5G NR - Dedicated SIB Request

Message Fact Sheet

Protocol	rrc	Network	5g
Spec	3GPP TS 38.331	Spec Section	6.2.2
Direction	UE -> gNB	Message Type	System Information Control

Full message name	5G NR - Dedicated SIB Request
Protocol	RRC
Technology	5G
Direction	UE -> gNB
Interface	Uu
Signaling bearer / channel	SRB1 / UL-DCCH
Typical trigger	Used when the UE needs SIB content that is delivered on demand instead of being continuously broadcast.
Main purpose	Lets the UE ask the network for specific broadcast SIB content through dedicated signaling when on-demand delivery is supported.
Main specification	3GPP TS 38.331, 6.2.2
Release added	Release 16
Procedures where used	On-Demand System Information Delivery, Connected-Mode SIB Acquisition
Related timers	T319 or implementation-specific connected-mode handling

What is Dedicated SIB Request in simple terms?

Dedicated SIB Request is the UE-to-network NR RRC message used to request on-demand system information while the UE is already in connected mode.

Lets the UE ask the network for specific broadcast SIB content through dedicated signaling when on-demand delivery is supported.

Why this message matters

Dedicated SIB Request lets a connected UE ask the network for specific SIBs instead of waiting for normal broadcast delivery.

Where this message appears in the call flow

On-Demand System Information Delivery

Call flow position: Sent after the UE has a connected RRC context and needs additional SIB content.

Typical state: UE is in RRC_CONNECTED.

Preconditions:

The UE already has an active RRC connection.
The network supports dedicated or on-demand SIB delivery.

Next likely message: Dedicated delivery of the requested SIB content

Call flow position

Previous message(s): RRC Reconfiguration

Next message(s): System Information, Dedicated system information delivery

Message direction and transport

Sender and receiver: UE -> gNB

Interface: Uu

Domain: Access-side radio control

Signaling bearer: SRB1

Logical channel: UL-DCCH

Transport / encapsulation: Dedicated RRC signaling on SRB1

Security context: Sent as protected connected-mode signaling after AS security activation.

Message Structure Overview

Dedicated SIB Request is a compact request message rather than a full system-information container.
The practical focus is which SIB types the UE asks for and whether the network answers with the expected dedicated delivery.

ASN.1 for 5G NR - Dedicated SIB Request

DedicatedSIBRequest ::= SEQUENCE {
  requestedSIB-List SEQUENCE (SIZE (1..maxSIB)) OF SIB-Type
}

How to read this ASN.1

The key field is the requested SIB list. Engineers mainly check whether the UE asked for the right SIB type and whether the network responded.

Decoded Message Dump

UL-DCCH-Message
  message: dedicatedSIBRequest
    requestedSIB-List: sib20, sib21

How to read this dump

The requested list should match the SIBs the UE is missing or explicitly needs on demand.

Important Information Elements

IE	Required	Description
`requestedSIB-List`	Yes	List of SIB types the UE wants the network to deliver through dedicated signaling.

Detailed field explanation

`requestedSIB-List`

List of SIB types the UE wants the network to deliver through dedicated signaling.

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.

What to check in logs and traces

Check which SIB types the UE requested.
Verify whether the network answered with dedicated delivery or normal SystemInformation.

Common Issues and Troubleshooting

The UE never receives the requested SIB.

Likely cause: The request may be missing, malformed, or the network may not support dedicated delivery for that SIB type.

What to inspect: Check requestedSIB-List and the follow-on downlink response.

Next step: Correlate the request with the later dedicated delivery or SystemInformation content.

LTE / 5G / Variant Comparison

Dedicated SIB Request versus System Information

Dedicated SIB Request asks for SIB content, while System Information delivers one or more SIBs.

FAQ

What does Dedicated SIB Request do in 5G NR?

It lets the UE request specific SIB content through dedicated signaling while already connected.

Related message pages

Related Procedures

5G Initial Registration

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.