T320 RRC Timer in 5G NR

T320 RRC Timer in 5G NR

The T320 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 Handover. Understanding the exact behaviour of this timer is essential for optimizing network performance, mobility, and radio link recovery.

Purpose and Overview

• Purpose: CHO (Conditional Handover) execution timer
• Description: Maximum time to execute conditional handover after condition is met

Timer Specifications

• Typical Value: 1000 ms
• Allowed Values (ms): 100, 200, 300, 400, 600, 1000, 1500, 2000, 4000, 8000, 10000
• Value Range: 100 ms to 10000 ms
• 3GPP Specification Reference: 3GPP TS 38.331 Section 5.3.5.8

Operational Triggers

Started When

CHO execution condition is met

From TS 38.331: Upon reception of t320 or upon cell (re)selection to NR with validity time configured.

Stopped When

CHO successfully executed or cancelled, or T320 expires

From TS 38.331: Upon entering RRC_CONNECTED, RRCRelease, PLMN selection, or cell (re)selection to another RAT.

At Expiry

CHO not executed within timer duration Actions Taken: CHO configuration cancelled, UE continues on serving cell

From TS 38.331: Discard the cell reselection priority information provided by dedicated signalling.

Related Parameters

• Related Timers: T304
• Related Counters: None
• Associated Procedures: Conditional Handover, Event-triggered Mobility, L3 Mobility

Troubleshooting Tips

If you are observing anomalies or frequent expirations related to the T320 timer, consider checking the following:

• Check CHO trigger conditions and thresholds
• Verify target cell preparation
• Analyze measurement report timing
• Review CHO configuration parameters
• Check for race conditions with regular HO
• Verify target cell SSB quality when condition met

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Further Reading

• 5G NR User Equipment Timers & Constants
• Interactive 5G NR RRC Timers Reference Tool