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To: Paul Lee who wrote (7254)11/22/1999 8:37:00 PM
From: SteveG   of 9236
 
<A> Why AM radio and ADSL are at odds

nwfusion.com

By TIM GREENE
Network World, 11/22/99

AM radio interferes with
ADSL because they try to use
the same electromagnetic
frequencies at the same time.

The nearly 5,000 AM radio stations licensed in
the U.S. broadcast at frequencies between 540
KHz and 1.7 MHz. ADSL service providers
use the 138-KHz to 1.1-MHz range to
download data to customers.

So as you can see, there's a sizable overlap.

It would seem that AM radio would wipe out
most of the ADSL range, but because stations
transmit at discrete frequencies, each station
affects just a targeted area of the ADSL
spectrum. AM stations in a given area don't
generally fill up the entire available spectrum.

ADSL modems have the ability to just stop
using that segment of the frequency spectrum
occupied by any nearby AM station. ADSL
transmissions are broken into frequency
chunks called "carriers." There are 256
carriers per ADSL line and 128 per line for
G.lite, a lower-speed version of ADSL. When
an AM signal interferes with a carrier, one
remedy is to stop using that carrier and to
drop the bandwidth available to carry data.

Only the download speed is affected, and
that's because the frequencies used to send
data to customers are the ones that overlap
with AM radio.

The longer the wire to the customer site, the
more susceptible an ADSL line is to
interference. That is because the signal gets
weaker as it travels down the wires and is
therefore more easily disrupted. The effect is
particularly pronounced if the AM transmitter
is near the customer at the end of a long line.

Twisting wires around each other makes the
signal on them less sensitive to interference,
and phone companies use twisted pairs of
wire to reach customers.
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