[ntp:questions] WiFi & NTP.

Kevin Oberman oberman at es.net
Fri Aug 31 03:10:51 UTC 2007

> Kevin Oberman wrote:
> >>From: "Richard B. Gilbert" <rgilbert88 at comcast.net>
> >>Date: Thu, 30 Aug 2007 10:45:01 -0400
> >>Sender: questions-bounces+oberman=es.net at lists.ntp.org
> >>
> >>
> >>Maarten Wiltink wrote:
> >>
> >>>"Guy" <gdelavil at ens.insa-rennes.fr> wrote in message
> >>>news:1188474891.630591.247300 at k79g2000hse.googlegroups.com...
> >>>
> >>>
> >>>
> >>>>Something important I forgot to mention is that my goal is not to
> >>>>have the most precise absolute time but to have the smallest offset
> >>>>as possible on a local network and using a WiFi connection.
> >>>>
> >>>>So I declared one of my devices as an NTP server and the other as a
> >>>>client (syncing to a single time source as a matter of fact).
> >>>
> >>>
> >>>The 'problem' with that is that you are then at the mercy of the
> >>>stability of your server. The stability of the crystal that ultimately
> >>>drives a local clock reference isn't great, it's quite temperature-
> >>>dependent.
> >>>
> >>>On the other hand, it _is_ quite predictable - people have produced
> >>>graphs where it is clearly visible when the airconditioning in the
> >>>server room started up, or when doors and windows were opened.
> >>>
> >>>The lesson as I read it is to make sure your server has a stable
> >>>environment. If the motherboard temperature is constant, so will be
> >>>the crystal speed. However, you are surrounded here by some rather
> >>>hardcore geeks, who read the lesson differently as requiring that you
> >>>have the One True Time, and stability will naturally follow from it.
> >>>
> >>>Groetjes,
> >>>Maarten Wiltink
> >>>
> >>>
> >>
> >>Let's hear it for "The One True Time"!
> > 
> > 
> > And which time is that? UTC, GMT, TAI, ...?
> UTC!
> > 
> > I hate leap seconds!!!
> Leap seconds do not affect my daily existence.  My computer takes care 
> of them.

Sorry, but that is beyond the capability of computers. It takes astronomers to 
determine when one is needed and humans to teach the computers and other 
equipment. Only your 'one true time', UTC has this problem. GMT continuously 
adjusts (also ugly) and TAI makes no attempt to stay in sync with the earth.

Note that GPS has no concept of the leap second There is a mechanism for 
handling this, but at some level, it requires human intervention. NTP is nice 
because it means that YOU don't personally have to deal with them, but some of 
us do and it is a royal pain. After the last one it was over a month before 
all of the CDMA and GPS systems I sync against to adjust their times, so I had 
to adjust my stratum 1s twice, once after the second was inserted and then, 
one at a time as the carriers got around to them.

It is all made worse because POSIX does not do leap seconds. To do it right, a 
system as to be able to run one 61 second minute and POSIX makes this 
impossible, so there is no really accurate way of making most computers do 
leap seconds. Assuming your stratum 1s are properly handling leap seconds, NTP 
has to drift the time, so it is off by quite a bit for quite a while.

As I said, I hate leap seconds.
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at es.net			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751

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