| Protocol | rrc | Network | 5g |
|---|---|---|---|
| Spec | 3GPP TS 38.331 | Spec Section | 5.2.2.4.21, 6.3.1 |
| Direction | gNB -> UE | Message Type | Broadcast System Information |
| Full message name | 5G NR - System Information Block 20 (SIB20) |
|---|---|
| Protocol | RRC |
| Technology | 5G |
| Direction | gNB -> UE |
| Interface | Uu |
| Signaling bearer / channel | Broadcast transport / BCCH-DL-SCH |
| Typical trigger | Broadcast when the cell offers MBS broadcast and the UE must learn MCCH / MTCH timing. |
| Main purpose | Provides the MCCH timing and related configuration the UE needs to acquire MBS broadcast control and data channels. |
| Main specification | 3GPP TS 38.331, 5.2.2.4.21, 6.3.1 |
| Release added | Release 17 |
| Procedures where used | System Information Acquisition, MBS Broadcast Reception |
| Related timers | SIB20 does not use a dedicated UE-specific transaction timer |
| Related cause values | SIB20 does not carry reject causes |
System Information Block 20 (SIB20) is an NR broadcast system information block used to provide the MCCH and MTCH acquisition configuration for MBS broadcast.
Provides the MCCH timing and related configuration the UE needs to acquire MBS broadcast control and data channels.
SIB20 tells the UE how to find the MCCH and start MBS broadcast reception.
Call flow position: Read as additional SI after the basic broadcast context is available.
Typical state: UE is camped and extending its broadcast context for MBS broadcast reception.
Preconditions:
Next likely message: UE starts MCCH monitoring according to SIB20
Previous message(s): MIB, System Information Block 1 (SIB1)
Next message(s): MCCH monitoring, 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 {
sib20-v1700 SIB20-r17,
...
},
...
}
SIB20-r17 ::= SEQUENCE {
mcch-Config-r17 MCCH-Config-r17,
cfr-ConfigMCCH-MTCH-r17 CFR-ConfigMCCH-MTCH-r17 OPTIONAL,
...
}
The most important engineering block is mcch-Config-r17 because it tells the UE when and how to monitor MCCH.
BCCH-DL-SCH-Message
message: c1
systemInformation
criticalExtensions
systemInformation
sib-TypeAndInfo
sib20
mcch-Config-r17
mcch-RepetitionPeriodAndOffset-r17: rf16
mcch-WindowStartSlot-r17: 8
mcch-ModificationPeriod-r17: rf64
| IE | Required | Description |
|---|---|---|
mcch-Config-r17 | Yes | Main MCCH configuration block, including repetition period, window start, and modification period. |
cfr-ConfigMCCH-MTCH-r17 | Optional | Optional additional configuration for MCCH and MTCH acquisition. |
mcch-Config-r17Main MCCH configuration block, including repetition period, window start, and modification period.
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.
cfr-ConfigMCCH-MTCH-r17Optional additional configuration for MCCH and MTCH acquisition.
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: SIB20 may be missing or its MCCH timing may not match what the UE expects.
What to inspect: Check mcch-Config-r17 and its repetition or window values.
Next step: Correlate SIB20 with observed MCCH monitoring windows and MBS reception behavior.
SIB20 is for MBS broadcast MCCH / MTCH acquisition, while SIB24 is for multicast MCCH / MTCH configuration in the relevant RRC_INACTIVE use case.
SIB20 is System Information Block 20, used to broadcast MCCH and MTCH acquisition configuration for MBS broadcast.
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.