This clause specifies the procedures supported in the 5G System to handle failures affecting Public Warning System (PWS). The stage 2 architecture and procedures for PWS are specified in TS 23.041.

The NG-RAN shall report that on-going PWS operation for one or more cells of the NG-RAN has failed by sending a PWS Failure Indication as specified in TS 23.041.

After a NG-RAN (i.e. gNB or ng-eNB) has restarted, it shall delete all its warning message data. If the warning message service is operational in one or more cell(s) of the NG-RAN, the NG-RAN shall send a PWS Restart Indication message, which shall include the identity of the NG-RAN, the identity of the restarted cell(s), and the TAI(s) and Emergency Area Id(s) with which the restarted cell(s) are configured, to request the CBC to re-load its warning message data if applicable. The NG-RAN should send the PWS Restart Indication message via two AMFs of the AMF Region, if possible, to ensure that the CBC receives the message even if one AMF cannot propagate it to the CBC (e.g. due to a path failure between the AMF and the CBC). If the AMF interfaces with multiple CBCs, the AMF shall forward the PWS Restart Indication to all CBCs. Upon receipt of a PWS Restart Indication message, the CBC shall consider that the warning message service is restarted in the reported cell(s), i.e. the service is operational and no warning messages are being broadcast in the cell(s). The CBC shall then re-send the warning message data (with the same message identifier and serial number) to the NG-RAN for these cells, if any. When doing so, the CBC:

• shall provide the identity of the NG-RAN received in the PWS Restart Indication when sending the Write-Replace-Warning-Request message(s) to the AMF, to enable the AMF to forward the message(s) only to the NG-RAN involved in the restart. The identity of the NG-RAN shall be included in:
  • the Write-Replace-Warning-Request message(s) sent to the PWS-IWF over the SBc interface; or
  • the globalRanNodeList IE of the NonUeN2MessageTransfer request message(s) sent to the AMF over the N50 interface (see clauses 5.2.2.4.1.3 and 6.1.6.2.9 of TS 29.518) to request the transfer of the Write-Replace-Warning-Request message(s).
• should set the warning area list to the identities of the cell(s) to be reloaded which are relevant to the warning message data being reloaded; and
• may update the number of broadcasts requested, if necessary.
  The CBC shall consider a PWS Restart Indication message received shortly after a preceding one for the same cell identity as a duplicate restart indication for that cell which it shall ignore.

Likewise, in other scenarios where the NG-RAN may need to reload its warning message data (e.g. when an individual cell is restarted), the NG-RAN shall send a PWS Restart Indication message (including the identity of the NG-RAN, the identity of the restarted cell(s), and the TAI(s) and Emergency Area Id(s) with which the restarted cell(s) are configured) to the CBC to request the CBC to re-load its warning message data if applicable. The NG-RAN, AMF and CBC shall then proceed as specified above for a NG-RAN restart.