| Protocol | rrc | Network | 5g |
|---|---|---|---|
| Spec | 3GPP TS 38.331 | Spec Section | 5.2.2.4.22, 6.3.1 |
| Direction | gNB -> UE | Message Type | Broadcast System Information |
| Full message name | 5G NR - System Information Block 21 (SIB21) |
|---|---|
| Protocol | RRC |
| Technology | 5G |
| Direction | gNB -> UE |
| Interface | Uu |
| Signaling bearer / channel | Broadcast transport / BCCH-DL-SCH |
| Typical trigger | Broadcast when the network needs to support MBS service continuity across frequencies. |
| Main purpose | Provides MBS FSAI mapping for the serving and neighboring frequencies so the UE can support service continuity for MBS broadcast reception. |
| Main specification | 3GPP TS 38.331, 5.2.2.4.22, 6.3.1 |
| Release added | Release 17 |
| Procedures where used | MBS Broadcast Reception, MBS Service Continuity |
| Related timers | SIB21 does not use a dedicated UE-specific transaction timer |
| Related cause values | SIB21 does not carry reject causes |
System Information Block 21 (SIB21) is an NR broadcast system information block used to provide MBS Frequency Selection Area Identity mapping for service continuity.
Provides MBS FSAI mapping for the serving and neighboring frequencies so the UE can support service continuity for MBS broadcast reception.
SIB21 helps the UE keep MBS broadcast service continuity when frequencies change.
Call flow position: Used after basic MBS acquisition context is known.
Typical state: UE is evaluating service continuity across current and neighboring frequencies.
Preconditions:
Next likely message: UE derives MBS frequencies of interest for continuity behavior
Previous message(s): MIB, System Information Block 1 (SIB1), System Information Block 20 (SIB20)
Next message(s): MBS continuity evaluation across frequencies
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 {
sib21-v1700 SIB21-r17,
...
},
...
}
SIB21-r17 ::= SEQUENCE {
mbs-FSAI-IntraFreq-r17 SEQUENCE OPTIONAL,
...
}
The practical focus is the FSAI mapping. Engineers use it to explain continuity behavior rather than raw access behavior.
BCCH-DL-SCH-Message
message: c1
systemInformation
criticalExtensions
systemInformation
sib-TypeAndInfo
sib21
mbs-FSAI-IntraFreq-r17: present
| IE | Required | Description |
|---|---|---|
mbs-FSAI-IntraFreq-r17 | Optional | List of MBS FSAIs for the current frequency. |
MBS FSAI mapping for neighboring frequencies | Optional | Neighbor-frequency continuity information used for MBS service continuity. |
mbs-FSAI-IntraFreq-r17List of MBS FSAIs for the current frequency.
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.
MBS FSAI mapping for neighboring frequenciesNeighbor-frequency continuity information used for MBS service continuity.
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: SIB21 may be missing or the FSAI mapping may not match the expected continuity domain.
What to inspect: Check mbs-FSAI-IntraFreq-r17 and any neighbor-frequency mapping.
Next step: Compare SIB21 with observed MBS continuity behavior before assuming a pure RF issue.
SIB20 helps the UE acquire MBS broadcast control, while SIB21 helps it maintain continuity across frequencies.
SIB21 is System Information Block 21, used for MBS service continuity 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.