When a high-speed wide area network service such as DSL is not available, you can connect your computer or LAN to the Internet, or directly to a remote computer, through the dial telephone system (the Public Telephone Switched Network or PTSN, also known as POTS, short for Plain Old Telephone Service). Dial-up network links are considerably slower than DSL, cable, or other high-speed services, but they're convenient because there's a POTS telephone line in just about every home and business, and because the PTSN often continues to work during power failures.
A network connection through a telephone line uses a modem to convert digital computer data to sounds that can pass through the PTSN. A second modem at the other end converts those sounds back to digital data. The communications programs in Windows and other operating systems send control codes that instruct the modem to transmit telephone numbers and adjust the data transfer speed and other configuration settings.
Most new laptops have built-in PTSN modems. Separate modems for desktop computers are available as internal expansion cards or external devices that connect to the computer through a serial data port or a USB cable.
Figure 2-9 shows the dial-up modem control panel used in Windows XP to connect a computer to a distant network or an Internet service provider (ISP). Figure 2-10 shows the setup screen for the HyperTerminal program. Other programs have different layouts, but they all do essentially the same thing: They dial a telephone number and log in to the computer that answers the call. Advanced properties specify the type of network connection, the data speed, and other configuration settings.
Figure 2-9. The Connect dialog in Windows specifies the telephone number that a modem will call and the login and password that the computer will send after the connection goes through.
Figure 2-10. The Connect To dialog in HyperTerminal includes space for an area code and telephone number.