5G NR - UE Capability Enquiry

Message Fact Sheet

Protocol	rrc	Network	5g
Spec	3GPP TS 38.331	Spec Section	5.6.1, 6.2.2, Annex B.1
Direction	gNB -> UE	Message Type	Capability Exchange

Full message name	5G NR - UE Capability Enquiry
Protocol	RRC
Technology	5G
Direction	gNB -> UE
Interface	Uu
Signaling bearer / channel	SRB1 / DL-DCCH
Typical trigger	The gNB needs the UE's NR capability information before applying feature-dependent or capability-dependent radio configuration.
Main purpose	Requests the UE's supported radio capability containers so the network can make correct configuration decisions for NR operation.
Main specification	3GPP TS 38.331, 5.6.1, 6.2.2, Annex B.1
Release added	Release 15
Procedures where used	UE Capability Transfer, Connected-Mode Capability Discovery, Capability-Aware RRC Reconfiguration

What is UE Capability Enquiry in simple terms?

Downlink NR RRC message used by the gNB to request UE capability information needed for feature, bearer, mobility, and radio configuration decisions.

Requests the UE's supported radio capability containers so the network can make correct configuration decisions for NR operation.

Why this message matters

UECapabilityEnquiry is the gNB asking the UE to report what radio features and capabilities it supports.

Where this message appears in the call flow

UE Capability Transfer

Call flow position: Network request that starts the capability exchange between gNB and UE.

Typical state: UE is already connected and able to send protected DCCH signaling.

Preconditions:

SRB1 and connected-mode signaling are active.
The network needs UE capability data for the next decision.

Next likely message: UECapabilityInformation

Capability-Aware RRC Reconfiguration

Call flow position: Capability request before feature-dependent radio configuration such as measurements, CA, EN-DC-style behavior, or advanced radio setup.

Typical state: UE is in RRC_CONNECTED and the gNB is preparing a configuration that depends on UE support.

Preconditions:

The network has not yet confirmed the needed capability details from the UE.

Next likely message: UECapabilityInformation followed by capability-based RRCReconfiguration

Call flow position

Previous message(s): SecurityModeComplete, Connected-mode procedure context

Next message(s): UECapabilityInformation, RRCReconfiguration

Message direction and transport

Sender and receiver: gNB -> UE

Interface: Uu

Domain: Access-side radio control capability handling

Signaling bearer: SRB1

Logical channel: DL-DCCH

Transport / encapsulation: RRC over DCCH on SRB1 during connected-mode capability exchange

Security context: Typically exchanged after AS security is active, so it is usually part of the protected connected-mode RRC signaling path.

Message Structure Overview

UECapabilityEnquiry is a request message. The practical heart of the message is the list of capability containers the gNB wants back from the UE.
The message itself does not carry the capability data; it tells the UE what capability information to return.
Engineers should read it together with the following UECapabilityInformation and the later RRCReconfiguration that depends on the reported support.

ASN.1 for 5G NR - UE Capability Enquiry

UECapabilityEnquiry ::= SEQUENCE {
  rrc-TransactionIdentifier RRC-TransactionIdentifier,
  criticalExtensions CHOICE {
    ueCapabilityEnquiry UECapabilityEnquiry-IEs,
    criticalExtensionsFuture SEQUENCE {}
  }
}

UECapabilityEnquiry-IEs ::= SEQUENCE {
  ue-CapabilityRAT-RequestList UE-CapabilityRAT-RequestList,
  lateNonCriticalExtension     OCTET STRING OPTIONAL,
  nonCriticalExtension         SEQUENCE {} OPTIONAL
}

How to read this ASN.1

The key practical field is the RAT request list, because it tells you exactly what capability information the gNB expects the UE to report next.

5G NR - UE Capability Enquiry - Example Dump

DL-DCCH-Message
  message: ueCapabilityEnquiry
    rrc-TransactionIdentifier: 3
    criticalExtensions:
      ueCapabilityEnquiry:
        ue-CapabilityRAT-RequestList:
          - rat-Type: nr
        lateNonCriticalExtension: absent
        nonCriticalExtension: absent

How to read this dump

The request list tells the UE which RAT capability containers to return.
A normal next step is UECapabilityInformation on UL-DCCH.
If the later RRCReconfiguration fails, check whether the network correctly interpreted the capability response.

Important Information Elements

IE	Required	Description
`rrc-TransactionIdentifier`	Yes	Transaction identifier used to correlate the enquiry with the UE response.
`ue-CapabilityRAT-RequestList`	Yes	List of RAT capability containers the network wants the UE to provide.
`lateNonCriticalExtension`	Optional	Extension container used for later release evolution and compatibility handling.
`nonCriticalExtension`	Optional	Forward-compatible extension branch for later additions.

Detailed field explanation

`rrc-TransactionIdentifier`

Transaction identifier used to correlate the enquiry with the UE response.

Presence: Required

In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.

`ue-CapabilityRAT-RequestList`

List of RAT capability containers the network wants the UE to provide.

Presence: Required

In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.

`lateNonCriticalExtension`

Extension container used for later release evolution and compatibility handling.

Presence: Optional

In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.

`nonCriticalExtension`

Forward-compatible extension branch for later additions.

Presence: Optional

In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.

What to check in logs and traces

Confirm the message is sent on SRB1 / DL-DCCH after connected-mode security is active.
Inspect the requested RAT list and verify it matches the intended scenario.
Correlate the transaction identifier with the following UECapabilityInformation.
Check whether the returned UE capability data is complete and consistent with the enquiry.
If later configuration fails, verify whether the network used capability information correctly.

Common Issues and Troubleshooting

UECapabilityEnquiry is sent but no UECapabilityInformation follows.

Likely cause: The UE may fail to process the request, the response may be missing from the trace, or the procedure may be interrupted.

What to inspect: Check transaction pairing, UL-DCCH activity, and whether the UE supports the requested capability reporting path.

Next step: Correlate with UE logs and verify whether the capability exchange completed or stalled.

Later RRC reconfiguration fails after capability exchange.

Likely cause: The gNB may have built configuration based on incorrect, incomplete, or misinterpreted UE capabilities.

What to inspect: Compare UECapabilityEnquiry, UECapabilityInformation, and the resulting RRCReconfiguration.

Next step: Validate that the reported UE support matches the feature being configured.

Capability exchange seems redundant or repeated.

Likely cause: The network may be re-requesting capabilities due to context loss, handover preparation, or implementation behavior.

What to inspect: Check whether the procedure context changed and whether earlier capability information was still valid.

Next step: Determine whether the repeat enquiry is expected for that scenario.

LTE / 5G / Variant Comparison

Compared with UECapabilityInformation

UECapabilityEnquiry is the network request. UECapabilityInformation is the UE response carrying the actual reported capability containers.

FAQ

What does UECapabilityEnquiry do in 5G NR?

It asks the UE to provide its supported radio capability information.

Who sends UECapabilityEnquiry?

The gNB sends UECapabilityEnquiry to the UE.

What comes after UECapabilityEnquiry?

The UE normally responds with UECapabilityInformation.

Does UECapabilityEnquiry contain the UE capability itself?

No. It only requests the capability data. The UE reports the actual capability in UECapabilityInformation.

Why is UECapabilityEnquiry important?

It helps the network avoid configuring features the UE does not support.

Related message pages

Related Procedures

5G RRC Reconfiguration

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.