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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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3 Answers 3

up vote 16 down vote accepted

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 '14 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 '14 at 13:47
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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 '14 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 '14 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 '14 at 22:26

You can use any channel you want, but as others have said channels 1, 6, and 11 are your best bet. The channels overlap each other such that 1, 6, and 11 are the only channels that don't overlap each other. A picture is worth a thousand words, this one is from the Android app Wifi Analyzer:

sample image of overlapping channels

Notice that SSID S, centred on channel 1, gets interference from not just SSID 2412s (which is also on channel 1), but from 2417s on channel 2 and from 2422s on channel 3! However 2347s on channel 6 doesn't interfere with it at all. 2347s also does not interfere with 2362s, on channel 11.

You may need to use a wifi analyzer to find out which channel has the least interference, but bear in mind a lot of devices will change channel automagically. It is possible, though, that you're in an area where the local internet provider uses equipment that all sits on one channel, so you could get lucky. At one point I was in an apartment with 24+ different wifi networks bleeding in... all on channel 1, all gear from same ISP. There were only 3 networks elsewhere. As long as I chose a channel 6 or higher, I had no trouble.

Also note channel 14 is an option, but must keep its transmit power below a certain level in the US (same for 13, though the power limit is a little higher). Otherwise you're committing a felony... so most wifi devices don't let you choose it.

TL;DR version: Yes, you can choose any channel, but assuming an essentially even distribution (a safe assumption in most cases), choosing 1, 6, or 11 will give you best performance since they don't overlap with each other.

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Channel 14 (2473-2495 MHz) in the US overlaps with the 2483.5-2495 MHz range, which is reserved for Federal or licensed use and does not allow the use of unlicensed Part 15 devices. Using it put you at risk of potential fines and/or prosecution no matter the power level you use. Channel 13 (2461-2483 MHz) is not approved for use in the US by the 802.11 standards because it abuts so closely with the 2483.5-2495 MHz range without any sort of "buffer" to prevent interference. If your device does "leak" signal above what it should, you could subject yourself to fines and/or prosecution. –  YLearn 2 days ago

Just randomly browsing the key terms "Wifi" and "Channel 3".. it's gonna be different for everyone. For me, channel 1, 6 and 11 simply do not work. There's way too much interference and my wireless mouse keeps skipping. My wireless keyboard also lags (A LOT). I set the router to channel 3 and the internet sped up about 5%, and also the wireless mouse and keyboard stopped lagging/skipping.

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It is answers like this that actually makes things worse. When you set your device to channel 3, you are now actually interfering with both channels 1 and 6, making them less usable for those around you. Unfortunately, this is all too common in many residential areas where those who don't know any better use any random channel (and often supported by consumer devices that auto-select channels other than 1, 6, and 11). –  YLearn Dec 12 '14 at 0:09

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