| Protocol | rrc | Network | 5g |
|---|---|---|---|
| Spec | 3GPP TS 38.331 | Spec Section | 5.2.2.4.25, 6.3.1 |
| Direction | gNB -> UE | Message Type | Broadcast System Information |
| Full message name | 5G NR - System Information Block 24 (SIB24) |
|---|---|
| Protocol | RRC |
| Technology | 5G |
| Direction | gNB -> UE |
| Interface | Uu |
| Signaling bearer / channel | Broadcast transport / BCCH-DL-SCH |
| Typical trigger | Broadcast when the cell supports MBS multicast reception and the UE needs multicast MCCH / MTCH acquisition information. |
| Main purpose | Provides multicast MCCH and MTCH acquisition information, especially for MBS multicast reception scenarios relevant to RRC_INACTIVE. |
| Main specification | 3GPP TS 38.331, 5.2.2.4.25, 6.3.1 |
| Release added | Release 18 |
| Procedures where used | MBS Multicast Reception |
| Related timers | SIB24 does not use a dedicated UE-specific transaction timer |
| Related cause values | SIB24 does not carry reject causes |
System Information Block 24 (SIB24) is an NR broadcast system information block used to provide the information required to acquire multicast MCCH and MTCH configuration.
Provides multicast MCCH and MTCH acquisition information, especially for MBS multicast reception scenarios relevant to RRC_INACTIVE.
SIB24 gives the UE the broadcast information it needs to acquire multicast MCCH and MTCH configuration.
Call flow position: Used when the UE acquires multicast MCCH / MTCH configuration.
Typical state: UE is preparing or maintaining multicast reception, including the relevant RRC_INACTIVE context.
Preconditions:
Next likely message: UE applies multicast MCCH / MTCH configuration locally
Previous message(s): MIB, System Information Block 1 (SIB1)
Next message(s): Multicast MCCH monitoring, Multicast MTCH acquisition
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 {
sib24-v1800 SIB24-r18,
...
},
...
}
SIB24-r18 ::= SEQUENCE {
... -- multicast MCCH / MTCH configuration
}
The practical point is whether the multicast MCCH / MTCH configuration is available and matches the multicast scenario being debugged.
BCCH-DL-SCH-Message
message: c1
systemInformation
criticalExtensions
systemInformation
sib-TypeAndInfo
sib24
mcch-MTCH-Config-r18: present
| IE | Required | Description |
|---|---|---|
multicast MCCH / MTCH configuration | Yes | Broadcast configuration needed for multicast MCCH and MTCH acquisition. |
multicast MCCH / MTCH configurationBroadcast configuration needed for multicast MCCH and MTCH acquisition.
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.
Likely cause: SIB24 may be missing or its multicast configuration may not match the service.
What to inspect: Check the multicast MCCH / MTCH configuration block.
Next step: Compare SIB24 with the expected multicast service and inactive-state behavior.
SIB20 focuses on MBS broadcast acquisition, while SIB24 focuses on multicast MCCH / MTCH acquisition.
SIB24 is System Information Block 24, used to broadcast multicast MCCH and MTCH acquisition information.
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.