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White Rabbit – sub-nanosecond synchronization for large distributed systems

ohwr.org

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Re: White Rabbit – sub-nanosecond synchronization for large distributed systems

#21

Post by nuccy »

skulk wrote: post #2

Haven't looked into this in depth but sub-nanosecond sync for systems up to 10km apart is interesting since 10km is about 33 light microseconds. There is some trickery going on.

In our lab tests phase lock jitter between WR client and master is about 10ps (picoseconds) over 50km fiber (with temperature change of the fiber, so WR actively compensates elongations), so relative clock of one system can be transmitted with about that accuracy to another.

P.S. There is WR workshop this week with some talks being publicly available on CERN's indico website.

Re: White Rabbit – sub-nanosecond synchronization for large distributed systems

#22

Post by Wololooo »

boguscoder wrote: post #12
LowLevelKernel wrote: post #9

Not on GitHub?

Its on gitlab but even there I failed to find sources, documentation/presentations are there though

If I remember everything is open hardware, CERN should have those repo accessible, last time I used it it was still very much in dev, especially their PCIe cards with custom kernel. This being said, I haven't touched it since ~6 years ago...

Re: White Rabbit – sub-nanosecond synchronization for large distributed systems

#24

Post by xxs »

nuccy wrote: post #21
skulk wrote: post #2

Haven't looked into this in depth but sub-nanosecond sync for systems up to 10km apart is interesting since 10km is about 33 light microseconds. There is some trickery going on.

In our lab tests phase lock jitter between WR client and master is about 10ps (picoseconds) over 50km fiber (with temperature change of the fiber, so WR actively compensates elongations), so relative clock of one system can be transmitted with about that accuracy to another. P.S. There is WR workshop this week with some talks being publicly available on CERN's indico website.

Even though you're commenting on While Rabbit post, it took some time to understand "WR" is white rabbit, esp. since describing the pico seconds in brackets.

Re: White Rabbit – sub-nanosecond synchronization for large distributed systems

#25

Post by RossBencina »

catoc wrote: post #14
ooterness wrote: post #6

Earlier quoted context omitted.

Two-way time transfer measures the round-trip propagation time. As a result, it's not directly relevant to the accuracy.

So then you need to know distance / roundtrip-length within centimeter precision as well (below 29.98 cm for sub-nanosecond precision… to be precise). Since cm precision is often not possible, is roundtrip-length an estimated average from prior roundtrips?

gPTP estimates the link delay and the peer clock ratio, see for example this random link I just found for you: https://blog.meinbergglobal.com/2024/03/27/what-is-gptp/

Re: White Rabbit – sub-nanosecond synchronization for large distributed systems

#27

Post by Gravityloss »

catoc wrote: post #14
ooterness wrote: post #6

Earlier quoted context omitted.

Two-way time transfer measures the round-trip propagation time. As a result, it's not directly relevant to the accuracy.

So then you need to know distance / roundtrip-length within centimeter precision as well (below 29.98 cm for sub-nanosecond precision… to be precise). Since cm precision is often not possible, is roundtrip-length an estimated average from prior roundtrips?

Hmm one would expect heat expansion to change the length of fiber over tens of kilometers. Does it also affect light speed in the fiber? I think consumer fiber is not buried very deep on average, but maybe for these use cases you use something hefty anyway.

Re: White Rabbit – sub-nanosecond synchronization for large distributed systems

#28

Post by Gravityloss »

brookst wrote: post #10
mike_hock wrote: post #8

Earlier quoted context omitted.

That means you get a free clock cycle every 2-3 hours on top of a mountain compared to sea level!

Datacenters in spaaaace!

But they travel at 8 km/s so actually that cancels benefits? EDIT: checked, not enough to cancel them completely.

Re: White Rabbit – sub-nanosecond synchronization for large distributed systems

#29

Post by numpad0 »

catoc wrote: post #14
ooterness wrote: post #6

Earlier quoted context omitted.

Two-way time transfer measures the round-trip propagation time. As a result, it's not directly relevant to the accuracy.

So then you need to know distance / roundtrip-length within centimeter precision as well (below 29.98 cm for sub-nanosecond precision… to be precise). Since cm precision is often not possible, is roundtrip-length an estimated average from prior roundtrips?

... why would cm precision be often not possible?
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