FreeSWITCH: mod_dmodem¶

FreeSWITCH is an open-source telephony platform. mod_dmodem is a FreeSWITCH module that answers a phone call with a modem. Once the caller's modem connects, the data goes to a server or program of your choice: a BBS, a login prompt, or pppd for dial-up internet. It also sends and receives faxes. It is the FreeSWITCH version of the Asterisk module.

For every option, the fax details and the tests, see the technical details.

How it works¶

In the dialplan, send a number to the dmodem application with a profile name:

<extension name="bbs">
  <condition field="destination_number" expression="^5551000$">
    <action application="dmodem" data="bbs"/>
    <action application="hangup"/>
  </condition>
</extension>

Each profile, in autoload_configs/dmodem.conf.xml, says where the call's data goes:

<profile name="bbs">
  <param name="mode" value="tcp"/>
  <param name="host" value="127.0.0.1"/>
  <param name="port" value="2323"/>
</profile>
<profile name="ppp">
  <param name="mode" value="exec"/>
  <param name="command" value="exec /usr/sbin/pppd nodetach noauth local lock passive 192.168.77.1:192.168.77.2"/>
</profile>

All the options are in dmodem.conf.xml.

dmodem <profile>,o does the opposite: the modem dials out to a remote modem on a call FreeSWITCH has placed, for example originate sofia/gateway/trunk/5559999 &dmodem(isp,o).

What the modem can do¶

It answers every common standard from 300 bit/s up to V.34 (33600 bit/s), with error correction and compression. With a fully digital call path (G.711 all the way), it can also be the server end of a V.90 call, up to 56000 bit/s to the caller.

Fax¶

dmodem_receive_fax answers a call as a fax machine, and dmodem_send_fax sends a fax. The dmodem application also notices a fax machine calling and receives the fax. Faxes go over the audio or, on SIP calls, over T.38 (fax over the internet).

Building¶

You need an installed FreeSWITCH that pkg-config can find:

cmake -B build
cmake --build build --target mod_dmodem
cmake --install build
fs_cli -x "load mod_dmodem"

Add <load module="mod_dmodem"/> to modules.conf.xml to load it at start-up.