Content for  TS 23.501  Word version:  19.1.0

For supporting time synchronization service, the 5GS is configured to operate in one or multiple PTP instances and to operate in one of the following modes (if supported) for each PTP instance:

1. as time-aware system as described in IEEE Std 802.1AS [104],
2. as Boundary Clock as described in IEEE Std 1588 [126], provisioned by the profiles supported by this 3GPP specification including SMPTE Profile for Use of IEEE Std 1588 [126] Precision Time Protocol in Professional Broadcast Applications ST 2059-2:2015 [127];
3. as peer-to-peer Transparent Clock as described in IEEE Std 1588 [126], provisioned by the profiles supported by this 3GPP specification including SMPTE Profile for Use of IEEE Std 1588 Precision Time Protocol in Professional Broadcast Applications ST 2059-2:2015 [127]; or
4. as end-to-end Transparent Clock as described in IEEE Std 1588 [126], provisioned by the profiles supported by this 3GPP specification including SMPTE Profile for Use of IEEE Std 1588 Precision Time Protocol in Professional Broadcast Applications ST 2059-2:2015 [127].

The configuration of the time synchronization service in 5GS for option 1 by TSN AF and CNC is described in clause 5.28.3, and for options 1-4 by AF/NEF and TSCTSF in clause 5.27.1.8 and clause 5.28.3. The 5GS shall be modelled as an IEEE Std 802.1AS [104] or IEEE Std 1588 [126] compliant entity based on the above configuration. The DS-TT and NW-TT at the edge of the 5G system may support the IEEE Std 802.1AS [104] or other IEEE Std 1588 [126] profiles' operations respective to the configured mode of operation. The UE, gNB, UPF, NW-TT and DS- TTs are synchronized with the 5G GM (i.e. the 5G internal system clock) which shall serve to keep these network elements synchronized. The TTs located at the edge of 5G system fulfil some functions related to IEEE Std 802.1AS [104] and may fulfil some functions related to IEEE Std 1588 [126], e.g. (g)PTP support and timestamping. Figure 5.27.1-1 illustrates the 5G and PTP grandmaster (GM) clock distribution model via 5GS.

Figure 5.27.1-1 depicts the two synchronizations systems considered: the 5G Clock synchronization and the (g)PTP domain synchronization.

• 5G Access Stratum-based Time Distribution: Used for NG RAN synchronization and also distributed to the UE. The 5G Access Stratum-based Time Distribution over the radio interface towards the UE is specified in TS 38.331. This method may be used to either further distribute the 5G timing to devices connected to a UE (using implementation-specific means) or to support the operation of the (g)PTP-based time distribution method.
• (g)PTP-based Time Distribution: Provides timing among entities in a (g)PTP domain. This process follows the applicable profiles of IEEE Std 802.1AS [104] or IEEE Std 1588 [126]. This method relies on the 5G access stratum-based time distribution method to synchronize the UE/DS-TT and on the 5GS time synchronization to synchronize the gNB (which, in turn, may synchronize the DS-TT) and the NW-TT.

The gNB needs to be synchronized to the 5G GM clock. The 5GS supports two methods for determining the grandmaster PTP Instance and the time-synchronization spanning tree.

• Method a), BMCA procedure.
• Method b), local configuration.

This is further described in clause 5.27.1.6.

This clause describes support for multiple domains for gPTP and PTP and for GM clocks connected to DS-TT and NW-TT and only applies if DS-TT and NW-TT support the related functionality. PTP support and support of gPTP for GM clocks connected to DS-TT by DS-TT and NW-TT may be determined as described in clause K.2.1. Each (g)PTP domain sends its own (g)PTP messages. The (g)PTP message carries a specific PTP "domainNumber" that indicates the time domain they are referring to. The PTP port in ingress TT makes ingress timestamping (TSi) for the (g)PTP event messages of all domains and forwards the (g)PTP messages of all domains to the UPF/NW-TT that further distributes the (g)PTP messages to the egress TTs as specified in clause 5.27.1.2.2. The PTP port in the egress TT receives the original PTP GM clock timing information and the corresponding TSi via (g)PTP messages for one or more (g)PTP domains. The PTP port in the egress TT then makes egress timestamping (TSe) for the (g)PTP event messages for every (g)PTP domain. Ingress and egress time stamping are based on the 5G system clock at NW-TT and DS-TT. The process described in clause 5.27.1.2.2 is thus repeated for each (g)PTP domain between a DS-TT and the NW-TT it is connected to.

This clause describes the support of Time Synchronization functionality supported by the 5G System. Synchronization between UPF/NW-TT and NG-RAN is outside scope of 3GPP. DS-TT and NW-TT may support the following PTP instance types:

• Boundary Clock as defined in IEEE Std 1588 [126] as described in clause 5.27.1.1;
• End-to-End Transparent Clock as defined in IEEE Std 1588 [126] as described in clause 5.27.1.1;
• Peer-to-Peer Transparent Clock as defined in IEEE Std 1588 [126] as described in clause 5.27.1.1;
• PTP Relay instance as defined in IEEE Std 802.1AS [104].

DS-TT and NW-TT may support the following transports for PTP:

• IPv4 as defined in IEEE Std 1588 [126] Annex C;
• IPv6 as defined in IEEE Std 1588 [126] Annex D;
• IEEE Std 802.3 [131] (Ethernet) as defined in IEEE Std 1588 [126] Annex E.

For operation as a Boundary clock or as a Transparent Clock, DS-TT and NW-TT may support the following path and link delay measurement methods:

• Delay request-response mechanism as described in section 11.3 of IEEE Std 1588 [126];
• Peer-to-peer delay mechanism as defined in section 11.4 of IEEE Std 1588 [126].

DS-TT and NW-TT may support acting as a PTP grandmaster, i.e. may support generating (g)PTP Announce, Sync and Follow_Up messages. DS-TT and NW-TT supporting (g)PTP shall support one or more PTP profiles as described in section 20.3 of IEEE Std 1588 [126], i.e.:

• Default PTP Profiles in IEEE Std 1588 [126], Annex I;
• IEEE Std 802.1AS [104] PTP profile for transport of timing as defined in IEEE Std 802.1AS [104] Annex F;
• SMPTE Profile for Use of IEEE Std 1588 [126] Precision Time Protocol in Professional Broadcast Applications ST 2059-2:2015 [127].

TSN AF and TSCTSF may determine the PTP functionalities supported by DS-TT and NW-TT and may configure PTP instances in DS-TT and NW-TT using port and user plane node management information exchange as described in Annex K, clause K.2.

The procedure described in this clause is applicable when the PTP instance in 5GS is configured to operate as a time-aware system or as a Boundary Clock, and the PTP grandmaster is external to the 5GS, and the BMCA procedure (Method a) is used as described in clause 5.27.1.6. The NW-TT processes Announce messages according to IEEE Std 1588 [126]. In particular, the NW-TT shall compute and maintain the time when the Announce and Sync timeout events occur for the PTP port in a Follower state. When the 5GS is configured to operate as a time-aware system, the NW-TT shall determine the Sync and Announce message interval for the PTP Port at the other end of the link to which the Follower PTP Port in 5GS is attached, as described in IEEE Std 802.1AS [104]. When the 5GS is configured to operate as a Boundary Clock, the NW-TT shall determine Announce interval based on the configuration of the Follower port in 5GS, as described in IEEE Std 1588 [126]. The configuration of PTP instances in DS-TT and NW-TT for Sync and Announce timeouts is described in clause K.2. Upon detection of the Sync or Announce timeout event, the NW-TT shall re-evaluate the DS-TT and NW-TT port states as described in clause 5.27.1.6.

The procedure described in this clause is applicable if DS-TT and NW-TT support operating as a Boundary Clock described in IEEE Std 1588 [126] or as a time-aware system (support of the IEEE 802.1AS [104] PTP profile) and when the PTP instance in 5GS is configured to operate as a time-aware system or as a Boundary Clock. Whether DS-TT/NW-TT support operating as a Boundary Clock or as a time-aware system may be determined as described in clause K.2.1. The externally-observable behaviour of the Announce message handling by 5GS needs to comply with IEEE Std 802.1AS [104] or IEEE Std 1588 [126], respective to the configured mode of operation. The DS-TT forwards the received Announce messages to NW-TT over User plane. The NW-TT port forwards the received Announce messages from N6 interface to NW-TT. The NW-TT maintains the PTP port state for each DS-TT port and NW-TT port. The PTP port states may be determined by NW-TT either via:

• Method a), BMCA procedure.
• Method b), local configuration.

When Method b) is used, the following applies:

• When the PTP GM is external to the 5GS, for one of the NW-TT or DS-TT ports (per each PTP domain) the PTP port state is Follower and for all other NW-TT and DS-TT ports of the same PTP domain the PTP port state is set either to Passive or Leader (depending on implementation).
• When the 5GS is configured as a grandmaster for a (g)PTP domain for the connected networks, all NW-TT ports and DS-TT ports are set to Leader state for that (g)PTP domain.

The local configuration of PTP port states in DS-TT and NW-TT for Method b is described in clause K.2. When the Method a) is used (PTP port states are determined by BMCA procedure), the NW-TT needs to process the received Announce messages (from NW-TT port(s) and over user plane from the DS-TT(s)) for BMCA procedure, determine port states within the 5GS, and maintain the Leader-Follower hierarchy. The DS-TT maintains the Disabled, Initializing and Faulty (if applicable) state for the PTP ports in DS-TT. While in Disabled, Initializing or Faulty state, the port in the DS-TT discards any (g)PTP messages it may receive from the upstream PTP instance or from the NW-TT via user plane. When the 5GS Clock is determined as a grandmaster for a (g)PTP domain, the Announce messages are distributed as described in clause 5.27.1.7. When the grandmaster is external to the 5GS, the NW-TT regenerates the Announce messages based on the Announce messages received from Follower port in NW-TT or DS-TT for the Leader ports in NW-TT and DS-TT(s). The NW-TT/UPF forwards the regenerated Announce messages to the PDU session(s) related to the Leader ports in the DS-TT(s).

The 5GS that is configured to operate as a time-aware system or Boundary Clock may support acting as a PTP grandmaster for a (g)PTP domain. The configuration of PTP instances in DS-TT and NW-TT for PTP grandmaster function is described in clause K.2. The following options may be supported (per DS-TT) for the 5GS to generate the Sync, Follow_Up and Announce messages for the Leader ports on the DS-TT:

1. NW-TT generates the Sync, Follow_Up and Announce messages on behalf of DS-TT (e.g. if DS-TT does not support this).
   The NW-TT/UPF forwards the generated Sync, Follow_Up and Announce messages to the PDU session(s) related to the Leader ports on the DS-TT(s). The NW-TT timestamps the (g)PTP event message when the event message is sent to the PDU Session, and adds TSi corresponding to the timestamp to the Sync message and the OriginTimestamp corresponding to the timestamp to Sync message (if one-step operation is used) or PreciseOriginTimestamp corresponding to the timestamp to Follow_Up message (if two-step operation is used), and sets the cumulative rateRatio value with 1. The OriginTimestamp or PreciseOriginTimestamp shall be set by NW-TT/UPF to the 5GS internal clock.
   When DS-TT(s) receive the Sync, Follow_Up messages, it modifies the payload of the Sync, Follow_Up message as described for the PTP port in the egress TT in clause 5.27.1.2.2.2.
2. DS-TT generates the Sync, Follow_Up and Announce messages in this DS-TT. The OriginTimestamp or PreciseOriginTimestamp shall be set by DS-TT to the 5GS internal clock.

In both options, the NW-TT generates the Sync, Follow_Up and Announce messages for the Leader ports on the NW-TT.

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 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. 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. 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. 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 and uses the procedure for providing the 5G access stratum time distribution as described in clause 4.15.9.4 of TS 23.502 for the UEs. The TSCTSF uses the Time Synchronization parameters (Table 4.15.9.3-1 in TS 23.502) 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) 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. 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. 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) (see also clause 5.27.1.10).

The AF may request a specific time synchronization error budget when requesting a time synchronization service employing the (g)PTP-based or 5G access stratum-based time distribution method. If the AF includes a time synchronization error budget in its request, the TSCTSF uses it to derive an error budget available for the NG-RAN to provide the 5G access stratum time via the Uu interface to each targeted UE (referred to as Uu time synchronization error budget hereafter). The Time Synchronization Subscription data may optionally contain the authorized Uu time synchronization error budget. When the TSCTSF receives an AF request with a specific time synchronization error budget for the time synchronization service, the TSCTSF validates the Uu time synchronization error budget as described in clause 5.27.1.11. To derive the Uu time synchronization error budget for each targeted UE, the TSCTSF takes the following into account:

• selected time synchronization distribution method (i.e. 5G access stratum-based time distribution or (g)PTP-based time distribution);
• Uu time synchronization error budget in the Time Synchronization Subscription data defined in clause 5.27.1.11;
• in the case of the (g)PTP-based time distribution:
  • whether 5GS operates as a boundary clock and acts as a GM;
  • whether a clock connected to the DS-TT/NW-TT acts as a GM;
• PTP port states;
• a CN part and a Device part of the time synchronization error budget (both parts may be predefined at the TSCTSF, or calculated by the 5GS using the implementation-specific means).

If the AF does not include a time synchronization error budget, the TSCTSF uses a preconfigured time synchronization error budget to derive the Uu time synchronization error budget. The TSCTSF provides a 5G access stratum time distribution indication and the derived Uu time synchronization error budget to NG-RAN as described in clause 4.15.9.4 of TS 23.502. Based on this, NG-RAN provides the 5G access stratum time to the UE according to the Uu interface time synchronization error budget as provided by the TSCTSF (if supported by UE and NG-RAN). During Handover, Service Request, mobility registration and AM policy modification procedure, the AMF may provide the 5G access stratum time distribution indication and the Uu time synchronization error budget to NG-RAN as described in clause 4.15.9.4 of TS 23.502, if needed.

This feature enables the AF to request time synchronization service for a UE or a group of UEs in a specific geographical area (so called coverage area). The requested coverage area contains a list of Tracking Area (TA) or a geographical area (e.g. a civic address or shapes that the NEF transforms to list of TAs based on pre-configuration). TSCTSF checks with the UDM if the UE is allowed to receive the time synchronization service requested by AF and may update the Time Synchronization Coverage Area as described in clause 5.27.1.11. The coverage area defines a spatial validity condition for the targeted UE(s) that is resolved at the TSCTSF. In order to do that, the TSCTSF may either:

1. discover the AMF(s) serving the list of TA(s) that comprise the spatial validity using Nnrf_NFDiscovery_Request service from the NRF and subsequently, the TSCTSF subscribes to the discovered AMF(s) to receive notifications about presence of the UE in an Area of Interest events (as described in clause 5.3.4.4). The subscription is targeted to Any UE. To reduce the number of Area of Interest reports (based on presence of UE) for the discovered AMFs, the TSCTSF may provide additional filtering information (e.g. List of UE IDs, DNN(s)/S-NNSAI(s)) to limit the subscription to the indicated UE identities, or UEs having a PDU Session with the given DNN/S-NSSAI as specified in clause 5.3.4.4.
2. determine the serving AMF for each of the targeted UE(s) using the UDM and subscribe to the serving AMF to receive notification about presence of the UE in an Area of Interest (as described in clause 5.3.4.4).

An Area of Interest (AoI) for each AMF is represented by a list of TA(s), wherein the Area of Interest is identical to the Time Synchronization Coverage Area. Based on the outcome provided by the AMF about the UE's presence in the AoI, the TSCTSF determines if the time synchronization service is activated or deactivated. For access stratum distribution activation/deactivation, the TSCTSF will enable/disable access stratum time distribution to the UE at the serving NG-RAN node reusing the procedures in clause 4.15.9.4 of TS 23.502. For (g)PTP distribution activation/deactivation, the TSCTSF will modify the PTP instance configuration by means of sending a PMIC to the impacted UE/DS-TTs and UMIC to the impacted UPF/NW-TT, as described in clause K.2.2. If the time synchronization service is modified based on the reports of UE presence in the Area of Interest, the TSCTSF informs the AF for the impacted UE(s) by indicating the PTP port state for the related DS-TT PTP port (in the case of (g)PTP based time distribution) or notifying the AF with the indication of 5G access status time distribution status (enabled or disabled, for ASTI based time distribution). If the Time Synchronization Coverage Area requested by the AF includes at least one TA that is a part of UE's Registration Area (RA), the 5GS may provide the AF-requested time synchronization service to the targeted UE within its RA, i.e. all TAs of the RA shall be treated as a Time Synchronization Coverage Area even if some of the TAs were not requested by the AF.

The distribution of timing information, 5G access stratum-based time distribution and (g)PTP-based time distribution, for a UE may be controlled based on subscription data stored in the UDM. The (g)PTP-based or 5G access stratum-based time synchronization service may be provided to a UE based on the UE's subscription which is specified in clause 5.2.3.3.1 of TS 23.502. The Access and Mobility Subscription data include the following information for the control of 5G access stratum-based time distribution:

• the Access Stratum Time Synchronization Service Authorization, which indicates whether the UE should be provisioned with 5G system internal clock timing information over access stratum as specified in TS 38.331.
• optionally, the Uu time synchronization error budget.
• optionally, one or more periods of start and stop times defining the times when the UE should be provisioned with 5G system internal clock timing information.
• optionally, a Time Synchronization Coverage Area comprising a list of TAs where the UE shall be provisioned with 5G system internal clock timing information.
• optionally, a clock quality detail level indicating whether and which clock quality information to provide to the UE. It comprises one of the following values: clock quality metrics or acceptable/not acceptable indication.
• optionally, the clock quality acceptance criteria for the UE. It may be defined based on one or more attributes listed in Table 5.27.1.12-1.

During the Registration procedure, the AMF retrieves the subscription from UDM. If the AMF receives 5G access stratum-based time synchronization service subscription for the given UE, the AMF controls the 5G access stratum-based time distribution:

• If the 5G access stratum-based time synchronization service is allowed for the UE, the AMF provides the 5G access stratum time distribution indication to the NG-RAN so that it can provide 5G timing information to the UE.
• The AMF may provide a Uu time synchronization error budget to the NG-RAN (as described in clause 5.27.1.9). If the UE's subscription contains a Uu time synchronization error budget, then AMF sends it to NG-RAN. Otherwise, the AMF uses the pre-configured Uu time synchronization error budget and sends it to NG-RAN.
• If the UE's subscription contains Coverage Area (defined as a list of TAs), the AMF configures the NG-RAN to provide the 5G timing information to UE only when the UE is in the Coverage Area as described in clause 5.27.1.10.
• If the AMF receives the start and stop times, then the AMF enables and disables the 5G access stratum time distribution indication to the NG-RAN according to the expiry of start and stop times if the UE is in CM-CONNECTED state. If the UE is in CM-IDLE state when a Start time condition is met, the AMF pages the UE and provides the 5G access stratum time distribution indication to NG-RAN as part of the subsequent service request procedure initiated by the UE in the response to the paging.
• If the AMF receives the clock quality detail level, then the AMF configures the NG-RAN to provide clock quality detail information reporting to UE as described in clause 5.27.1.12. The AMF may instruct the UE to reconnect to the network when the UE detects that the RAN timing synchronization status has changed while the UE is in RRC_INACTIVE or RRC_IDLE, as described in clause 5.27.1.12.
• If the AMF receives the same parameters both in the Access and Mobility Subscription data from UDM and in the AM Policy from PCF, the AMF shall use the value received from the AM policy.

The Time Synchronization Subscription data is the subscription data for the control of (g)PTP-based time distribution and 5G access stratum-based time distribution and includes the following information:

• the "AF request Authorization", indicating whether the UE is authorized for an AF-requested 5G access stratum-based time distribution and (g)PTP-based time distribution services. The indication is provided separately for each service:
  • "allowed" or "not allowed" for (g)PTP based time synchronization service (per DNN/S-NSSAI and UE identity),
  • "allowed" or "not allowed" for ASTI based time synchronization services (per UE identity).
  • If the "AF request Authorization" is set to "allowed", the Time Synchronization Subscription data may contain additional information for authorization of (g)PTP- and ASTI- based time synchronization services: this information is optional and may be provided separately for authorization of each service:
    • optionally, a list of TA(s) which specifies an area (a so-called Authorized Time Synchronization Coverage Area) in which an AF may request time synchronization services;
    • optionally, one or more periods of authorized start and stop times, which indicates the allowed time period during which an AF may request time synchronization services;
    • optionally, authorized Uu time synchronization error budget, which indicates the limit the AF may request.
    • optionally, "allowed" or "not allowed" for clock quality detail level equals to "clock quality metrics", which indicates whether clock quality metrics information may be provided to the UE.
    • optionally, "allowed" or "not allowed" for clock quality detail level equals to "acceptable/not acceptable indication", which indicates whether an acceptable/not acceptable indication may be provided to the UE.
    • optionally, one or more sets of clock quality acceptance criteria for the UE using the TSS attributes as defined in clause 5.27.1.12.
• one or more Subscribed time synchronization service ID(s), each containing the DNN/S-NSSAI and a reference to a PTP instance configuration pre-configured at the TSCTSF (e.g. PTP profile, PTP domain, etc.):
  • optionally, for each PTP instance configuration, one or more periods of start and stop times defining active times of time synchronization service for the PTP instance.
  • optionally, for each PTP instance configuration, a Time Synchronization Coverage Area defining a list of TAs where the (g)PTP-based time synchronization is available for the UEs in the PTP instance.
  • optionally, for each PTP instance configuration, Uu time synchronization error budget.

The TSCTSF retrieves the Time Synchronization Subscription data from UDM. If the TSCTSF receives the Time Synchronization Subscription data for a UE, the TSCTSF controls the Time Synchronization Service including (g)PTP-based time distribution and 5G access stratum-based time distribution:

• The TSCTSF retrieves the Time Synchronization Subscription data from the UDM when the TSCTSF receives an AF request for the time synchronization service (either ASTI or (g)PTP). According to the "AF request Authorization" in the UE's Time Synchronization Subscription data, the TSCTSF determines whether the UE is authorized for an AF-requested time synchronization service:
  • If the UE's Time Synchronization Subscription data contains an Authorized Time Synchronization Coverage Area (i.e. a list of TA(s) defining the restricted area for AF request), TSCTSF checks whether the AF requested Coverage Area satisfies the authorized area: If the requested Coverage Area (see clause 5.27.1.10) is within the Authorized Time Synchronization Coverage Area, the TSCTSF uses the requested Coverage Area. If the Authorized Time Synchronization Coverage Area is inside of the requested Coverage Area, the TSCTSF uses the Authorized Time Synchronization Coverage Area. If the requested Coverage Area partly overlaps with the Authorized Time Synchronization Coverage Area, the TSCTSF uses the intersection of them. If there is no overlap between them, the TSCTSF shall reject the AF request.
  • If the AF requested Coverage Area satisfies the authorized area totally or partly, TSCTSF notifies to AF with the Time Synchronization Service information based on the Authorized Time Synchronization Coverage Area. TSCTSF subscribes to UE's presence in the Area of Interest at the discovered AMF(s), if the UE(s) moves out of the Time Synchronization Coverage Area requested by AF or updated by TSCTSF, the TSCTSF shall disable Time Synchronization Service and notifies to AF.
  • If the UE's Time Synchronization Subscription data contains authorized Uu time synchronization error budget, the TSCTSF checks whether the Uu time synchronization error budget derived from AF request satisfies (i.e. equal or larger than) the authorized Uu time synchronization error budget.
  • If the UE's Time Synchronization Subscription data contains periods of authorized start and stop times, the TSCTSF checks whether the AF requested temporal validity condition satisfies (i.e. within) any of the periods of authorized start and stop times. If such period is found, the TSCTSF uses the start and stop times of the AF request.
  • If the UE's Time Synchronization Subscription data contains a clock quality detail level (and clock quality acceptance criteria, if applicable), the TSCTSF checks whether the AF-requested clock quality information to be reported complies with the authorized clock quality detail level and the clock quality acceptance criteria (if applicable) in the UE's Time Synchronization Subscription data.
    Each parameter in the AF-requested clock quality is evaluated individually, if the TSCTSF determines that at least one parameter is "not acceptable", the TSCTSF rejects the AF request.
    If the "Traceable to GNSS" in the subscription data is "Yes", the AF request for Traceable to GNSS is allowed. Otherwise, the AF request for Traceable to GNSS is not allowed.
    If the "Traceable to UTC" in the subscription data is "Yes", the AF request for Traceable to UTC is allowed. Otherwise, the AF request for Traceable to UTC is not allowed.
    For Frequency stability and Clock Accuracy, AF is not allowed to request value which is lower than value in subscription data.
  • If the AF request is authorized, the TSCTSF proceeds as specified in clause 5.27.1.8 and in TS 23.502. Otherwise, the TSCTSF rejects the AF request.
• The TSCTSF retrieves the Time Synchronization Subscription data from the UDM when it receives notification from the PCF that a UE has established a PDU Session that is potentially impacted by (g)PTP-based time synchronization service:
  • The TSCTSF retrieves the PTP instance configurations referenced from the "Subscribed time synchronization service ID(s)". The PTP instance configurations are stored locally in the TSCTSF. The TSCTSF determines if one or more of the PTP instance configurations match with the DNN/S-NSSAI of the given PDU Session. If no PTP instance exists for the given PTP instance configuration, the TSCTSF initializes the PTP instance in 5GS as described in clause K.2.2.
  • The TSCTSF configures a PTP port in DS-TT and adds it to the corresponding PTP instance in NW-TT as described in clause K.2.2.
  • If the PTP instance configuration referenced by UE's Time Synchronization Subscription data contains an Uu time synchronization error budget, then the TSCTSF uses it to derive an Uu time synchronization error budget available for the NG-RAN to provide the 5G access stratum time for the UE as specified in clause 5.27.1.9.
  • If the PTP instance configuration referenced by the Time Synchronization Subscription data for the UE contains start and stop times, the TSCTSF, upon expiry of start time, creates the PTP instance and adds the PTP port in DS-TT to the PTP instance. Upon expiry of stop time, if this is the last period of start and stop times in the PTP instance configuration, the TSCTSF deletes the PTP instance, otherwise the TSCTSF temporarily removes the PTP port in DS-TT from the corresponding PTP instance as specified in clause 4.15.9.3 of TS 23.502.
  • If the PTP instance configuration referenced by the Time Synchronization Subscription data for the UE contains a Time Synchronization Coverage Area, the TSCTSF subscribes to UE's Presence in Area(s) of Interest corresponding to the Time Synchronization Coverage Area at the discovered AMF(s). When the TSCTSF determines that the UE has moved inside or outside of the Time Synchronization Coverage Area, the TSCTSF adds or temporarily removes the PTP port in DS-TT from the corresponding PTP instance.

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 TSCTSF may provide a list of gNB IDs or a list of TAs in the subscription request for RAN timing synchronization status reporting, and the AMF controls the gNB node level reporting and subscription using NGAP messages (see TS 38.413).
• 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 Association Modification). If the UE indicates its support for reconnection to the network due to changes in RAN timing synchronization status as specified in clause 5.4.4a and the AMF receives Clock Quality Reporting Control Information (CQRCI) either from TSCTSF via PCF or from the UDM as part of the subscription data, the AMF instructs the UE to transition to the RRC_CONNECTED state when the UE 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. gNBs and TSCTSF 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. The RAN timing synchronization status information includes the gNB node-level information about timing synchronization operation status, as described in TS 38.300. When the gNB is monitoring RAN timing synchronization status, the gNB determines the reporting triggers as described in TS 38.401. The gNB notifies the TSCTSF (either using N2 node level signalling via AMF, or via OAM) about timing synchronization status changes based on the pre-configured thresholds 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 may support only one or more network timing synchronization status attributes. Upon reception of the notification, the TSCTSF determines whether the TSS attribute meets (status improvement) or exceeds (status degradation) the preconfigured threshold value. 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 or meets the threshold again, 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.
• 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. 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 or some, or none of the information elements from Table 5.27.1.12-1.

The TSCTSF determines the UEs impacted by gNB's timing synchronization status change or UPF timing synchronization status change (only for the case when UPF/NW-TT is involved in providing time information to DS-TT) by comparing the received timing synchronization status information with a clock quality acceptance criteria provided by the AF. The TSCTSF evaluates whether the received timing synchronization status is acceptable or not acceptable for a UE. Each TSS attribute in the clock quality acceptance criteria is evaluated individually, if the TSCTSF determines that at least one reported value is "not acceptable" or there is at least one attribute present in the clock quality acceptance criteria is not reported, then the TSCTSF shall consider the timing synchronization status as "not acceptable" for the UE.

• 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 section 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).
    • 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 (CQRCI) manages the gNBs timing synchronization status reporting to the UE. The TSCTSF may retrieve CQRCI from UDM or receive CQRCI from AF request. The TSCTSF needs to check whether the AF requested CQRCI is allowed as defined in clause 5.27.1.11. 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 via AM policy or retrieved from UDM. If the AMF receives CQRCI both in the Access and Mobility Subscription data from UDM and in the AM Policy from PCF, the AMF shall use the value received from the AM policy. 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. 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.

TSC Assistance Information (TSCAI) is defined in Table 5.27.2-1 and describes TSC traffic characteristics for use in the 5G System. It can also be used for services such as XR services (AR/VR applications) and interactive media services as specified in clause 5.37. TSCAI may be used by the 5G-AN, if provided by SMF. The knowledge of TSC traffic pattern is useful for 5G-AN as it allows more efficiently scheduling of QoS Flows that have a periodic, deterministic traffic characteristics either via Configured Grants, Semi-Persistent Scheduling or with Dynamic Grants. TSCAI can be provided for both GBR and non-GBR QoS flows. The TSCTSF determines the TSC Assistance Container (defined in Table 5.27.2-2) based on information provided by an AF/NEF or a DetNet controller as described in clause 5.27.2.3 and provides it to the PCF for IP type and Ethernet type PDU Sessions. In the case of integration with IEEE TSN network, the TSN AF determines TSC Assistance Container as described in clause 5.27.2.2 and provides it to the PCF for Ethernet PDU Sessions. The PCF receives the TSC Assistance Container from the TSCTSF or the TSN AF and forwards it to the SMF as part of PCC rule as described in clause 6.1.3.23a of TS 23.503. The SMF binds a PCC rule with a TSC Assistance Container to a QoS Flow as described in clause 6.1.3.2.4 of TS 23.503. The SMF uses the TSC Assistance Container to derive the TSCAI for that QoS Flow and sends the derived TSCAI to the NG-RAN. The Periodicity, Periodicity Range, Burst Arrival Time (BAT), BAT Window and Survival Time components of the TSCAI are specified by the SMF with respect to the 5G clock. The SMF is responsible for mapping the Burst Arrival Time, BAT Window, Periodicity and Periodicity Range from an external clock (when available) to the 5G clock based on the time offset and cumulative rateRatio (when available) between the external clock time and 5GS time as measured and reported by the UPF. The SMF determines the TSCAI as described in clause 5.27.2.4. A Survival Time, which indicates the time period an application can survive without any data burst, may be provided by TSN AF/AF or by the TSCTSF either in terms of maximum number of messages (message is equivalent to all packets of a data burst) or in terms of time units. Only a single data burst is expected within a single time period referred to as the periodicity. The SMF may send an update of the TSCAI to the NG-RAN as defined in clauses 4.3.3.2, 4.9.1.2.2 and 4.9.1.3.2 of TS 23.502 or as defined in clause 5.37.8.2.

In the case of integration with IEEE TSN network, the TSN AF determines a TSC Assistance Container (defined in Table 5.27.2-2) and provides it to the PCF. The determination of TSC Assistance Container based on Per-Stream Filtering and Policing (PSFP) information applies only to Ethernet type PDU Sessions. The TSN AF may be able to identify the ingress port and thereby the PDU Session as described in clause 5.28.2. The TSN AF interfaces towards the CNC for the PSFP (IEEE Std 802.1Q [98]) managed objects that correspond to the PSFP functionality implemented by the DS-TT and the NW-TT. Thus, when PSFP information is provided by the CNC, the TSN AF may extract relevant parameters from the PSFP configuration. The TSN AF calculates traffic pattern parameters (such as burst arrival time with reference to the ingress port and periodicity). TSN AF also obtains the flow direction as specified in clause 5.28.2. Survival Time may be pre-configured in TSN AF. TSN AF may enable aggregation of TSN streams if the TSN streams belong to the same traffic class, terminate in the same egress port and have the same periodicity and compatible Burst arrival time. When Survival Time information is provided for a TSN stream, then it should not be aggregated with other TSN streams into a single QoS Flow, or if they are aggregated, then the Survival Time parameter shall not be provided. One set of parameters and one TSC Assistance Container are created by the TSN AF for multiple TSN streams to enable aggregation of TSN streams to the same QoS Flow. Annex I describes how the traffic pattern information is determined. For a UE-UE TSC stream, the (TSN) AF divides the stream into one uplink stream and one or more downlink streams as defined in clause 5.28.2. The TSN AF binds the uplink and downlink streams to the PDU Sessions, and provides the streams on AF Session basis to the PCF(s). The TSN AF calculates traffic pattern parameters for the UL and the DL stream using the PSFP configuration (if provided) respectively:

• For the uplink stream, the Flow Direction is set to uplink and traffic pattern parameters (such as burst arrival time with reference to the ingress port and periodicity) is determined as described in Annex I.
• For downlink stream, the Flow Direction is set to downlink, the burst arrival time is set to sum of burst arrival time of the UL stream and 5GS Bridge delay of PDU Session carrying the UL stream, and the periodicity is determined as described in Annex I.

The TSCTSF constructs TSC Assistance Container (defined in Table 5.27.2-2) based on information provided (directly or via NEF) by the AF for IP or Ethernet type PDU Sessions, or by the DetNet controller for IP type PDU Sessions. In the case of an AF request, the AF may provide Flow Direction, Burst Arrival Time (optional) at the UE/DS-TT (uplink) or UPF/NW-TT (downlink), Maximum Burst Size, Periodicity, Survival Time (optional), and a Time Domain (optional) to the TSCTSF. If the AF is able to adjust the burst sending time, the AF may in addition provide a BAT Window or the Capability for BAT adaptation to the TSCTSF. Addtionally if the AF is able to adjust the periodicity, the AF may also provide the Periodicity Range along with the Periodicity to the TSCTSF. Based on these parameters, the TSCTSF constructs a TSC Assistance Container and provides it to PCF. If the AF provides to the TSCTSF a Burst Arrival Time or Periodicity without corresponding Time Domain, the TSCTSF sets the Time Domain = "5GS" in the TSC Assistance Container. If the AF is able to adjust the transmission time and periodicity then in addition to above parameters, it may provide a BAT Window (optional) or the capability for BAT adaptation (optional), or Periodicity Range (optional), to the TSCTSF. The AF provides these parameters to the NEF and the NEF forwards these parameters to the TSCTSF. The AF trusted by the operator provides these parameters to the TSCTSF directly. In the case of Deterministic Networking, the TSCTSF constructs the TSC Assistance Container based on information provided by the DetNet controller as defined in clause 6.1.3.23b of TS 23.503. The TSCTSF sends the TSC Assistance Container to the PCF as follows:

• The TSCTSF uses the UE IP address/DS-TT port MAC address to identify the PCF and N5 association related to the PDU Session of a UE/DS-TT.

The SMF determines the TSCAI (defined in Table 5.27.2-1) for the QoS Flow based on the TSC Assistance Container of the PCC rule bound to the QoS Flow. This clause is applicable irrespective of whether the TSC Assistance Container is determined by the TSN AF or by the TSCTSF. The Burst Arrival Time and Periodicity component of the TSCAI that the SMF sends to the 5G-AN are specified with respect to the 5G clock. The SMF is responsible for mapping the Burst Arrival Time and Periodicity in the TSC Assistance Container from an external clock to the 5G clock based on the time offset and cumulative rateRatio (when available) between external time and 5GS time as measured and reported by the UPF. The SMF may correct the TSCAI based on the UPF report for time offset and cumulative rateRatio between external PTP time and 5GS time as measured and reported by the UPF. The TSCAI parameter determination in SMF is done as follows:

• For traffic in downlink direction, the SMF corrects the Burst Arrival Time in the TSC Assistance Container based on the latest received time offset measurement from the UPF and sets the TSCAI Burst Arrival Time as the sum of the corrected value and the static value for the CN PDB (as defined in clause 5.7.3.4), representing the latest possible time when the first packet of the data burst arrives at the AN.
• For traffic in uplink direction, the SMF corrects the Burst Arrival Time in the TSC Assistance Container based on the latest received time offset measurement from the UPF and sets the TSCAI Burst Arrival Time as the sum of the corrected value and UE-DS-TT Residence Time, representing the latest possible time when the first packet of the data burst arrives at the egress of the UE. How the SMF corrects the Burst Arrival Time if the UE-DS-TT Residence Time has not been provided by the UE is up to SMF implementation.
• The SMF corrects the Periodicity in the TSC Assistance Container using the cumulative rateRatio if the cumulative rateRatio was previously received from the UPF and sets the TSCAI Periodicity as the corrected value. Otherwise, the SMF sets the received Periodicity in the TSCAI without any correction.
• The SMF sets the TSCAI Flow Direction as the Flow Direction in the TSC Assistance Container.
• If Survival Time is provided in terms of maximum number of messages, the SMF converts maximum number of messages into time units by multiplying its value by the TSCAI Periodicity, and sets the TSCAI Survival Time to the calculated value. If Survival Time is provided in time units, the SMF corrects the Survival Time using the cumulative rateRatio if the cumulative rateRatio was previously received from the UPF and sets the TSCAI Survival Time to the corrected value. Otherwise, SMF sets the TSCAI Survival Time without correction.
• If the TSC Assistance Container contains a BAT Window, the SMF sets and corrects the indicated earliest and latest possible arrival time of the first packet in the same way it is described for the correction of the Burst Arrival Time above.
• If the TSC Assistance Container contains a Capability for BAT adaptation, the SMF sets the Capability for BAT adaptation in the TSCAI. When the SMF determines that the TSCAI contains the Capability for BAT adaptation without a BAT, the SMF enables notification control for the QoS Flow in order to receive the BAT offset along with the "GFBR can no longer be guaranteed" notification described in clause 5.7.2.4.
• If the TSC Assistance Container contains a Periodicity Range, the SMF sets and corrects the Periodicity Range in the same way it is described for the correction of the Periodicity above.

If a dynamic value for the CN PDB is configured in NG-RAN or received from the SMF (as defined in clause 5.7.3.4), the NG-RAN shall adjust the Burst Arrival Time in the TSCAI for the downlink direction by adding the dynamic value for the CN PDB and deducting the static value for the CN PDB of the corresponding 5QI. Depending on whether the Time Domain is provided in the TSC Assistance container, SMF may perform the following:

• the SMF provisions the UPF/NW-TT to report the clock drifting between 5G clock and the external GM clock for the (g)PTP time domain number that is configured to the NW-TT.
• the SMF provisions the UPF/NW-TT to report the clock drifting between 5G clock and the external GM clock for the given Time Domain number.

The SMF uses the N4 Association Setup or Update procedures as described in clause 4.4.3 of TS 23.502 to provision the UPF to report the clock drifting. If the SMF has clock drift information for a Time Domain and if the Time Domain matches with the Time Domain in the TSC Assistance Container (i.e. clock drift between 5G timing and AF supplied Time Domain determined based on UPF reporting), or Time Domain information is not provided in the TSC Assistance Container, then the SMF may adjust the TSCAI information so that it reflects the 5GS Clock as described in clause 5.27.2.1. If the SMF does not have synchronization information for a requested Time Domain in the TSC Assistance Container, or the Time Domain in the TSC Assistance Container is set to a value = "5GS", then the TSCAI information will be used without adjustment. In the case of drift between external GM clock and 5G clock, the UPF updates the offset to SMF using the N4 Report Procedure as defined in clause 4.4.3.4 of TS 23.502. If the cumulative rateRatio is available and in the case of change of cumulative rateRatio between external PTP time and 5G time, the UPF updates the cumulative rateRatio to SMF using the N4 Report Procedure as defined in clause 4.4.3.4 of TS 23.502. The SMF may then trigger a PDU Session Modification as defined in clause 4.3.3 of TS 23.502 in order to update the TSCAI to the NG-RAN without requiring AN or N1 specific signalling exchange with the UE.