On Fri, 22 Aug 1997 00:09:36 +-200 Glenn Johansson <glenn@WRITEME.COM>
writes:

>
>>        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!

Harold