[time-nuts] question about multi-way measurement

[Hal Murray]

Stromeko at Nexgo.DE said:
> Over wired Ethernet you can expect to synchronize a bunch of systems to
> within a ~200µs envelope of absolute time and maybe a factor of 2x-3x  lower
> if you can control certain things more tightly than usual if you  run those
> system on a single hop switched LAN that have a GPS-based  stratum-1 NTP
> server.  For anything better than that you'd need PTP,  which unfortunately
> the rasPi is incapable of.  Or do you plan to use  the rasPi itself as the RX/
> TX?  The few  papers I've just looked up all  use Atheros 9k hardware. 

On the Raspberry Pi, the Ethernet is on USB so there is another source of 
timing error.  It's full speed USB rather than low speed USB so the timing 
noise isn't as bad as it could be - 1/8 ms vs 1 ms.  (I could be off on the 
numbers or names, but the general idea is correct.)

If you think you can avoid that by averaging, be sure you understand hanging 
bridges.

If you need good times on a Pi, consider a GPS hat.  The PPS goes in via a 
GPIO pin with an interrupt so the timing can be pretty good.



-- 
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