White Rabbit – sub-nanosecond synchronization for large distributed systems
ohwr.orgIf you run "make" in the papers/IBIC2013 directory you'll get this paper: https://cds.cern.ch/record/1743073/files/thbl2.pdf
It's quite interesting - this isn't ethernet as we know it. Instead of each NIC using its own free-running clock, all the physical layers are sync'ed to each other at layer 1. (note that gigabit ethernet, which is what it uses, sends data at all times - when idle it sends the idle symbol)
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.
It's totally possible to achieve synchronization better than light transmission time. For the purposes of synchronization, the speed of light delay, and any other delay are indistinguishable, and need not be distinguished.
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.
The roundtrip time is measured and compensated. Even NTP does this. Knowing the distance is not necessary for time synchronization.
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/
... why would cm precision be often not possible?
delay is easy
jitter kills
The gravity well time dilation is about 3.5 nanoseconds per meter per year near the surface of the earth. (time changes rate with altitude in a gravity well)
Sub-nanosecond synchronization is getting into the relativity is measurable realm.
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.
Yes, it uses phased locked loops and measures phase difference between the master clock and the local clock.
Similar to the much smaller scale Wi-Wi setup that was on HN a few days ago (which came after WR): https://news.ycombinator.com/item?id=48209055
yes, it needs custom built hardware to work.
Some may find https://gitlab.com/ohwr/project/white-rabbit/-/wikis/home an easier starting point. Particularly the "Synchronization" page.
In short, it's about giving PTP and SyncE some extra smarts.
Probably even better to start at the collaboration website https://www.white-rabbit.tech/wr-technology/
Haven't dug in on the technicals, but this is coming out of CERN, it looks like - and in that light, the links to "We're hiring" on that page almost feel like a flex...
What is significance of this?
it is useful e.g. to align the phase of signals being sent from different locations
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...
https://www.white-rabbit.tech/wr-technology/
Note that this is also for a large part a hardware-based technology