The focus for this use-case description is power sub-stations where robust time synchronization is needed for e.g., synchro phasor and other Smart Grid applications.1
Power sub-systems use wireless means to achieve time synchronization, as is the case today leveraging GNSS receiver capabilities. In today's power sub-system, the timing resiliency is based on e.g., having multiple grandmasters leveraging multiple GNSS receivers. In this use case the 5G system is used as a supplement to, e.g., integrated as alternative radio in the grandmaster clock or as an alternative grandmaster clock with 5G capability in the power sub-system, or alternative to the GNSS, e.g., integrated with PTP grandmaster clock avoiding installation of external GNSS antenna and receiver at the power sub-station.
In this use case, the smart grid is subscribed to and authorized to use the 5G timing resiliency service to ensure reliable timing signals are always available. The 5G timing resiliency service monitors the accuracy and availability of timing signals from a designated timing source and is able to provide an alternate source (e.g., 5G holdover capacity, atomic clock) in case of failure in the primary source. An SLA may be in place to establish the primary and any alternate timing sources that can be used.
This section should also describe how the timing resiliency is enhanced when the 5G system is used in conjunction with the GNSS, how the system is notified of a failure of GNSS, how the 5G system picks up the slack to keep the system running smoothly in the absence of GNSS. The UEs are preconfigured to enable the provision of time synchronization service from the 5G system. In addition to the initial pre-configuration the UEs may have, additional information may be used for dynamically configuring the management of the 5G system time synchronization network within the smart grid system.
The configuration of the smart grid system includes timing service from GNSS with resiliency support provided by the 5G system. Based on the 5G time resilience requirements, the time distribution method used, and type of time synchronization device connected to the UE (e.g., DS-TT), the 5G system may configure different network entities (i.e., gNBs, UEs/DS-TTs, UPFs/NW-TTs) and the behaviour when an issue is detected (e.g., notification towards the UE or application, back-up configuration).
The 5G system detects that the GNSS is experiencing a problem (e.g., loss of satellite access, detection of inconsistent timing information).
The 5G system communicates the loss of GNSS to UEs or applications subscribed to the timing resilience service. Additionally, the 5G system may indicate the UEs should use the timing service provided by the 5G system until further notice instead of GNSS signal. The 5G system is configured to provide an accurate timing service for a specified holdover time. Depending on the detected problem and the impacted area, the 5G system may reconfigure its own network to ensure the distribution of an accurate timing service to the UEs.
The 5G system provides timing information to the UEs using a secure mechanism.
The UEs are able to verify the integrity and accuracy of the timing information provided by the 5G system
The 5G system continues to monitor for a return to service by the GNSS. When the GNSS service recovery is detected, the 5G system informs the UEs or application subscribed to the resilience timing service that they can again receive GNSS timing information.
The Smart Grid can use 5G system as a resiliency backup to their GNSS receiver based (or wired) time synchronization systems to improve availability and reliability of the time synchronization. Alternatively, existing GNSS receiver-based solutions may be replaced with a 5G system-based solution where it improves efficiency and/or reliability.
Many 5G system features specified in Rel-16 and expected in Rel-17 will be required for the use-case:
Time synchronization to UTC leveraging 5G system C-Plane and/or U-Plane
Support for IEEE 1588 PTP
Propagation delay compensation, to ensure that time synchronization can be conducted accurately across wide area deployments needed for power sub-station support
Exposure framework supports time synchronization based on the needs of the service
The 5G system shall monitor for timing source failure.
The 5G system shall be able to indicate to devices (e.g., UEs, applications) that they need to use an alternate time source (e.g., use 5G system with internal holdover capability or an alternate source, e.g. atomic clock, Sync over Fiber, TBS), taking into account the holdover capability of the devices.
The 5G system shall support a holdover capability (e.g., maintaining required accuracy to UTC) of up to 24h.
The 5G system shall be able to detect when a timing source fails or is restored for network time synchronization.
The 5G system shall be able to collect charging information per UE for use of a timing resiliency service (e.g., start/stop time and source used by a UE, timing service used by UE, holdover capability of the service).
The 5G system shall be able to collect charging information on 5G system timing resiliency service (e.g., service KPIs, holdover capability, number of UEs using the service).
|Power grid (5G network)||Up to 24 hour||UTC (note 1)||
< 250ns-1000ns ,  (note2)||< 20 km2||low||When 5G System provides direct PTP Grandmaster capability to sub-stations|
|Power grid (time synchronization device)||> 5 s||UTC (note 1)||
< 250ns-1000ns ,  (note2)||< 20 km2||Low||When 5G sync modem is integrated into PTP grandmaster solution (with 24h holdover capability) at sub-stations)|
A different synchronization target is acceptable as long as the offset is preconfigured when an alternatively sourced time differs from GNSS. In this case, a 5G end device shall provide PPS output which can be used for measuring the difference.
Use case [New] in 
illustrates the different accuracy measurements based on different configurations needed to support the underlying requirements from IEC 
. The range is between 250 ns and 1000 ns. The actual requirement depends on the specific deployment.