 RRC Timer in 5G NR
T300 RRC Timer in 5G NR
The T300 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 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 Request timeout
- Description: Time to wait for RRCSetup message after sending RRCSetupRequest
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 7.3
Operational Triggers
Started When
UE transmits RRCSetupRequest message
From TS 38.331: Upon transmission of RRCSetupRequest.
Stopped When
RRCSetup or RRCReject is received, or T300 expires
From TS 38.331: Upon reception of RRCSetup or RRCReject message, cell re-selection and upon abortion of connection establishment by upper layers.
At Expiry
RRCSetup not received within timer duration Actions Taken: UE returns to RRC_IDLE, informs upper layers (connection failure), releases MAC/L1 configuration
From TS 38.331: Perform the actions as specified in 5.3.3.7.
Related Parameters
- Related Timers: T301, T319
- Related Counters: N300
- Associated Procedures: RRC Connection Establishment, Cell Selection, Random Access, Initial Access
Troubleshooting Tips
If you are observing anomalies or frequent expirations related to the T300 timer, consider checking the following:
- Check PRACH configuration and success rate
- Verify gNB MSG2 (RAR) transmission
- Analyze MSG4 (RRCSetup) delivery
- Check for high interference or poor coverage
- Review RRC connection reject causes
- Verify beam management in initial access
Further Reading
WirelessBrew Team
Technical expert at WirelessBrew, specializing in 5G NR, LTE, and wireless system optimization. Committed to providing accurate, 3GPP-compliant engineering tools.
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