| 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 SIB7 - System Information Block Type 7 |
|---|---|
| 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 SIB7 when idle mobility may involve GERAN, during initial camping, inter-RAT reselection evaluation, system-information refresh, or other LTE-to-GSM mobility analysis. |
| Main purpose | Provides the GERAN carrier priorities, thresholds, and reselection detail the UE needs when deciding whether to leave the serving LTE frequency and camp on a GSM carrier. |
| 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, Inter-RAT Mobility Evaluation, System Information Change Notification |
Broadcast LTE RRC system information block carried inside System Information that provides inter-RAT reselection information for GERAN frequencies in idle mode.
Provides the GERAN carrier priorities, thresholds, and reselection detail the UE needs when deciding whether to leave the serving LTE frequency and camp on a GSM carrier.
SIB7 is the LTE broadcast block that tells an idle UE how to judge GSM frequencies for reselection.
Call flow position: Inter-RAT mobility preparation step used after the UE already has the LTE serving-frequency context and now needs GERAN reselection information.
Typical state: UE is camped in idle mode on LTE and is building the broader mobility view toward GSM frequencies.
Preconditions:
Next likely message: Continued camping or GERAN candidate-frequency ranking
Call flow position: Inter-RAT candidate-frequency evaluation step where the UE compares the current LTE serving frequency against GERAN frequencies.
Typical state: UE is idle and is deciding whether a GSM carrier offers a better camping choice than the serving LTE carrier.
Preconditions:
Next likely message: Stay on LTE or camp on a better GERAN frequency
Call flow position: Refresh step where the UE reacquires SIB7 because LTE-to-GERAN reselection data may have changed.
Typical state: UE must refresh stored GERAN mobility assumptions before continuing to rely on them.
Preconditions:
Next likely message: Idle mobility continuation using refreshed SIB7 inter-RAT data
Previous message(s): MasterInformationBlock, SystemInformationBlockType1, System Information carrying SystemInformationBlockType3, System Information carrying SystemInformationBlockType5
Next message(s): Inter-RAT candidate-frequency ranking and reselection decision, Idle continuation on the serving LTE frequency, Camping on a better GERAN frequency
Security context: Broadcast message with no dedicated SRB or AS security. The UE reads it after MIB, SIB1, and the earlier LTE broadcast context.
SystemInformationBlockType7 ::= SEQUENCE {
carrierFreqsInfoList CarrierFreqsInfoListGERAN,
t-ReselectionGERAN T-Reselection,
t-ReselectionGERAN-SF SpeedStateScaleFactors OPTIONAL,
lateNonCriticalExtension OCTET STRING OPTIONAL,
nonCriticalExtension SystemInformationBlockType7-v890-IEs OPTIONAL
}
CarrierFreqsInfoListGERAN ::= SEQUENCE (SIZE (1..maxGERAN-Carrier)) OF CarrierFreqsInfoGERAN
CarrierFreqsInfoGERAN ::= SEQUENCE {
carrierFreqs CarrierFreqsGERAN,
commonInfo CommonInfoGERAN,
cellReselectionPriority CellReselectionPriority OPTIONAL,
ncc-Permitted NCC-Permitted OPTIONAL,
q-RxLevMin Q-RxLevMin,
threshX-High ThreshX-High,
threshX-Low ThreshX-Low
}
This page focuses on the stable SIB7 reading model used in practice: which GERAN frequencies are advertised, how they are prioritized, and what thresholds govern LTE-to-GERAN reselection.
BCCH-DL-SCH-Message
message c1 : systemInformation : {
criticalExtensions systemInformation-r8 : {
sib-TypeAndInfo {
sib7 {
carrierFreqsInfoList {
{
carrierFreqs {
startingARFCN 62
},
cellReselectionPriority 3,
q-RxLevMin -70,
threshX-High 6,
threshX-Low 4
}
},
t-ReselectionGERAN 2
}
}
}
}
| IE | Required | Description |
|---|---|---|
carrierFreqsInfoList | Yes | List of GERAN carrier groups with their inter-RAT reselection parameters. |
cellReselectionPriority | Optional | Priority used when ranking the GERAN frequency group against LTE and other candidate RAT or frequency choices. |
threshX-High / threshX-Low | Optional | Thresholds that govern movement between the serving LTE frequency and the candidate GERAN frequency group. |
ncc-Permitted | Optional | Permitted network colour codes used to filter valid GERAN candidates. |
q-RxLevMin | Optional | Minimum received level assumption for the candidate GERAN frequency group. |
t-ReselectionGERAN | Yes | GERAN reselection timer used when evaluating movement from LTE toward GERAN. |
lateNonCriticalExtension | Optional | Late extension branch for later release additions. |
nonCriticalExtension | Optional | Release-extension branch used for newer inter-RAT mobility additions. |
carrierFreqsInfoListList of GERAN carrier groups with their inter-RAT reselection parameters.
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.
cellReselectionPriorityPriority used when ranking the GERAN frequency group against LTE and other candidate RAT or frequency choices.
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.
threshX-High / threshX-LowThresholds that govern movement between the serving LTE frequency and the candidate GERAN frequency group.
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.
ncc-PermittedPermitted network colour codes used to filter valid GERAN candidates.
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-RxLevMinMinimum received level assumption for the candidate GERAN frequency group.
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.
t-ReselectionGERANGERAN reselection timer used when evaluating movement from LTE toward GERAN.
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.
lateNonCriticalExtensionLate 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.
nonCriticalExtensionRelease-extension branch used for newer inter-RAT mobility 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.
Likely cause: The SIB7 priorities or threshX values may legitimately favor staying on LTE longer than expected.
What to inspect: Check the GERAN carrier list, cellReselectionPriority, threshX-High, threshX-Low, and the measured radio conditions.
Next step: Validate the LTE-to-GERAN mobility policy before blaming the UE behavior.
Likely cause: The advertised GERAN frequency may carry higher priority or more favorable thresholds than assumed.
What to inspect: Compare the LTE serving-frequency view from SIB3 with the GERAN policy in SIB7.
Next step: Use SIB7 as the main explanation path for the inter-RAT move.
Likely cause: The refreshed SIB7 may have updated GERAN priorities, thresholds, or permitted target conditions.
What to inspect: Compare the old and new SIB7 content and correlate the changed values with the new mobility outcome.
Next step: Treat it as a broadcast inter-RAT mobility-policy change until proven otherwise.
SIB6 extends LTE mobility toward UTRA. SIB7 extends the mobility view toward GERAN.
SIB5 is about other LTE carriers. SIB7 is about leaving LTE and evaluating GERAN candidates.
System Information is the container message. SIB7 is the LTE-to-GERAN mobility payload inside that container.
It is the LTE broadcast block that gives the UE inter-RAT reselection information for GERAN frequencies.
Because it often explains why an idle UE stayed on LTE or moved to GSM.
Start with the advertised GERAN frequencies, their priorities, and the threshX values.
SIB6 extends LTE mobility toward UTRA. SIB7 extends the mobility view toward GERAN.
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.