RRC 5G UE -> gNB 3GPP TS 38.331

5G NR - UL Information Transfer

UL Information Transfer is the NR RRC message the UE uses to send upper-layer information, most commonly NAS messages, to the network over the established RRC connection.

Message Fact Sheet

Protocol	rrc	Network	5g
Spec	3GPP TS 38.331	Spec Section	5.6.1, 6.3.2
Direction	UE -> gNB	Message Type	Dedicated Information Transfer / NAS Transport

Full message name	5G NR - UL Information Transfer
Protocol	RRC
Technology	5G
Direction	UE -> gNB
Interface	Uu
Signaling bearer / channel	SRB1 or SRB2 / DCCH
Typical trigger	Sent when the UE needs to deliver NAS signaling such as registration, session, mobility, or service-related messages after the RRC connection exists.
Main purpose	Provides the radio-side RRC container that carries NAS or other dedicated upper-layer information from the UE toward the gNB and core network.
Main specification	3GPP TS 38.331, 5.6.1, 6.3.2
Release added	Release 15
Procedures where used	5G Initial Registration, 5G PDU Session Establishment, 5G Mobility Registration Update, Service Request
Related timers	UL Information Transfer itself does not define a standalone transaction timer, Timing is usually analyzed through the NAS procedure it carries, such as registration or session management
Related cause values	UL Information Transfer does not carry reject causes, Failures are usually inferred from missing NAS payload, transport issues, or no corresponding core-network response

What is UL Information Transfer in simple terms?

UL Information Transfer is the NR RRC message the UE uses to send upper-layer information, most commonly NAS messages, to the network over the established RRC connection.

Provides the radio-side RRC container that carries NAS or other dedicated upper-layer information from the UE toward the gNB and core network.

Why this message matters

UL Information Transfer is the 5G NR RRC message the UE uses to send NAS information upward through the radio network after the RRC connection is already established.

Where this message appears in the call flow

Initial Registration

Call flow position: Used after access is established when the UE sends NAS information toward the AMF.

Typical state: UE is connected and capable of dedicated RRC signaling.

Preconditions:

RRC setup is complete.
The UE has NAS payload to send.

Next likely message: DL Information Transfer or other network response carrying NAS content

PDU Session Signaling

Call flow position: Used when the UE sends session-related NAS payload through the RRC layer.

Typical state: UE is connected and NAS session signaling is active.

Preconditions:

The UE has session-related NAS information to deliver.

Next likely message: DL Information Transfer, NAS response, or subsequent session procedure signaling

Mobility / Service Signaling

Call flow position: Relevant when the UE sends NAS mobility or service-related information while already connected.

Typical state: UE is connected and upper-layer signaling must reach the core network.

Preconditions:

Relevant NAS signaling has been generated by the UE.

Next likely message: Network-side NAS handling and corresponding downlink response

Call flow position

Previous message(s): RRC Setup Complete, Security Mode Complete, RRC Reconfiguration

Next message(s): DL Information Transfer, NAS processing in AMF, Core-network procedure continuation

Message direction and transport

Sender and receiver: UE -> gNB

Interface: Uu

Domain: Access-side radio control bridging UE upper-layer signaling toward the core

Signaling bearer: SRB1 or SRB2

Logical channel: DCCH

Transport / encapsulation: Dedicated RRC signaling on DCCH carrying UL dedicated NAS information or other upper-layer payloads

Security context: Dedicated UE-specific AS-protected signaling once access security is active in connected mode.

Message Structure Overview

UL Information Transfer is a lightweight wrapper message. Its main engineering value is the payload it carries.
For most traces, the first question is which NAS message is inside the dedicatedNAS-Message container.
This message should be interpreted together with the NAS procedure and the following network response, not as an isolated RRC event.

ASN.1 for 5G NR - UL Information Transfer

UL-DCCH-Message ::= SEQUENCE {
  message CHOICE {
    c1 CHOICE {
      ulInformationTransfer ULInformationTransfer,
      ...
    },
    ...
  }
}

ULInformationTransfer ::= SEQUENCE {
  criticalExtensions CHOICE {
    ulInformationTransfer ULInformationTransfer-IEs,
    ...
  }
}

ULInformationTransfer-IEs ::= SEQUENCE {
  dedicatedNAS-Message       DedicatedNAS-Message,
  lateNonCriticalExtension   OCTET STRING OPTIONAL,
  ...
}

How to read this ASN.1

At the RRC level, UL Information Transfer is simple. The important practical question is what NAS payload sits inside dedicatedNAS-Message and how that payload relates to the current core-network procedure.

Decoded Message Dump

UL-DCCH-Message
  message: c1
    ulInformationTransfer
      criticalExtensions
        ulInformationTransfer
          dedicatedNAS-Message
            7E 00 41 79 00 0D 01 F0 71 C0 ...

How to read this dump

The outer RRC decode is usually short.
The key trace step is to decode the NAS payload carried inside dedicatedNAS-Message.
If the NAS payload is missing, malformed, or does not match the expected procedure stage, the whole uplink signaling path can fail.

Important Information Elements

IE	Required	Description
`dedicatedNAS-Message`	Yes	The main payload container used to carry NAS information from the UE toward the network.
`lateNonCriticalExtension`	Optional	Optional extension field for later release additions.
`payload container context`	Optional	Operationally important because engineers usually care more about the carried NAS message than the outer RRC wrapper.

Detailed field explanation

`dedicatedNAS-Message`

The main payload container used to carry NAS information from the UE toward the network.

Presence: Required

`lateNonCriticalExtension`

Optional extension field for later release additions.

Presence: Optional

`payload container context`

Operationally important because engineers usually care more about the carried NAS message than the outer RRC wrapper.

Presence: Optional

What to check in logs and traces

Check which NAS procedure is expected at the moment UL Information Transfer is sent.
Verify that dedicatedNAS-Message is present and decodes to the expected NAS message.
Correlate the uplink RRC transfer with the corresponding NGAP or core-side signaling.
If there is no response, check whether the payload reached the core network correctly.
Separate an RRC transport issue from a NAS rejection or core-network handling issue.
Compare the carried NAS message with the current registration, mobility, or session state.

Common Issues and Troubleshooting

UL Information Transfer is sent but the procedure does not continue.

Likely cause: The NAS payload may be malformed, unexpected for the current state, or not forwarded correctly.

What to inspect: Check the dedicatedNAS-Message content and correlate with NGAP and AMF behavior.

Next step: Debug the carried NAS procedure before assuming an RRC failure.

The UE appears connected, but NAS signaling never reaches the core.

Likely cause: The UL Information Transfer may be missing, incorrectly formed, or lost across the access-to-core path.

What to inspect: Check uplink RRC traces, NGAP forwarding, and AMF-side receipt.

Next step: Trace the message end-to-end from RRC to NGAP to NAS.

The downlink response does not match the uplink message.

Likely cause: The wrong NAS payload may have been sent, or the core interpreted it differently than expected.

What to inspect: Decode the NAS payload inside UL Information Transfer and compare it with the resulting DL Information Transfer or core response.

Next step: Validate the NAS state and payload contents before tuning radio-side behavior.

Detailed explanation

5G NR - UL Information Transfer Explained

UL Information Transfer is the 5G NR RRC message the UE uses to send NAS or other upper-layer information toward the network after a dedicated RRC connection already exists.

For beginners, the simple meaning is: this is how the UE sends important core-network signaling upward through the radio connection.
For experienced engineers, UL Information Transfer is usually the RRC wrapper around a NAS message and should be debugged together with the carried NAS payload and the matching NGAP/core traces.

What is UL Information Transfer in simple terms?

UL Information Transfer is a dedicated UE-to-gNB RRC message carried on DCCH. The message itself is usually simple. Its main value is the payload inside it.

In practical terms, UL Information Transfer helps answer:

what NAS message did the UE send?
did that NAS payload leave the UE correctly?
did the radio network and core network continue the expected procedure?

Why this message matters

UL Information Transfer matters because many registration, mobility, and session procedures pass through it:

NAS signaling must leave the UE correctly
the network must forward that payload correctly
the following downlink NAS response depends on it

Where this message appears in the call flow

A simplified signaling path is:

UE is already connected.
UE has NAS information to send.
UE sends UL Information Transfer.
gNB forwards the payload toward the core network.
The network responds, often through DL Information Transfer.

Mini sequence:

UE                 gNB                 AMF
|--- UL Information Transfer --------->|
|                    |------ NAS ----->|
|<--- DL Information Transfer ---------|

So UL Information Transfer sits between connected-mode RRC and core-network procedure continuation.

Message direction and transport

sender: UE
receiver: gNB
interface: Uu
signaling bearer: SRB1 / SRB2
signaling channel: DCCH
security context: dedicated protected signaling

Important Information Elements

IE	Why it matters
`dedicatedNAS-Message`	Main carried payload.
`lateNonCriticalExtension`	Optional later-release extension field.
NAS payload contents	Usually the real troubleshooting focus.

Example message dump

UL-DCCH-Message
  message: c1
    ulInformationTransfer
      criticalExtensions
        ulInformationTransfer
          dedicatedNAS-Message
            7E 00 41 79 00 0D 01 F0 71 C0 ...

FAQ

What is UL Information Transfer in 5G NR?

UL Information Transfer is the NR RRC message the UE uses to send NAS or other upper-layer information to the network after the RRC connection is established.

Who sends UL Information Transfer?

The UE sends UL Information Transfer to the gNB.

What does UL Information Transfer usually carry?

Most commonly it carries a dedicated NAS message related to registration, mobility, service, or session procedures.

What comes after UL Information Transfer?

Usually a network-side NAS response, often delivered back through DL Information Transfer.

Is UL Information Transfer mainly an RRC message or a NAS wrapper?

In practice it is mostly an RRC wrapper around NAS payload.

Why is UL Information Transfer useful in troubleshooting?

Because it helps engineers confirm whether the correct NAS payload left the UE and whether the procedure failure is really radio-side or core-side.

Summary

UL Information Transfer is the NR RRC message the UE uses to send upper-layer information, most commonly NAS messages, to the network over the established RRC connection.

Advanced Engineer Notes

This message is often best treated as an RRC wrapper around NAS rather than as a complex RRC procedure on its own.
Many troubleshooting cases that look like RRC problems are really NAS payload or core-network interpretation problems inside UL Information Transfer.
Always correlate UL Information Transfer with the matching NGAP and AMF-side procedure state.

LTE / 5G / Variant Comparison

UL Information Transfer versus DL Information Transfer

UL Information Transfer carries NAS or upper-layer payload from the UE to the network. DL Information Transfer carries the corresponding payload from the network back to the UE.

UL Information Transfer versus RRC Setup Complete

RRC Setup Complete may also carry NAS in the early access stage, while UL Information Transfer is the general dedicated wrapper used after the connection is already established.

Related message pages

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.