HakTip 13 – WiFi 101: Channels and interference

Today as we continue on our WPA cracking adventure it’s all about WiFi Channels and a little fun with a 2.4 GHz Spectrum Analyzer, BackTrack 5 Linux and a microwave.

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In terms of WiFi a Channel is merely a band of spectrum. Whether it’s A, B, G or N the basic idea is that a range of frequencies are allocated to a channel and seperated from their neighbor channels just a bit, typically just a few MHz, which we refer to as whitespace.

For example in the case of 802.11 protocols using the 2.4ghz spectrum the channel width is 22 MHz. Each of these channels or bands, with the exception of channel 14, are separated by 5 MHz of unused spectrum.

For instance Channel 1 is centered at 2.412 GHz and since it’s 22 MHz wide it begins at exactly 2.400 GHz and ends at 2.422 GHz. Then channel 2 centered at 2.417 GHz so it begins just 5 MHz past where Channel 1 started. This continues ever 5 MHz with Channel 3 being centered at 2.422 GHz, and so on, and so on. Until channel 14 that is, which has a 12 MHz spacing.

And as you can see there is quite a bit of overlap so the general recommendation is to use channels 1, 6, 11 and 14 as they are discrete — that is to say they do not overlap each other at all.

Now channel availability is regulated by country. Here in North America we’re supposed to use channels 1 through 11 while the rest of the world get channels 1 through 13. Japan is special, because, well, they’re Japan — so they get all 14 channels.

Watch the video for a spectrum analyzer demonstration of microwave interference on the 2.4ghz band.

The Ubertooth One is available in our HakShop.

Last week I asked “what 802.11b channel is only allowed in Japan?” and youtube commenter markpinegar answered: “Channel 14 is the 802.11b channel that can only be used in Japan, but only in direct-sequence spread spectrum (DSSS) and complementary code keying (CCK)? modes.” Thanks for answering and look out for a direct message so we can send you our favorite USB WiFi radio.

This week I’d like to know what 802.11 standard enables high powered WiFi equipment to operate, with license in the United States, on the 3.6 GHz spectrum.

Answer in the comments to be randomly selected to receive the radio I use here on HakTip.

And as always we value your feedback and suggestions. If you have a tip to share with me, email [email protected]. And be sure to check out our sister show Hak5 for more great stuff, just like this. I’ll be there reminding you to trust your technolust.


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