5G NR - RRC Reconfiguration

Message Fact Sheet

Protocol	rrc	Network	5g
Spec	3GPP TS 38.331	Spec Section	5.3.5, 6.2.2
Direction	gNB to UE	Message Type	Control-plane signaling

Full message name	5G NR - RRC Reconfiguration
Protocol	RRC
Technology	5G
Direction	gNB to UE
Interface	Uu
Signaling bearer / channel	SRB1 / DL-DCCH
Typical trigger	Bearer setup, measurement updates, mobility execution, or secondary-cell configuration changes while the UE is already connected.
Main purpose	Update the UE with new radio parameters, DRBs, measurement objects, mobility rules, or secondary cell changes.
Procedures where used	PDU Session Establishment, Measurement Reporting, Connected Mode Handover
Related timers	T304 in handover-related cases
Related cause values	None highlighted
Main specification	3GPP TS 38.331, 5.3.5, 6.2.2

What this message does in simple terms

Reconfigures the UE radio connection after setup, including bearers, measurements, and mobility.

Update the UE with new radio parameters, DRBs, measurement objects, mobility rules, or secondary cell changes.

Why this message matters

RRCReconfiguration is the main message the gNB uses to change a UE's radio behavior after the connection is already up.

Where this message is used

PDU Session Establishment

Call flow position: Connected-mode radio update used to add or modify radio resources after the initial setup phase.

Typical state: UE is already in RRC_CONNECTED.

Preconditions:

SRB1 is active.
The network has decided to update bearer or radio configuration.

Next likely message: RRCReconfigurationComplete

Connected Mode Handover

Call flow position: Mobility-related configuration step that can prepare or execute connected mobility.

Typical state: UE is in connected mode with measurement and mobility handling active.

Preconditions:

A mobility decision or related radio update has already been made.

Next likely message: RRCReconfigurationComplete

Full Procedure Context

Previous message(s): Connected-state RRC/NAS procedure context

Next message(s): RRCReconfigurationComplete

Message Direction and Transport

Sender and receiver: gNB to UE

Interface: Uu

Domain: Access-side radio control

Signaling bearer: SRB1

Logical channel: DL-DCCH

Transport / encapsulation: RRC over DCCH on SRB1 in connected mode

Security context: Typically delivered after AS security is active, so integrity and ciphering context should be considered during trace analysis.

Message Structure Overview

RRCReconfiguration is the main connected-mode workhorse message for changing radio behavior without releasing the connection.
Many important sections are optional, so troubleshooting starts by identifying which configuration blocks are actually present.
radioBearerConfig, measConfig, mobilityControlInfo, and secondaryCellGroup each point to a different engineering purpose.

ASN.1 for 5G NR - RRC Reconfiguration

RRCReconfiguration ::= SEQUENCE {
  rrc-TransactionIdentifier RRC-TransactionIdentifier,
  criticalExtensions CHOICE {
    rrcReconfiguration RRCReconfiguration-IEs,
    criticalExtensionsFuture SEQUENCE {}
  }
}

RRCReconfiguration-IEs ::= SEQUENCE {
  radioBearerConfig     RadioBearerConfig OPTIONAL,
  secondaryCellGroup    OCTET STRING OPTIONAL,
  measConfig            MeasConfig OPTIONAL,
  lateNonCriticalExtension OCTET STRING OPTIONAL,
  nonCriticalExtension  SEQUENCE {} OPTIONAL
}

How to read this ASN.1

In practice, the important question is which optional configuration blocks are populated and whether they match the current procedure, UE capabilities, and target radio behavior.

Decoded Message Dump

DL-DCCH-Message ::= {
  message c1 : rrcReconfiguration : {
    rrc-TransactionIdentifier 1,
    criticalExtensions rrcReconfiguration : {
      measConfig {
        measObjectToAddModList {
          {
            measObjectId 1,
            measObjectNR {
              ssbFrequency 631968,
              ssbSubcarrierSpacing kHz30
            }
          }
        }
      }
    }
  }
}

How to read this dump

measConfig is a common sign that the message is arming the UE for future mobility or reporting behavior.
If radioBearerConfig is present, inspect it closely when bearer setup or modification fails.
mobilityControlInfo changes the interpretation toward mobility execution or handover-related behavior.

Information Elements

IE	Required	Description
`radioBearerConfig`	Optional	Adds, modifies, or releases SRBs and DRBs.
`measConfig`	Optional	Configures measurement objects, report rules, and gap settings.
`mobilityControlInfo`	Optional	Carries target cell details when the reconfiguration is used for handover.
`secondaryCellGroup`	Optional	Delivers secondary cell configuration for advanced radio features.

Detailed field explanation

`radioBearerConfig`

Adds, modifies, or releases SRBs and DRBs.

Presence: Optional

`measConfig`

Configures measurement objects, report rules, and gap settings.

Presence: Optional

`mobilityControlInfo`

Carries target cell details when the reconfiguration is used for handover.

Presence: Optional

`secondaryCellGroup`

Delivers secondary cell configuration for advanced radio features.

Presence: Optional

What to check in logs and traces

Check which optional configuration blocks are present in this instance.
Verify the transaction identifier and correlate with RRCReconfigurationComplete.
Inspect radioBearerConfig, measConfig, and mobilityControlInfo according to the active procedure.
Confirm the delivered parameters are supported by the UE capability profile.
In mobility cases, correlate the message with measurement reports and target-cell details.

Common Issues and Troubleshooting

UE receives RRCReconfiguration but does not complete the procedure.

Likely cause: Unsupported or inconsistent configuration in one of the optional blocks.

What to inspect: Decode each populated section and compare with UE capability and serving-cell context.

Next step: Compare with a known-good reconfiguration trace or simplify the configuration to isolate the failure.

Handover fails after reconfiguration delivery.

Likely cause: The mobility-related configuration may not match target-cell preparation.

What to inspect: Check mobilityControlInfo, target frequency, PCI, and related measurement assumptions.

Next step: Validate target cell preparation and neighbor configuration.

Detailed explanation

What this message does

RRCReconfiguration is one of the most common NR RRC messages. The gNB sends it whenever the connected UE needs new radio instructions without tearing down the session.

Troubleshooting notes

Check whether rrc-TransactionIdentifier increments correctly when repeated reconfigurations are sent.
Validate that measConfig and radioBearerConfig match UE capability support.
In mobility cases, confirm that target frequency and PCI align with neighbor configuration.

FAQ

Is RRC Reconfiguration only used for handover?

No. It is a general-purpose connected-state update message and is also used for bearer setup and measurements.

Why is measConfig often present in this message?

Because the gNB uses this message to arm the UE with the measurement logic needed for mobility decisions.

Summary

Reconfigures the UE radio connection after setup, including bearers, measurements, and mobility.

Advanced Engineer Notes

The same message name covers many different connected-mode actions, so context is essential.
Troubleshooting should start by identifying whether the message is mainly about bearers, measurements, mobility, or advanced radio features.

LTE / 5G / Variant Comparison

Compared with RRC Setup

RRC Setup creates the first connected-mode control configuration, while RRC Reconfiguration handles later connected-state changes such as DRBs, measurements, mobility, and secondary-cell updates.

Related Messages

Related Procedures

Related Tools

Use This Reference in Practice

Decode this message with the 3GPP Decoder, inspect the related message database, or open the matching call flow to see where this signaling step fits in the full procedure.