Newbie question on legacy faxes and modems

Hey Folks,

We’re looking at rolling our own telephone system with Asterisk on Digium hardware. I’ve read ‘Practical Asterisk’ and the O’Reilly book and it sounds like fax machines and computer modems can be problematic. I hope someone would be kind enough to field a question about keeping users with legacy hardware happy. We are located in the United States. Our interface to the PSTN will be a PRI/T1 through a card like the Digium 1TE121BF.

Would a 2400 series analog card with FXS ports in the same server permit the use of legacy computer modems and fax machines out across the PRI? I realize they would have to wired directly to the card POTS-style.

Is there a better approach?


I have never tried your solution, but it could work OK. YMMV.

I have PRIs coming in to a Digium card and Fax machines on the LAN via ATAs. Some fax machines are remote to the PRIs, but within the same city, with Asterisk at each site doing IAX2 trunking over dedicated T1. I have had pretty good luck with faxing, probably because I have a low latency in my network and I do not get my outside trunks via SIP.

Note that BT and probably other PTTs are using VoIP technology in their “circuit switched” networks (BT call this 21CN). Some modems or modem applications are particularly sensitive to network delays, so you may have to consider the cumulative effect of your system and VoIP in the PSTN.

Wow. Based on reading forums and the books, I had the impression that faxing was way more fragile. Do you mind if I ask what ATAs you’ve had the best luck with? Did it require T.38 compatible fax machines?

That’s one of the reasons I was leaning toward keeping everything on the server’s bus. One of the Asterisk books mentioned future cards that might draw timing over a wire connected to the T1 card but I’m not sure if they’re out yet.


If we’re talking faxes, that’s a fine approach - faxes aren’t typically up for half a day or more at a time, so the buffers in place to handle any slips that might occur across the PCI bus are sufficient.

If we’re talking modems, then your best bet, better than any timing cable, would be to use a multi-port Digium T1 card. Put the PRI on the first port, drop an analog channel bank on the second port. Construct a dialplan that doesn’t cause Asterisk to sit on the channel (don’t use transfer flags). And, the call will be bridged directly inside the T1 card, so there’s no escaping across the PCI bus, so there’s no danger of junk on the bus (blocking) interfering with the fax.


Believe it or not, I use Grandstream 24 port gateways. Same old fax machines we had been using. I am not saying it has been flawless, but it works for our needs… until I can get them eliminated, or at least 90% eliminated.