Subject: [ntpwg] Timing Definitions

From: anthony.flavin at bt.com (anthony.flavin at bt.com)
Date: Wed, 21 Feb 2007 08:56:03 -0000

This is an interesting discussion, but it should be well understood by
most people.

We are dealing with nothing more that sampling theorem. The fact that the
quantity we are sampling is time is irrelevant. Terms such as precision
and resolution are well understood here. As are error (noise).

If these terms have been re-defined within NTP, then that's fine and those
definitions should hold.

So what was the original question that was trying to be answered? Because
I have lost track....

Regards,

	Tony Flavin
	Core Networks Engineering Manager

	e-mail: anthony.flavin at bt.com
	tel: +44(0) 1473 609570
	mobile: +44(0) 7801 759596
	fax: +44(0) 1908 862752

	Orion 5,  pp11 , Adastral Park, Martlesham, Ipswich IP5 3RE, UK

P.s "Big Ben" is the bell not the clock:-

The Great Clock
Hands: Minute (copper) Length 4.3m (14 ft) Weight 101.6kg (2 cwt)
Hour (gunmetal) Length 2.7m (9 ft) Weight 304.8kg (6 cwt)
Pendulum: Total Length 4.4m (14 ft 5 in)
Length of Roman numerals: 61cm (2 ft)
Minute squares: 30.5cm (1 ft)
Number of panes of glass 312
in each clockface
The Bells
Big Ben (the Great Bell) Weight 13.8 tonnes 13 tons 10 cwt 99 lb
Note E: Hammer Weight 203.2kg 4 cwt
Quarter Bells
1 Note G sharp Weight 1 ton 1 cwt 23 lb 1.07 tonnes
2 Note F sharp Weight 1 ton 5 cwt 30 lb 1.28 tonnes
3 Note E Weight 1 ton 13 cwt 69 lb 1.71 tonnes
4 Note B Weight 3 tons 10 cwt 69 lb 3.59 tonnes
Time between Strikes:
>From start of chime to 12th strike = 95 seconds
Big Ben: From 1st strike to 12th strike 54 seconds, 5 seconds between
strikes

-----Original Message-----
From: ntpwg-bounces+anthony.flavin=bt.com at lists.ntp.isc.org
[mailto:ntpwg-bounces+anthony.flavin=bt.com at lists.ntp.isc.org] On Behalf
Of Kurt Roeckx
Sent: 20 February 2007 23:14
To: David L. Mills
Cc: ntpwg at ntp.isc.org
Subject: Re: [ntpwg] Timing Definitions

Hi Dave,

On Tue, Feb 20, 2007 at 12:56:05AM +0000, David L. Mills wrote:
>
> Stand next to Westminister Abbey and set your watch to Big Ben. Big
> Ben
> clockkeepers keep the clock in tune by listening to the BBC and putting
> coinage on the pendulum. Presumably, the "accuracy" of Big Ben depends
> on the residual frequency error and the interval between coinage
> updates. You set your watch to Big Ben and inherit its characteristics
> and some of your own. How "accurate" is your watch?

Since I have no idea about what the accuracy or precision is of Big Ben,
it's going to be rather hard to say anything about it.  And I think that's
the point we're both trying to make.

Anyway, there are 2 ways to synchronize your clock to Big Ben.  One is
looking at it and reading the time.  This is as far as I know with a
resolution of 1 minute, so a precision of atleast 1 minute.  But the clock
might be set with an accuracy in the order of a few seconds.  The other is
listening to it's bells which is going to have a higher precision but a
different accuracy.

To say anything useful about any of those 2 ways, you need to do
statistics by comparing it to a clock that has a small enough error.

If I'm going to set my watch by any of those 2 methods, what matters is
which way has the smallest total error.  The accuracy of my watch is going
to depend on that, and not how accurate the clock is or how precise I can
"read" it.

You can go and reduce the error caused by the precision by taking more
samples, but you can't compensate for the accuracy error.

Anyway, the definition of precision and accuracy I use might be a little
bit different than what you're used to when talking about NTP. If you take
some samples and compare it to a reference, the accuracy would be the
average difference between the samples and the reference, the precision
would be the standard error.

What you use in NTP for the precision is the highest of the resolution and
the time it takes to read the clock.  This is a reasonable estimate for
how precise you can measure the local system clock.  It's not a good
estimate for how precise a remote ntp daemon can come and read your system
clock since it doesn't only depend on your own system now.  It's this
"remote" precision that is used to synchronize the clock.  And I think you
try to estimate that by the jitter.

Kurt

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From: mills at udel.edu (David L. Mills)
Date: Wed, 21 Feb 2007 14:57:54 +0000

Tony,

Watch out for the sampling theorem. That has to do with the minimum 
frequency for sampling time-varying signals with specified bandwidth.

What set this discussion off was the notion on the part of some folks 
that accuracy could be defined in some arbitrary manner. Not so, and my 
carefully crafted error budget is the counter. I would take serious bets 
even now that few have actually seen and considered the draft 
specification. I have received no comments on the draft of any kind.

Dave

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