Q My I.S.P is telling me that i can't connect at 56k because my phone lines are to old.My house was built in 1962 and as far as i know no phone line work has ever been done. Is this true or are they just feeding me a bunch of B.S.?

- Haulgas

A BS? From a phone company? Nonsense, Haulgas! Let's talk about the realities of telecommunications, infrastructure, Shannon's Limit, and the deepest mysteries of POTS. (and what a great topic this is for the Road Runner page, eh?)

POTS, or Plain Old Telephone Service, is an interesting little beast. When the phone system was built, a hundred years ago, give or take (regardless of the age of your particular house), it was designed to carry nothing but voices. Voice communication, it turns out, is pretty easy, compared to various other methods of communication. The band is pretty narrow, and all the data are filtered through the greatest computer ever created - the human brain. One of the things the brain is best at is pulling information out of "noise." Your phone has to get pretty doggoned crackly before you can't figure out what the person at the other end is saying. And if there's a hum on the line, or you've picked up a local radio station, or the signal drops out for a quarter of a second, it doesn't matter. You can still figure out what the other person is saying. So, you don't need a truly incredible infrastructure for voice communications, because all the heavy lifting is being done by that great computer between your ears.

The computer on your desk isn't as good. You have to have a pretty clean phone line to do data communications, since the computer on your desk gets really confused by the radio playing on the wires, or crackling, or even little dropouts. The big problem is that the bandwidth in the phone line isn't all that great. Remember, it was designed for voice, which is strictly analog information in a fairly narrow band. The modem takes the digital data you have in the computer, modulates it to an analog signal, and sends it down the line. The other modem takes those analog data, and demodulates them back to digital. Hence, the name "modem" - MOdulate/DEModulate. But, since an analog signal is so unnatural for computer data, and since the line is so noisy, there are a lot of resends of bad data. Also, data that move slower are less susceptible to noise. So, by using an analog signal, you automatically limit your speed.

Also, there's something called Shannon's Limit. Shannon's Law states that for any kind of data transmission medium, there is a theoretical limit to the transfer speed beyond which the signal-to-noise ratio becomes so low that no useful data can be extracted. The demodulation of the signal at the receiver's modem introduces quantization noise, one of the limiting factors on standard modem transmission speed. The so-called Shannon's Limit (theoretical maximum transmission speed) under such conditions is in fact around 35kbps. Which explains 33Kbps modems. 56K modems depend on a good set of digital connections to avoid one analog-to-digital conversion, which is where much of the problem occurs. If it can do that, you can get around 53Kbps (not 56, never 56 - there has been some talk about whether or not the "56K" tag is false advertising).

So, if you can get a really, really clean line, you can get up near that 53K limit. But the older your infrastructure (and it's not just what's in your neighborhood, it's what's between you and the Central Office that counts), the lower that limit is going to be, since the modems will slow down until they can find a speed that gives them a clean connection.

Now, note that everything said above relates only to analog signals. When you have pure digital signals, as you do with Road Runner or a DSL connection, none of the above applies. At that point, it becomes more of a matter of the quality of the lines and of th

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