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It seems that everything I read about wifi says there are only three usable channels in the 2.4 GHz band; however, there are eleven 2.4Ghz wifi channels allowed in the US.

If everyone is using 1, 6 or 11, wouldn’t it make sense to use an unused channel, say, channel 3 for my wifi infrastructure? Or to put the question differently, why can’t I use channels other than 1, 6 and 11?

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The 2.4GHz band is one of many portions of radio spectrum, called the Industrial, Scientific and Medical (ISM) bands that are allocated for unlicensed use. As long as you operate within the power and antenna limits, you can pretty much do what you want. So the short answer is, you can. But there are very good reasons why you shouldn’t.

Part of the confusion regarding wifi channels comes from the allocation of the frequency spectrum. The ISM band was first allocated in 1958, before most of us were born and well before anyone even imagined wireless networking. The channel definitions were made before wifi was invented, and they assumed 5 MHz spacing.

802.11b and g transmissions require 22MHz bandwidth. Because they’re 22MHz wide, the signal covers two channels above and below the center frequency. So if you use channel 6, your signal spreads across channels 4-8. There is only room in the entire band for three 22MHz wide signals (in the US) without overlapping if they center on channels 1, 6 and 11.

If you transmit your wifi signal between two of these channels, say centered on channel 3, two things happen: your signal interferes with other wifi users on 1 and 6, and their signals interfere with you. This will greatly increase the number of data errors, which in turn will cause retransmissions and significantly reduce your throughput.

It’s as if there are a number of parallel bicycle lanes, and you try to drive a bus down one of them. Although you drive down one lane, your bus will occupy several of the adjacent lanes. If someone happens to be driving their bus in one of those adjacent lanes when your bus goes by, well…it won’t be pretty.

If you only want to use one access point in a remote area with no other wifi signals, then you can probably get a away with using a different channel. But in most urban commercial environments, the 2.4GHz band is pretty crowded. If you use an overlapping channel, you are likely to experience (and cause) interference. If your wireless system is large with many access points, then you will need all three non-overlapping channels to get good coverage. Using something other than 1, 6 or 11 will limit the density of your access points, further reducing throughput.

In summary, it’s good practice to use 1, 6 and 11 to get the maximum use of the radio spectrum with a minimum of interference.

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I live in bigger complex -- 10 stories/40 flats -- almost every flat has own home wifi. Wouldn't it be even worse if everyone is using channell 1/6/11? Using you analogy, there would be 1000 cyclists on lane designed for 10. –  jnovacho Mar 21 at 8:55
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@jnovacho, No. It's always better to use 1, 6, or 11 in the U.S. The channel you pick is the center channel used by your signal, not the only channel it uses. As Ron said, wifi signals take up 5 channels (the center channel + 2 on each side of that.) So if, say, you pick channel 4, you're really using channels 2, 3, 4, 5, and 6. As such, you're interfering with (and receiving interference from) networks on both channel 1 (which use channels 1-3 [plus some below channel 1]) and channel 6 (which uses channel 4, 5, 6, 7, and 8.) The only 3 non-overlapping channels are 1, 6, and 11 (in the U.S.) –  reirab Mar 21 at 13:47
    
if everyone in the neighbourhood reduced the transmit power levels on their routers and wifi clients (instead of cranking it to maximum, as many people do), wouldn't that also significantly reduce interference with each other's wireless networks? –  user1082 Mar 27 at 21:03
    
@user1082: Depends. If two wifi devices on the same channel see and ears each other perfectly, then they will not interfere. They will happily share the channel by trying to not transmit at the same time using CSMA/CA (CA as in collision avoidance, pretty accurate with regard to the lane analogy). If you reduce their power and they don't see themselves as much, CSMA/CA will not work anymore, so they will transmit at the same time and so will interfere. However, if you reduce transmit power level even more, they will interfere less and less. –  BatchyX Mar 27 at 22:26
    
@user1082 Yes. It’s a friendly move to reduce your transmit power to the lowest setting that still gives you good enough coverage everywhere you need it. This reduces the noise level for your neighbours, giving them better “air quality”. –  Leon Weber Mar 27 at 22:26

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