5 ms·
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 t
by skulk 5mo ago
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.
- UltraSane 5mo agoYes, it uses phased locked loops and measures phase difference between the master clock and the local clock.
- geerlingguy 5mo agoSimilar 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 https://news.ycombinator.com/item?id=48209055
- elromulous 5mo agoIt'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.
- colechristensen 5mo agoThe 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.
- mike_hock 5mo agoThat means you get a free clock cycle every 2-3 hours on top of a mountain compared to sea level!
- brookst 5mo agoDatacenters in spaaaace!
- Gravityloss 5mo agoBut they travel at 8 km/s so actually that cancels benefits? EDIT: checked, not enough to cancel them completely.
- pwndByDeath 5mo agoI wonder I'd that's the math for the ludicrous space data center ideas "floating" around...
- OutOfHere 5mo agoJust because you don't understand how something can work doesn't make it ludicrous. People like you are hell bent on destroying what's left of the Earth by turning it into a computer. If we left progress to those without an imagination, we wouldn't even have had a working calculator.
- dboreham 5mo agoThat might be true if the proponents had an argument of any sort in support of their plan, other than "we need idiots to give us money".
- OutOfHere 4mo agoYou misused a comma; it doesn't belong where you used it. Just because there are ninety nine ways to do something wrong doesn't mean there couldn't exist one way to do it right. In the case of space datacenters, there absolutely exists a way to do it effectively. Dumb VCs will never know the difference.
- ted_dunning 5mo ago
- ooterness 5mo agoTwo-way time transfer measures the round-trip propagation time. As a result, it's not directly relevant to the accuracy.
- catoc 5mo agoSo 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?
- netjiro 5mo agodelay is easy jitter kills
- fsh 5mo agoThe roundtrip time is measured and compensated. Even NTP does this. Knowing the distance is not necessary for time synchronization.
- RossBencina 5mo agogPTP 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/ https://blog.meinbergglobal.com/2024/03/27/what-is-gptp/
- Gravityloss 5mo agoHmm 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.
- tempay 5mo agoIt doesn't matter if the length changes provided: * you measure the round trip time often enough * the shift affects light in both directions equally
- SiempreViernes 5mo agoyes, it needs custom built hardware to work.
- nuccy 5mo agoIn 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.
- xxs 5mo agoEven 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.
- kcexn 5mo agoIf both systems have a good clock. Then the synchronization messages only need to contain the time delta to correct the time (phase?) drift to achieve full synchronization.