3 ms·

“sub-nanosecond synchronization” “typical distances of 10 km between nodes” Light travels 30cm in a nanosecond. How do they achieve sub-nanosecond accuracy ov

• Xarodon 3y ago
  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.
  • bestouff 3y ago
    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.
  • dphidt 3y ago
    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.).
    • willis936 3y ago
      And even better: they suggest the use of a single medium for both transmission and receiving (1000BASE-BX10) to minimize asymmetry. https://ohwr.org/project/white-rabbit/wikis/SFP https://ohwr.org/project/white-rabbit/wikis/SFP
    • tgingold 3y ago
      Only over fiber. Copper SFPs are not deterministic enough to precisely synchronize networks.
  • tgingold 3y ago
    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/2b9d42b664f8e2fd9856c042bd5f6652/WR_Maciej_ALBA.v0.3.pdf 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/6a357829064b9e27a46fbce4cb4398b4/mgr.pdf https://ohwr.org/project/white-rabbit/uploads/6a357829064b9e...
    • Aune 3y ago
      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?
      • tgingold 3y ago
        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.
      • rcxdude 3y ago
        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)
      • ooterness 3y ago
        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: https://ohwr.org/projects/white-rabbit/wiki/SFP https://ohwr.org/projects/white-rabbit/wiki/SFP
    • 3y ago
• faisalhackshah 3y ago
  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 3y ago
    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.
• deleted 3y ago
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