The overall goal of the project was to demonstrate all the metrological steps necessary for the industrial adoption of PTP-WR, including improvement of the devices and the study of an effective implementation in ordinary industrial IT infrastructures without degradation of the performance of the technique compared to the results achieved in controlled research laboratories or using dedicated fibre infrastructures.

The specific objectives of the project were:

1. To develop improved and scalable calibration techniques for PTP-WR fibre links that are applicable to both existing telecommunication configurations with either a single fibre or with duplex fibres, enabling the delay asymmetry of the propagation time to be accurately known for a time service competitive with GNSS systems. The target uncertainty for device calibrations was 200 ps, and the target uncertainty for propagation calibration was 1 ns for fibre link lengths up to 1000 km.
2. To develop validated techniques for redundant and resilient time transfer to industrial end users that met the recommendations for the timing characteristics of primary reference clocks (ITU T Primary Reference Time Clock (G.8272)) and enhanced Primary Reference Clocks in Telecommunications Networks (PRTC) performance levels during a switch of PTP-WR-GrandMasters and in hold-over situations. Redundancy within the industrial time-service would be ensured by the use of multiple time-links from source to user, and resilience achieved by providing alternative clock-sources, e.g. time-links, local clocks, and GNSS signals.
3. To develop a next generation of PTP-WR devices with improved performance, that interface better with existing industrial protocols and standards such as IEC61850 for Smart Grids. The target frequency instability, characterised by an Allan deviation (ADEV), was <1×10^-13 over an observation time of 100 s.
4. To demonstrate the use of PTP-WR to deliver Universal Coordinated Time UTC(k) time scales and frequency in the radio frequency (RF) domain from NMIs to industrial users within a specified market segment, and to evaluate the end-to-end uncertainty of the established time transfer.
5. To facilitate the take up of the technology and measurement infrastructure developed in the project by the measurement supply chain, calibration laboratories, standards developing organisations (in particular, IEEE PNCS – Precise Networked Clock Synchronization Working Group and ITU) and end users, in particular the telecommunication industry and the National Research and Education Networks.

Progress beyond the state of the art

This project advanced the state of the art in PTP-WR by establishing the metrological capacities required to accelerate the industrial adoption of PTP-WR for time transfer. Cost effective solutions hare proposed, which can be integrated with existing fibre networks devoted to internet communication, allowing industry to receive a time reference signal traceable to UTC without extra costs related to new infrastructures and with the resilience and reliability typical of telecommunication services.

Improved and scalable calibration techniques for PTP-WR optical fibre links have been developed, including the calibration and characterisation of the internal delays within devices and their dependence on external factors (e.g. temperature), and differential propagation delays through the optical fibre. The work within WRITE demonstrated that PTP-WR surpasses state-of-the-art Two way Satellite Time and Frequency Transfer time transfer, compared with the current state-of-the-art uncertainty in industrial applications of around 100 ns. WRITE demonstrated reliable PTP-WR dissemination at sub-nanosecond level, down to 200 ps accuracy.

Many industrial applications require high reliability and resilience. This project developed redundant and resilient features for time transfer for industrial use, such as redundant links from multiple time sources to one user, and improved resilience by providing alternative clock sources (e.g. time link, local clock, and GNSS signals) and enhanced performance levels. Although similar redundancy and resilience measures have been implemented with classical PTP, the PTP‑WR implementation by this project has, for the first time, enabled reliable and standardised time distribution with better performance than GNSS methods.

Two important interfaces for PTP‑WR are the time and frequency outputs, and in addition low phase noise, low timing jitter and fast rise time are highly desirable features. WRITE investigated the use of low-noise local oscillators and improved circuitry to obtain sharply defined waveforms, allowing a frequency instability of ADEV <6×10^-15 at 100 s integration time. A third essential interface with industrial applications is formed by the PTP‑WR optical outputs themselves, which are backward compatible with PTP (IEEE 1588-2008-2019). The project pursued the development of devices that are broadly backwardly compatible with existing industrial protocols and standards.

WRITE enabled uptake of PTP-WR UTC traceable services, proving their viability by connecting four NMIs to industrial users active in the space industry and telecommunications through (redundant) fibre-optic links that also carry regular transport data, in order to demonstrate a durable and robust time and frequency transfer process, to test the real Technology Readiness Level, to accelerate the uptake of the IEEE1588-2018 High Accuracy profile to be published, and to demonstrate the routine use of PTP-WR by a high-level European industry.