• Published on | Aug 16, 2014 | by FozzTexx

Using modems without phone lines

Ever since I did BBS Week on RetroBattlestations, I’ve been wondering if there’s a way to use a modem without having to use a land line. It seemed like it should be possible since modems work with T1 lines, and T1 lines are digital. However when I experimented with it during BBS week, I didn’t have a PBX that I could use to interface to an analog line, and I didn’t have any modems with an acoustic coupler.

I decided to setup an Asterisk PBX and get the cheapest card I could find that would support analog lines. I got a clone TDM400P card  and I had it configured with a single FXO module and three FXS modules. The FXO is for connecting to a land line, the FXS modules are meant to connect to old fashioned analog phones. In my case however I’ll be connecting modems to the FXS modules.

I’m using a very old junker Pentium 4 computer as the Asterisk machine. Setup was actually pretty easy, I downloaded Elastix 2.4.0 which is the stable version, booted the ISO, and let it run through the install. It detected the card with no issues, and I added the three FXS ports as extensions using the default ulaw (G.711) codec.

With the extensions setup, I connected an old USRobotics 56k modem to the one port. The other modem I want to try is an old Radio Shack TRS-80 300 baud acoustic modem, so I’m hooking up a phone to another of the FXS ports. From the phone I dialed the main extension, and put the handset on the cradle. The modems connected at 300 baud with no problem!

Well if 300 baud works, what about something faster? I hooked up another 56k modem to the remaining extension and gave it a try. It connected at 31200 bits, which is pretty close to the limit of what modems could do over an analog line. They won’t go 56k without a special kind of digital modem that actually only does 56k one way anyway.

Calling from one extension to another is nice for testing things and seeing that modems work, but what about making it so other people could call in? Would using VoIP work? I added a trunk to my PBX using a SIP provider and gave it a real phone number. My first test with using the real phone number was to make a simple call to myself, which worked just fine.

Using a modem located off-site and connected to a land line, I was able to dial in and connect. Again I had no problem getting 31.2kbit. There was the occasional pause while some error correcting kicked in, but the connection held. I am really amazed at how well it works, and for now I’m using it to run a BBS on my Apple IIgs.

Another thing I tried was using a Snom 320 with the 300 baud acoustic modem. This also worked perfectly. I could direct dial an extension, or dial out through the SIP provider back to myself. Works just fine.

I was really surprised when I saw the modems connect the first time. My only goal was to enable the use of 300 baud modems just because I thought it would be fun for people to use retro tech and connect to an old school BBS. I was hoping I might be able to get 2400, but instead I’m getting nearly the maximum that analog lines could go!

To the rest of the internet: you’ve got it wrong. It is possible to connect modems without a land line!

Join The Discussion

+1  Posted by TCPMeta • Sep.24.2015 at 04.39 • Reply

Where did you get the cheap TDM400P card? I'm trying to get this working myself for a small BBS project.

+1  Posted by FozzTexx • Sep.24.2015 at 14.02 • Reply


+1  Posted by CodeAsm • Jan.11.2016 at 22.15 • Reply

im almost ready to empty my pockets for one of these, found them here: http://www.aliexpress.com/w/wholesale-TDM400P.html? hope they are the same "clones". (not yet buying, low on money atm. but ill go dor 1 FXO and 2 FXS i think)

+1  Posted by HsR • Dec.01.2020 at 13.40 • Reply

Thank you for that interesting article. Do you think it would be possible to get a modem to 'answer' or at least decode a recorded dialling played from the soundcard?