followup <A> AM radio creates ADSL static
AM signals can cause a dramatic drop in download speeds.
nwfusion.com
By TIM GREENE
Network World, 11/22/99
Now there's one more reason to
hate AM radio: It cuts the
bandwidth on asymmetric digital
subscriber lines (ADSL).
Equipment maker Nortel Networks claims
interference from AM stations can slash
high-speed bandwidth by 40%, and that the
problem arises on approximately 15% of
ADSLs.
While service providers say there are more
signifi- cant problems in real-world ADSL
deployments, the International
Telecommunications Union is considering
requiring ADSL modem makers to test how well
their gear deals with AM interference.
Customers could then look at the results to
compare one ADSL modem with another.
Several causes cited
The AM radio interference problem varies from
place to place, depending on a number of
factors, including how close ADSL lines are to
AM broadcast antennas and the quality of the
inside wiring at customer sites.
BellSouth says customers at the end of very
long ADSL lines, where the signal is weak
anyway, and who happen to be near an AM
radio transmitter, have been unable to get ADSL
service at all.
The problem is worse if untwisted or poorly
twisted wiring is part of the connection, a
common problem with wiring in homes. Twisted
wiring tends to protect the ADSL signal.
While the AM radio frequencies interfere with
ADSL download speeds, they do not affect
upload speeds. And AM radio does not affect
all types of DSL, only ADSL and its subsets.
These include rate adaptive DSL, which has
adjustable speeds, and G.lite, which has a
maximum download speed of 1.5M bit/sec.
Wayne Getchell, director of access products for
Nortel Networks, says AM radio interference is
a relatively minor problem when compared with
other issues, such as the length of DSL lines,
whether other lines run off customers' lines and
even poorly attached wires.
Chip Ach, chief technology officer for DSL
provider HarvardNet, says his company has not
identified AM radio as a problem in its
deployment of 700 DSL lines. "When you start
with 8M bit/sec downstream, dropping a little bit
on that side, especially when trying to provide
asymmetric service as we are, is not a big deal. I
worry much more about upstream bandwidth
being compromised, which is not the case
here," Ach says.
A spokesman for BellSouth says in the cases in
which the carrier has diagnosed AM radio as a
problem, the impact was severe.
In at least one case, the interference made
ADSL ser vice impossible. But the company
says less severe degradation may go unnoticed
or be attributed to other factors. BellSouth
offers ADSL as a best-effort service, so the
carrier does not doggedly track down every
factor that might affect the bandwidth of
individual lines.
Nortel Networks says it has ways to blunt the
effects of AM radio interference. In full-rate
ADSL, the company's modems can stop using
frequencies where the radio waves cause
interference.
With G.lite, also known as DSL-Lite, Nortel
Networks is working on chips for customer-end
modems that can filter out some of the AM
waves to eliminate the disruption they cause,
Getchell says. |
