There ARE modems that send a short beep now and then when they
are originating a call to let the other end know this is a data call
instead of fax or voice. My MaxTech 14.4 voice modem does this. If the
people calling your system have modems that do this, you can detect this
tone prior to sending the modem answer tone.
A friend has a fax machine with a very clever feature. I think
it automatically answers after maybe 6 rings, but it is always listening
to the line. If it hears the fax CNG tone, it goes off hook, sends
answer tone and continues with receiving a fax. What typically happens
when someone is trying to fax her is the phone rings, either she or the
answering machine picks up the line, the distant fax machine is sending a
CNG beep. Her fax machine hears the CNG beep the same time she does (and
her fax machine is still on-hook), so her fax machine goes off hook and
starts the fax transaction. She or the answering machine hang up on
hearing all the noise.
You could do something like this to identify data calls.
Another approach is the use of distinctive ringing. Some modems
will give you a code for the ring pattern. This allows multiple phone
numbers on one local loop.
Finally, there's caller ID. If the calls are coming from a
specific number (or group of numbers), they can be indentified before the
line is picked up as data calls.
First I'll admit that I know nothing about how a calling modem and an
answering modem will react.
But, chances are if a person answers the phone, they'll say hello within
about 10 seconds, or less. If your calling modem is gonna stay quiet,
until the ansering modem whines it's funky whine, then couldn't you just
monitor the line for the noise someone would make when they say "hello"
and a conversation starts. And if there is no noise, you can assume it's
a modem calling.
Of course, none of this will work if a calling modem doesn't keep quiet.
But if it doens't keep quiet, I think you should be able to monitor for
the noise it will make.
Just musing...
-Shane.
On Fri, 22 Aug 1997, William Chops Westfield wrote:
> There's always the ring hack...
>
> Dial up, let the phone ring once.
>
> Hang up, have the pic (which detected the one-ring situation) answer the
> next call IFF it occurs within the next minute...
>
> BillW
>
part 0 1862 bytes Even if the modem doesn't have the tone "data calling", you can simply use "ATDnumberR" to dial with reverse channel (meaning the call originating modem becomes the B-channel instead of the A-channel as normal). This means that the calling modem beeps just as the answering modem would normally have done. This enables the answering party to distinguish between modem and voice (and of course fax) calls.
ALL modems support reverse channels, but both modems of course have to be prepared that the channels will be reversed.
> A friend has a fax machine with a very clever feature. I think
>it automatically answers after maybe 6 rings, but it is always listening
>to the line. If it hears the fax CNG tone, it goes off hook, sends
>answer tone and continues with receiving a fax.
I don't think it works that way, because such a fax machine would only work on special phone systems. The answering fax cannot hear the CNG tone from the calling fax without lifting the hook, and then the call is already answered! The only audio which is on the line before the call is answered, is caller-ID tones (if any).
Most "fax switches" simply answer the call at the first ring and play an audio ring tone to the caller. If it then hears a "fax calling" CNG tone, it enters fax receiving mode. Otherwise it starts the ringer in the fax telephone, so the human can answer the voice call manually.
> You could do something like this to identify data calls.
For this to work, all phone switches need to be aware of the tone, so they can listen to it when a potential fax makes an outgoing call, and send it to the dialed fax if it's a fax calling. CNG is not implemented in the switches used in Sweden and I don't think it's implemented in any country. Neither is the tone "data calling".
> > A friend has a fax machine with a very clever feature. I think
> >it automatically answers after maybe 6 rings, but it is always =
> listening
> >to the line. If it hears the fax CNG tone, it goes off hook, sends
> >answer tone and continues with receiving a fax.
>
> I don't think it works that way, because such a fax machine would only =
> work on special phone systems. The answering fax cannot hear the CNG =
> tone from the calling fax without lifting the hook, and then the call is =
> already answered! The only audio which is on the line before the call is =
> answered, is caller-ID tones (if any).
If nobody answers the call, then there won't be audio on the line--true. But
I think what he's suggesting is that the fax machine is looking for the case
where someone ELSE picks up the line and CNG tones are then audible. If no-
thing else grabs the line before ring #6, then the fax machine answers the
line itself in case a fax is calling.
The idea here is that you can share the line between an answering machine,
fax, and (optional) person with the only caveat being that the person will
have to put up with fax calls (they'll answer the phone, then the other fax's
beep will come through, then the fax machine will grab the line and the person
can hang up). While such switching is generally only useful for fax handling,
it may still be useful to build a box to add that ability to fax machines
without it. Such a box could be built with a PIC 16C62x to detect the CNG
tone; the biggest difficulty would be triggering the fax machine to answer
the phone once CNG was detected. Unfortunately, wiring into the fax's "start"
button is the only way I can see to accomplish that...
>
>> A friend has a fax machine with a very clever feature. I
>think
>>it automatically answers after maybe 6 rings, but it is always =
>listening
>>to the line. If it hears the fax CNG tone, it goes off hook, sends
>>answer tone and continues with receiving a fax.
>
>I don't think it works that way, because such a fax machine would only
>=
>work on special phone systems. The answering fax cannot hear the CNG =
>tone from the calling fax without lifting the hook, and then the call
>is =
>already answered! The only audio which is on the line before the call
>is =
>answered, is caller-ID tones (if any).
This fax machine appears to "snoop" the line prior to picking it
up in case someone else (a person or answering machine) picks it up
first. Once that person or machine has picked it up, the CO switch sends
the CNG or data tone on through (just like voice!) and the snooping fax
machine picks it up. I thought it was a clever idea since it does not
need to generate ringing like a fax switch or even answer the line unless
it either detects CNG when someone else picks up the line, or when its
ring counter hits some number (typically set higher than the answering
machine).
>CNG is not
>implemented =
>in the switches used in Sweden and I don't think it's implemented in
>any =
>country. Neither is the tone "data calling".
The CNG or Data Calling tone need not be handled any differently
than voice by the CO switch since it is just passed through after the
terminating line goes off-hook (again, either by a person, an answering
machine, or a fax machine).
The ideas about originating the call in answer mode and doing the
"ring back" are also clever solutions!
What about this 'distinctive ringing' or whatever it's called where the
telco sends two or three different ringing patterns to a single line -
depending on which exchange 'number' was dialled ? I know this is
available in several countries - perhaps your friend's too....
WRITEME.COM>