7 thoughts on - Is Possible To Use FXO Digium Card Like A Fax Modem?

• Yes, in theory it is entirely possible to use an FXO card driven by software as a modem (and indeed, this is exactly what Winmodems do); although you will have to do all the hard work of generating the outgoing tones, and decoding the incoming tones, yourself. This is a highly non-trivial task, and there is almost certain to be a better way than this of achieving whatever you want.

• Humm thanks for your reply,

  Do you know whats is step for I can transform this card link a fax modem?

  2016-03-30 9:36 GMT-03:00, A J Stiles :

• There are also cheap USB fax modems that you can attach to an FXO port and that works fine. All you have to do then is configure asterisk to detect incoming faxes and route them to that port (faxdetect=yes?).

  This worked great for me when I had all my incoming calls coming over a Century Link POTS line. As I approach retirement and want to save money, I
  switched from the $44/month POTS line to a pennies-per-month VOIP service via IAX registration. So now I’m wondering whether this setup would still work. The question undoubtedly shows my ignorance of telephony stuff. I’m willing to do my homework, I just want to know if it’s even possible to do this, or if there are better ways to handle fax over VOIP.

  –Greg

• I am going to say this with tongue only partially in cheek. The better way to do fax over VOIP is not to do it. It is finicky and unless you have a real need for it, it isn’t worth the time it takes to make it all work. Even working, you still have complaints every time a fax fails to send or receive as people somehow have this expectation that faxes should never fail. To quote the movie War Games, “The only winning move is not to play.”

  It would be preferable to use a scanner and email to send documents if at all possible. If you still need the occasional fax, I would recommend using a fax service and letting that be someone else’s headache.

  That said, my company still has plenty of people who insist that faxes are the greatest thing since sliced bread, so I get the fun of supporting them. Your options, depending on scale are to use one the solutions you can integrate right into the Asterisk server or to use an external package and then you just forward the calls from your asterisk box over to your fax software (this is the one I use).

  Make sure that your SIP/IAX provider supports T.38 faxing (specifically transcoding) as this will make your life much easier. You have to be careful here as many providers will happily pass T.38 along if it comes in that way, but if someone with an analog line/fax setup sends you a fax, it will hit their system as audio and pass on to you as audio, which with SIP
  can be fraught with danger unless you have a really excellent connection to your sip provider. With transcoding, they can convert it as it enters their system to T.38 and then just pass the T.38 to you, which results in greater successes. T.38 passthrough is common, transcoding less so, but it is getting more common as time goes on.

  Also, if your provider does not support T.38 transcoding, plan on sticking with ulaw or alaw for faxing. The compressed codecs do not allow the audio signal to pass properly and faxes will not work.

• Start with the specification document for the modulation scheme you want to implement, and the DAHDI Source Code for the card you want to use.

  But if you have to ask how to do it, it’s probably too hard for you …..

• With the sole exception of the X100P, which, IIRC, was basically an existing fax/modem card. If you dig deep enough you may find existing drivers for it.

  Assuming what is now called X100P is close enough to what was originally that modem.

• Asterisk + iaxmodem gives you a bunch of soft FAX modems. Add one of the analogue PSTN interface cards you listed and you have a multi-channel PSTN connected FAX modem. This arrangement is widely used with HylaFAX, although people do use it with other FAX software, such as the stuff built into Windows (using ethernet virtual terminals to connect the windows box to the linux box).

  Regards, Steve