This clause specifies the procedures supported in the 5G System to restore network timing synchronization status monitoring upon a failure affecting the AMF, NG-RAN or TSCTSF. The stage 2 procedures for network timing synchronization status monitoring are specified in clause 4.15.9.5 of TS 23.502.

When an AMF instance fails with or without restart, the subscriptions for non-UE specific N2 information that were created at this AMF instance before the AMF failure are lost. The TSCTSF may detect the failure of the AMF instance with or without restart by using the procedures defined in clauses 6.2 and 6.3. Upon sending an NG SETUP REQUEST to the restarted AMF to re-establish the N2-connection, the NG-RAN shall delete any subscription for TSS information reporting from any TSCSTF that was created by the AMF before the failure. Upon detecting that the AMF instance has failed with or without restart, a TSCSTF that had subscribed to TSS information reporting via the failed or restarted AMF instance should subscribe again to TSS information reporting (by sending an Namf_NonUeN2InfoSubscribe request and a Namf_NonUeN2MessageTransfer request as specified in clause 4.15.9.5 of TS 23.502) via the restarted AMF instance or via an alternative AMF instance from the same or a different AMF set serving the target area (e.g. TAIs or NG-RAN IDs) that was served by the failed or restarted NG-RAN node.

When an NG-RAN node fails with or without restart, the subscriptions for TSS information reporting that were created at the NG-RAN node before the NG-RAN failure are lost. Upon detecting that an NG-RAN node fails with or without restart, the AMF shall notify the TSCSTFs subscribed to TSS information reporting about the failure of the NG-RAN node with or without restart by sending an Namf_NonUeN2InfoNotify request with the tssInfo IE including the NG-RAN node ID and the ngranFailureInfo IE indicating that the NG-RAN has failed with or without restart. Upon being notified by the AMF of the NG-RAN failure without restart, the TSCSTF may log the information and should consider that no more TSS information can be received from the failed NG-RAN. Upon being notified by the AMF of the NG-RAN failure with restart, the TSCTSF should subscribe again to TSS information reporting (by sending an Namf_NonUeN2MessageTransfer request as specified in clause 4.15.9.5 of TS 23.502 but without sending a Namf_NonUeN2InfoSubscribe request since the subscription already exists at the AMF) to the restarted NG-RAN.

The procedure specified in this clause is optional to support. When a TSCTSF instance fails with or without restart, the subscription(s) for TSS information reporting activated in one or more NG-RANs, including the non-UE specific N2 information subscription in the AMF(s), are lost at the TSCTSF if the TSCTSF is not deployed within an TSCTSF set. The AMF may detect the failure of the TSCTSF instance with or without restart by using the procedures defined in clauses 6.2 and 6.3. Upon detecting that a TSCTSF instance which subscribed to the AMF to receive subsequent TSS reports, has failed with or without restart, and no alternative TSCTSF instance can be reselected, the AMF may:

• delete the non-UE specific N2 information subscription from the TSCTSF instance locally and stop forwarding any subsequent report to the failed TSCTSF; and
• send an NGAP Timing Synchronization Status Request message with the Routing Id set to the failed TSCTSF NF ID to stop TSS reporting on behalf of the failed TSCTSF, towards NG-RAN(s) from which the AMF receives an NGAP TSS Report message intended for the failed TSCTSF (i.e. for a TSCTSF for which no non-UE specific N2 information subscription exists at the AMF), to avoid possible hanging TSS reporting in the NG-RAN(s).

Alternatively, based on the AMF implementation, the AMF may do the above (i.e. delete the non-UE specific N2 information subscription and request the NG-RAN to stop reporting TSS information for the failed TSCTSF) upon forwarding TSS reports to the TSCSTF and receiving no responses from the TSCTSF or receiving application errors "RESOURCE_CONTEXT_NOT_FOUND" or "SUBSCRIPTION_NOT_FOUND" in the NonUeN2Info Notify Response message.