[asterisk-users] how to set iaxmodem receiving speed

[Lee Howard]

On 05/17/2012 07:40 AM, gincantalupo wrote:
> That could seem counter-intuitive but it is not. Not to mention the 
> fact that information technology is not science, the solution to 
> broken faxes is to lower down speed.

The DSP algorithms change slightly between bitrates and considerably 
between modulations.  Changing the modulation/demodulation algorithms 
can many times avoid problems caused by some type of line audio 
disturbance.  This is why fax machines and fax applications are 
programmed to try various bitrates and modulations when training is 
failing at the default settings.

Because historically most fax machines have supported V.17 14400 bps as 
a default it became customary for support technicians to suggest 
"slowing" it down to 9600 bps.  (I think that the real intent here was 
to switch to V.29 9600 bps, but in practice it often results in V.17 
9600 bps.)  The purpose in this isn't really so that the communication 
takes longer (you can imagine that stretching-out data over a longer 
period would increase the likelihood of some audio disturbance affecting 
the demodulation), but instead I believe that the purpose in this 
recommendation is to cause a change in the DSP algorithms.

Now, disabling ECM (error correction) is just plain wrong as long as the 
ECM protocol is implemented properly on both ends.  If ECM protocol is 
implemented properly on both ends (and most are implemented well-enough 
that this applies to them) then ECM should be left enabled.  By 
disabling a well-implemented ECM feature you're essentially making the 
claim that the remote-side ECM protocol is broken.

If disabling ECM actually makes things work and you never get a 
corrupted fax image come through after that then it only means that one 
or both of the endpoints had faulty ECM protocol.

Some technicians (including those working for fax machine manufacturers) 
will recommend disabling ECM if faxes aren't getting through.  While 
this may have originated with the purpose of avoiding problems in faulty 
ECM protocol I think that any regular use of this suggestion is simply 
to get the customer to go away.  The customer will see a page come 
through with streaks and lines, but it will be "successful", and so 
they'll unfortunately be happy with that enough to let the technician 
off-the-hook with the "disable ECM" advice instead of actually fixing 
the real problem (either getting the line audio quality problem 
corrected or fixing the broken ECM protocol).

> My idea was to tell iaxmodem not to accept fast speed rates so the fax 
> machine on the other side should be forced to negotiate a slower speed 

And what you were doing with the HylaFAX modem config file for the 
iaxmodem should have worked to do this.  Why it wasn't working can only 
be determined by investigating your installation.

Thanks,

Lee.