| Protocol | rrc | Network | 5g |
|---|---|---|---|
| Spec | 3GPP TS 38.331 | Spec Section | 5.2.2.4.19, 6.3.1 |
| Direction | gNB -> UE | Message Type | Broadcast System Information |
| Full message name | 5G NR - System Information Block 18 (SIB18) |
|---|---|
| Protocol | RRC |
| Technology | 5G |
| Direction | gNB -> UE |
| Interface | Uu |
| Signaling bearer / channel | Broadcast transport / BCCH-DL-SCH |
| Typical trigger | Broadcast when the cell needs to expose GIN-related network-selection or onboarding information. |
| Main purpose | Provides GIN-related broadcast information so the UE can perform the correct network-selection or onboarding behavior for SNPN-related scenarios. |
| Main specification | 3GPP TS 38.331, 5.2.2.4.19, 6.3.1 |
| Release added | Release 17 |
| Procedures where used | System Information Acquisition, SNPN Network Selection / Onboarding |
| Related timers | SIB18 does not use a dedicated UE-specific transaction timer |
| Related cause values | SIB18 does not carry reject causes |
System Information Block 18 (SIB18) is an NR broadcast system information block used to provide Group IDs for Network selection (GINs) to support credentials-holder based access and UE onboarding.
Provides GIN-related broadcast information so the UE can perform the correct network-selection or onboarding behavior for SNPN-related scenarios.
SIB18 gives the UE GIN-related information used for private-network selection and onboarding.
Call flow position: Read as additional SI after MIB and SIB1.
Typical state: UE is camped and extending broadcast context for private-network selection or onboarding.
Preconditions:
Next likely message: UE applies the GIN information locally
Call flow position: Used when the UE evaluates GIN-based support for credentials-holder access or onboarding.
Typical state: UE is in selection behavior rather than a dedicated transaction.
Preconditions:
Next likely message: Network selection decision or later RRC access
Previous message(s): MIB, System Information Block 1 (SIB1)
Next message(s): Network selection using GIN information
Security context: Broadcast information. It is cell-common and not protected by dedicated AS security.
SystemInformation-IEs ::= SEQUENCE {
sib-TypeAndInfo SEQUENCE (SIZE (1..maxSIB)) OF CHOICE {
sib18-v1700 SIB18-r17,
...
},
...
}
SIB18-r17 ::= SEQUENCE {
gin-ElementList-r17 SEQUENCE (SIZE (1..maxGIN-r17)) OF GIN-Element-r17 OPTIONAL,
...
}
The practical payload is the GIN-related list and support indication. Engineers use it to explain SNPN selection and onboarding behavior.
BCCH-DL-SCH-Message
message: c1
systemInformation
criticalExtensions
systemInformation
sib-TypeAndInfo
sib18
gin-ElementList-r17: present
| IE | Required | Description |
|---|---|---|
gin-ElementList-r17 | Optional | Contains one or more GIN elements used for group-based network selection. |
supportedGINs bitmap | Optional | Indicates which GINs are supported by the SNPN. |
gin-ElementList-r17Contains one or more GIN elements used for group-based network selection.
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.
supportedGINs bitmapIndicates which GINs are supported by the SNPN.
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: SIB18 may be missing or its GIN data may not match the intended SNPN scenario.
What to inspect: Check gin-ElementList-r17 and any support indication bitmap.
Next step: Compare SIB18 with SIB1 and the intended private-network identity mapping.
SIB10 provides human-readable network naming, while SIB18 provides GIN-based network-selection or onboarding information.
SIB18 is System Information Block 18, used to broadcast GIN information for network selection and onboarding.
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.