RRC Connection Re-establishment in LTE

Query_1: Why RRCConnectionReestablishment is needed?
  • This procedure is needed in order to re-establish RRC Connection. Also, it helps to re-establish the SRB1 operation and re-activate the Security algorithms (the security algorithms doesn't change)
  • This procedure is successful only when eNBl has a valid UE context. If eNB doesn't have the UE context then UE moves to RRC_IDLE state

Query_2: When RRCConnectionReestablishment is triggered?
1. This procedure is triggered when any of the following conditions are met:
  • upon detecting radio link failure (To know more about RLF, click here)
  • upon handover failure
  • upon mobility from E-UTRA failure (To know more about Mobility from EUTRA failure, click here)
  • upon integrity check failure indication from lower layers
  • upon an RRC connection reconfiguration failure (To know more about Reconf failure, click here)
2. Also, an important point is that this procedure is triggered only when AS security has been activated, otherwise, UE doesn't initiate this procedure and moves to RRC_IDLE state

Query_3: What happens when RRCConnectionReestablishment procedure is triggered?
Once, UE enters any of the state as mentioned in "Query_2", it will initiate the RRCConnectionReestablishment procedure. This procedure will perform the following steps:
  • stop timer T310, if running (This timer is started when the UE detects consecutive out-of-sync indications from lower layers -related to N310 constant-). This will be a case of RADIO LINK FAILURE
  • start timer T311 (This timer is started when UE initiates the RRC connection re-establishment procedure)
  • suspend all RBs except SRB0 (SRB0 is needed in order to transmit the RRC connection re-establishment message; and this procedure is needed to re-establish SRB1 operatation)
  • reset MAC (removed the already applied MAC configurations)
  • apply the default physical channel configuration
  • apply the default semi-persistent scheduling configuration
  • apply the default MAC main configuration
  • release reportProximityConfig and clear any associated proximity status reporting timer
  • perform cell selection in accordance with the cell selection process

Query_4. What does UE includes in RRCConnectionReestablishmentRequest message?
  • include the ue-Identity "c-RNTI" as used in the Scell
  • include the PHY Cell ID of the source cell 
  • include the ue-Identity "c-RNTI" as used in the Scell
  • include the shortMAC-I for Security algorithms
  • include the reason of the reestablishmentCause as 'reconfigurationFailure' OR 'handoverFailure' OR  'otherFailure'
Important TIP:
Now, UE RRC constructs this message and passes it to lower layers for transmission. Here, Random Access procedure will be triggered to send the RRCConnectionReestablishmentRequest message. Note that the contention resolution of the RA procedure will be PDCCH order based on C-RNTI. 

Query_5. What happens when RRCConnectionReestablishment is received by the UE?
  • stop timer T301 (This timer is started when UE sends the RRCConnectionReestabilshmentRequest)
  • re-establish PDCP for SRB1
  • re-establish RLC for SRB1
  • perform the radio resource configuration procedure in accordance with the received radioResourceConfigDedicated 
  • resume SRB1
  • generate security keys as specified by nextHopChainingCount IE
  • submit the RRCConnectionReestablishmentComplete message to lower layers for transmission
Important Points:
  • eNB should not transmit any message on SRB1 prior to receiving the RRCConnectionReestablishmentComplete message
  • UE shall configure lower layers to activate ciphering and integrity protection, and 
  • UE should apply ciphering and integrity protection to all subsequent messages received and sent by the UE
  • UE shall use Security algorithms while transmitting the RRCConnectionReestablishmentComplete msg.

Query_6. Which Channels are used for RRCConnectionReestablishment procedure?

1. The following messages 
  a. RRCConnectionReestablishment 
  b. RRCConnectionReestablishmentRequest and,
  c. RRCConnectionReestablishmentReject will use the following:

Signalling radio bearer: SRB0
Logical channel: CCCH

2. RRCConnectionReestablishmentComplete will use the following:

Signalling radio bearer: SRB1


Logical channel: DCCH

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