[time-nuts] Symmetricom TS2100-GPS - 1PPS has 10uS offset

Chris Albertson albertson.chris at gmail.com
Sun May 12 18:35:32 EDT 2013

On Sun, May 12, 2013 at 1:34 AM, Esa Heikkinen <tn1ajb at nic.fi> wrote:
> Jason Rabel kirjoitti:
>> I think nobody ever really bothered to fix anything since the NTP time was
>> "good enough".
> Yes - definitely... If it's only used for NTP then the 10 usec error is
> totally insignificiant.

A startum one NTP server typically runs with abot 2 uSec error fro
UTC.   ButA stratum two server, that means one that gets time from the
strum one over the network

NTP solution seems to be quite inaccurate, if I poll
> my server from http://support.ntp.org/ntpq.php the results can vary couple
> of milliseconds every time.

Most Statum One NTP server are better then 10 uSec.  2 uSec is about
what you should shoot for.

It shouldn't work that way.  What NTP does is discipline a local
clock.  It take hours or days before it settles and a lot depends on
how good the local oscillator is.  Mostly your local oscillator is
just a TTL can that cost about 50 cents solder to a PC mother board.
The the error in one "poll" does not matter to much because hundreds
of them will be done and NTP wil make tiny adjustments to the locals
clock's RATE.  NTP is not adjusting the time, it adjusts the rate. (OK
there is  one time exception if the time is found to tbe far off)
It does not do an adverse of servers.  It compares them to find the
best subset.  This stops a broken server from polluting the average.

Yes there might be a 10ms error in a polled time  Bt what NTP is doing
is noteing the local time, letting the local clock run for say 20
minutes then comparing to the server(s) to see if the local clock is
fast or slow.  So that 10ms error is only parts per million.  It is
not a 10ms absolute error in UTC time.

If your local oscillator were better then NTP would open up that 20
minutes window and if the results aren't good it closes it down.   It
tries to keep several of those 20 minute intervals open at once.

On the other hand SMTP, to thing that Windows uses simply gets the
time and jumps the local clock to match and that's it.  It does not
discipline the rate.  SNTP is a one-time thing


Chris Albertson
Redondo Beach, California

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