5.27.1.9 Support for derivation of Uu time synchronization error budget

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. TheTSCTSF 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 [3]. 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 [3], if needed.

NOTE: This release of the specification assumes that deployments ensure that the targeted UEs and the NG-RAN nodes serving those UEs support Rel-17 propagation delay compensation as defined in TS 38.300 [27].