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. Thus the 56k needs a lesser limiting phone line.

    It has long been thought that the "theoretical limit" on modem speed over an ordinary phone line was 33.6 kbps. 56k modems achieve their speed by avoiding a conversion from digital to analog lines in the connection between user and service provider. Ordinary connections begin over an analog line, are converted to digital by the phone company and are converted back to analog in the final segment before arriving at the service provider. 56k connections begin analog, are converted to digital and are not converted back to analog at the service provider. This requires the service provider to have a direct digital connection and therefore avoids one conversion of the signal. By avoiding this second conversion, speeds of up to 56k and faster are possible. Therefore, modems users need to know that they can only achieve 56k if their service provider supports it.

    Interestingly, FCC regulations limit the speed to 53k, but modem makers are fast at work to sidestep and/or waive this rule. Just because 56k is possible, doesn't mean that every user will achieve it. Poor local phone lines and other conditions may limit speed. Users who are unable to achieve 28.8 or 33.6 with their current service are unlikely to achieve faster connections with a 56k modem.

    56k modems download at speeds up to 56kbps, but can upload at only 33.6kbps.

There are three things needed to operate 56k modems.

1. A 56k modem. These are becoming the norm and are not that hard to find. The more modems produced the cheaper they are becoming to purchase.
2. Once you have the 56k modem, you need a place to call. Nearly all of the Internet service providers have pledged support for 56k technology.
3. The third requirement is the one that may prevent some people from using 56k speed. For 56k to work, you can only have one analog-to-digital conversion between your home or office and the Internet service provider. If there is more than one, you can't use 56k, and your connection will be limited to V.34 speeds (a maximum of 28.8k or 33.6k).