History
Traditional phone service (sometimes called POTS for "plain old telephone service") connects our home or small business to a telephone company office over copper wires that are wound around each other and called twisted pair. Traditional phone service was created to let you exchange voice information with other phone users and the type of signal used for this kind of transmission is called an analog signal. An input device such as a phone set takes an acoustic signal (which is a natural analog signal) and converts it into an electrical equivalent in terms of volume (signal amplitude) and pitch (frequency of wave change). That's why our computer has to have a modem - so that it can demodulate the analog signal and turn its values into the string of 0 and 1 values that is called digital information.
Because analog transmission only uses a small portion of the available amount of information that could be transmitted over copper wires, the maximum amount of data that you can receive using ordinary modems is about 56 Kbps (With ISDN, which one might think of as a limited precursor to DSL, you can receive up to 128 Kbps)
The ability of our computer to receive information is constrained by the fact that the telephone company filters information that arrives as digital data, puts it into analog form for our telephone line, and requires our modem to change it back into digital. In other words, the analog transmission between our home or business and the phone company is a bandwidth bottleneck.
