[time-nuts] Transmitting a time sync signal - what's the trick?

[Matthew Smith]

Hi Folks

I am looking at getting a number of wireless devices set up in and
around the house.  These will all be transmitting on 433MHz (the Aussie
ISM band, although they don't call it that) and will use an addressing
system to determine sender and destination.

As part of this system, I would like to send a time signal from the time
server that I am building as some of the devices will be actual clocks
and others will have RTCs for various purposes.

What's the trick though?  If I send a signal to say "it is 12:33:45", by
the time that the signal has got there (at a mere 4800bps) and been
interpreted, it won't be any more.

My thought was to use something like the old British speaking clock
whereby "at the third stroke, the time will be...".  However, if I am
broadcasting in packets, it's not just a case of waiting for a carriage
return, it's still a case of waiting for several bytes, the receipt of
which takes quite a long time, relatively speaking.

I suppose that the issue exists with all asynchronous communications,
packet-based or no.  Only when you've got a dedicated wire called
'clock' can I see how this can be made to work.

I know that this must be happening out there in many shapes and forms -
but how?

Cheers

M

-- 
Matthew Smith
Smiffytech - Technology Consulting & Web Application Development
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