RRC Reestablishment request in 5G NR

In the 5G-NR RRC Reestablishment procedure is a way for UE to inform the network that it needs to reestablish the connection with the network due to any error of failure on the UE. UE can further inform the network of the reason for sending a reestablishment request.

What is RRC Connection Re-establishment? RRC Connection Re-establishment is a mechanism allowing the UE to re-establish its connection with the network after encountering issues such as signal degradation or handover failures. During this process, the UE informs the network about the failure, enabling it to take corrective action to resume communication.

When is Re-establishment Triggered? According to 3GPP Specification TS 38.331, the UE can trigger the RRC Re-establishment procedure only under specific conditions:

• The UE is in the RRC_CONNECTED state.
• Access stratum (AS) security is activated with SRB2 and at least one data radio bearer (DRB).

What is the Purpose of Re-establishment? The primary goal of this procedure is to:

• Reconnect the UE to the network.
• Restore data and signaling connections.
• Ensure secure communication by reactivating security protocols.

In the case of successful re-establishment, the network applies the procedure e.g. as follows:

• When AS security has been activated and the network retrieves or verifies the UE context:
  • to re-activate AS security without changing algorithms;
  • to re-establish and resume the SRB1;

There are certain limitations when the UE can and cannot trigger a reestablishment request. 3GPP Spec 38.331 specifies that UE can trigger the reestablishment procedure when it is RRC_CONNECTED and Access stratum security has been activated with SRB2 and one data radio bearer (DRB).

RRC Reestablishment fallback mechanism

UE initiating the RRC Connection reestablishment is one part of the process, but the network should also be able to retrieve the UE context for the RRC connection reestablishment to be successful. If the network is unable to retrieve a valid UE context or, if the UE context cannot be retrieved, then it will send an RRC Setup to UE, from which UE gets an indication that the RRC reestablishment procedure is not successful. UE has to perform the Registration procedure again.

When UE is re-establishing an RRC connection, and the network is not able to retrieve or verify the UE context:

• to discard the stored AS Context and release all RBs;
• to fallback to establish a new RRC connection.

How Does the Procedure Work?

The RRC Re-establishment process involves several steps between the UE and the network:

1. Initiation by the UE

The UE sends an RRCReestablishmentRequest message to the network, providing:

• UE Identity: Includes parameters like C-RNTI (Cell Radio Network Temporary Identifier) and physical cell ID.
• Re-establishment Cause: Indicates the reason (e.g., handover failure, reconfiguration failure).

2. Network Response

• If the network retrieves the UE’s context successfully, it sends an RRCReestablishment message to the UE.
• If the UE context cannot be retrieved, the network initiates an RRC Setup process, signaling the UE to perform re-registration.

3. Resumption of Secure Communication

Upon receiving the RRCReestablishment message:

• The UE resumes integrity protection and ciphering for signaling bearers.
• Security keys are derived or updated based on the existing context.

Actions During Re-establishment

On the UE Side:

• Re-establishes signaling connections (SRB1) and resumes data communication.
• Configures lower layers for resumed security (ciphering and integrity protection).
• Sends an RRCReestablishmentComplete message to the network.

On the Network Side:

• Restores or verifies the UE’s context.
• If the context cannot be verified, it discards the stored AS context and falls back to establishing a new RRC connection.

Limitations and Restrictions

• Security Activation: The UE cannot initiate the re-establishment procedure unless AS security is activated.
• Connection State: The procedure is limited to UEs in the RRC_CONNECTED state.
• Failure Scenarios: If the re-establishment fails, the UE transitions to RRC_IDLE with specific release causes (e.g., “RRC connection failure”).

When can UE initiate the RRC Re-establishment Procedure?

3GPP has specified a number of scenarios where UE can perform re-establishment on NR RAT. Below is the list of RRC Reestablishment reasons:

• Radio link failure on MCG and T316 is not configured.
• Radio link failure on MCG while SCG transmission is suspended.
• Radio link failure on MCG while SCG transmission is suspended.
• Radio link failure on MCG while Pscell Change is ongoing.
• Radio link failure on SCG while MCG transmission is suspended.
• RRC connection reconfiguration failure.
• Reconfiguration with sync failure on MCG.
• Reconfiguration with sync failure on SCG while MCG transmission is suspended.
• SCG change failure while MCG transmission is suspended.
• SCG configuration failure while MCG transmission is suspended.
• Integrity check failure indication from SCG lower layers concerning SRB3 while MCG is suspended.
• Integrity check failure indication from lower layers concerning SRB1 or SRB2.
• mobility from NR failure.
• T316 timer expiry.

RRC Re-establishment causes codes

3GPP has specified below Cause indications that UE can use when it sends RRC Re-establishment message:

• reconfigurationFailure
• handoverFailure
• otherFailure

In the Asn.1 code of the ReestablishmentCause, there is one more option Spare1, which is used for future enhancements. As you can see there are so many triggers for the Reestablishment message but only a few ReestablishmentCause mentioned in the specification that UE can use.

RRCReestablishmentRequest

• The RRCReestablishmentRequest message is used to request the reestablishment of an RRC connection.
• Signalling radio bearer: SRB0
• RLC-SAP: TM
• Logical channel: CCCH
• Direction: UE to Network

-- ASN1START
-- TAG-RRCREESTABLISHMENTREQUEST-START
 
RRCReestablishmentRequest ::=       SEQUENCE {
    rrcReestablishmentRequest           RRCReestablishmentRequest-IEs
}
 
RRCReestablishmentRequest-IEs ::=   SEQUENCE {
    ue-Identity                         ReestabUE-Identity,
    reestablishmentCause                ReestablishmentCause,
    spare                               BIT STRING (SIZE (1))
}
 
ReestabUE-Identity ::=              SEQUENCE {
    c-RNTI                              RNTI-Value,
    physCellId                          PhysCellId,
    shortMAC-I                          ShortMAC-I
}
 
ReestablishmentCause ::=            ENUMERATED {reconfigurationFailure, handoverFailure, otherFailure, spare1}
 
-- TAG-RRCREESTABLISHMENTREQUEST-STOP
-- ASN1STOP

Tools to checkout

• 5G NR RRC Timers
• Beam Failure Recovery
• SSB Configuration