[plug] baud rates and modem dropouts

[Matt Kemner]

On Sun, 22 Nov 1998, David Buddrige wrote:

> I would assume that if the modem had been disconnected, then the light's
> would all turn off... at present, it appears to remain connected, but the
> ping's are no longer working.... I'm going to check out the suggestions
> made by Christian...

I would say you're experiencing line noise of some sort.  Usually when a
modem "hears" line noise, it will "retrain" down to a lower speed..
Usually it will go from say 33.6 -> 28.8 -> 14.4 -> 9600 -> 4800 -> 2400
-> 1200 -> 600 -> 300 -> dropout. (overly simplified - there are a few
baud rates in between that it will use)  Most people that have line noise,
have a lot of it, so their symptons are just that the modem drops out
completely, because it has run through the speeds in fast succession..
However, others have line noise at irregular intervals, and the modem
cycles down to a speed below 1200, with the result being that it appears
there is no connection between the two modems, but there is one that is
very very slow. (read >20second ping times.)
You may be experiencing the latter effect.  Since you've spoken to Telstra
(and presumably your ISP) I assume you've checked the main causes of line
noise, but just in case I'll list the main 2 culprits again.

1. Disconnect ANY other equipment that's in parallel on your phone line
   (especially Telstra Touchphones)
2. Make sure your phone line doesn't run past any electrical wires because
   they can cause interference.

With the advent of 56k modems things are slightly different, because they
don't just cycle down through the speeds, at regular intervals they will
try to cycle back up through the speeds as well, to try and give you the
fastest possible connection at all times.. However this can cause problems
because often while the modem is retraining, if it gets some noise on the
line, it may not be able to negotiate a speed and will drop out
altogether.  To counter this you may want to choose a speed that your
modem can happily attain every time (eg. 44k, 48k or even 50k if you're
lucky) and force your modem to not connect to a higher speed than that.
To do this on most modems you use the AT+MS init string.. Eg. to limit
your modem to 44k, use AT+MS=56,1,300,44000

Anyway, enough of my ramblings :)

 - Matt