1.7 What is a Modem ?
A modem (or analog modem) is a device that lets one send digital signals over an ordinary telephone line not designed for digital signals. If telephone lines were all digital then you wouldn't need a modem. But sometimes, a substitute for an analog modem, connected to a digital phone line, is imprecisely called a "digital modem". A modem permits your computer to connect to and communicate with the rest of the world. When you use a modem, you normally use a communication program or web browser to utilize the modem and dial-out on a telephone line. Advanced modem users can set things up so that others may phone in to them and use the computer remotely. This is called "dial-in".
Oversimplified, there are four basic types of analog modems for a PC: external serial (RS-232), USB (= external USB), internal, and built-in. The external serial and USB set on your desk outside the PC while the other two types are not visible since they're inside the PC. The external serial modem plugs into a connector on the back of the PC known as a "serial port". The USB modem plugs into a USB cable. See USB Modems. The internal modem is a card that is inserted inside the computer. The built-in modem is a chip on the motherboard used primarily in laptops. What is said in this HOWTO regarding internal modems will generally apply also to built-in modems. Internal modems are further subdivided into PCI, ISA, and AMR, depending on whether they are designed for the PCI or ISA bus, or for an AMR slot.
For an external vs internal comparison see External vs. Internal. When you get an internal or built-in modem, you also get a dedicated serial port (which can only be used with the modem and not with anything else such as an external modem or console terminal). In Linux, the common serial ports are named ttyS0, ttyS1, etc. (or tts/0, tts/1 for the device file system (devfs). These ports usually corresponding respectively to COM1, COM2, etc. in Dos/Windows). But in special cases, the names are longer such as: ttySHCF0 is the 0th serial port for a type of winmodem (HCF = Host Controlled Family). New types of serial ports just add some more letter to ttyS.
See Modem & Serial Port Basics for more details on how modems and serial ports work. With a USB modem, the driver simulates a serial port at for example /dev/ttySHCFUSB or /dev/usb/asm/0 (for devfs).
Modems usually include the ability to send Faxes (Fax Modems). See Fax for a list of fax software. "Voice" modems can work like an automatic answering machine and handle voicemail. See Voicemail Software.
The v.92 protocol can put the modem "on hold" when someone makes an ordinary voice call to your telephone, provided that you have "call waiting" from your telephone company. Thus you can get a phone call while online. As of Jan. 2003 Linux doesn't seem to support it. If this is the latest version of this HOWTO, let me know about any Linux support for it. Some linmodem drivers may support it (but what if you have a hardware modem that doesn't use any linmodem driver?).
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