T3411, T3402
Timer T3411 is used during the Attach Procedure and Tracking Area Update (TAU) Procedure to manage retries when the UE encounters issues such as network failures, rejected messages, or failed attempts. T3411 ensures that the UE does not retry excessively and provides a structured mechanism for initiating reattempts after a predefined waiting period.
Similarly, Timer T3402 is used when the retry limit for T3411 has been reached (typically after 5 failed attempts). T3402 introduces a longer backoff period before the UE can retry the Attach or TAU procedure. This timer is particularly important for mitigating signaling congestion on the network by spreading out retry attempts over time. The duration of T3402 is determined by the network and is communicated to the UE via the Attach Reject or TAU Reject messages.
Together, T3411 and T3402 ensure a balanced and efficient retry mechanism:
- T3411 manages immediate, short-delay retries to quickly recover from transient issues.
- T3402 enforces a longer waiting period after multiple failures to avoid excessive signaling and allow the network to stabilize.
These timers work in tandem to optimize retry behavior, protect network resources, and enhance the overall user experience.
Timer T3411 is crucial for:
- Managing retries in a controlled manner to avoid overloading the network with frequent attempts.
- Introducing a delay before retries to allow the network to recover from transient issues.
- Structuring the UE’s behavior to comply with operator policies and regulatory requirements for retry mechanisms.
Timer T3402 is crucial for:
- Introducing a longer backoff period after multiple failed retries to prevent network congestion.
- Spacing out retry attempts to optimize resource usage and reduce signaling load.
- Allowing the network sufficient time to recover from overloads or issues.
- Aligning retry behavior with network operator policies and configurations.
- Enhancing the likelihood of successful retries by ensuring appropriate waiting times.
T3411 and T3402 can be illustrated as below (Refer to UE EPS Timer page for formal description in 3GPP). Based on the specification, the default value of T3411 is 10 seconds and N (number of reties) is 5, but as far as I tested various devices, these values seems to vary depending on device implementation. However, T3402 seems to be quite consistant
(10 min by default). When you are testing this with test equipment, it would be important not to send RRC Connection Release before T3411 expires. If Network (Test requipment) send RRC Connection Release before T3411 expires, the counter value (N) may get to be initialized back to 0 and you may not get the expected result.
NOTE : Check out this tutorial in Amarisoft Tech-Academy on how to test T3402.
Key Scenarios Involving T3411
Attach Procedure:
- Start of T3411:
- The UE starts T3411 when:
- The Attach Procedure fails due to network failure.
- The Attach Request is rejected for reasons not considered critical (e.g., EMM cause values indicating retry is allowed).
- The attach attempt counter is less than 5 (indicating retries are still permissible).
- Once T3411 expires, the UE retries the attach procedure.
- Stopping T3411:
- T3411 is stopped if:
- The attach procedure is retried due to expiration.
- The network successfully completes the attach procedure.
- The attach procedure is replaced by another higher-priority procedure (e.g., for emergency services).
Tracking Area Update (TAU) Procedure:
- Start of T3411:
- The UE starts T3411 when:
- A TAU Request fails due to network failure.
- The TAU attempt counter is less than 5.
- The normal or combined TAU procedure needs to be retried after a delay.
- Expiration of T3411:
- When T3411 expires, the UE triggers a retry of the TAU procedure.
Abnormal Cases:
- Attach Reject:
- For non-critical EMM cause values, the UE starts T3411 to delay the next attach attempt.
- If the attach attempt counter reaches 5, the UE switches to using T3402 for longer backoff before retrying.
- Lower Layer Failure:
- If the UE detects lower layer failures during attach or TAU procedures, T3411 is started to delay retries.
Retries and Limits:
- The UE is allowed up to 5 retries (managed by the attempt counter).
- After 5 failed attempts, T3411 is no longer used, and the UE switches to a longer backoff timer (e.g., T3402) for retries.
Combined Procedures:
- For combined attach or TAU procedures involving EPS services and SMS:
- If EPS attach is successful but SMS is not accepted, the UE starts T3411 to retry the combined procedure after expiration.
Key Scenarios Involving T3402
Starting T3402:
- Attach or TAU Rejection:
- When the UE receives an ATTACH REJECT or TRACKING AREA UPDATE REJECT message with specific causes (e.g., congestion), T3402 is started based on the value provided by the network.
- Tracking Area Update Attempt Limit Reached:
- If the TAU attempt counter reaches 5 (maximum retries), the UE starts T3402 before initiating another attempt.
- Attach Attempt Limit Reached:
- Similarly, if the attach attempt counter reaches 5, T3402 is triggered to delay further attach requests.
Expiry of T3402:
- On expiration, the UE:
- Retries the Attach Procedure or TAU Procedure depending on the scenario.
- Resets the attach or TAU attempt counter, enabling a fresh series of retries.
Network-Indicated Value for T3402:
- The network can set a custom duration for T3402 in ATTACH REJECT or TRACKING AREA UPDATE REJECT messages. If no value is provided, the UE uses a default value.
Behavior During T3402:
- The UE avoids initiating attach or TAU procedures while T3402 is running, except for:
- Emergency bearer services.
- Upper-layer requests for critical data transmission, such as for exceptional events or RLOS.
- The UE may attempt other recovery actions, such as selecting a new PLMN if allowed.
Abnormal Cases:
- Congestion (EMM Cause #22):
- If the UE receives a reject message indicating congestion, T3402 is started immediately to enforce a delay before retrying.
- Extended Wait Time Ignored:
- In cases where "Extended wait time" is ignored by the UE, and retries are allowed, T3402 ensures proper backoff.
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