T.38 H323 or SIP FAX transmission/ tx fax SpanDSP

Hi all,

We have whatever the latest beta version of Asterisk is installed, H323 (latest version) with T.38 and SpanDSP (v. 0.0.2 311??) - and are trying to transmit fax over SIP or H323. Have no interest in receiving faxes, just broadcasting tiff files to fax machines.

On both H323 and SIP when calling the receiving fax machine (calling both windows fax client and conventional fax machine), the fax answers, but on our end it keeps ringing… After a minute or so the call is terminated from the server and the fax (using winfax) says "reception error).

We have once sent a fax that was received but simply came across as a black page, although that was the only one that was ever received…

We don’t have any additional hardware, and once this is setup it will have a direct connection to a 20MB line, so bandwidth isn’t an issue.

The only other issue is that we are testing with two gateways, one is SIP which is all fine compatibility wise, but the other isn’t sure if thier carrier can pass on H323 traffic - apparentley this can be confirmed and activated on Monday if required.

I’m purely looking for suggestions, as I know not many people have had any luck with this… Maybe you can think of something we haven’t tried???

Oh, if you can help this is kind of urgent! Will gladly share info with you when it works!



Managed to fax over oH323 and can receive fine, trouble is when I try anything with more than 2/3 lines of text it fails halfway through the transmit…

Once tried cutting out blocks of a full page to make it smaller but it still wouldn’t work when there were one or two lines of text left, when I moved the text an inch down from the top of the page it went straight through… Tried this with the full page text again (moving its position down on the page) but it still didn’t work.

The format is tif 3, and resolution of 204x197 - although I have created a tif with a resolution of just 197 and it sent fine… Any ideas???

Faxing over VOIP is generally difficult. VOIP codecs tend to destroy fax messages because of the incomplete sampling and compression they do to sound.

If you must, the best thing to do is use a toll quality codec like ulaw or alaw. (g.711a or g.711u)

They will give you your best results.

Otherwise my best advice is to NOT use VOIP to transmit your faxes. Use a telco style analog line to receive faxes.

If you’re intent is to receive them by network, then scan and email the documents. VOIP is very problematic when working with faxes.


Thanks for that but we have to use foip as will be sending faxes in the millions if it works… Nothing dodgy mind, just moving current business up a gear. Have been using G.711 A and U law.

Looking around T.38 seems to cure the problem and is used by FOIP gateway hardware such as those made by Cisco and Belkin (although they start at £4000 and are a little out of my budget!) - oH323 is meant to have T.38 as part of it, however it doesn’t seem to work (T.38) in asterisk…

Any other ideas?

Sorry, no.

No one has yet made FAX over IP work reliably. Especially in the volumes that you are asking it to work.

My best advice is still to use TDM. Fax is a really a TDM solution. It wasn’t designed for IP networks, and none of the codecs really take it or modem calls into account. (Fax being essentially a modem message.)

The gateway protocols you mention are really intended for private networks.


In these systems, the fax is really completing delivery of it’s message to the nearside gateway. It’s then sent by the nearside gateway as an IP message (packets) to the farside gateway. The farside gateway then switches to fax delivery mode and pretends that it’s the sending fax to finish off the delivery to the intended destination fax machine.

It pretty much assumes you have control over the distant end. (Such as might be in a multi site corporation.) The nearside gateway would not use the IP network unless the destination of the fax was somewhere on the corporate network. It would just re-direct the fax to the PSTN using standard telco lines.

This is how companies like Cisco managed to make fax work over IP networks.

Good luck anyway. You might someday crack this nut.

I’m going to keep trying!

But you’ve led me to my next question, I can use a direct connection via TDM at the colo, but… What do I do???

I’m not entirely sure how to connect the Asterisk server to the TDM connection - is it just ethernet into their switch, or do I need some kind of card to connect, and how does Asterisk (spanDSP?) know I’m trying to fax through TDM? I’ve spent the last few weeks engrossed in FOIP that I haven’t a clue about anything else, and as I mentioned previously, time is of the essence here!

Are there other codecs/protocols I need to get spanDSP to go through TDM, and/or do I need any of these Digium cards to connect to the TDM gateway?

Please help, I’ve seen less info on this than I have on fax over IP!



Just grab one of the Digium cards. Spandsp should see it as any other channel. It shouldn’t care if it’s an IP channel or a TDM type.


It’s going to cost you a thousand quid, but it’s much more certain to work than an IP channel.

IDEALLY though, you would use an Intel/Dialogic board with fax capability. Those are designed to BE fax machines. (Not emulate them.) It also has the benefit of offloading the processing of fax images to a DSP located on the board, allowing higer density port/server.

Much pricier but a permanent solution.