Re: OT: time and computer networks
On Thu, Dec 02, 2004 at 12:59:57AM -0500, Matt Price wrote:
> hi folks,
>
> ok... this is way OT. but I thought I'd put this question to the
> most knowledgable group of people I know...
>
> I have to give a lecture on the history of timekeeping technologies.
> I want to end with late c.20/ early c21 technologies of synchronized
> timekeeping. GPS is one obvious example, NTP is another. But
> puttingthe lecture together I realized I don'trelaly understand why
> it's important for computer networks to have fine-grain
> synchronization. So I thought I'd ask some geeks (as my sig says, I'm
> only a hemi-geek): why does a network need careful clock
> synchronization? Are packets like railroad cars -- in the sense that
> it's VERY important to know which got sent first, and which is ocming
> next -- and if you screw up the timeable, you get a catastrophe? Or
> is there more flexibility in the system?
>
In San Francisco, in a museum of local history, there is a large clock
which is described as the first official clock of the SF business
community. Public time is important to business, as it is the way
people (including business people, not just geeks) sequence events,
decide cause and effect, discover defects in the data that is available,
etc. This easily pre-dates the internet, and indicates why people are
interested in knowing the 'correct' time.
Real technologists, i.e. people like Edison and Westinghouse, know this
and design systems that have built in sequencing and synchronization.
For example, Y2K was a real big deal for the billing department of the
local electric company, but had nothing to do with keeping the various
AC generators on the grid in sync. (Please, no flames about hidden chips
inside essential boxes. Those were not designed by real technologists.)
The best internet time is many orders of magnitude inferior to the
best NIST laboratory clock time. The time available from GPS is in
between in quality, i.e. far better than internet time, but far
inferior to the best NIST lab. time. GPS time is 'good enough' for it
job, namely determining position from signal time of arrival
measurements to an accuracy of a few meters.
Also, GPS time can never be as good as NIST lab. time, because the
motion of the GPS satelites makes synchronizing two clocks to NIST
lab. accuracy a meaningless exercise.
--
Paul E Condon
pecondon@mesanetworks.net
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