[ADSL Artcle at InternetTelephony; Tac Berry Extensively Quoted]
Good Morning Pat!
A good article over at InternetTelephony. It discusses power requirement concerns, interference, and a little bit of line-code discussion.
Steve
(Sunny, Hot, Humid)
The price of high performance
Telcos implementing high-speed DSL technology must be prepared for increased power requirements
By PAT BLAKE
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"One of the big problems with DSL service is the impulse noise at the home or business," says Tac Berry, Amati Communications' vice president of marketing. Amati develops equipment for ADSL platforms using the DMT chipset (Figure 2). It also holds patents for the ADSL/DMT standard.
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"One of the main differences between DMT and some of the other solutions is instead of considering the entire bandwidth for the signal processing, DMT splits it up into 4 kHz subchannels. Each one is like an individual modem, if you will. And each of those subchannels is looked at on a signal-to-noise and noise-to-floor basis.
"If there's sufficient noise margin in that subchannel to carry a number of bits, then we use it. If, for some reason, that 4 kHz subchannel does not carry enough bits, we don't use it. We just notch it out. So if an impulse noise appears on one of the subchannels, a DMT system is able to recover very, very rapidly because it can actually ignore that subchannel. In a broad spectrum-type system, that's not necessarily true because any noise source raises the noise
floor of the entire sampling frequency range."
When power is increased to extend the reach of a transmission, it also may increase the potential for interference. Noise is another major detractor. "There are two major constraints that we all have to work under," says Michaels.
"Number one is to minimize the amount of noise that we create. The less noise you generate the better.
"The other aspect is in the loop plant. There's all manner of noise that shows up, especially when you're running at very high speeds, things that we don't notice when we're talking on a phone line. So we've learned to develop our algorithms to take into account noise that can create aberrations."
FULL TEXT AT:
internettelephony.com |
