White-Rabbit -> Synchronization

White Rabbit provides synchronization

• of sub-nanosecond accuracy
• of picoseconds precision
• among few thousands nodes
• over large distances (tens of kilometers)

White Rabbit synchronization is a combination of:

• Precision Time Protocol (PTP)
• Synchronous Ethernet (SyncE)
• Digital Dual-Mixer Time Difference (DDMTD) phase detection

White Rabbit synchronization method is

• defined as a Profile of the Precision Time Protocol
• compatible with the Precision Time Protocol and interoperable with standard PTP gear
• referred to as WR PTP
• described in White Rabbit Specification
• implemented in software as a "WR PTP Daemon" (ppsi)

Short description of WR PTP

The accuracy of the PTP synchronization is implementation-dependent. The standard is foreseen for sub-nanosecond accuracies. However, such performance is not achieved in typical PTP implementations. WR achieves sub-nanosecond accuracy by basing its time distribution on PTP standard and addressing the following issues limiting PTP’s performance:

1. Limited precision and resolution of timestamps
2. Unknown link asymmetry
3. The quality of the PTP-syntonization depending on the exchange rate of PTP messages

White Rabbit addresses these limitations to achieve sub-nanosecond accuracy of synchronization. It uses SyncE to distribute the common notion of frequency in the entire network over the physical medium. It casts the problem of timestamping into a phase detection measurement (using DDMT). The results of these precise measurements are used both during normal PTP operation and for quantifying physical link asymmetry* during the calibration phase. The improved performance of the synchronization is accomplished without increasing PTP message traffic (it can be actually decreased) since PTP is only governing the synchronization, while the syntonization is done by SyncE. The solution enables for network-wide sub-nanosecond accuracy of synchronization but requires additional (to standard PTP) logic and data exchange. These add-ons are incorporated into PTP within WR Profile, called WR PTP.

Detailed description of WR PTP

• An article about White Rabbit Protocol
  • White Rabbit: a PTP application for robust sub-nanosecond synchronization
• The specification of the White Rabbit Protocol
  • White Rabbit Specification
• Very detailed description of White Rabbit Protocol implementation (Master Thesis)
  • Precise time and frequency transfer in a White Rabbit network

Performance Results:

• ~0.5 ns accuracy and *< 10 ps* precision (sdev) of a cascade of 4 WR Switches over 15 km - measurements and results described in White Rabbit: a PTP application for robust sub-nanosecond synchronization
• *< 100 ps* drift under temperature-stress test (+12.5 to 85.0 Celsius degrees over 2.5h) - measurements and results described in White Rabbit: sub-nanoseconds synchronization for embedded systems
• sub-ns synchronization (long-term measurements) of a deployed White Rabbit-based timestamping system - measurements and results in Performance results of the first White Rabbit installation for CNGS time transfer

2 July 2012