[orourke]

“sub-nanosecond synchronization”

“typical distances of 10 km between nodes”

Light travels 30cm in a nanosecond. How do they achieve sub-nanosecond accuracy over long distances?

[Xarodon]

Because they know the speed of light and the distance between nodes so they can account for the propagation delays of light due to distance.

They're not talking of sub millisecond latency in communications.

[tgingold]

No, we don't know the distance between nodes (although we could deduce it). But using timestamps, we can know the round-trip time.

See https://www.ohwr.org/project/white-rabbit/uploads/2b9d42b664... (page 9 and later for the principle).

If you want all the details, see https://ohwr.org/project/white-rabbit/uploads/6a357829064b9e...

[dphidt]

Even better, the actual in situ delays are measured and compensated for, and it works independent of the physical connection (and through fiber/copper, switch layers, etc.).

[bestouff]

Indeed. It's exactly the same (albeit on a different scale) as NTP synchronization, where you can frequently (ha!) reach a few ms accuracy over a hundred ms latency network.

[faisalhackshah]

It seems that you're implying that nodes cannot be synchronized within the time it takes for light to travel between the nodes.

Images both nodes having their own atomic clocks. Now allow them to timestamp transmitted and received messages with very high precision.

[tgingold]

In a white-rabbit network you don't need atomic clocks on each node. One atomic clock is enough, its frequency is distributed over the network.