[189294] in North American Network Operators' Group
Re: Cost-effectivenesss of highly-accurate clocks for NTP
daemon@ATHENA.MIT.EDU (Baldur Norddahl)
Sat May 14 15:39:33 2016
X-Original-To: nanog@nanog.org
In-Reply-To: <CAPkb-7Bn__UnaThbJ8W+QUxjm8GOGVEFHtDbFmZ4Y9yCy=jWfg@mail.gmail.com>
Date: Sat, 14 May 2016 21:39:29 +0200
From: Baldur Norddahl <baldur.norddahl@gmail.com>
To: "nanog@nanog.org" <nanog@nanog.org>
Errors-To: nanog-bounces@nanog.org
On 13 May 2016 at 23:01, Baldur Norddahl <baldur.norddahl@gmail.com> wrote:
> Ok how many hours or days of holdover can you expect from quartz,
> temperature compensated quartz or Rubidium? Should we calculate holdover as
> time until drift is more than 1 millisecond, 10 ms or more for NTP
> applications?
>
> I am thinking that many available datacenter locations will have poor GPS
> signal so we can expect signal loss to be common. Some weather patterns
> might even cause extended GPS signal loss.
>
>
>
I found some data points here: https://en.wikipedia.org/wiki/Crystal_oven
Assuming that acceptable drift is 10 milliseconds due that being the
expected accuracy from NTP.
The common crystal oscillator can be as bad as 1E-4 => holdover time is 2
minutes.
TCXO is listed as 1E-6 => holdover time is 3 hours.
OCXO is listed as multiple values, I will use 1E-7 => holdover time is 1
day.
Rubidium is listed as 1E-9 => 3 months
Caesium is listed as 1E-11 => 30 years
Hydrogen Maser 1E-15 => 300 millennia
I clearly need three of those maser things for my network.
Regards,
Baldur
