What is the difference between timers t301 and t311?

Hello Experts,

Both the timers t301 and t311 are related to RRC Connection ReEstablishment procedure. Can anyone tell what is the difference between them?

Hello @rafaeldamasio.

  • Both timers can NOT start at same time. If one of them is running then other timer will be stopped.

  • If UE detects handover failure, RLF, etc… then it needs to perform the RRC Reestablishment. For performing Reestablishment, first UE needs to select suitable cell, so it starts t311 timer.

  • Before expiring t311, if UE finds suitable cell then it proceeds to RRC Reestablishment procedure so it stop t311 and starts t301 timer.

  • If UE gets RRC Reestablishment from eNB then UE stop the t301 time.

  • If any of the time expire before next step as explained above, or UE gets RRC Reestablishment Reject, then UE leaves the RRC Connected mode with Release Cause as RRC Connection Failure.

RRC Connection Reestablishment is a mechanism to avoid UE from moving to Idle State and remaining in RRC Connected State.
Then why are you saying that UE finds a suitable cell first and does cell selection and then send RRC connection reestablishment request?

3GPP TS 36.331 5.3.11 Radio link failure related actions
3GPP TS 36.331 5.3.7 RRC connection re-establishment
3GPP TS 36.213 4.2.1 Radio link monitoring
3GPP TS 36.133 7.6 Radio Link Monitoring

Once things go bad in your phone and it gets out-of-sync (after N310 times), it starts T310 to wait for PHY get back in shape and return to in-sync.

If PHY doesn’t recover and T310 expires, the UE starts T311 in order to reselect into another cell, or even the same cell if in the meantime RF gets back to acceptable ranges. T311 is a supervision timer for the cell selection procedure after a OoS scenario.

If before T311 expires the UE finds a suitable cell, it stops T311, starts T301, send RRCConnectionReestablishmentRequest and wait for the eNodeB response. The T301 is a supervision timer for the RRCConnectionReestablishment procedure.

If T311 expires before the UE finds a suitable cell, or the T301 expires before the RRCConnectionReestablishment, the UE goes to RRC_IDLE.

If success is achieved in the Reestablishment procedure, UE happily goes back to RRC_CONNECTED.