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Radio over a telephone line may seem bizarre, even contradictory, after all, we're talking telephones, not broadcasting. But have you ever considered how your cable television system works, how they get dozens and dozens of bandwidth hogging television channels onto a single piece of coax? Ever thought how much bandwidth that takes up? Hmm? And you know that many people have cable modems to connect to the internet. So, how is this all done? With equipment that sets up radio channels within the wire, so that instead of broadcasting into space, you transmit that information down a wire.

An ADSL modem from Westell, the equipment at your house which opens a big communication channel to your local switch and from there to the internet. Yes, it is an analog based device. Read on . . .

Think of a radio station signal going not to a tower, but instead going down a piece of insulated wire, and out from there to a person's home. Remove the insulation and the wire acts as an antenna. It's the same way with cable TV, in fact, in older systems you can pick up the C.A.T.V stations on a scanning radio, since when the insulation gets worn away, or if there are bad connections, the coax strung on utility poles starts radiating into space. This doesn't happen with fiber optic based cable T.V. but it's common with the old copper cable.

Why would signals leak out? A channel was set up within the wire using a radio frequency. We create radio spectrum in the twisted pair for greater capacity, far more than what is available using the normal voice channel, the one a conventional modem uses. Read more about these terms by clicking here. Telephone companies have been using radio based techniques for over fifty years, the first schemes using analog carrier, equipment that made telephone lines more efficient by putting several calls on one wire, spacing them out by frequency, like the way radio stations are spaced out on a dial.

ADSL schemes vary somewhat. In this example an 80,000 (80Khz) path exists for uploading and a one million hertz (1Mhz) path is set up for downloading. Don't confuse bandwidth with bits, by the way, you can be downloading a file at 1.3 Mbs, even though the channel is only 1 meg wide. (I'll write more on this later, to clear things up.)

There are many, many ways to put a signal on a wire. We call this subject modulation. First, we must discuss a new term, carrier. We talked about variable resistance on the last page. In that technique the telephone varies line current to represent voice. Audio signals are in the voice frequency range, in telephony, from about 0 to 4,000 cycles per second.

Carrier doesn't use this method. It generates instead a constant radio frequency which is then varied. This is quite different from varying the background line current. Once you create a radio frequency, on a wire, or off a wire, such as in broadcasting, you have a carrier and you are discussing modulation.

Radio signals start in the tens and tens of thousands of cycles per second. Much, much higher than voice signals. We're putting or impressing audio frequency signals, low ones, on a very high radio frequency. A two step process. Get it?

Take a look at the diagram below. Can you intuitively understand carrier and modulation?

The strength or the amplitude of your voice varies the carrier, be it a radio carrier for broadcast or along a broadband cable. The diagram above illustrates Amplitude Modulation, what an A.M. radio station uses. This is not so difficult, so far, is it?

ADSL uses Discrete Multi-Tone modulation or DMT. Don't tune out or give up because of that name. We know about modulation already. And we know DSL sets up radio frequencies within in a wire to pass information. We'll get to specifics later on, for now, know this: DMT up sets 256 radio channels to send information on! As it works, the DSL system is constantly checking each of these sub-channels for the best line conditions. Too much interference on channel 7? Send the information down channel 8. The DSL system is at all times trying to send as much information as possible over the most channels it can. Very cool. And what's even slicker is that equipment at the telephone's central office can separate voice traffic from data, putting each on its own network.

A more advanced look at ADSL, with much on DMT, from Broadband Access Technology, Interfaces, and Management by Alex Gillespie (194K, 12 pages in .pdf)

Read Ramblin Rode's article below and consider my diagram of all the parts involved. The filter you see pictured is just a small, non-powered device, that plugs between your regular telephone equipment and the wall jack; all it does is eliminate the hiss or noise generated by the DSL equipment.