ADSL, asynchronous digital subscriber link.
I've been hooked up over two years now on ADSL.
Let's start with what you are looking at, these words on your screen, right now. You wish to reply to me so you use the mouse to push the "Post Reply" button. When your finger clicks the mouse button, a digital signal courses through the computer, is used to change the image on the screen, and is sent out through the phone line to the world wide web.
The digital signal from your mouse click stays digital from your computer through the phone line and on into the "cloud" of the internet on the ADSL connection. The modern dial up modems beginning with the transition to V.90 and later standards also do a direct digital transmission. The real difference is in the width of the band of frequencies allowed by the phone company for your specific line. Dial up is limited to a band of frequencies (bandwidth) that works well for carrying the human voice, and little else (that's why the music-on-hold always sound tinny). Dial up bandwidth can't be adjusted. So dial up simply can't get faster, now that we've used up all the mathematical compression and conversion tricks, with out a wider band of frequencies to send the signal into.
ADSL service provides a seperate band of frequencies above the frequencies used for voice. The higher band is used by the ADSL router for the digital information to go to and from the cloud of the internet and can be made wider or narrower, depending on how much you wish to pay for each month. As this band of frequencies doesn't interfer with the voice signal, both can be on the same single phone line. The voice line in your house is kept free of the whistle of the higher frequency digital "noise" with simple low-bandpass LC filters at each phone, or at the head of a wire that goes only to (a) phone(s). The computer ADSL router doesn't need a filter as it simply ignores the lower voice frequencies.
The phone wire that runs out the back of your computer, through the wall and down the road to the closest phone company office is metaphorically called "The Last Mile" and is where the greatest signal disruption occurs. The condition of your Last Mile is what determines whether the phone company can even supply ADSL to you, specifically, at all. The wire was manufactured and installed for low frequency voice signals. ADSL is higher frequency, and therefore, more susceptable to capacitive losses along the length of The Last Mile. Sopmetimes, the condition of The Last Mile restricts the speeds availabel to the slower ADSL rates. Sometimes, substituting a different line pair in the trunk will help. Sometimes, bits of hardware must be removed from your specific line. Sometimes, one must wait for the company to build an entirely new section of The Last Mile (if they ever will).
