Remco Barende wrote:
On Sat, 15 Apr 2006, Kevin P. Fleming wrote:
Actually, I did. During a FAX transmission, there are many shifts to
different carriers and signaling rates as pages are transmitted and
acknowledged. It is _not_ as simple as a single carrier, like a normal
data modem connection. In addition to those shifts occurring, they are
very strictly timed and must occur within fairly short windows.
Hi Kevin,
I think the biggest problem is that almost any more modern fax machine
persistently tries to connect at the highest possible speed.
The only place speed is an issue is when someone tries to use V.34 Super
G3 FAX through an analogue->digital->analogue path. Otherwise, if a path
works for V.29 is should certainly work for V.17. These things are not
failing because of the modem speed. If the modem cannot be carried by
the path, FAX machines negotiate the speed downwards, anyway.
To solve the problem I suggested a workaround to this earlier on the
list, no idea if it is technically possible or dfficult to implement,
this is what I wrote :
record the sound fax machines make when negotiating (specifically the
part where they try to negotiate anything above
9600 baud) and make a provision in asterisk (an extra letter added to
the Dial command?) that will make Asterisk monitor
the channel and listen for the fax nego sounds and have Asterisk
distort or mute the audio. This way all fax machines
would be forced to lower their speeds.
Complex, clunky, and solves nothing.
I suspect that such a solution would greatly improve reliability for
faxing without the need for drastic changes in the
way asterisk works. If you could lower the speed further down to 4800
or even 2400 baud that might even be an interesting
option. Instead of faxing at 9600 or 14k4 through a normal (expensive)
landline it could be cheaper to fax even at 2400 baud
via a voip line depending on where you need to fax to.
The V.27ter modem used for 4800bps FAXing (2400 is not a supported speed
for G3 FAX) is no more or less demanding than V.29 or V.17 in this
context. It will work with worst line distortion, but that is not a
problem we have in Asterisk.
None of my fax machines are able to reduce their TX/RX speeds, if any
devices capable of capping the speed it would be a
nice addition to the wiki, I would instantly buy some all-in-one
machines that could do that
Actually, they are all able to reduce their speed when they need to.
They figure out for themselves what the path is capable of. Often in a
slightly quirky way, but bugs are the mainstay of the FAX industry.
The lower connection speeds wouldn't bother me, reliable faxing would
make up for the lost connection speed!
Regards,
Steve
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