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[rob4manhere at gmail.com]

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

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[rob4manhere at gmail.com]

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: