4.28 Subscription-based distribution of timing information

23.5023GPPProcedures for the 5G System (5GS)Release 18TS

4.28.1 General

The distribution of 5G Clock (5G Access Stratum-based Time Distribution) and the (g)PTP domain synchronization ((g)PTP-based Time Distribution) may be controlled based on subscription data.

4.28.2 5G Access Stratum-based Time Distribution

4.28.2.1 Control of access stratum time synchronization service without AF request

The control of 5G Access Stratum-based Time Distribution for a UE is performed by the AMF according to parameters retrieved in the Access and Mobility Subscription data.

Figure 4.28.2.1-1: Subscription-based control of 5G access stratum-based time distribution

1. The UE performs the registration procedure as described in clause 4.2.2.2.2. The AMF retrieves the Access and Mobility Subscription data including the Access Stratum Time Synchronization Service Authorization. The Access and Mobility Subscription data may further include the Uu time synchronization error budget, one or more periods of start and stop times defining active times, and a coverage area [2].

2. The AMF performs the control according to the subscription data as follows.

If the AMF receives start and stop times then the AMF sends the message to the NG-RAN to enable or disable the 5G access stratum time distribution according to the expiry of start and stop times if the UE is in CM-CONNECTED state. If the AMF receives start and stop times, then the AMF pages the UE to enable or disable the 5G access stratum time distribution according to the expiry of start and stop times if the UE is in CM-IDLE state.

3. The AMF then sends the N2 message to enable or disable the 5G access stratum time distribution accordingly.

4. The AMF sends N2 message (UE Context Modification Request) to the NG-RAN indicating whether the time distribution should be enabled or disabled. If the AMF received the Uu time synchronization error budget, the AMF includes the Uu time synchronization error budget in the message.

Step 2 and step 4 may be performed during or after the general registration procedure.

4.28.3 (g)PTP-based Time Distribution

4.28.3.1 Control of (g)PTP time synchronization service without AF request

The TSCTSF may use the subscription data to control the (g)PTP-based time distribution without AF request.

Figure 6.18.3.3-1: Subscription based control of (g)PTP time synchronization service without AF request

1. The UE performs the UE-requested PDU Session Establishment.

2. The PCF determines if the PDU Session is potentially impacted by time synchronization service and invokes Npcf_PolicyAuthorization_Notify service operation to the TSCTSF discovered and selected for time synchronization to indicate there is a UE connected to a specific DNN/S-NSSAI configured for (g)PTP-based time distribution.

3. The TSCSTF uses the SUPI to retrieve the Time Synchronization Subscription Data available at the UDM.

4. If the Time Synchronization Subscription Data contains:

a) one or more Subscribed Time Synchronization Service ID(s) that can be mapped to PTP instance configuration(s), the TSCTSF determines if one or more of the PTP instance configurations match with the DNN/S-NSSAI of the given PDU Session. The TSCTSF stores information that the time-synchronization service cannot be controlled by an AF for the given SUPI.

b) An indication that an AF-requested (g)PTP time synchronization service is allowed for the given UE and DNN/S-NSSAI, the TSCTSF adds the given SUPI to the list of SUPIs for which the time-synchronization service can be controlled by an AF.

c) If TSCTSF receives neither a) nor b), the TSCTSF assumes that the time-synchronization service cannot be controlled by an AF for the given SUPI. The TSCTSF releases the AF-session with the PCF.

5. For each matching PTP instance configuration determined in step 4, 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 of TS 23.501 [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 of TS 23.501 [2].

If the Time Synchronization Subscription data for the UE in the UDM contains start and stop times, the TSCTSF sets a timer for the expiration of start and stop time. Upon expiry of start time, the TSCTSF adds the PTP port in DS-TT to the corresponding PTP instance. Upon expiry of stop time, the TSCTSF temporarily removes the PTP port in DS-TT from the corresponding PTP instance.

If the Time Synchronization Subscription data for the UE in the UDM contains a coverage area, the TSCTSF subscribes to UE’s presence in Area of Interest at the discovered AMF(s). When the TSCTSF determines that the UE has moved inside or outside of the coverage area, the TSCTSF adds or removes the PTP port in DS-TT from the corresponding PTP instance.