Section: 5.27.1.10 Support for coverage area filters for time synchronization service
Content: 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. 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: a) 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. b) 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 requested 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[ Procedures for the 5G System (5GS) ]  [3]. 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. NOTE 1: This ensures that (1) there is no impact on the Registration Area (RA) of UE if/when the AMF receives a time synchronization service request with a spatial validity condition (i.e. for specific geographical area); and (2) the UE can continue receiving the time synchronization service when it moves within the RA in the RRC_IDLE state. NOTE 2: Since the RA can be specific to the UE, the result can be different Time Synchronization Coverage Area for different DS-TT ports of different UEs within a PTP instance.