Section: 5.27.1.12 Support for network timing synchronization status monitoring
Content: While the time synchronization service is offered by the 5GS, based on 5G access stratum-based time distribution or (g)PTP-based time distribution, the network timing synchronization status of the nodes involved in the operation (e.g. gNBs and/or UPF/NW-TTs) may change. gNBs and UPF/NW-TT can detect timing synchronization degradation or improvement locally. The support for network timing synchronization status monitoring enables the 5GS to modify time synchronization service for a UE or a group of UEs depending on the current synchronization status and notify service updates. There may be three consumers of this information: - TSCTSF may receive node-level information about timing synchronization status from gNB and/or UPF/NW-TT directly from OAM or alternatively, if supported by a node, using control plane signalling at node level. Node level signalling uses UMIC for UPF/NW-TT case and an AMF service to report N2 node level information for the gNB case. In the latter case, the AMF controls the gNB node level reporting and subscription using NGAP messages (see TS 38.413[ NG-RAN; NG Application Protocol (NGAP) ]  [34]). - AF may subscribe to time synchronization status notifications for a UE or group of UEs for which the AF requests or has requested time synchronization service (for 5G access stratum time distribution or (g)PTP services). - For 5G access stratum time synchronization service, the UE may receive clock quality information from the gNB based on UE subscription data stored in the UDM (see clause 5.27.1.11) or AF request for clock quality reporting to the UE. When activating time synchronization for a UE, TSCTSF forwards the clock quality detail level (if available) to the AMF (via PCF using AM policy). The AMF instructs the UE to transition to the RRC_CONNECTED state in the case when the UE later detects that the gNB timing synchronization status has changed while the UE is in the RRC_INACTIVE or RRC_IDLE state. When the UE wants to access the 5GS, the UE shall perform Unified Access Control as defined in TS 38.331[ NR; Radio Resource Control (RRC); Protocol specification ]  [28]. gNBs may be pre-configured with thresholds for each Timing Synchronization Status (TSS) attribute, if supported, that is described in Table 5.27.1.12-1. gNBs may include a reference report ID in SIB information, if supported. A reference report ID consists of a scope of the TSS and an Event ID. A scope of the TSS supports providing TSS information for all cells or a group of cells within a single gNB. Event ID is an integer indicating that the gNB's clock quality has changed, resulting in at least one TSS attribute exceeding or meeting again the pre-configured threshold. Uniqueness of Event ID value is ensured by combining it with a gNB ID as specified in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [27]. When the network TSS attribute exceeds the thresholds (i.e. status degradation), or the network TSS attribute meets the thresholds again (i.e. status improvement), the gNB notifies the TSCTSF (either using N2 node level signalling via AMF, or via OAM) with the scope of the timing synchronization status (i.e. gNB ID or a list of Cell IDs within a single gNB) and the corresponding network timing synchronization status attributes as described in Table 5.27.1.12-1. The gNB indicates the status change to the UEs via the reference report ID change in SIB information: - When the network timing synchronization status exceeds any of the pre-configured thresholds (i.e. status degradation) or meets the threshold again (i.e. status, improvement), the gNB changes the reference report ID in SIB information. Either event serves as a notification for the UEs reading the SIB information that there is new TSS information available. NOTE 1: NG-RAN is assumed not to provide clock quality metrics better than the pre-configured threshold, i.e. if a clock quality metric is better than the corresponding threshold, the NG-RAN reports the threshold value to the UE in an RRC message instead. NOTE 2: It is assumed the pre-configured thresholds in the gNB(s) are sufficient to meet UE time sync performance requirement which are configured by the operator. - If supported, the UE in the RRC_INACTIVE or RRC_IDLE state compares the reference report ID in SIB information with its locally stored reference report ID to determine whether it has the latest available clock quality information already or it needs to transit to the RRC_CONNECTED state to retrieve it. - If the UE is instructed by AMF (via the Registration procedure, or the UE Configuration Update procedure) to reconnect to the network in the case when the UE determines that the reference report ID has changed, the UE in the RRC_INACTIVE or RRC_IDLE state, if supported by the UE, reconnects to the network. RAN may delay or prioritize UE's transition to the RRC_CONNECTED state using the UAC framework [28], i.e. UEs are not expected to transition to the RRC_CONNECTED state immediately after determining that the clock quality information has changed and receiving instructions from the AMF. After the UE has reconnected to the network, the gNB uses unicast RRC signalling to provision the clock quality information to the UEs. The network timing synchronization status information from gNB or UPF/NW-TT to the TSCTSF may contain the following information as described in the Table 5.27.1.12-1. The details for gNB timing synchronization status information are specified in TS 38.413[ NG-RAN; NG Application Protocol (NGAP) ]  [34]. However, it is up to gNB to determine whether to provide its timing synchronization status reporting and which of the information elements to include in the TSS report to the TSCTSF, i.e. based on the implementation gNB may report all, some, or none of the information elements from Table 5.27.1.12-1. Table 5.27.1.12-1: Information elements that gNB or UPF/NW-TT timing synchronization status information may contain (all optional) The TSCTSF determines the UEs impacted by gNB's timing synchronization status change (i.e. degradation, failure or improvement) or UPF timing synchronization status change (only for the case when UPF/NW-TT is involved in providing time information to DS-TT). - For the gNB case, when the TSCTSF receives information about timing synchronization status change, the TSCTSF uses the NRF to discover the AMFs serving the impacted gNBs and subscribes to receive notifications for UE's presence in Area of Interest information from AMF as described in clause 5.3.4.4. The Area of Interest is set to the scope of the timing synchronization status (i.e. gNB ID or a group of cells within the gNB specified with a list of Cell IDs that has reported status degradation (i.e. the pre-configured thresholds are exceeded in the gNB). The subscription is targeted to any UE in the AMF, the TSCTSF may provide additional filtering information as specified in clause 5.3.4.4 (e.g., List of UE IDs, DNN(s)/S-NNSAI(s)) to limit the subscription to the indicated UE identities, UEs having a PDU Session with the given DNN(s)/S-NSSAI(s). The TSCTSF correlates information about impacted gNBs and the UE location information received from the AMF. If the gNB notifies the TSCTSF for the status improvement (i.e. the pre-configured thresholds are met in the gNB), the TSCTSF modifies the subscription to remove the corresponding Area of Interest from the subscription. - For UPF case, the TSCTSF determines the UEs for which the impacted UPF/NW-TT is configured to send (g)PTP messages on behalf of DS-TT (see clause 5.27.1.7). If the gNB's or UPF's timing synchronization status change, the TSCTSF may perform the following: - For AFs that subscribe for 5G access stratum time synchronization service or (g)PTP time synchronization service status update (i.e. change in support status of the clock quality acceptance criteria provided by the AF and specified using TSS attributes from Table 5.27.1.12-1), the TSCTSF may provide notification towards the AF when there is a change in support status for a UE or group of UEs. - Deactivating/reactivating/updating time synchronization services: - (g)PTP time synchronization service case: For UEs that are part of a PTP instance and which are impacted by NG-RAN or UPF time synchronization status degradation or improvement: - If TSCTSF determines that the clock quality acceptance criteria provided by AF can still be met, then TSCTSF may update the clock quality information sent in Announce messages (see clause 7.6.2 of IEEE 1588 [8]) for the PTP instance using existing procedures and existing PMIC/UMIC information. The handling of Announce messages follows existing procedures as described in clause 5.27.1.6. - If TSCTSF determines that the clock quality acceptance criteria provided by AF cannot be met, then TSCTSF informs the AF for the corresponding PTP ports being inactive due to the result of fulfilling the clock quality acceptance criteria; and the TSCTSF temporarily removes the UE/DS-TT from the PTP instance using the procedure in clause K.2.2.1 and clause K.2.2.4. The AF may send a service update or delete request (see clause 4.15.9.3 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3]). - If TSCTSF determines that the clock quality acceptance criteria provided by AF can be met again then TSCTSF informs the AF about the result, adds the DS-TT PTP port to the PTP instance again and re-activates the Grandmaster functionality. For 5G access stratum time synchronization service, clock quality reporting control information manages the gNBs timing synchronization status reporting to the UE. When AMF provides the 5G access stratum time distribution indication and the Uu time synchronization error budget to gNB, the AMF also includes the clock quality reporting control information (CQRCI) provided by the TSCTSF or retrieved from UDM. CQRCI may be a part of Access and Mobility Subscription data at the UDM, or AF may include CQRCI in its request. CQRCI contains the following fields: - Clock quality detail level. It indicates whether and which clock quality information to provide to the UE and can take one of the following values: "clock quality metrics" or "acceptable/not acceptable indication". - If the clock quality detail level equals "clock quality metrics", the NG-RAN provides clock quality metrics to the UE that reflect its current timing synchronization status. i.e. one or more of the following information elements: clock accuracy, traceability to UTC, traceability to GNSS, frequency stability, parent time source, synchronization state as defined in Table 5.27.1.12-1. NG-RAN is locally configured which of the clock quality metrics supported by NG-RAN are provided to UE(s). - If the clock quality detail level equals "acceptable/not acceptable indication", NG-RAN provides clock quality acceptance criteria for the UE. The gNB provides an acceptable indication 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. If AF includes clock quality acceptance criteria in its request towards TSCTSF, the AF shall be notified about the result once TSCTSF determines whether the clock quality acceptance criteria can be met or not. Based on the notification, the AF may decide to modify the service if preferred (e.g., disable the service upon status degradation or enable it again upon status improvement). When determining the clock quality metrics for a UE and when determining whether clock quality is acceptable or not acceptable for a UE, the gNB considers whether propagation delay compensation is performed. NOTE 3: In this Release, UE capabilities and internal inaccuracies are assumed to be budgeted by the client network operator when agreeing the required clock accuracy with the 5G network operator. To provision clock quality information to the UEs, a gNB uses unicast RRC signalling: - For UEs in the RRC_CONNECTED state, the gNB uses unicast RRC signalling. - UEs that are not in the RRC_CONNECTED state first need to establish or resume the RRC connection to receive the clock quality information from the gNB via unicast RRC signalling. During N2 Handover and Xn handover, Service Request, mobility registration and AM policy modification procedure, the AMF may provide the CQRCI to NG-RAN.