3GPP introduced features like CMAS and ETWS as Public Warning System (PWS) services using cell broadcast capabilities.
CMAS and ETWS effectively can provide text based alerts and warnings to the public via a broadcast message sent to an alerting area defined by cell site coverage.
Cellular devices with in that area will receive and display the alert to the user if those devices are enabled to receive warning messages.
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Short Tech Notes : 5G CMAS - Commercial Mobile Alert Service
Call Flow Message :
Emergency Broadcast Report: CBE entiry prepares the Emergency broadcast report as per information Source such as PSAP or Regulator e.g. “warning type”, “warning message”, “impacted area”, “time period”
Non-UeN2MessageTransfer (Write-Replace Warning Request) : The CBCF sends a WriteReplaceWarningRequest message to AMF containing the warning message to be broadcast and the delivery attributes like Message Identifier, Serial Number, Tracking Area ID list, Warning Area, CWM Indicator, Send Write-Replace-Warning-Indication, Global RAN Node ID to AMFs. CBCF selects the AMFs based on the impacted area mapping.
Non-UeN2MessageTransfer (WriteReplaceWarningConfirm): The AMF sends a WriteReplaceWarningConfirm message o the CBCF indicating that the AMF has started to distribute the warning message to gNodeBs nodes.
Emergency Broadcast Response: Upon reception of the WriteReplaceWarningConfirm messages from the AMFs, the CBCF may confirm to the CBE that it has started to distribute the warning message using the Emergency Broadcast Response.
WireReplaceWarningRequest: The AMF forwards WriteReplaceWarningRequest to gNBs. The AMF shall use the Tracking Area ID list to determine the gNB in the delivery area. If the Tracking Area ID list is not included and no Global gNB ID has been received from the CBCF, the message is forwarded to all gNBs connected to the AMF. If a Global gNB ID has been received from the CBCF, the AMF shall forward the message only to the gNB indicated.
SystemInfromationBlockType1: Upon recieving WriteReplaceWarningRequest message from AMF, gNB identify the cells and broadcast the updated SIB#1 including the SIB#8 scheduling information. UE needs to decoded the updated SIB1
SystemInformation: The system information includes SIB8 information having the CMAS alert message
WireReplaceWarningResponse: gNBs sends WriteReplaceWarning Response messages to AMF and the AMF determines the success or failure of the delivery and creates a trace record.
NonUeN2InfoNotify (Write-Replace Warning Indication): AMF notify the CBCF nodes with Write-Replace Warning Indication. This message exchange is optional.
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