I would second the suggestion about using ulaw or alaw. I really need to keep to one of these codecs so that the fax tones don’t get mangled in transcoding. I’ve been using fax machines on Asterisk systems using Linksys PAP2T’s. Mind you I’ve recently started using Schmooze Com’s Fax Pro module for receiving faxes and e-mailing them as PDF attachments and I really love it!

If you have t.38 setup properly there is no CODEC involved.

This article describes the process accurately:

It is key to understand that you guys keep talking about g.711 pass through, a spotty at best method. t.38 provides some relief with a more robust protocol.

The only way to get any better is to jump up to proprietary store and forward servers such as RightFax.

Or use a soft t38modem much as the old iaxmodem which still works under g711, I have never been able to make it work under Centos but under Debian, an easy compile and it does with a little work. . . . .

The distro includes SPANDSP already compiled that does the same thing.


I was talking of t38modem, both use spandsp but iaxmodem only support t30 over g711, t38modem is a fully functional t38 soft modem that supports t30 also both over udp or tcp, consider it more like digium’s res_fax, add hylafax+ and Avantfax and you have a nice gui/portal and as many fax modems as you want and it is all opensource.

What is the most reliable way of setting up a hardware fax machine in the following scenario:

ATA: Cisco/Linksys SPA8000.

Using EPM and Setting SPA8000 for Fax.

The SPA8000 specs indicate t38 support.

How is t38 initiated between the FreePbx/Asterisk host and the ATA.