[asterisk-users] Faxing with Asterisk & SpanDSP [Was Fax Problems with SpanDSP]

[Gordon Henderson]

On Wed, 31 Oct 2007, Alan Lord wrote:

> Steve Underwood wrote:
> <snip /> ...
>> SpanDSP handles faxes within Asterisk, through the app_rxfax and
>> app_txfax applications. It handles faxes outside Asterisk when used with
>> iaxmodem (there is actually a copy inside the iaxmodem package).
>>
>> SpanDSP cannot be used by the standard distribution of Asterisk, as it
>> is GPL code. However, if you are using Asterisk within the restrictions
>> of the GPL you can add app_rxfax and app_txfax to Asterisk quite easily.
>>
>> Regards,
>> Steve
>
> Thanks Steve,
>
> I was a bit lazy when I posted that question sorry - I just got a bit
> excited. A quick google and looking around the voip-info.org site
> pointed me to some interesting pages.
>
> Could anyone who has experience confirm if:
>
> * this will work with an x100p card so I can have a single FXO line that
> can detect incoming faxes and/or voice calls and enable me to use it for
> outgoing voice and fax?

I've used it in exactly this mode with TDM400 cards. It worked mostly OK. 
You might need to fiddle with the gains in the zapata.conf file. (although 
I now steer people away from this way of doing it and use a separate ATA 
if they have a real fax machine, or an external fax to email service)

> Also, I read somewhere that Asterisk should "never" be used with fax.
> IIRC correctly it was by the moderator on the Trixbox forum discussion
> about the trial of OSLEC. Yes, here:
> http://www.trixbox.org/forums/trixbox-forums/open-discussion/need-people-echo-problems
>
> "I really don't know how many times I need to say this but you should
> never run faxes through Asterisk. It was not designed to handle it.
> --
> Kerry Garrison
> trixbox Community Director"
>
> Is he right?

Dunno, but it works. FAX data is nothing special - however modems are very 
critical of jitter - you and me and tolerate a bit of packet loss, or the 
odd duplicate, etc. in an audio stream, a modem can't. (NO CARRIER :)

So your asterisk box shouldn't have jitter internally, but if it's running 
cpu intensive applications, it might have, I guess...

A lot of people have success with running iaxmodem - which is just spanDSP 
connected to some fancy code to understand AT commands, and they then 
plumb this into hylafax running on a separate server over ethernet.. I've 
tried ATAs over ethernet to send/receive faxes to real fax machines with 
good results too. (Make sure the codec is G711 though, as nothing else 
will work)

Gordon