[HN Gopher] White Rabbit Project (2020)
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White Rabbit Project (2020)
Author : kimburgess
Score : 95 points
Date : 2023-06-23 11:57 UTC (11 hours ago)
(HTM) web link (ohwr.org)
(TXT) w3m dump (ohwr.org)
| throwoutway wrote:
| Now getting a Gitlab 500 "Whoops, something went wrong on our
| end" error page on even the root domain
| mitchbob wrote:
| https://web.archive.org/web/20230623115746/https://ohwr.org/...
| twic wrote:
| Not mentioned in their list of users, but at least one stock
| exchange uses it internally, and exposes it to participants:
|
| https://www.eurex.com/ex-en/support/initiatives/archive/high...
| tracker1 wrote:
| Not to be confused with:
| https://en.wikipedia.org/wiki/White_Rabbit_Project_(TV_serie...
| orourke wrote:
| "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?
| faisalhackshah wrote:
| 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 wrote:
| 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]
| Xarodon wrote:
| 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.
| dphidt wrote:
| 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 wrote:
| 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
| tgingold wrote:
| Only over fiber. Copper SFPs are not deterministic enough
| to precisely synchronize networks.
| tgingold wrote:
| 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...
| Aune wrote:
| 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?
| rcxdude wrote:
| 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 wrote:
| 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
| tgingold wrote:
| 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.
| ramdac wrote:
| What happens when the roundtrip time isn't consistent?
| nickez wrote:
| The roundtrip time is never consistent. Light travels
| with different speed in fiber depending on the
| temperature. This is why you calibrate every second.
| bestouff wrote:
| 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.
| willis936 wrote:
| Important and relevant piece of information: IEEE 1588-2019 HA
| (default profile) is interoperable with White Rabbit.
| Unfortunately I haven't seen any commercial support for this yet.
| cbm-vic-20 wrote:
| [flagged]
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(page generated 2023-06-23 23:01 UTC)