Section: 16.8.2.3 Network timing synchronization monitoring towards UE
Content: The gNB may receive clock quality reporting control information for a UE from the AMF, see TS 23.501[ System architecture for the 5G System (5GS) ]  [3]. The clock quality reporting control information contains the clock quality detail level (i.e., "metrics" or "acceptable/not acceptable indication") and clock quality acceptance criteria for the UE (if the clock quality detail level equals "acceptable/not acceptable indication"). Based on the clock quality reporting control information, the gNB determines how to provision clock quality information to the UE: - If the clock quality detail level equals "clock quality metrics", the gNB provides clock quality metrics supported by the gNB to the UE, i.e., one or more of the following information elements: synchronization state, traceability to UTC, traceability to GNSS, clock frequency stability, clock accuracy, parent time source, as defined in Table 5.27.1.12-1 in TS 23.501[ System architecture for the 5G System (5GS) ]  [3]. - If the clock quality detail level equals "acceptable/not acceptable indication", the gNB indicates "acceptable" to the UE if the gNB's timing synchronization status matches the acceptance criteria received from the AMF; otherwise, the gNB indicates "not acceptable" to the UE. Clock quality acceptance criteria can be defined based on one or more information elements listed in Table 5.27.1.12-1 in TS 23.501[ System architecture for the 5G System (5GS) ]  [3]. To provision clock quality information to the UEs: - For UEs in the RRC CONNECTED state, the gNB uses unicast RRC signalling. The RRC signalling includes Event ID and clock quality information. - For UEs that are not in the RRC_CONNECTED state, the UE first needs to establish or resume the RRC connection to receive the gNB timing synchronization status information from the gNB via unicast RRC signalling. The gNB broadcasts Event ID in SIB9 to notify its timing synchronization status. Event ID or gNB ID change serves as a notification for the UEs reading the SIB information that there is new RAN timing synchronization status information available. The following figure describes the signalling procedure of gNB reporting clock quality information to a UE: Figure 16.8.2-1: Signalling procedure of gNB reporting clock quality information to a UE 0. The gNB node is pre-configured for the thresholds for each timing synchronization status attribute as described in clause 5.27.1.12 in TS 23.501[ System architecture for the 5G System (5GS) ]  [3]. If there is a change on its primary source so that the thresholds are exceeded or met again, the NG-RAN node detects a change on its timing synchronization state (e.g., degradation, failure, recovery). 1. The gNB notifies a change on its timing synchronization operation using Event ID in SIB9. The Event ID scope is local to gNB. 2. The UE in RRC_INACTIVE or RRC_IDLE determines if there is clock quality information update available at the gNB based on SIB9 information. For a UE in RRC_CONNECTED state, steps 2-3 can be skipped. 3. If there is a RAN timing synchronization status update available, the UE's RRC layer indicates this to the NAS layer which may request the RRC layer to initiate RRC Setup or RRC Resume procedure. 4. The gNB determines clock quality information reporting to the UE (e.g., metrics or "acceptable/not acceptable"). 5. The gNB sends the clock quality information to the UE via unicast RRC signalling.