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Old 08-12-2010, 08:07 AM   #1
Registered: Jul 2010
Distribution: Gentoo, Debian, Mint, Xubuntu
Posts: 150

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Working around wireless interference (wifi.)

A few weeks ago, I noticed that our internet was really slow. Now that I have a laptop (running Hardened Gentoo,) I no longer use a wired connection to our router, and it was painful to download anything. Other family members experience the same problem.

When I ran 'sudo iwlist scan', I came up with more than one network. In fact, at one time, I was able to pick up at least 11 networks (I forgot exactly how many). These included clearly-visible and prominent ones on channels 1 and 11, as well as ones on channel 9 and a few on channel 6 (which I suspect to be 108 Mbps systems) that turned up more rarely in the scans.

I looked at the iwconfig man page, as I remembered some potentially useful settings there. I set up this configuration for my system (I used the same settings on our router, except all I could configure was the channel, fragmentation, and RTS

mode: Managed (not sure what Windows calls this)
channel: 5 (this had the least overlap with the most affected channels.)
Access Point: <our AP mac address>
Transmit power: max (for me, 100mW, or 20 dBm)
sensitivity: 1
RTS: 256
Fragmentation: 256
Power: all (power management setting.)

I have only tested for a few minutes during interference, but it seems to work well now. I emailed this list of settings to the Windows person in our household. It goes without saying that if (big if) these settings also exist on Windows, they'll be hard to find.

I hope this helps others who run into the same problem.
Old 08-12-2010, 09:06 AM   #2
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Registered: Aug 2003
Distribution: CentOS, OS X
Posts: 5,131

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The slowness is probably due to multiple systems using the same channels. I've had the problem here, and had to manually force an unused channel to get my wireless working fine; I still have to alter it at times, because neighbours seem to switch channels from time to time (for a reason I don't understand). Luckily there are free channels.

It's not just other wireless networks that can cause this; other devices work on the same frequencies too, for example some wireless audio equipment. If they're positioned in a suitable way, they may interfere with the wireless traffic if the frequencies are close to each other, and result in a connection that doesn't work well or at all. While this is a rare problem these days, at least here, I suspect that it will become worse, because more and more devices connect wirelessly to other devices, and the more people have them, the more crowded the frequencies become.
Old 08-13-2010, 12:15 PM   #3
Registered: Jul 2010
Distribution: Gentoo, Debian, Mint, Xubuntu
Posts: 150

Original Poster
Rep: Reputation: 35
Since the channels overlap (google it if you don't know about this,) every channel was blocked for me (or illegal in the US, without an amateur radio license). I selected the most open one.

I have yet to run into more interference, so I can't be sure how reliable this method is.
Old 08-13-2010, 04:25 PM   #4
Senior Member
Registered: May 2009
Location: WV, USA
Distribution: Slackware, Ubuntu, Amazon Linux
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Channels 1, 6, and 11 make up the largest non-overlapping set for USA channels. Channels 2 to 5 overlap 1 and 6 (and each other). Channels 7 to 10 over 6 and 11 (and each other). Basically, each channel overlaps with those as much as 4 channels above and below (so channel 4 overlaps channel 8 but not channel 9).

It might seem intuitive for everyone to just use channels 1, 6, and 11, only. But that's actually not the best in all cases. Other kinds of interference sources, such as cordless phones, and impact various channels selectively. You might find that channel 3 works where 1 and 6 won't (or won't work as well). Also, signal reflections, and antenna issues, can affect what frequencies work better or worse. The simple direction is to try them all and use what works best.

The explanation of why a neighbor's network changes channels is that the are exploring what works better, or just decided the one they are on does not work as well. They may wonder why you changed.

Going to 5.8 GHz might be an idea for some ... either for wifi or for cordless phones or other gadgets.


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