Section: 5.27.1.8 Exposure of Time Synchronization
Content: 5G System supports time synchronization service that can be activated and deactivated by AF. Exposure of time synchronization comprises the following capabilities: - The AF may learn 5GS and/or UE availability and capabilities for time synchronization service. - The AF controls activation and deactivation of the time synchronization service for the target UE(s). - The AF may subscribe to time synchronization service status for the target UE(s). The AF may use the service-specific parameters to control the time synchronization service for targeted UE(s). These parameters are specified in clause 4.15.9.3 and 4.15.9.4 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3] for (g)PTP-based and 5G access stratum-based time synchronization services, respectively. The AF may subscribe for 5GS and/or UE availability and capabilities for time synchronization service. The AF indicates in the request the DNN, S-NSSAI, and in addition the AF may indicate a list of UE identities or group identity to limit the subscription only to corresponding UEs. If the AF does not indicate DNN, S-NSSAI, the NEF determines the DNN, S-NSSAI based on the AF Identifier. The TSCTSF (directly or via NEF) exposes the 5GS and/or UE availability and capabilities for synchronization service to the AF as described in clause 4.15.9.2 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3]. The exposed information includes the list of user plane node identities, the list of UE identities and may include the supported capabilities for (g)PTP time synchronization service per user plane node and UE. The AF request to control the (g)PTP time synchronization service is sent to the TSCTSF (directly or via NEF). The request is targeted to a set of AF-sessions that are associated with the exposure of UE availability and capabilities for synchronization service. The AF may request to use a specific PTP instance type when requesting the (g)PTP-based time synchronization distribution method (IEEE Std 1588 [126] or IEEE Std 802.1AS [104] operation (i.e. as a Boundary Clock, peer-to-peer Transparent Clock, or end-to-end Transparent Clock or as a PTP relay instance)). The request to control the (g)PTP time synchronization service may contain other service parameters as specified in Table 4.15.9.3-1 in clause 4.15.9.3 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3]. The AF may request to use the 5G access stratum as a time synchronization distribution method. In this case, the time source is provided by the 5GS. 5G-AN provides the 5GS time to the UE via 3GPP radio access; UE/DS-TT may provide 5G access stratum timing information to end stations using implementation specific means. The request to control the 5G access stratum time distribution (including the parameters such AF requests may contain) is described in clause 4.15.9.4 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3]. The AF or NEF selects the TSCTSF as specified in clause 6.3.24. The AF request may include a time synchronization error budget (see also clause 5.27.1.9). The time synchronization error budget defines an upper bound for time synchronization errors introduced by 5GS. The AF uses the procedure for configuring the (g)PTP instance in 5GS as described in clause 4.15.9.3 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3] and uses the procedure for providing the 5G access stratum time distribution as described in clause 4.15.9.4 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3] for the UEs. The TSCTSF uses the Time Synchronization parameters (Table 4.15.9.3-1 in TS 23.502[ Procedures for the 5G System (5GS) ]  [3]) as received from the AF (directly or via NEF) to control the (g)PTP time synchronization service. When IEEE Std 1588 [126] or IEEE Std 802.1AS [104] operation have been selected, the TSCTSF determines the necessary (g)PTP parameters to activate and control the service in DS-TT(s) and NW-TTs. For this purpose, the TSCTSF uses the PMIC or UMIC to manage the IEEE Std 1588 [126] or IEEE Std 802.1AS [104] operation in the DS-TT(s) or NW-TTs, respectively (see clause 5.27.1.4). The TSCTSF may indicate whether it can support the service or not as per the requested acceptance criteria (e.g. based on the known timing synchronization status attribute thresholds also pre-configured at gNB) and provide notification when there is a service status update if the AF subscribes to service status updates (see also clause 5.27.1.12). The TSCTSF uses the Time Synchronization parameters (Table 4.15.9.4-1 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3]) as received from the AF (directly or via NEF) to control the 5G access stratum time synchronization distribution as described in clause 4.15.9.4 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3]. For handling (g)PTP traffic, the PCF, according to PCC rule authorization, chooses a 5QI and dynamically set the PDB and/or MDBV according to requirements for (g)PTP protocol. The PCF provides the SMF with a PCC rule generated based on the AF request to control the (g)PTP time synchronization service. The SMF may take the information in the PCC rule to modify a PDU Session to create or modify or release a QoS Flow for transmitting the (g)PTP messages. The PCF acknowledges the policy request to the TSCTSF. The TSCTSF may report the result of the time synchronization request to the AF (directly or via NEF). The AF may provide a temporal validity condition to the TSCTSF (directly or via NEF) when the AF activates or modifies the time synchronization service. Temporal validity condition contains the start-time and stop-time (in absolute time value) attributes that describe the time period when the time synchronization service is active for the targeted AF sessions. The TSCTSF manages the temporal validity condition as described in clauses 4.15.9.3 and 4.15.9.4 of TS 23.502[ Procedures for the 5G System (5GS) ]  [3]. The AF may provide clock quality detail level and clock quality acceptance criteria to the TSCTSF (directly or via NEF) when the AF activates or modifies the time synchronization service. For ASTI based time synchronization services, the TSCTSF provides the clock quality reporting control information to AMF (see also clause 5.27.1.12). The AF may provide a requested coverage area for the time synchronization service to the TSCTSF (directly or via NEF) when the AF activates or modifies the time synchronization service. The requested coverage area defines a spatial validity condition for the service using a geographical area (e.g. a civic address or shapes), or a list of Tracking Area Identities (TAIs).