[ntp:questions] Converting from Y-m-d h:m:s
mills at udel.edu
Mon May 17 04:19:48 UTC 2010
In the very early history of the Internet, the ubiquitous Fuzzball
routers (and the time they provided to other hosts) were synchronized to
the power grid. There are three grids in the country, East of the
Rockies, West of the Rockies and Texas. Each is an island loosely
synchronized to civil time, more or less. I have graphs of the Eastern
grid in August, in which the time slips by up to five seconds during the
day and makes it up overnight. Since the Fuzzballs ran off the same
power grid, the only thing the time synchronization protocol (Hellospeak
at the time) was to set the clock from the WWVB receiver once at the
server and slew the offsets of the clients to the same value. Frequency
discipline was not necessary. The folks in Ohio managed the grid
generators as the load waxed and waned to keep the average offset from
civil time small, so the Fuzzballs in principle needed only to
synchronize only once. The Fuzzballs ran this way for several years
until the line-frequency clock was replaced by one driven from a crystal.
Since the Fuzzballs at the Earth stations in the US, UK, Norway, Germany
and Italy were synchronized to the various national grids, I had the
chance to measure the offset of each grid over a period of days. Norway,
which largely relies on hydro, was, excuse me, rather awful and did not
cling closely to civil time. The UK was somewhat better, but still
varied up to ten seconds over the day. The UK doesn''t have such wide
load variations as the US in August, so I inquired about the reason. I
was told the principal contributor to load variations was TV sets, where
the highest load is in the evening. With this understanding, I
volunteered to the BBC to provide a rating service where the number of
viewers could be calibrated by the time lag of the power grid. They
thought this was pretty funny, but didn't take me up on the offer.
Uwe Klein wrote:
> Terje Mathisen wrote:
>> Marc Leclerc wrote:
>>> I am looking to stay on true time, the real issue here in those
>>> installation is not realy having the perfect time but that all the
>>> device in the field have the same time. There are new ways to acheive
>>> this such as IEEE 1588 but since no one is ready for this using GPS
>>> device is our closest friend. But it's funny you asked, soon we will
>>> tackel simple synchronous switching of breaker and then we'll have to
>>> keep up with the AC cycles
>> This is actually somewhat easier:
>> You only need 8 or 10 ms accuracy to do this, in order to determine
>> the start of each 50 or 60 Hz cycle, then you phaselock your switch
>> to the AC cycles.
> There is a (massive?) hitch.
> The grid 24h average is held to the nominal frequency.
> Shortterm frequency is load dependent.
> so you need to sync all boxes to the _same_ zerocrossing in a
> "variable frequency" domain with the help of a "true time" source.
> not easy.
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