8.8 Connecting to the Internet
8.8.2 Modems
The modem is an electronic device that is used for computer communications through telephone lines. It allows data transfer between one computer and another. The UARTs convert byte-oriented data to serial bit streams. Blocks of data are handled by software. Internal modems combine a UART and a modem on board. The modems convert digital data to analog signals and analog signals back to digital data. The term "modem" actually derives from the function of the device. The process of converting analog signals to digital and back again is called modulation/demodulation (hence the term modem). Modem-based transmission is remarkably accurate, despite the fact that telephone lines can be quite noisy due to clicks, static, and other problems.

There are four main types of modems:

  • Expansion cards are the most common. They plug into the motherboard expansion slots (ISA or PCI) and are called internal modems.
  • PCMCIA modems are a variation of modems that are designed for easy installation in notebook computers. Also known as PC cards, they look like credit cards and are small and very portable.
  • External modems can be used with any computer. They plug into a serial port (COM1 or COM2) on the back of the computer.
  • Built-in modems are used in some notebook or laptop computers.

Internal modems simply plug into one of the expansion slots on the motherboard. These internal modem varieties, therefore, do not take up extra space on the desktop. They usually cost a little less than the ones that are plugged externally into the computer. To configure them, jumpers may have to be set in order to select the IRQ and I/O addresses. No configuration is needed for a plug-and-play (PnP) modem, which is installed on a motherboard that supports PnP. A modem using a serial (COM) port that is not yet in use must be configured. Additionally, the software drivers that come with the modem must be installed for the modem to work properly.

External modems are typically a bit more expensive than the internal varieties. Plugging them into one of the serial ports will connect them to the computer. The computer case does not have to be opened. Newer USB modems are plugged into a USB port or hub. An external modem uses the IRQ and I/O address assigned to the serial port. A status light on the modem indicates whether or not the modem is online. Software must be installed in order for the external modem to work properly.

In most current modem types, a phone line is plugged into a standard RJ-11 telephone jack. Other modem types and devices, such as cable modem and DSL, are discussed separately in a later section.