[time-nuts] Low-long-term-drift clock for board levelintegration?

Attila Kinali attila at kinali.ch
Wed Feb 22 09:05:06 UTC 2012


On Tue, 21 Feb 2012 23:23:23 -0800
Chris Albertson <albertson.chris at gmail.com> wrote:

> Seems the problem is possible asymmetry in the speed of network
> connections.   You really can not detect this with simple "pings".
> But what if you send 100MB messages?    Even using NTP  at the tens of
> millisecond level you can measure the one way speed accurately if the
> message is long enough.   Send one large message in each direction and
> you'll have the forward to reverse speed ratio.     Now rather than
> divide the round trip time by two.  We know the speed ratios for the
> forward and reverse link

The speed (or bandwidth) of a  link is independent of its delay.
Eg a 10Gbit/s fiber will have 10Gbit/s no matter whether it's 1m
or 100km long. But its delay will vary from ~3ns to ~300us.

Usually, for internet links, their speed is known (especially if you
own the link yourself). What you want to know is whether the delay
induced by the time in flight, routers, repeaters, etc changes over
time.

			Attila Kinali

-- 
The trouble with you, Shev, is you don't say anything until you've saved
up a whole truckload of damned heavy brick arguments and then you dump
them all out and never look at the bleeding body mangled beneath the heap
		-- Tirin, The Dispossessed, U. Le Guin



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