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posted by martyb on Monday November 20, @11:02PM   Printer-friendly
from the good-question dept.

If you've ever had dialup internet service, or still do, or just know someone that does, you have probably heard terms like "56k modem". "56k" has become almost synonymous with dialup Internet access. But it's such an arbitrary number. It's not divisible by ten, it's not a power of two... so why was it chosen as the fastest dialup speed? For the answer, we will have to travel back in time quite a while.

Our visitors from Google should be warned that this is not a "stripped down" explanation; it is intended for relatively technical readers. But if you really want to know where this magic number comes from, you need to understand some of the technical background. As we shall see, "56k" was not just pulled out of a hat.

[...] Anyone that has ever used a dialup modem knows full well that they don't actually get to connect at that speed, though. And that their connection speed varies each time they dial in. There are two factors at work here.

The first is the FCC. If you are in the United States, the FCC places a restriction on the power output of devices connected to the phone network. The result is that you will never be able to connect at a speed faster than 53.3 kbit/s.

The second is the overall complexity of the phone network. 56 kbit/s (or 53.3 kbit/s) requires very good operating conditions, as it is really operating beyond the paramaters of what the phone network is required to be capable of. Operating at these speeds requires that there only be one ADC between the user and their ISP (which is not guaranteed to be true, but typically is), and that the copper wiring in the user's "local loop" have very good electrical properties. Part of the dialup process that is used to initiate a connection is an evaluation of the overall quality of the connection; if it is determined to be lacking, the modem will automatically drop down to a lower data rate.

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  • (Score: 1) by pTamok on Tuesday November 21, @11:11AM

    by pTamok (3042) on Tuesday November 21, @11:11AM (#1333713)

    The government wasn't regulating the modems at the customer end, rather the FCC restricted the outputs on the line from the CO equipment (radiated power limitations or somesuch for some reason) so that the modems, as designed, could never reach the theoretical maximum speed of the communication standard.

    The limits imposed by the FCC did indeed mean that you could never actually get more than 53333 in the field.

    I believe it was to minimize/mitigate crosstalk. There are some standards for the number of twists per foot for unshielded twisted pair [] telephone cable which set a limit for the allowable power levels. To complicate matters, in a properly built bundle, different pairs will have different numbers of twists per foot (on purpose), so it comes down to averaging a lot of unknowable but guessable parameters, Putting too much signal power down one pair will affect others and vice-versa.

    Other countries had different standards for the cabling, or different tolerances for crosstalk, so allowed higher power levels. which would have allowed for theoretically higher data rates.