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56K Basics
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How it worksTo understand 56K, it's necessary to take a brief look at how traditional analog modems work. Information inside your computer is in digital format: all of the data is stored as 1s and 0s. Normal phone lines are analog: they transmit data as a series of peaks and valleys. Your "modem" is a MOdulator/DEModulator: it modulates outgoing data from digital to analog, and demodulates incoming data from analog to digital. Above a certain threshold (called Shannon's Limit) the signal-to-noise ratio of any medium becomes too low to reliably transfer data. The analog phone line is the limiting factor in the speed of data transmission because of the inherent noise it contributes. Today's telephone network is increasingly digital. In particular, the portion of the phone connection between the phone company and the Internet Service Provider (ISP) is often digital. Digital lines still have noise, and are still subject to Shannon's limit, but they have less noise and a higher ceiling. Several companies have created techniques that take advantage of the digital portion of the phone network to achieve higher speeds than were possible with a purely analog pathway. These new techniques treat the phone system as a mostly digital network that just happens to have an analog portion. There are several consequences to the reliance on a half-digital connection. Your Internet service provider (ISP) must have digital phone lines to the public switched telephone network (PSTN). That's the easy part: if your ISP offers 56K, they've got the digital lines.
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