Hello all,
I am having a problem with external modems. I have some locations -
separate phone lines, where I am having a hard time getting my
external modems to get a dialtone. On these lines when I connect an
analog landline telephone I get a dialtone just fine but the modems
only intermittently see a dialtone. I was wondering if there were
maybe any differences in the physical specifications between analog
voice phones and external data modems that would explain this? The
teleco keeps telling me the lines are fine and they keep telling me
that the fact I get a dialtone and the fact that I can dial the long
distance number via an analog voice phone means the line is good - the
modems we are having problems with are brand new Motorola Codex 3260
Fast modems so I am not yet willing to accept we have bad modems since
modems that fail work fine consistently on other, known good, lines.
Any ideas?

-Robert

in_like_flynt@yahoo.com (Robert B. Phillips II) wrote:
An interesting problem. You can pretty much bet that the modems
are in fact doing certain things.

1) They do go off hook.

If a modem fails in that respect, it won't likely be
intermittent.

2) They probably do get a dial tone.

If you consistently get a dial tone with a telset, the modem
isn't going to be any different. The parameters are that the
telephone line will provide dial tone if it is in an "offhook"
state and a digit receiver is available in the telco switch. An
offhook state means that 20 or more milliamps of loop current is
flowing. And a loop has to be provisioned such that any
resistance less than something like 325 ohms (the value isn't
important so I'm not going to look it up) must cause an offhook.

Such things can be intermittent (e.g., not enough digit
receivers in the switch), but you'd see it with the telset just
as readily as with the modem. And the modem most certainly
provides less than 325 ohms across the telephone line for DC
current, and is almost certain to be less than most telephone
sets.

Which brings us to the one difference between a telset and a
modem!

3) Your modem is probably *not* able to detect every possible
variation on dialtone that might be heard.

And of course a human listening to a telephone set can detect
dialtone that a modem cannot.

So my bet is that your modems always get dialtone, but don't
always know it. That could be caused by faulty modem design, by
an odd dialtone, or by noise or other distortion on the line.
For example, some modems will not recognize stutter dial tone,
dialtone that is not loud enough, or dialtone where one
frequency is not quite right.

You can also program your modems to just ignore lack of
dialtone. You probably want to try that. If the problem goes
away or is hugely diminished, then dialtone recognition is the
problem. On the other hand, if it really isn't getting a
dialtone that will mean there will be an increase in what are
called "no ring / no answer" calls. When your modem ignores the
fact that there is no dialtone and just dials merrily away, the
"call" will go high and dry every time. So being able to detect
that will allow you to determine if there is actually a fix or
if it just moved the problem.

--
FloydL. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@barrow.com