Telecom engineering reference for protocols, messages, call flows, troubleshooting, releases, and tools.
Menu
RRC5GgNB -> UE3GPP TS 38.331
5G NR - System Information Block 2 (SIB2)
System Information Block 2 (SIB2) is an NR broadcast system information block used mainly for cell reselection and idle-state mobility-related behavior after the UE has already acquired the essential access information from MIB and SIB1.
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 2 (SIB2)
Protocol
RRC
Technology
5G
Direction
gNB -> UE
Interface
Uu
Signaling bearer / channel
Broadcast transport / BCCH-DL-SCH
Typical trigger
Broadcast by the cell according to SI scheduling so UEs can obtain additional mobility and reselection-related information after essential system information acquisition.
Main purpose
Provides additional non-essential-but-operationally-important broadcast information, especially reselection-related parameters, so the UE can make correct idle and mobility decisions on and around the serving cell.
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, Cell Reselection, Idle Mobility, Inactive Mobility
Related timers
SI acquisition timing is driven by broadcast scheduling and UE behavior rather than a single dedicated RRC transaction timer
Related cause values
SIB2 does not carry a reject cause IE, Problems are usually inferred from missing acquisition, stale broadcast information, or reselection behavior that does not match deployment expectations
What is System Information Block 2 (SIB2) in simple terms?
System Information Block 2 (SIB2) is an NR broadcast system information block used mainly for cell reselection and idle-state mobility-related behavior after the UE has already acquired the essential access information from MIB and SIB1.
Provides additional non-essential-but-operationally-important broadcast information, especially reselection-related parameters, so the UE can make correct idle and mobility decisions on and around the serving cell.
Why this message matters
SIB2 is an additional 5G broadcast information block that mainly helps the UE make better idle-mobility and reselection decisions after it already knows the essential access information from MIB and SIB1.
Where this message appears in the call flow
System Information Acquisition
Call flow position: Additional broadcast information read after SIB1 when the UE needs more cell-common information beyond the essential access layer.
Typical state: UE is camped or preparing to remain on the cell and is expanding its broadcast-information context.
Preconditions:
The UE has synchronized to the cell and decoded MIB.
The UE has already acquired SIB1.
The UE knows how and when the additional SI is scheduled.
Next likely message: Idle mobility or reselection behavior using the newly acquired parameters
Cell Reselection
Call flow position: Used as supporting broadcast information while the UE evaluates whether to stay on the serving cell or move to another candidate.
Typical state: UE is in idle-like mobility behavior rather than dedicated connected-mode signaling.
Preconditions:
The UE is camped and monitoring cell-common information.
Next likely message: Continued camping, reselection decision, or later access on a chosen cell
Idle / Inactive Mobility
Call flow position: Provides additional mobility-related broadcast parameters referenced during non-connected mobility handling.
Typical state: UE is not in a dedicated connected-mode RRC exchange.
Preconditions:
Current cell system information has been acquired successfully.
Next likely message: Paging monitoring, reselection action, or later access attempt depending on UE state
Next message(s): Cell reselection evaluation, Idle or inactive mobility decisions, Initial Access
Message direction and transport
Sender and receiver: gNB -> UE
Interface: Uu
Domain: Access-side radio control and broadcast system information
Signaling bearer: Broadcast transport
Logical channel: BCCH-DL-SCH
Transport / encapsulation: RRC system information carried on BCCH-DL-SCH after the UE has already acquired MIB and SIB1 and knows how additional SI is scheduled
Security context: Broadcast information. It is cell-common and not carried under dedicated AS security.
Message Structure Overview
SIB2 is part of the additional NR system-information layer rather than the first essential broadcast step.
In practice, engineers use SIB2 to understand why idle mobility and reselection behavior looks different from what SIB1 alone would suggest.
SIB2 is often more relevant to mobility, camping stability, and paging-area behavior than to the very first access attempt.
ASN.1 for 5G NR - System Information Block 2 (SIB2)
SIB2 is usually carried inside the broader SystemInformation container rather than as a standalone first-stage broadcast anchor like MIB or SIB1. For engineers, the key point is not only the ASN.1 name but whether the UE acquired the scheduled SI and applied the reselection-related content correctly.
The key practical point is that the UE is no longer at the 'can I read the cell at all?' stage. It already has essential broadcast context and is now reading additional mobility-related information.
cellReselectionInfoCommon sets the baseline reselection behavior and hysteresis logic.
intraFreq and interFreq entries help explain why the UE stayed on the serving cell, reselected away, or ignored a candidate frequency.
If the field values look technically valid but user behavior is still wrong, compare them with neighbor-cell configuration and measurement conditions.
Important Information Elements
IE
Required
Description
cellReselectionInfoCommon
Optional
Carries common reselection-related information that directly affects how the UE evaluates staying on the serving cell or moving away from it.
intraFreqCellReselectionInfo
Optional
Provides intra-frequency reselection parameters that engineers often inspect when a UE does not stay on the expected NR cell.
interFreqCarrierFreqList
Optional
Provides inter-frequency reselection context that influences how the UE evaluates neighboring NR carriers.
ssb-PositionsInBurst or related mobility context
Optional
May appear as part of broader broadcast context depending on release and configuration interpretation.
threshold and priority-related parameters
Optional
Operationally important fields that influence reselection aggressiveness, candidate ranking, and idle mobility behavior.
Detailed field explanation
cellReselectionInfoCommon
Carries common reselection-related information that directly affects how the UE evaluates staying on the serving cell or moving away from it.
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.
intraFreqCellReselectionInfo
Provides intra-frequency reselection parameters that engineers often inspect when a UE does not stay on the expected NR cell.
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.
interFreqCarrierFreqList
Provides inter-frequency reselection context that influences how the UE evaluates neighboring NR carriers.
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.
ssb-PositionsInBurst or related mobility context
May appear as part of broader broadcast context depending on release and configuration interpretation.
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.
threshold and priority-related parameters
Operationally important fields that influence reselection aggressiveness, candidate ranking, and idle mobility behavior.
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 already acquired before looking for SIB2 problems.
Verify that the UE actually attempted to acquire additional SI according to the scheduling announced in SIB1.
Check whether SIB2 decode is stable across repeated reads or whether only partial / stale acquisition is occurring.
Inspect reselection thresholds, priorities, hysteresis, and timers against the observed camping behavior.
Correlate serving-cell SIB2 values with neighbor-cell configuration and real RF conditions.
If the issue appears during paging or idle mobility, verify that the UE is applying the expected broadcast information version.
Common Issues and Troubleshooting
The UE camps on the cell but idle mobility behavior looks wrong.
Likely cause: SIB2 reselection parameters may be misconfigured, stale, or different from what the planner expects.
What to inspect: Check hysteresis, reselection timers, priorities, and thresholds in the decoded SIB2.
Next step: Compare serving and neighbor cells together instead of reviewing only one SIB2 in isolation.
The UE does not seem to use expected neighbor frequencies.
Likely cause: Inter-frequency reselection information may be missing, incomplete, or not aligned with deployment intent.
What to inspect: Check inter-frequency carrier entries and threshold values.
Next step: Correlate SIB2 with measurement behavior and the neighbor database.
Initial access works, but idle mobility later behaves unexpectedly.
Likely cause: The issue may be in additional system information such as SIB2 rather than in MIB, SIB1, PRACH, or RRC setup.
What to inspect: Check whether SIB2 acquisition completed and whether the UE applied the latest content.
Next step: Analyze MIB, SIB1, and SIB2 together as one broadcast-information chain.
LTE / 5G / Variant Comparison
SIB2 versus SIB1
SIB1 gives the essential access and cell-identity context the UE needs before real access. SIB2 extends the broadcast picture with more mobility and reselection-oriented information.
SIB2 versus dedicated RRC
SIB2 is cell-common broadcast information. It does not configure one specific UE the way dedicated messages such as RRC Setup or RRC Reconfiguration do.
FAQ
What is SIB2 in 5G NR?
SIB2 is System Information Block 2, an additional broadcast system information block mainly used for idle mobility and cell reselection behavior.
Who sends SIB2?
The gNB broadcasts SIB2 to UEs as part of additional system information.
Is SIB2 read before first access like SIB1?
Not in the same way. SIB1 is the essential first access-related broadcast message, while SIB2 is additional information often more relevant to later idle and mobility behavior.
On which channel is SIB2 sent?
SIB2 is carried in system information on BCCH-DL-SCH.
Why is SIB2 important for troubleshooting?
Because it can explain strange reselection, camping, and idle mobility behavior that is not obvious from MIB, SIB1, or RF measurements alone.
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.