AM radio creates ADSL static

Now there's one more reason to hate AM radio: It cuts the bandwidth on asymmetric digital subscriber lines (ADSL). Nortel Networks claims interference from AM stations can slash high-speed bandwidth by 40%.

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 significant 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.

Why AM Radio and ADSL Are at Odds

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 540KHz and 1.7MHz. ADSL service providers use the 138KHz to 1.1MHz 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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