LTE RRC LTE eNodeB -> UE 3GPP TS 36.331

LTE RRC System Information

Broadcast LTE RRC message on BCCH and DL-SCH that carries one or more SIBs such as SIB2 and later system-information blocks after the UE has already read MIB and SIB1.

Message Fact Sheet

Protocol	lte-rrc	Network	lte
Spec	3GPP TS 36.331	Spec Section	5.2.1, 5.2.2.4, 5.2.2.8, 6.2.2
Direction	eNodeB -> UE	Message Type	Broadcast System Information

Full message name	LTE RRC System Information
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 SystemInformation when SIB1 scheduling indicates the required SIBs are present, during initial acquisition, after cell reselection, after system information change notification, or whenever stored broadcast information is no longer valid.
Main purpose	Carries the scheduled SIB content the UE needs for common radio configuration, mobility, idle behavior, warning handling, and other broadcast LTE functions beyond the MIB and SIB1 entry point.
Main specification	3GPP TS 36.331, 5.2.1, 5.2.2.4, 5.2.2.8, 6.2.2
Release added	Release 8
Procedures where used	System Information Acquisition, Initial Access, Idle-mode Camping, System Information Change Notification, Paging Preparation

What is LTE RRC System Information in simple terms?

Broadcast LTE RRC message on BCCH and DL-SCH that carries one or more SIBs such as SIB2 and later system-information blocks after the UE has already read MIB and SIB1.

Carries the scheduled SIB content the UE needs for common radio configuration, mobility, idle behavior, warning handling, and other broadcast LTE functions beyond the MIB and SIB1 entry point.

Why this message matters

SystemInformation is the LTE broadcast message that carries the real SIB set the UE needs after MIB and SIB1.

Where this message appears in the call flow

LTE system information acquisition

Call flow position: Scheduled broadcast container read after MIB and SIB1 when the UE needs the remaining required SIBs.

Typical state: UE is building the wider broadcast view of the serving cell before or during idle behavior and before later dedicated access signaling.

Preconditions:

MIB and SIB1 have already been read successfully.
SIB1 scheduling indicates that the required SIBs are present.

Next likely message: RRCConnectionRequest or idle-mode continuation using the acquired SIB content

LTE initial access

Call flow position: Broadcast follow-on step where the UE reads SIB2 and any other required SIBs before normal connection establishment can proceed.

Typical state: UE is still in the broadcast-acquisition part of access and has not yet moved into dedicated SRB signaling.

Preconditions:

The UE already has the MIB and SIB1 context for the cell.
The serving cell schedules the required SystemInformation content.

Next likely message: RRCConnectionRequest

System information change handling

Call flow position: Reacquisition step entered when paging or validity logic tells the UE that the stored broadcast context changed.

Typical state: UE is refreshing the relevant SIB content before relying on the cell's idle, access, paging, or mobility assumptions again.

Preconditions:

The UE already has stored broadcast context for the cell.
A system-information change indication or validity trigger requires refresh.

Next likely message: Idle continuation or later access using refreshed system information

Call flow position

Previous message(s): MasterInformationBlock, SystemInformationBlockType1, Paging with systemInfoModification in change-notification cases

Next message(s): RRCConnectionRequest after required system information is available, Idle-mode camping, reselection, or paging monitoring using the acquired broadcast context

Message direction and transport

Sender and receiver: eNodeB -> UE

Interface: Uu

Domain: Access-side radio control for broadcast cell context beyond the BCH and SIB1 entry layer

Signaling bearer: Broadcast system information transport

Logical channel: BCCH mapped to DL-SCH

Transport / encapsulation: SystemInformation carried on BCCH and transmitted on DL-SCH according to SIB1 scheduling

Security context: Broadcast message with no dedicated SRB or AS security. The UE reads it using the scheduling and validity rules already learned from SIB1.

Message Structure Overview

SystemInformation is a broadcast container message rather than a single-purpose dedicated control step.
Its practical meaning comes from which SIBs are included in sib-TypeAndInfo and why the UE needed them at that point.
In real traces, the most common first read is whether the message carries SIB2 for common radio behavior, then whether later SIBs explain mobility, inter-frequency reselection, warnings, or feature-specific context.

ASN.1 for LTE RRC System Information

SystemInformation ::= SEQUENCE {
  criticalExtensions CHOICE {
    systemInformation-r8 SystemInformation-r8-IEs,
    criticalExtensionsFuture-r15 CHOICE {
      posSystemInformation-r15 PosSystemInformation-r15-IEs,
      criticalExtensionsFuture SEQUENCE {}
    }
  }
}

SystemInformation-r8-IEs ::= SEQUENCE {
  sib-TypeAndInfo SEQUENCE (SIZE (1..maxSIB)) OF CHOICE {
    sib2 SystemInformationBlockType2,
    sib3 SystemInformationBlockType3,
    sib4 SystemInformationBlockType4,
    sib5 SystemInformationBlockType5,
    sib6 SystemInformationBlockType6,
    sib7 SystemInformationBlockType7,
    sib8 SystemInformationBlockType8,
    sib9 SystemInformationBlockType9,
    sib10 SystemInformationBlockType10,
    sib11 SystemInformationBlockType11,
    sib12-v920 SystemInformationBlockType12-r9,
    sib13-v920 SystemInformationBlockType13-r9,
    sib14-v1130 SystemInformationBlockType14-r11,
    sib15-v1130 SystemInformationBlockType15-r11,
    sib16-v1130 SystemInformationBlockType16-r11,
    sib17-v1250 SystemInformationBlockType17-r12,
    sib18-v1250 SystemInformationBlockType18-r12,
    sib19-v1250 SystemInformationBlockType19-r12,
    sib20-v1310 SystemInformationBlockType20-r13,
    sib21-v1430 SystemInformationBlockType21-r14,
    sib24-v1530 SystemInformationBlockType24-r15,
    sib25-v1530 SystemInformationBlockType25-r15,
    sib26-v1530 SystemInformationBlockType26-r15,
    sib27-v1610 SystemInformationBlockType27-r16,
    sib28-v1710 SystemInformationBlockType28-r17,
    sib29-v1710 SystemInformationBlockType29-r17,
    spare3 NULL,
    spare2 NULL,
    spare1 NULL
  }
}

How to read this ASN.1

This page uses the Release 18 TS 36.331 V18.3.1 ASN.1 for the regular LTE SystemInformation message. In practice, the outer wrapper matters less than which entries appear inside sib-TypeAndInfo.

LTE RRC System Information - Example Dump

BCCH-DL-SCH-Message
  message c1 : systemInformation : {
    criticalExtensions systemInformation-r8 : {
      sib-TypeAndInfo {
        sib2 {
          radioResourceConfigCommon present
        },
        sib3 {
          cellReselectionInfoCommon present
        },
        sib4 {
          intraFreqNeighCellList present
        }
      }
    }
  }

How to read this dump

The first practical question is which SIBs are present in sib-TypeAndInfo, not just whether the SystemInformation wrapper exists.
SIB2 is often the most important early block because it provides common radio configuration needed for access assumptions.
Later SIBs explain idle mobility, inter-frequency behavior, warning handling, or release-specific features depending on the cell.

Important Information Elements

IE	Required	Description
`criticalExtensions`	Yes	Versioned wrapper that carries the SystemInformation payload for regular LTE or future extension branches.
`sib-TypeAndInfo`	Yes	List of one or more included SIBs such as SIB2, SIB3, SIB4, SIB5, and later broadcast blocks scheduled for this SI message.
`nonCriticalExtension`	Optional	Future-extension branch used by later releases through the criticalExtensions structure.

Detailed field explanation

`criticalExtensions`

Versioned wrapper that carries the SystemInformation payload for regular LTE or future extension branches.

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.

`sib-TypeAndInfo`

List of one or more included SIBs such as SIB2, SIB3, SIB4, SIB5, and later broadcast blocks scheduled for this SI message.

Presence: Required

`nonCriticalExtension`

Future-extension branch used by later releases through the criticalExtensions structure.

Presence: Optional

What to check in logs and traces

Confirm MIB and SIB1 were acquired before interpreting the SystemInformation message.
Check which SIBs are present in sib-TypeAndInfo.
Verify that the required SIBs for the active scenario are actually scheduled and acquired.
If the UE behaves differently after a change indication, compare the old and new SystemInformation content.
Move from the wrapper into the individual SIB payloads instead of diagnosing from the container name alone.

Common Issues and Troubleshooting

The UE reads MIB and SIB1 but still cannot continue normal LTE access.

Likely cause: The required later SystemInformation content such as SIB2 may be missing, not scheduled as expected, or not decoded reliably.

What to inspect: Check SIB1 scheduling, SystemInformation delivery timing, and whether the needed SIBs are present in sib-TypeAndInfo.

Next step: Validate the required SIB set before debugging later RRCConnectionRequest or access behavior.

Idle behavior or reselection looks wrong even though the cell can be camped.

Likely cause: The relevant mobility or idle-mode SIBs inside SystemInformation may differ from expectations.

What to inspect: Check which later SIBs are carried, especially the ones that govern common radio, reselection, or inter-frequency behavior.

Next step: Compare the serving cell's SystemInformation content with a working cell or an earlier trace.

The UE refreshes broadcast information after paging or a change indication.

Likely cause: systemInfoModification or validity logic triggered reacquisition of the scheduled SystemInformation content.

What to inspect: Check the paging indication, current SIB1 scheduling, and what changed inside the reacquired SystemInformation message.

Next step: Treat the issue as a broadcast refresh path, not just a paging decode problem.

LTE / 5G / Variant Comparison

Compared with MasterInformationBlock

MasterInformationBlock is the BCH entry anchor. SystemInformation is the later BCCH-on-DL-SCH container that carries the scheduled SIB set after the UE already has MIB and SIB1 context.

Compared with SystemInformationBlockType1

SIB1 gives identity, access, and scheduling context. SystemInformation carries the scheduled later SIBs such as SIB2 and mobility-related broadcast content.

Compared with Paging

Paging may tell the UE that broadcast information changed. SystemInformation is the message family the UE then reacquires to refresh the actual SIB content.

FAQ

What is SystemInformation in LTE?

It is the LTE RRC broadcast message that carries one or more scheduled SIBs after the UE has already read MIB and SIB1.

What is inside LTE SystemInformation?

Its main payload is sib-TypeAndInfo, which contains one or more SIBs such as SIB2, SIB3, SIB4, and other release-dependent system-information blocks.

What comes after SystemInformation in LTE initial access?

Once the UE has the required broadcast information such as SIB2 and the needed later SIBs, it can continue normal access and send RRCConnectionRequest.

What should I inspect first in LTE SystemInformation?

Start with which SIBs are present in sib-TypeAndInfo and whether they match the active scenario or problem under investigation.

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.