Now it wants to communicate with the eNB for any purpose (maybe data, voice call, tracking area update).

For the same it has to initiate RRC connection request message.

The purpose of this procedure is to re-establish the RRC connection to continue RRC connection, which involves the resumptions of SRB1 operation and the re-activation of security.

In 5G NSA EN-DC mode, RLF is declared separately for the MCG (E-UTRA LTE Cell) and for the SCG cells (NR).

If radio link failure is detected for MCG (LTE eNB), the UE initiates the RRC connection re-establishment procedure, but when RLF is detected for SCG (NR cell) failure, the UE suspends SCG transmissions for all radio bearers and reports the SCG Failure Information to the eNB, instead of triggering re-establishment.

UE start the T310, upon receiving N310 consecutive out-of-sync indications from lower layer and stop this Upon receiving N311 consecutive in-sync from lower layer before expiry of this timer.

In this case if there is some uplink traffic then UE will start RACH procedure and during this time if RACH gets failed on NR cell then UE will trigger the SCG failure.

This is similar to LTE case, the only difference is in case of LTE (MN node) re-establishment would be triggered, here in case of NR (SN node), SCG failure triggered.

If reconfiguration received over SRB1 is invalid then UE trigger the re-establishment.

Here SRB1 associated with Master Node (EUTRA) whereas SRB3 is associated with Secondary node (NR).

If same condition occur for the reconfiguration message received over SRB1 then UE trigger the re-establishment here.

Following figure shows the key RLF configuration parameters, which includes the timers and counters.

In addition, the failureDedectionResources parameter also includes a purpose parameter, which can indicate: RLF, Beam Failure or both.

There can be multiple instances of the failureDedectionResources parameter.

The measurements of the RS (Reference Signal) BLER can be configured for the SSB-RS or CSI-RS.

Here, the measurement evaluation time will also vary depending on whether DRX is configured or not, as well as the Frequency Range (FR1 or FR2).

Assuming the UE receives n311 In-Sync indication prior to the timer expiring, the t310 timer will stop.

Lte Rach Radio Link Timeout