5G NR - System Information Block 6 (SIB6)

Message Fact Sheet

Protocol	rrc	Network	5g
Spec	3GPP TS 38.331	Spec Section	5.2.1, 5.2.2, 5.4.3, 6.3.1
Direction	gNB -> UE	Message Type	Broadcast System Information

Full message name	5G NR - System Information Block 6 (SIB6)
Protocol	RRC
Technology	5G
Direction	gNB -> UE
Interface	Uu
Signaling bearer / channel	Broadcast transport / BCCH-DL-SCH
Typical trigger	Broadcast by the cell as additional system information so camped UEs can apply inter-RAT reselection policy toward UMTS where that legacy mobility path still matters.
Main purpose	Provides UTRA-related reselection information so the UE can make correct idle or inactive mobility decisions when neighboring UMTS systems are part of the operator mobility strategy.
Main specification	3GPP TS 38.331, 5.2.1, 5.2.2, 5.4.3, 6.3.1
Release added	Release 15
Procedures where used	System Information Acquisition, Inter-RAT Cell Reselection, Idle Mobility, Inactive Mobility
Related timers	Reselection timing is driven by broadcast mobility parameters rather than a dedicated UE-specific RRC transaction timer
Related cause values	SIB6 does not carry reject causes, Problems are inferred from missing SI acquisition or inter-RAT behavior that does not match the configured UTRA mobility policy

What is System Information Block 6 (SIB6) in simple terms?

System Information Block 6 (SIB6) is an NR broadcast system information block used for inter-RAT mobility context toward UTRA, helping the UE evaluate reselection from NR toward UMTS when that mobility path is part of the deployment.

Provides UTRA-related reselection information so the UE can make correct idle or inactive mobility decisions when neighboring UMTS systems are part of the operator mobility strategy.

Why this message matters

SIB6 is the NR broadcast block that mainly tells the UE how to behave with UMTS targets when that legacy RAT is still part of the network mobility design.

Where this message appears in the call flow

System Information Acquisition

Call flow position: Read as additional broadcast information after the essential access layer is already available.

Typical state: UE is camped and building the broader mobility view across legacy radio technologies.

Preconditions:

MIB and SIB1 have been acquired.
The UE knows the scheduling for additional system information.

Next likely message: UTRA reselection evaluation

Inter-RAT Cell Reselection to UTRA

Call flow position: Used when the UE evaluates mobility from NR toward UMTS.

Typical state: UE is not in dedicated connected-mode signaling.

Preconditions:

The serving-cell broadcast information is valid.
UTRA neighbors are part of the mobility design.

Next likely message: Stay on NR or reselect to UTRA

Idle / Inactive Mobility

Call flow position: Provides broadcast rules for mobility decisions from NR toward UTRA.

Typical state: UE is camped or inactive and monitoring mobility-related broadcast information.

Preconditions:

The UE is already operating with valid serving-cell system information.

Next likely message: Paging monitoring, RAT search, or reselection depending on conditions

Call flow position

Previous message(s): MIB, System Information Block 1 (SIB1), System Information Block 5 (SIB5)

Next message(s): UTRA reselection evaluation, Idle mobility decisions toward UMTS, Later access on a reselected UTRA cell

Message direction and transport

Sender and receiver: gNB -> UE

Interface: Uu

Domain: Access-side radio control and broadcast mobility information

Signaling bearer: Broadcast transport

Logical channel: BCCH-DL-SCH

Transport / encapsulation: RRC system information carried on BCCH-DL-SCH after the UE has acquired the essential system information and the scheduling for additional SI

Security context: Broadcast information. It is cell-common and not protected by dedicated AS security.

Message Structure Overview

SIB6 focuses on NR-to-UTRA mobility rather than NR-only mobility or NR-to-LTE mobility.
For engineering work, the main question is whether the legacy UMTS mobility policy matches the intended deployment strategy.
SIB6 matters mostly in networks where UMTS still exists as part of coverage or fallback planning.

ASN.1 for 5G NR - System Information Block 6 (SIB6)

BCCH-DL-SCH-Message ::= SEQUENCE {
  message CHOICE {
    c1 CHOICE {
      systemInformationBlockType1 SystemInformationBlockType1,
      systemInformation          SystemInformation,
      ...
    },
    ...
  }
}

SystemInformation ::= SEQUENCE {
  criticalExtensions CHOICE {
    systemInformation SystemInformation-IEs,
    ...
  }
}

SystemInformation-IEs ::= SEQUENCE {
  sib-TypeAndInfo SEQUENCE (SIZE (1..maxSIB)) OF CHOICE {
    sib6 SystemInformationBlockType6,
    ...
  }
}

SystemInformationBlockType6 ::= SEQUENCE {
  ...
}

How to read this ASN.1

SIB6 is typically carried inside the broader SystemInformation container. In practice, engineers care most about whether the UTRA target list and thresholds explain the observed legacy inter-RAT mobility behavior.

Decoded Message Dump

BCCH-DL-SCH-Message
  message: c1
    systemInformation
      criticalExtensions
        systemInformation
          sib-TypeAndInfo
            sib6
              utra-CarrierList
                carrierFreq: 10688
                cellReselectionPriority: 4
                threshX-High: 8
                threshX-Low: 4
                q-OffsetFreq: dB0

How to read this dump

The key practical block is the UTRA carrier list.
Carrier and priority information tell you whether UMTS is really part of the mobility strategy.
Thresholds and offsets help explain why the UE stays on NR or moves toward UTRA.

Important Information Elements

IE	Required	Description
`utra carrier list`	Yes	The main SIB6 content. It defines the candidate UTRA carriers or neighbor information relevant for mobility from NR toward UMTS.
`priority and threshold-related parameters`	Optional	Help explain when UTRA should become attractive enough to be considered for reselection.
`q-Offset or equivalent biasing information`	Optional	Changes how favorable or unfavorable the UMTS target appears in idle mobility decisions.
`search and timing controls`	Optional	Operationally important parameters that affect when the UE searches and when it commits to a move toward UTRA.

Detailed field explanation

`utra carrier list`

The main SIB6 content. It defines the candidate UTRA carriers or neighbor information relevant for mobility from NR toward UMTS.

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.

`priority and threshold-related parameters`

Help explain when UTRA should become attractive enough to be considered for reselection.

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.

`q-Offset or equivalent biasing information`

Changes how favorable or unfavorable the UMTS target appears in idle mobility decisions.

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.

`search and timing controls`

Operationally important parameters that affect when the UE searches and when it commits to a move toward UTRA.

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 MIB and SIB1 were decoded successfully before reviewing SIB6 behavior.
Verify that the UE actually acquired the additional SI carrying SIB6.
Check the configured UTRA targets against the expected legacy mobility plan.
Inspect thresholds, offsets, and priorities against the observed leave-NR or stay-on-NR behavior.
Compare NR serving conditions with UTRA attractiveness rather than reviewing one side only.
If users report missing or unexpected legacy mobility, correlate SIB6 with RF measurements and broadcast versions.

Common Issues and Troubleshooting

The UE does not reselect to UMTS when legacy continuity is expected.

Likely cause: UTRA thresholds or priorities may not make UMTS attractive enough.

What to inspect: Check SIB6 carrier list, threshold values, and priority settings.

Next step: Compare the configured UMTS continuity strategy with real radio conditions and neighbor planning.

The UE leaves NR unexpectedly for UMTS.

Likely cause: The UTRA target may be too highly prioritized or insufficiently penalized.

What to inspect: Check priority and q-Offset related values for the UTRA target.

Next step: Correlate SIB6 with field logs to see whether the mobility policy is too aggressive.

LTE fallback behavior is fine, but UMTS-directed mobility is wrong.

Likely cause: The issue may sit specifically in SIB6 rather than in SIB5.

What to inspect: Separate NR-to-LTE and NR-to-UTRA behavior clearly in the trace analysis.

Next step: Analyze SIB5 and SIB6 together only after separating the target RAT path.

LTE / 5G / Variant Comparison

SIB6 versus SIB5

SIB5 covers broader inter-RAT mobility such as NR to LTE. SIB6 is more specific to NR to UTRA mobility.

SIB6 versus dedicated RRC

SIB6 is broadcast cell-common mobility information, not UE-specific connected-mode configuration.

FAQ

What is SIB6 in 5G NR?

SIB6 is System Information Block 6, an NR broadcast block used mainly for inter-RAT reselection toward UTRA.

Who sends SIB6?

The gNB broadcasts SIB6 as additional system information.

What is the main purpose of SIB6?

To provide inter-RAT reselection parameters for idle and inactive mobility toward UMTS.

On which channel is SIB6 sent?

SIB6 is carried in system information on BCCH-DL-SCH.

Why is SIB6 useful in troubleshooting?

Because it helps explain why the UE does or does not move from NR to UMTS where that legacy path still exists.

Does SIB6 contain UE-specific information?

No. SIB6 is broadcast cell-common information.

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.