Radio Link Failure in LTE

Triggering points of RLF

• upon indication from RLC that the maximum number of re transmissions has been reached
• upon expiry of Timer T310 (this timer is started when physical layer problems are detected i.e. upon receiving N310 consecutive out-of-sync indications from lower layers)
• upon random access problem indication from MAC while neither T300, T301, T304 nor T311 is running

Actions when RLF is detected

• UE shall initiate RRC Connection Re-establishment procedure. (click here to know about the RRC CONNECTION RE-ESTABLISHMENT procedure in detail)
• if AS security has not been activated then inform upper layers about the release of RRC connection with release cause 'other'

In-sync Indications (N311) received:
1. UE will stop T310

2. UE maintains the RRC connection (no configurations are changed)

N311 is triggered when in-sync indications are received. Once when the condition of N311 is fulfilled, UE won't go for RLF. Upon expiry of this constant, UE will  UE maintains the entire radio resource configuration

Timers and constants used in RLF:

T301	Transmission of RRCConnectionReestabilshmentRequest
T310	Upon detecting physical layer problems i.e. upon receiving N310 consecutive out-of-sync indications from lower layers
T311	Upon initiating the RRC connection re-establishment procedure
N310 (constant)	Maximum number of consecutive "out-of-sync" indications received from lower layers
N311 (constant)	Maximum number of consecutive "in-sync" indications received from lower layers