[lug] Dialup modems and Linux

[J. Wayde Allen]

On Thu, 5 Sep 2002, Justin-lists wrote:

> I put my isa modem back in the linux box and I'm stuck at the point
> where minicom dials the isp and disconnects just as the tones stop. I
> couldn't get any further than that and logs don't tell me anything and
> nothing diagnostic wise gets printed to the minicom window. I put my
> login/passwd in pap/chap-secrets and tried all the /dev/ttyS* ports.
> It seems that the modem only works (in minicom)  under /dev/ttyS1,
> which makes sense because the modem comes up as com2 in windows.

That minicom can dial the modem and get a connection is a good thing.  It
also probably explains why you don't get anything printed in the logs.  
(There really is nothing printed in the logs right?) As far as the Linux
system is concerned there isn't a problem.

What is particularly interesting is that you are getting nothing in the
minicom window.  That to me means that the problem you are having is
between the modem and the dialup server and not between Linux and the
modem control.  Either the modem settings don't match what the server
expects (usually you'd get some unreadable garbage in this case though),
or the server isn't answering you back for whatever reason.  

Do you have a manual for this modem and have you checked the
initialization string that is sent to the modem?  That would configure
things like flow control, how the modem handles negotiation of the dial
link speeds, etc..  Have you been able to connect to this server before,
perhaps running one of these modems under MSWindows?  If so, you may be
able to take a look at what that software is using in its initialization
string.

The kind of behavior you are describing could occur if the modem settings
don't match what the dialup server expects.  Linux could control the modem
and dial it, but the modem wouldn't be able to negotiate the connection
protocol with the server (incompatible speeds or incompatible compression
algorithms, etc.).  That would probably result in a dropped connection
attempt, and relatively little logging information.  If you don't see any
kind of activity in the minicom screen then you never actually get a
communications link established with the server.  It never asks for a
login and so your system never even gets to the authentication stage.

(One thing is nagging in the dark recesses of memory about a newer login
procedure that some dialup servers were starting to use.  That might not
create the classical login challenge used by chat scripts, but I can't
quite place what that might be?  Perhaps someone knows, or perhaps this is
one of those vague not quite correct recollections...?)

- Wayde
  (wallen at lug.boulder.co.us)