A simple v.23 modem including the data link layer. This is an application for Asterisk. Place a call over SIP and it'll connect via Telnet to a pre-configured service in the dialplan.
V.23 Softmodem for Asterisk with some Bildschirmtext-specific stuff in it. Pretends to be a modem but it actually sets up a telnet-like TCP session to an IP address. Like many things with Asterisk, you have to compile everything from source with this module in a particular location.
DIRE WOLF (Decoded Information from Radio Emissions for Windows Or Linux Fans) is a new software implementation of the APRS protocol. Rather than using a hardware TNC, this software implementation lets you plug the soundcard jacks into your radio to work APRS. The page could stand to be better organized, but you'll find pre-compiled binaries and source code at the Download tab.
minimodem is a command-line utility which implements a virtual FSK (forward shift keying) modem. Hook it into an audio source and it will act just like a modem, sending and receiving data transmissions. Can also interpret recordings of transmissions and generate new ones from traffic you send it. Who needs a serial modem when you've got this? Includes support for various standard FSK protocols such as Bell103, Bell202, RTTY, TTY/TDD, NOAA SAME, and Caller-ID.
Maybe I can use this to write my VoIP wardialer?
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