A random survey across the city of Chicago reveals a surprising detail about the growth and compatibility of 802.11b/g networks: The non-overlapping channels, 1/6/11, are not at all uniformly used. Instead 70% of all networks are on channel 6, leaving 16% on channel 11, and only 5% on channel 1! I would bet that most problems associated with wireless networking nowadays have nothing to do with signal strength but rather with interference from co-channel networks. In a separate survey, in a single location, some 30+ networks were detected from inside a brick apartment building.
The survey itself was a bus ride from Midway Airport to Hyde Park via 55th street. The laptop running Netstumbler was closed and sitting in a backpack, with no attempt to improve the signal in any way. 367 networks were detected. It is clear that significant amounts of networks were not found because of the location of the antennas--a previous survey of a few blocks in a well-to-do neighborhood near the University of Chicago (Hyde Park) showed over a hundred networks with a five block walk and a sensitive USB wireless adapter.
22% of the networks did not have encryption turned on. This is not really a bad thing--you shouldn't rely on a completely broken encryption (WEP) to protect your communications. Instead, run a SSH tunnel or a VPN to ensure all communications are secure.
I suppose at this point, from the results of this look, if someone asked me for help about their wireless problems, I'd immediately tell them to switch channels on their access point. A smarter (and probably paid) consultant would scan their location and pick the least populated channel, whatever it was (and odds are it wouldn't be channel 6). That, or switch to the much less popular 802.11a, which runs at 5.8GHz.
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