System Information is downlink broadcast information transmitted periodically by base station. SIB#8 is used to provide a Commercial Mobile Alert Service (CMAS) notification. CMAS is used as Public Warning System (PWS) to deliver multiple, concurrent warning text messages. The FCC established CMAS to allow operators to send emergency alerts as text messages to their users. This emergency alerts broadcast can be either within the whole network or within certain geographical areas, down to the size of a single cell.
The Emergency alret messages are basicaly generated by an entity named as Cell Broadcast Entity (CBE) managed by national authorities, dealing with public safety or crime prevention or both in that country where network is deployed. Following figure shows a high level broadcast flow for CMAS alerts.
- SIB#8 can be charchersised as Other System Information (OSI)
- It is wrapped with generic RRC message System Information means when you check RRC message for SIB#8 in UE logs, you will not see it as SystemInformationBlockType8 rather you need to look for SystemInforamtion RRC message and look inside that if it contain SIB#8
- SIB#8 scheduling information is provided by SIB1
- SIB#8 is mapped to BCCH logical channel, DL-SCH transport channel and PDSCH physical channel
- SIB#8 can be recived and decoded by UE in both RRC_IDLE as well as RRC_Connected state
- SIB#8 is mostly commonaly across North America for Emergency Alerts
- Emergency Alerts can includes highest level Presidential Alerts to lowest normal alerts
- Signalling radio bearer: Not Applicable
- RLC Mode: TM
- Logical channels: BCCH
- Transport Channel: DL-SCH
- Physical Channel: PDSCH
- Scrambling RNTI : SI-RNTI
- Direction: gNB to UE
Following show the Content for System Information Block Type #8 used for CMAS alerts in 5G NR.
- messageldentifier: It is a string of 16 bits string which defines the type of CMAS message broadcast in SIB#8. These types are specified within 3GPP TS 23.041 and some of these include ‘Presidential Alerts’, ‘Extreme Alerts with Severity of Extreme, Urgency or Immediate, and Certainty of Observed’ and ‘Severe Alerts with Severity of Severe and Urgency of Expected,
- serialNumber: It is a string of 16 bits string used to track any changes in the CMAS notification. The serial number is updated every time a CMAS message with a specific Message Identifier is updated.
- warningMessageSegmentType: It an enumarated data, used to indicate whether the message sent with CMAS alert is the last segment of the message or not.
- warningMessageSegmentNumber: It represents the number of the message segment with a range from 0 to 63. This number is used when reconstructing the complete message.
- warningMessageSegment: This IEs actually contain the CMAS message and allows to carry single or multi OCTET information.
- dataCodingScheme: It identifies the alphabet, coding and language used for the CMAS message. The allowed values are specified within 3GPP TS 23.038.
- warningAreaCoordinatesSegment: It provides information regarding the area across which the CMAS warning message is valid. The UE uses this infonnation to determine whether or not the warning message is displayed.
Sample SIB#8 Content
Following figure shows a sample content for SIB#8.
- 3GPP TS 38 331 : 5G NR, Radio Resource Control (RRC)
- 3GPP TS 23.041 : Technical realization of Cell Broadcast Service (CBS)
- 3GPP TS 23.038: Alphabets and Language-specific Information
- 5G NR Cell Search and Synchronization
- 5G NR System Information
- 5G Master Information Block (NR-MIB)
- 5G NR System Information Type 1 (NR-SIB1)
- 5G NR System Information Type 2 (NR-SIB2)
- 5G NR System Information Type 3 (NR-SIB3)