[time-nuts] Timing on Ethernet

Bill Hawkins bill at iaxs.net
Sun Aug 5 17:25:36 EDT 2007

There are many replies to this thread. I have not read them all,
but I sense some diversion from the truth.

Ethernet is inherently imprecise. Once a message is started, it
runs to completion, taking chunks of time as it goes. This delays
other messages, such as time.

NTP by David Mills is the best way to apply statistics to many
Internet transactions to find the best time transmission.

SNTP does not use statistics, but does estimate the round trip
delay, assuming equal out and back delays.

Broadcast time does not estimate the delay, but assumes that any
delay is negligible.

And then there is Mister Softee's time sync, which, at best, may
be good to the nearest few minutes.

So, if you are using long baseline telescopes, Internet is just
not suitable. You can't get nanosecond resolution.

If you are doing a sequence of physical events, NTP will separate
almost all of them.

If your events are detected by applications that run ten or more
times per second, then SNTP should be adequate, if the network
is not large.

If you detect events once per second or more, broadcast is fine.

IMHO, MS time sync is not useful if you are interested in the
correct time to less than a minute or two.

Bill Hawkins

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