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LTE RRCLTEeNodeB -> UE3GPP TS 36.331
LTE RRC SIB3 - System Information Block Type 3
Broadcast LTE RRC system information block carried inside System Information that gives the UE the serving-frequency cell reselection rules used in idle mode after the common SIB1 and SIB2 context is already known.
Message Fact Sheet
Protocol
lte-rrc
Network
lte
Spec
3GPP TS 36.331
Spec Section
5.2.2, 5.2.4, 6.2.2
Direction
eNodeB -> UE
Message Type
Broadcast System Information
Full message name
LTE RRC SIB3 - System Information Block Type 3
Protocol
LTE-RRC
Technology
LTE
Direction
eNodeB -> UE
Interface
Uu
Signaling bearer / channel
Broadcast system information transport / BCCH mapped to DL-SCH
Typical trigger
The UE reads System Information carrying SIB3 when it needs valid idle-mode serving-frequency reselection rules during initial camping, reselection, return from out of coverage, or after a system-information refresh.
Main purpose
Provides the serving-frequency cell reselection priorities, thresholds, and mobility behavior the UE uses when deciding whether to stay on the current LTE cell or move to another candidate while idle.
Main specification
3GPP TS 36.331, 5.2.2, 5.2.4, 6.2.2
Release added
Release 8
Procedures where used
System Information Acquisition, Idle-mode Camping, Cell Reselection, Serving-frequency Mobility Evaluation, System Information Change Notification
What is System Information Block Type 3 in simple terms?
Broadcast LTE RRC system information block carried inside System Information that gives the UE the serving-frequency cell reselection rules used in idle mode after the common SIB1 and SIB2 context is already known.
Provides the serving-frequency cell reselection priorities, thresholds, and mobility behavior the UE uses when deciding whether to stay on the current LTE cell or move to another candidate while idle.
Why this message matters
SIB3 is the LTE broadcast block that tells an idle UE how to judge the serving frequency for reselection.
Where this message appears in the call flow
LTE idle-mode camping
In LTE idle-mode camping, System Information Block Type 3 gives the serving-frequency reselection rules the UE applies while deciding whether the current LTE cell is still the best place to stay camped.
Call flow position: Serving-frequency reselection step used after the UE already has the basic access and common-radio context from earlier broadcast blocks.
Typical state: UE is camped or trying to camp in idle mode and needs the serving-frequency reselection rules for the current LTE carrier.
Preconditions:
MIB, SIB1, and the required earlier system information were acquired.
System Information containing SIB3 is present and decodable.
Next likely message: Continued camping or same-frequency reselection decision
LTE cell reselection
In LTE cell reselection, the UE combines the serving-cell SIB3 policy with candidate-cell broadcast information to decide whether to leave the current LTE cell and camp on a better one.
Call flow position: Mobility-evaluation step where the UE applies serving-frequency reselection thresholds and priorities before staying or moving.
Typical state: UE is idle and is comparing serving-cell quality against candidate-cell behavior.
Preconditions:
The UE is already camped on the LTE serving frequency.
Reselection evaluation has been triggered by radio conditions or mobility logic.
Next likely message: Serving-cell stay decision or move toward a better candidate cell
System information change handling
When system information changes, the UE refreshes SIB3 so that serving-frequency reselection behavior follows the updated idle-mobility policy.
Call flow position: Refresh step where the UE reacquires SIB3 because serving-frequency reselection rules may have changed.
Typical state: UE must refresh its idle mobility assumptions before continuing to rely on stored serving-frequency behavior.
Preconditions:
A system-information refresh was triggered.
Refreshed SIB1 and the relevant System Information container are available.
Next likely message: Idle continuation using the refreshed reselection rules
Domain: Access-side radio control for idle-mode mobility and serving-frequency cell reselection
Signaling bearer: Broadcast system information transport
Logical channel: BCCH mapped to DL-SCH
Transport / encapsulation: System Information carried on BCCH and transmitted on DL-SCH with SIB3 included in sib-TypeAndInfo
Security context: Broadcast message with no dedicated SRB or AS security. The UE reads it after MIB, SIB1, and the earlier common-system-information context.
Message Structure Overview
System Information Block Type 3 is the first LTE SIB that is primarily about idle mobility rather than access or common radio setup.
Its practical meaning comes from the reselection thresholds and serving-frequency criteria that explain why the UE stayed, moved, or failed to move in idle mode.
In troubleshooting, SIB3 is most useful when read together with SIB4 and SIB5, because the serving-frequency rules are only one part of the broader mobility picture.
ASN.1 for LTE RRC SIB3 - System Information Block Type 3
This page focuses on the stable top-level SIB3 structure that remains the main operational entry point in Release 18 LTE troubleshooting: common reselection info, serving-frequency reselection info, and intra-frequency reselection info.
LTE RRC SIB3 - System Information Block Type 3 - Example Dump
The most useful first question is whether SIB3 is present at all in the serving cell's scheduled System Information.
If idle mobility looks wrong, do not stop at the wrapper. Move straight into the serving-frequency reselection thresholds and compare them with the observed radio behavior.
SIB3 often needs to be read together with SIB4 or SIB5 because serving-frequency rules alone do not explain every candidate-cell decision.
Important Information Elements
IE
Required
Description
cellReselectionInfoCommon
Yes
Common cell reselection information used as the base idle-mobility context.
cellReselectionServingFreqInfo
Yes
Serving-frequency-specific reselection parameters that shape how the UE evaluates the current LTE carrier.
intraFreqCellReselectionInfo
Yes
Intra-frequency-specific reselection configuration for same-frequency idle mobility behavior.
lateNonCriticalExtension
Optional
Late extension branch for later release additions.
nonCriticalExtension
Optional
Release-extension branch used for newer reselection-related additions.
Detailed field explanation
cellReselectionInfoCommon
Common cell reselection information used as the base idle-mobility context.
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.
cellReselectionServingFreqInfo
Serving-frequency-specific reselection parameters that shape how the UE evaluates the current LTE carrier.
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.
intraFreqCellReselectionInfo
Intra-frequency-specific reselection configuration for same-frequency idle mobility behavior.
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
Late extension branch for later release 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.
nonCriticalExtension
Release-extension branch used for newer reselection-related 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 MIB, SIB1, and the required earlier system information were acquired before interpreting SIB3.
Check whether SIB3 is present in the relevant System Information message.
Inspect the serving-frequency and intra-frequency reselection fields before diagnosing unexpected idle mobility.
Compare SIB3 against SIB4 and SIB5 when same-frequency or inter-frequency movement is involved.
If mobility changed after a system-information refresh, compare the new SIB3 values with the old ones.
Common Issues and Troubleshooting
The UE stays on the serving LTE cell longer than expected.
Likely cause: The SIB3 serving-frequency reselection thresholds or priorities may legitimately keep the UE on the current cell despite radio degradation.
What to inspect: Check cellReselectionServingFreqInfo and intraFreqCellReselectionInfo against the measured serving and candidate-cell conditions.
Next step: Validate the idle-mobility policy before treating the behavior as a UE defect.
The UE reselects unexpectedly or too aggressively.
Likely cause: The SIB3 reselection criteria may favor earlier movement, or the serving-cell quality may fall below the configured expectations sooner than assumed.
What to inspect: Compare the configured SIB3 reselection thresholds with field measurements and a working reference cell.
Next step: Use SIB3 as the main explanation path before blaming neighbor definition problems.
Mobility behavior changed after a broadcast refresh.
Likely cause: The refreshed SIB3 may have updated the serving-frequency reselection rules.
What to inspect: Compare the pre-refresh and post-refresh SIB3 content, then correlate the changed values with the new idle behavior.
Next step: Treat it as a system-information-driven mobility change until proven otherwise.
LTE / 5G / Variant Comparison
Compared with System Information Block Type 2
SIB2 gives the UE common radio and access-side configuration. SIB3 gives the serving-frequency idle reselection rules used after the UE is already camped.
Compared with System Information Block Type 4
SIB3 explains serving-frequency reselection logic. SIB4 adds same-frequency neighbor-cell details that help the UE interpret specific candidate cells.
Compared with System Information Block Type 5
SIB3 is about the current LTE serving frequency. SIB5 extends the mobility view toward other LTE carrier frequencies.
FAQ
What is System Information Block Type 3 in LTE?
It is the LTE broadcast block that gives the UE serving-frequency cell reselection rules for idle-mode mobility.
Why is LTE SIB3 important?
Because it often explains why an idle UE stayed on the serving cell, reselected, or behaved differently across cells even when access itself looked normal.
What should I inspect first in LTE SIB3?
Start with the serving-frequency and intra-frequency reselection information, then compare those rules with the observed idle radio conditions.
How does SIB3 relate to SIB4 and SIB5?
SIB3 gives the serving-frequency rules. SIB4 adds same-frequency neighbor-cell detail, and SIB5 covers inter-frequency LTE mobility.
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.
