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Dueling with Microwave Ovens
Part 2
Cell Tower
Life Bluetube Headsets
Cell Phone Towers Health Effects
EM Field Meter
Cell Phone Sensitivity
Real world testing uncovers some issues
Recently, I ran some tests to see how much impact a typical
microwave oven has on WLAN operation. I did this within
offices inside the building of my company, Wireless-Nets,
Ltd., a three-story facility with typical wood and drywall
construction for inside walls. A single access point doesnt
have any problems covering the entire facility.
The access point was a Cisco Aironet 350 Series, which was
initially set to its maximum transmit power of 100mW. As a
wireless client, I used a laptop with AirMagnet monitoring
software. AirMagnet is able to continually transmit packets
of a specified size for a period of time while measuring
signal strength, noise levels, packet retransmission rates,
and throughput.
The microwave included in the testing is made by GoldStar
and resides in a break room. The label on the back of the
microwave indicates that it consumes 1,200 watts of power
and operates at 2,450 MHz, which is close to 802.11b channel
9.
Before turning on the microwave, I set the access point to
channel 9 (a worst case situation), and took some
measurements within the break room to use as a baseline. The
access point signal level resulting from the beacons within
the break room was -63dBm, sufficient for solid 11Mbps
associations. Throughput tests indicated 667 packets per
second (pps) while sending 1,532 byte frames.
While holding the wireless client within one foot of the
microwave, I recorded some measurements with AirMagnet while
the microwave was set to high and heating up a large bowel
of water. The throughput fell to 90pps, a drop of more than
85 percent.
This is a substantial reduction in performance, but its the
worst-case situation. The access point was set to the same
frequency of the microwave, and its unlikely that someone
would use a wireless client so close to the microwave.
A more realistic distance from the microwave is from one of
the break tables, which is about eight feet away from the
microwave. At this range, I reran the throughput tests,
resulting in 178pps. This still equates to around a 75
percent decrease, something that would likely make users
unhappy.
In order to see what a 75 percent decrease in throughput
feels like, I tried surfing to a web site having a few
graphics. With the microwave running, the pictures would
come in painfully slow. I also surfed around a bit to other
pages, and sometimes the pages would freeze.
After turning off the microwave, I cleared the browser
cache, and found no problems surfing the same pages. The
pages loaded lightening fast.
I also repeated the tests down the hall about 20 feet away
with the microwave running and still experienced fairly
sluggish responses. In fact, throughput from there was still
only 260pps. Obviously, the microwave was making the WLAN
crawl at surprisingly great distances from the microwave.
Something to consider is that these tests were run with only
one active wireless client. The results would have been much
worse if there were more users on the network.
Curious to know what channels the microwave would affect the
most, I ran the throughput tests again with the access point
set to different channels. On channels 1, 2, 3, 4, 5, 6, 7,
8, 9, 10, and 11, the throughput was 660pps, 658pps, 655pps,
651pps, 643pps, 574pps, 434pps, 258pps, 178pps, 191pps, and
210pps, respectively. Based on these numbers, the microwave
was most critically impacting channels 8, 9, 10, and 11.
I also found that the impact is more severe near edges of
the range boundary of the access point where signal levels
are lower. To test this, I lowered the transmit power of the
access point to 1mW, which caused the signal power to
decrease to -77dBm. This level is close to the range
boundary of the WLAN.
As one would expect, the hit on throughput was even more
with the weaker signals. The resulting throughput was about
10 percent lower than the case where the signal levels from
the access point were higher at -63dBm.
Dueling with Microwave Ovens Part 1
Moldova Chisinau
Austria Vienna
Athens Georgia USA
Spain, Madrid,
Peru, Lima, City,
Austria, Vienna,
Poland, Warsaw,
Toowoomba, Queensland,
Egypt, Cairo: city limits,
Comoros, Moroni (on Grande Comoro)
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