[asterisk-users] FAX reliability

Steve Underwood steveu at coppice.org
Sun Apr 12 23:19:44 CDT 2009


Most people using iaxmodem + HylaFAX or spandsp from an Asterisk 
application get satisfactory results for both transmitting and receiving 
FAXes. However, quite a few people find FAX receive is reliable, but FAX 
transmit can be flaky. Its box dependent. The cause of this has been 
known since the early days of spandsp. Asterisk + zaptel/dahdi is unable 
to produce a consistent audio stream on transmit, even though it usually 
receives OK.

I suspected Digium might have finally done something about this, as they 
are releasing a FAX module similar to app_fax. It looks like they have. 
In chan_dahdi.c there is now code that extends the buffering inside 
dadhi when a FAX is detected, and puts the buffering back to normal at 
the end. This isn't really a cure - its more of a bandaid. However, I 
expect it has the desired effect if they have put it into the trunk code.

You need to enable this feature in chan_dahdi.conf. The current default 
chan_dahdi.conf has the following line


You will need to remove the ";" to make the feature work. I doubt that 6 
is a big enough number, though. 8 might be more successful, based on 
audio logs I have received in the past.


In the early days of developing spandsp, a number of people reported 
they could receive FAXes reliably, but could not transmit them reliably. 
As this did not happen on my test machines, I asked a few people to try 
a test, and send me the resulting audio files. The test was as follows:

- Loop 2 E1/T1 ports together
- Use patched app_rxfax and app_txfax apps, which records the audio in files
- Set up a FAX call, so it dials out through one E1/T1 and back into the 
- Send me the resulting audio files

What I found was occasional gaps of 20ms, 40ms, 60ms, or sometimes even 
80ms in the received audio. The gaps all appeared to be an exact 
multiple of 20ms. If these gaps were edited out by hand, the received 
signal precisely matched the transmitted signal. One would expect this 
problem to be on the transmit side, as a problem on the receive side 
would likely loose some of the signal, not inject silences. This was 
basically confirmed by tests between a flaky box and a reliable one. 
Reports said the problem happened on lightly loaded machines, so it does 
not appear to relate to heavily loaded machines.

This problem was reported, but nobody seemed to care. Without a machine 
displaying the effect in my possession, it was impractical for me to 
investigate the root cause properly. Very interestingly, many people who 
have problems sending from app_txfax or app_fax have no problem sending 
from iaxmodem + HylaFAX. It seems Asterisk can feed a zaptel/dahdi 
channel much more reliably when passing a signal through, than when 
generating on in an app.

After all these years it looks like Digium have finally had to face up 
to this problem to get a commercial FAX package out the door. Everyone 
should benefit.


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