[asterisk-users] Fax detection ...

[Lee Howard]

Jay R. Ashworth wrote:

>My personal experience is that I've never seen a consumer-grade fax
>machine with send-CNG turned off, and I don't *think* I've ever seen
>one on which there was a knob *to* turn it off; I would be less sure
>about fax modems -- those may have a knob, but I would expect it to
>default on.
>  
>

On fax modems the way to silent-dial (and I believe that this was a norm 
from early-on) to to add an "@" at the end of the dialstring:  
"ATDT5551212@".  I would be very surprised to find any modern fax modem 
that does not have this capability.

I don't know of any specific fax machine that has such a "knob" to turn 
CNG off.  But my contention wasn't that it was consumer-grade fax 
machines that were the main culprit here, but rather fax servers (PCs 
with fax modems in them).  And depending on what industry you are 
sampling, those may actually consitute a fair amount of the caller 
pool.  (For example, some industry software - like insurance agent 
application software - will have built-in fax features that will use the 
PC's fax modem - and the application vendor may insist on that feature 
being used.)

I cannot cite specific software that does it, but I suspect that most 
fax application developers are aware of the ability to silent-dial, and 
the reasons why it may be employed.  As I've said before, one reason, as 
an example, is to make the modem capable of hearing ringback - so that 
it knows if the call has been answered or not (which itself is a 
unreliable endeavor).  Another reason is to avoid annoying the receiver 
moreso on a call to a wrong-number.

It's off the topic of silent dialing but on the topic of this thread... 
Brother fax machine manuals state that it is possible for them to 
erroniously detect certain voices or music as CNG tones, and if that 
becomes a problem to disable fax detection.  And that's basically 
another point along the lines that say that fax detection is not 
completely reliable.

Lee.