[Freeswitch-users] Mod fax best practices

[Rob Forman]

Hi all,

Made some progress on the success rate of faxes and question about  
mod_fax default settings that might help others just getting starting.

ECM is turned on by default- which proved problematic, at least for my  
setup.  ECM, if I understand correctly, is the Error Correction Mode  
which allows fax machines to detect errors in small blocks of data on  
a page (via check sums) and request that just those blocks be re- 
transmitted (http://en.wikipedia.org/wiki/Error_correction_mode).

Unfortunately, I was seeing a build up of resend requests in the debug  
logs which eventually, on 10+ page faxes, exceeded the limit and  
caused the fax to fail.  Disabling ECM prevented this failure, though,  
as I understand it, possibly at the expense of quality.  Certain  
blocks could have errors that aren't corrected.  I haven't seen any  
visible loss in quality though and now am transmitting and receiving  
with a very good rate of success.

I guess the next question is why there were so many resend requests.   
Is that just part of the fax-over-ip territory?  Or is there more than  
can be tuned?  Thoughts anyone?

Cheers,
Rob

On Sep 8, 2009, at 10:29 AM, Rob Forman wrote:

> Hi all,
>
> I built a mod_fax setup which is working well.  Hats off to the  
> authors for the module.  I receive faxes regularly without issue.  I  
> can also send faxes, but seem to have a higher failure rate,  
> especially with faxes over 10 pages, usually with error 21: No  
> response after sending a page.
>
> I know fax-over-IP can be dicey  sometimes (don't know if T.38 would  
> help but see that mod_fax doesn't support yet anyway), but here's my  
> question:
>
> Are there any best practices when using mod_fax?  Codecs to use or  
> avoid, jitter settings, OS tuning, etc?  Things that you guys have  
> learned through live use.  Sorry if its a newbie question but I've  
> read through the documentation and wiki but haven't seen much in this.
>
> About the system:  Dedicated hosted server, Debian 5, Freeswitch  
> 1.0.4.  Nothing else running on the system.
>
> Any thoughts or lessons learned would be greatly appreciated.
>
> Cheers,
> Rob Forman