Failure handling
Radio link failure and RRC re-establishment
Out-of-sync handling with N310, N311 and T310, then re-establishment supervised by T311 and T301.
Overview
The UE counts consecutive out-of-sync indications (N310) and starts T310; consecutive in-sync indications (N311) stop it. The RRC re-establishment procedure is in TS 38.331 clause 5.3.7, and radio link failure related actions in clause 5.3.10.
During re-establishment the UE starts T311, performs cell selection, and sends RRCReestablishmentRequest with a reestablishmentCause of reconfigurationFailure, handoverFailure or otherFailure. If the network answers with RRCSetup instead, the UE treats it as a fallback to RRC establishment (TS 38.331 5.3.7.8). T311 expiry (5.3.7.6) and T301 expiry (5.3.7.7) have their own actions.
In NR, a gNB that receives the request from a UE it does not hold can use the XnAP Retrieve UE Context procedure to fetch the context from the old node (TS 38.423 8.2.4).
Call flow
- 1UE
T310 running after N310 out-of-sync indications
- 2UE
Radio link failure declared
Start T311 and select a cell.
- 3UEtoSelected cell
RRCReestablishmentRequest
reestablishmentCause as above. T301 supervises the request.
- 4Selected celltoUE
RRCReestablishment, or RRCSetup
RRCSetup means fallback to RRC establishment.
- 5UEtoSelected cell
RRCReestablishmentComplete
Timers and parameters
| Name | Where | Role |
|---|---|---|
| T310 | UE | Radio link failure detection. NR values run from 0 ms to 6 s. |
| T311 | UE | Supervises cell selection in re-establishment. Values 1 s to 30 s. |
| T301 | UE | Supervises the re-establishment request. Values 100 ms to 2 s. |
| N310 / N311 | UE | Out-of-sync (n1 to n20) and in-sync (n1 to n10) counters. |
Specifications
Changes by release
| Release | Change |
|---|---|
| Rel-16 | DAPS handover: N310 handling for the source SpCell when a DAPS bearer is configured. |
Failure behaviour in the specs
- T304 expiry (reconfiguration with sync failure) leads to re-establishment with cause handoverFailure; a reconfiguration failure leads to reconfigurationFailure (TS 38.331 5.3.7.4).
Related procedures
Connected mode
5G Xn-based handover
Inter-gNB handover prepared over the Xn interface, with an NG path switch toward the core.
Connected mode
LTE handover
LTE connected-mode handover: RRCConnectionReconfiguration with mobilityControlInfo and timer T304.
Connected mode
Conditional handover (CHO)
The network pre-configures candidate target cells with execution conditions and the UE moves when a condition is met.
Optimisation
Mobility robustness optimisation (MRO)
How NG-RAN detects too-late, too-early and wrong-cell handovers from RLF reports and successful handover reports.