The WiFi Adjacent Channel Myth
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Dirty little Airwaves! Are you interfering with your neighbors? Or even your own Wireless Access Points at your location?

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Everyone operating in 802.11b and g (and pre-n) wireless systems is operating on a fixed or finite resource. Most of them don't even realize how limited it really is.

Why do we have really so few WiFi 802.11 channels in reality to use when we have multiple access points on a yacht (or in a building, or a home) in some close proximity?

Many wireless networking professionals advocate using what they call non-adjacent channels. By this they mean you need to arrange your site using only channels that don't overlap arranged in a sort of cellular arrangement (use the building plan or yacht plan.) They usually recommend using only channels 1, 6, and 11 (assuming those are legal in your area.) Access points with the same channel number should not be reaching each others territory. A channels territory can overlap another channels territory as long as frequency-wise they don't overlap. See below some simple channel bandwidth math. So is this a myth or a reality? (I say it is a reality.)

[10/05/2009] - There's a neat utility that you can download and run on your WiFi-capable laptop called inSSIDer Get inSSIDer Here that really does a good job of illustrating the channel overlap. Someone can show you a graph or a chart (like I've always had below in this article) but it's a WHOLE LOT BETTER to be able to see your *Real* WiFi Environment on your laptop screen - right in front of you - in *Real Time*. You can see what other WiFi Access Points are on your chosen channel and on adjacent channels that could be slamming you right to Mars. It could be intentional or unintentional. Lots of people just throw up wireless access points with complete ignorance of what's around them. Professionals do not do that. If you've ever been on CB Radio or any other Two-way Radio system - you may know what it's like to get your radio transmission stepped on. If someone else has a stronger signal on the same channel (frequency) then they usually win. Now WiFi does have some resilient features, it will retry, it will resend, it will even back off and slow down (downshift) - but you really don't want it to do that. If you use WiFi only to access the Internet - then you might not notice it downshifting in speed, but if you do multimedia stuff on your local network - you will...

inSSIDer screen shot from Metageek

Channel Frequency
 
1 2412    US/Europe
2 2417  US/Europe
3 2422  US/Europe
4 2427  US/Europe
5 2432  US/Europe
6 2437  US/Europe
7 2442  US/Europe
8 2447  US/Europe
9 2452  US/Europe
10 2457 US/Europe/France/Spain
11 2462 US/Europe/France/Spain
12 2467 Europe/France
13 2472 Europe/France
14 2484 Japan

Notice that the channels are showing in reality the "center" frequency not the actual spectrum or airwave frequency space that these channels occupy when a wireless card or Access Point (e.g. a Hotspot or WLAN in a premises) when operating. 802.11 (WiFi) channels are 5 Mhz apart, but that's the center frequency. Each channel actually occupies 30 Mhz of "bandwidth". Ahah! Now you know what bandwidth really means! That is 15 Mhz above and 15 Mhz below the channel center frequency.

(Special Note: I realize that the 802.11 specification is for 22 Mhz channels, but have a look at:

http://www.wi-fiplanet.com/tutorials/article.php/972261
which says

An important concept to note regarding channel assignments is that the channel actually represents the center frequency that the transceiver within the radio and access point uses (e.g., 2.412 GHz for channel 1 and 2.417 GHz for channel 2). There is only 5 MHz separation between the center frequencies, and an 802.11b signal occupies approximately 30 MHz of the frequency spectrum. The signal falls within about 15 MHz of each side of the center frequency. As a result, an 802.11b signal overlaps with several adjacent channel frequencies. This leaves you with only three channels (channels 1, 6, and 11 for the U.S.) that you can use without causing interference between access points.

and also:

http://www.cisco.com/univercd/cc/td/doc/product/wireless/airo1200/accsspts/techref/channel.htm in regards to the actual spectrum occupied.)

Here is a graphical representation of the US channels by US Robotics Company (taken from a PDF of theirs on their 100 Mb/s Product.) Even at 22 Mhz it doesn't look very good for any co-channels other than 1, 6, and 11.

802.11 overlapping channels graphic


* 15 Mhz on either side of center frequency

Channel 1 2412 (no care about - as no channel below 1) + 15 = 2427

That wipes out also channels 2 through 4

Channel 6 2437 - 15 = 2422

That wipes out also channels 5,4, and 3

Channel 6 2437 + 15 = 2452

That wipes out also channels 7,8, and 9

Channel 11 2462 - 15 = 2447

That wipes out also channels 10,9, and 8

Channel 11 2462 + 15 = 2477

And there goes channels 12, 13, and the US doesn't even allow above channel 11. Other Countries Europe, France can use 12 and 13.

* From this you can see how easy it would be to interfere with another access point or even an individual users wireless card. This is why it's so important to do a Site Survey when installing Wireless. But more of a site survey than many actually do. You need to do an actual R.F. - Radio Frequency (channel usage) site survey. If you truly will have seemless roaming in your site a laptop has to be able to walk from one access point onto another one without a "dead spot" and switch over to it. If the signal from one is reaching the other one then they cannot be on the same or on an overlapping (adjacent) channel.

I wonder if anyone has done this on a larger scale. For example it would be interesting to RF Site Survey a busy Yacht Marina. See how many 802.11 tranceivers are banging away transmitting on channels that actually interfere between boats and between boats and WiFi Hotspots.

That might be a good show for the Myth Busters on the Discovery Channel. Instead of  killing balistics gel and human replica dummies they could see how many WiFi channels they could kill with a few (legal power) mobile amplifiers, antennas and WiFi Cards.

It seems like you could wipe out a US site with just 3 amplified wifi cards... Channels 1, 6, and 11 would occupy all 11 US channels. Well you probably wouldn't  totally wipe it out but there would be a lot of contention for the RF air space. Some systems would surely slow down. Or is this a myth? What would be stronger a
1 watt amplifier on an external antenna on the next boat over or your laptop and your access point that are closer?

Some related interesting information...

http://www.wi-fiplanet.com/tutorials/article.php/1428941 says:

The spectrum regulatory body of each country restricts signal power levels of various frequencies to accommodate needs of users and avoid RF interference. Most countries deem 802.11 wireless LANs as license free. In order to qualify for license free operation, however, the radio devices must limit power levels to relatively low values. (Basically you are supposed to get 4 watts EIRP out of the system when omni-directional antennas are used including the gain of the antenna, remembering that every 3db of gain doubles that signal power.) Have a look also at: http://www.wlanantennas.com/wlan_faq_fcc.htm.

Some commercial sites and marine vessel networks might want to switch to 802.11a for more channels and less interference since almost everybody else will be using 802.11b/g using common off the shelf (COTS) equipment. The following diagram comes from:

http://searchnetworking.techtarget.com/tip/1,289483,sid7_gci970726,00.html and the Part 2 in this article is titled Moving to 802.11a. Notice that 802.11a has 12 Channels provided reportedly non-overlapping.


IEEE 802.11 radio link interfaces
Standard Maximum bit rate Fallback rates Channels provided Band Radio technique
802.11 2 Mbps 1 Mbps 3 2.4 GHz ISM FHSS or DSSS
802.11b 11 Mbps 5.5 Mbps
2 Mbps
1 Mbps
3 2.4 GHz ISM DSSS
802.11a 54 Mbps 48 Mbps
36 Mbps
24 Mbps
18 Mbps
12 Mbps
9 Mbps
6 Mbps
12 5 GHz U-NII OFDM
802.11g 54 Mbps Same as 802.11a, plus 2 Mbps 3 2.4 GHz ISM OFDM


The following identifies the center frequency and maximum output power of each of the U-NII bands:

Frequency

Channel Number

Transmit Frequency

Maximum Transmit Power

U-NII lower band

40

5.200 GHz

40mW


36

5.180 GHz



44

5.220 GHz



48

5.240 GHz


U-NII middle band

52

5.260 GHz

200mW


56

5.280 GHz



60

5.300 GHz



64

5.320 GHz


U-NII upper band

149

5.745 GHz

800mW


153

5.765 GHz



157

5.785 GHz



161

5.805 GHz


This chart is taken from: http://www.wi-fiplanet.com/tutorials/article.php/2109881

See also: http://www.qsl.net/kb9mwr/projects/wireless/allocations.html Amateur Radio Band allocations ... also thrown into the mix. (Not that Amateur Radio Operators are bad, I'm one! KA4UDX.)

[end of article]
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