Path: utzoo!attcan!uunet!husc6!bloom-beacon!tut.cis.ohio-state.edu!rutgers!ucsd!ucbvax!decwrl!sgi!daisy!david
From: david@daisy.UUCP (David Schachter)
Newsgroups: comp.periphs
Subject: Re: precise time for systems?
Keywords: wwv, WWV, precise time, PSTI, time-stamping, time, timing
Message-ID: <2268@daisy.UUCP>
Date: 28 Dec 88 01:31:59 GMT
References: <11615@cup.portal.com> <12812@cup.portal.com>
Reply-To: david@daisy.UUCP (David Schachter)
Distribution: na
Organization: Daisy Systems Corp., Mountain View, Ca.
Lines: 90

In article <12812@cup.portal.com> Michael_J_Ward@cup.portal.com writes:
>[...] PSTI, a company
>which makes a WWV receiver for synchronizing traffic lights and also for
>computer systems.
>
>How many systems would need such precise time-stamping? Do they put one one
>of these on each server, or somewhere in the middle of a network? Is anyone
>using one of these?

I wrote the software and helped design the hardware for the PSTI clock, so
I'm qualified to answer Mr. Ward's questions, I hope!


1. How many systems need precise time-stamping?

The PSTI clock provides a specified accuracy of 10 milliseconds and a
typical accuracy of 3 milliseconds.  This means the time you get is within
3 milliseconds of the WWV/WWVH time.  WWV/WWVH are within a few nanoseconds
of official world-wide time, as determined by the BIH (International Time
Bureau) in France.

Accuracy to the tens of milliseconds level is overkill for traffic light
control.  For synchronizing computer networks, it is just right: it is
accurate enough to be used for synchronization of distributed databases and
filesystems.

A radio-controlled clock has advantages besides accuracy.  It is reliable:
there are no batteries to change, no problems with daylight savings time,
no change of being set incorrectly.  If the power fails, the clock resets
itself when power returns, by receiving the radio broadcasts.

Radio-controlled clocks are traceable: you can trace the timing in your system
back to the BIH, and have a good level of confidence that your timestamps are
accurate.  This can be important in legal affairs, e.g. lawsuits over police
response speed.  If you must prove in court that an event occurred at a partic-
ular time, or that two events were separated by a particular interval, a radio-
controlled clock can be useful.

Radio-controlled clocks are more secure: the signal is not easy to forge and
PSTI clocks have further security features I won't discuss here.  Such clocks
can impede computer crime, and this is a Good Thing.


2.  Where do radio-controlled clocks go?

For traffic-light control, the alternative to radio-controlled clocks is
to trench the streets, connecting all the intersection controllers to
some central location.  This is not cheap, nor is it reliable.  Cables
break, get waterlogged, or accidentally dug up and ripped.  Central sites
go down.  WWV and WWVH never go down.  Each transmitter site is triply
redundant.  Moreover, the PSTI clocks maintain accurate time in the
absence of radio signals by digitally trimming the local crystal timebase.
Several transportation districts now use a PSTI clock at each intersection,
installed in the traffic light controller box.

For computer applications, PSTI clocks can be installed in the computer.
They are tolerant of high levels of RFI and EMI.  PSTI clocks maintain
higher accuracy if they are operated at a stable temperature.  The clocks
need a 15 volt DC power supply and they consume only a few watts.

One application I'm proud of is to use PSTI clocks for earthquake measurement.
This application requires low power, high reliability, and high accuracy.
The PSTI clock was the only device which met the requirements.

PSTI has software to connect PSTI clocks into VAXen and some other
computers.  On the VAX, the software permits one VAX to serve as a time
master for other VAXen on a network or in a cluster.  I didn't do the
software so I don't know what levels of time-transfer accuracy is achieved.
For higher accuracy, one could put a clock on each node.  An RS-232 port is
required, and money.


3. Who uses them?

Digital Equipment Corporation, Los Angeles County, and The White House are
three PSTI customers.  Contact PSTI for a more complete list.

Precision Standard Time, Inc. is at 105 Fourier Avenue, Fremont, CA 94539.
Their phone number is (415) 656-4447.  There is a nationwide 800 number,
but I don't know what it is.

Disclaimers: I used to work for them.  I have stock in the company.  I like
the product a lot.  (It fulfills a childhood dream of always having the correct
time.)

						-- David Schachter

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