IEEE1588 Standard

The Precision Time Protocol (PTP) is specified in the IEEE 1588 Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems. The currently active 2019 edition was preceeded by 2008 and 2002 editions. The standard is developed by the P1588 Working Group.

PTP is a packet-based protocol used to synchronize devices interconnected by a computer network. On a local area network, it allows achieving clock accuracy in the sub-microsecond range, making it suitable for measurement and control systems. The range of PTP applications is wide and includes: controls and measurement, finance, telecommunication, power, industry plants, broadcast, cars...

PTP can be used in such a wide variety of applications thanks to its mechanism of PTP Profiles that allows high level of customisation of the protocol. While the IEEE1588 standard specifies generic "default" PTP Profiles, industry-specific PTP Profiles are defined by Standard Defining Organisations relevant to different industries, such as ITU-T for telecom, SMPTE for broadcast, IEC for power.

White Rabbit and IEEE1588-2008

White Rabbit (WR) is an extension of the IEEE1588-2008 edition of the PTP protocol. The WR extension is referred to as WR-PTP and it is defined in the "White Rabbit Specification" in a form of a PTP Profile that is compliant with IEEE1588-2008 and its default profiles .

WR-PTP allows synchronization of networked devices with sub-ns accuracy. Achieving such high accuracy results from the combination of IEEE1588-2008 with the following further extensions:

Clock syntonization over the physical layer (similar to SyncE),
Enhancement of timestamps precision through phase detection,
Automatic precise evaluation of link asymmetry.

WR-PTP is implemetned by the WR Technology.

White Rabbit standardisation

The P1588 Working Group worked on revising the IEEE1588-2008 standard starting in 2013 and finishing with the publication of IEEE1588-2019 in 2020.

The work was arranged in 5 sub-committees focusing on different aspects of the PTP revision, namely: Architecture, Management, Upkeep, Security and High Accuracy Enhancements. The High Accuracy sub-committee was dedicated to generalising White Rabbit to a broader range of applications and include these generalised solutions in the IEEE1588 standard. A wide range of companies and institutes were involved in this work. The following parts of WR were distinguished:

Control of L1 syntonization by PTP
Estimation of delay asymmetry and correction of PTP calculations for delay asymmetry and hardware delays
Calibration of hardware delays and relative delay coefficient (needed to estimate the delay asymmetry)
Assignment of fixed master/slave roles to ports
Dedicated WR Profile compatible with Delay Request-Response Default PTP Profile

The project of standardizing WR is documented here.

High Accuracy Default PTP Profile of IEEE1588-2019

WR-PTP was generalised into the High Accuracy Default PTP Profile (HA Profile) of the IEEE1588-2019 edition. It is the third Default PTP Profile that is specified in the IEEE1588 standard.

The HA Profile requires implementation of a set of optional features introduced the IEEE1588-2019. These features provide protocol support for the WR enhancements and hardware to achieve sub-ns accuracy of synchronisation. The features based on WR and required by the HA Profile are:

Layer-1 based synchronization performance enahancements (Annex L)
Calculation of delayAsymmetry (Clause 16.8)
Configuration of correction of timestamps (Clause 16.7)
Mechanism for external configruration of PTP Port state (Clause 17.6)
MasterOnly mode (Clause 9.2.2.2)

Additionally, the following informative annexes are defined:

Calibration Procedures (Annex N)
Syb-ns synchronization using the High Accuracy Default PTP Profile (Annex M)

Annex M describes how to implement the HA Profile such that it achieves the sub-ns performance of WR.

While from the protocol point of view the WR-PTP and HA Profile are not compatible, both are comaptible with the WR hardware and provide the same level of synchronization performance.

The details of the WR integration into IEEE1588-2019 as HA Profile are described here.

The HA Profile is currently implemented by WR Technology as Beta version. Progresivelly, it will become the solution to be used by default while maintaining backward compatibility with the WR-PTP.

The Collaboration

The Technology

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