Re:White Rabbit Project
by Aune on Friday June 23, 2023 @09:48AM
(#36446789)
When synchronizing two nodes A and B, where there is a persistent difference in the travel times A->B and B->A, how do you achieve synchronization when knowing A->B->A or B->A->B?
[ Parent
| on HN ]
you can't. You can only assume that they are equal and attempt to make them as equal as possible. (the same issue arises when measuring the speed of light: it's actually not possible to distinguish if the speed of light is different in one direction to another, we only know accurately the average of each direction)
For a first approximation, you can assume A->B and B->A travel times are equal.
And because optical links are used, the asymmetry is mainly due to the wavelength difference which is known.
Delay symmetry is a critical assumption in any two-way time transfer process. White Rabbit goes to extreme lengths to maintain that property.
This includes mandating use of cables that share a single optical fiber, with specific wavelength pairs and fiber types so you can calibrate for unavoidable differences in propagation time.
More info on their wiki: