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44.10. Win Is a Modem Not a Modem?

The word "modem" is a contraction of "modulator-demodulator." The fundamental job of a modem is to turn a digital signal into an analog signal and send that analog signal across a phone line (modulation) and to receive an analog signal from a phone line and turn it back into the original digital signal (demodulation).

Controller-based modems do all of the digital signal processing, D/A and A/D conversion, and phone-line interfacing in hardware. Generally, these modems either are external modems that plug into a serial port or have a serial port chip included and thus just look like an extra serial port to the CPU. Configuring these modems under Unix is easy; just set up whatever program uses the serial port to use the port speed and serial options you want.

Host-based modems, often called "Winmodems," provide some level of hardware support (at a minimum, the physical phone line interface) and then emulate some or all of the hardware modulation and demodulation in software. There are a variety of specifications related to "soft" modems, and current information on things like available drivers, issues, standards, and whether a modem is a hard or soft modem are available at http://www.idir.net/~gromitkc/winmodem.html and http://www.linmodems.org.

The problem that soft modems present to Unix is that the software that makes up the fundamental functionality of the modem is almost always Windows software. These modems are widely available and cheap and do have some advantages, though, so there are efforts to provide Unix software for some set of them. Unix soft-modem software is highly in flux at the time of this writing. Before you buy a modem, be sure that you check the current information on that modem and available drivers for the Unix platform you want to use before you buy. Or spend a bit more and buy a modem that doesn't have these issues.


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