[time-nuts] Re: dial-up time source for NTP

[Bob kb8tq]

Hi

Traditionally, the gotcha with doing it dial up was getting 
the timing somewhere near close to correct. The obvious
stuff internal to the PC is similar to what you go through with
a PPS input. With a serial port, you don’t have an interrupt 
based on an edge. This makes things a bit more convoluted. 

The modem side of things also has delays. Working out just
what they are is a bit exciting. Like a lot of things, you got it
close and then moved on. 

You then have telecom network delays. This is fundamentally
the same as internet delays. 

All of the stuff above can be / is / might be asymmetric.  most 
cases all the delays turned out to be a bit variable. In an era of 
tenth second timing, that probably was not a big deal. These 
days with GPS giving you < 1 us the competition is in a different 
league. 

You have the same sort of issue with WWVB. The driver needs
to know the propagation delay. One high profile public server
in PA ran WWVB and got the sign on the delay backwards. 
Not a good thing ……Since the delay varies night vs day, you
would pick one or the other.  That might have changed in newer
drivers. 

Yes, all that info is a bit dated……

Bob

> On Sep 13, 2022, at 12:48 PM, Hal Murray via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> 
> Richard Laager said:
>> I believe Hal is incorrect. NTPsec has not removed this driver. It's the
>> "modem" driver. 
> 
> Thanks for the correction.
> 
> Does anybody have any data?  How well does it work these days?
> 
> What do I need in the way of a modem or PCI card so that I can test/use that 
> driver and service?
> 
> Will an old modem from eBay work if I plug it into the phone socket on the 
> side of my internet phone box?  Are there any models or buzzwords to get or 
> avoid?
> 
> 
> 
> -- 
> These are my opinions.  I hate spam.
> 
> 
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