5.27.2.4 TSCAI determination based on TSC Assistance Container

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 [3] 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 [3]. 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 [3]. The SMF may then trigger a PDU Session Modification as defined in clause 4.3.3 of TS 23.502 [3] in order to update the TSCAI to the NG-RAN without requiring AN or N1 specific signalling exchange with the UE.

NOTE 4: In order to prevent frequent updates from the UPF, the UPF sends the offset or the cumulative rateRatio only when the difference between the current measurement and the previously reported measurement is larger than a threshold as described in clause 4.4.3.4 of TS 23.502 [3].