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Understanding 20 MHz Versus 40 MHz
802.11n Channel Configuration

An explanation of 802.11n 40 MHz channels and their capabilities and limitations

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The Allocation of Transmission Channels for 802.11b/g, 802.11a and 802.11n

The 2.4 GHz ISM Band Used by 802.11b/g
In North America channels 1 through 11 are available for use. Each is 22 MHz wide which means that Channel 1, Channel 6 and Channel 11 are the only grouping of 3 (22 MHz) channels that don't overlap.

The Use of 40 MHz Channels in the 2.4GHz Band is Not Reasonable for Commercial Networks

The 2.4 GHz band is often heavily congested, particularly in dormitories, apartments and condominiums, and multi-story office buildings. This alone precludes effective use of 40 Mhz channels. A home user, or a commercial user requiring only a single access point may obtain improved throughput (relative to 802.11g) using a single 40 MHz channel. Interior space larger than 3000 square feet will probably need more than one 802.11n access point to provide maximum bit-rate connectivity to 100% of the area. When adding a second access point to an 802.11n network is considered in the 2.4 GHz band the problem with 2.4GHz 40 MHz channels becomes evident.

Defining 40 MHz Channels for 802.11n Used in the 2.4 GHz ISM Band
Only one 40 MHz channel will "fit" into the 2.4 GHz band. This is why commercial
802.11n design focuses on deployment in the 5 GHz U-NII bands.

Only One 40 Mhz 802.11n Channel Is Available in the 2.4 GHz ISM Band

The 5.8 GHz Channel Space

The 5.8 GHz ISM Band Used by 802.11a and 802.11n
The band is divided into four sections. U-NII-1, 2 and 3 each have 4 non-overlapping 20 MHz channels. U-NII-2 has an additional 8 20 MHz non-overlapping channels but is not supported by all manufacturers due to requirements for automatic military radar detection capability in this frequency range.

802.11n 40 MHz Channels As They Relate to Dynamic Frequency Selection (DFS) and Radar Avoidance

Military radar and some weather radar can operate in the U-NII-2 band. To avoid conflict between unlicensed transmitters (like Wi-Fi access points) operating in the U-NII-2 band the FCC requires [Rule #15.407(h)(2)] that all unlicensed transmitters operating in U-NII-2 implement Dynamic Frequency Selection (DFS). DFS allows the radio to detect the presence of radar signals and to dynamically and automatically change to a different transmit frequency if radar is discovered. Because this requirement demands that a manufacturer of a Wi-Fi access point implement additional, specialized detection mechanisms some manufacturers have simply decided to not provide support for U-NII-2 operation. While almost all major manufacturers do support U-NII-2 it's important to check for U-NII-2 capability when selecting a particular brand of equipment.

Notice that without U-NII-2 support there are only four available channels for indoor use and four for outdoor use. That's a non-issue when 20 MHz channels are being used (i.e. for 802.11a). In mathematics, the "Four Color Theorem" states that if a plane (a floorplan or area map, for example) is divided into contiguous regions (analogous to access point coverage cell areas) then the regions can be colored using at most four colors so that no two adjacent regions have the same color. If we substitute "channel" for "color" then the Four Color Theorem tells us that if you have four channels to work with then a design can be created that avoids detrimental channel overlap (where two or more access points cover an area with the same channel resulting in system degradation).

When considering 40 MHz channels it can be seen that use of the U-NII-2 band is critical. You can't get four (or even three) 40 MHz channels for either indoor or outdoor use unless you take channel space out of the U-NII-2 band. The channel map shown below indicates how

Defining 40 MHz Channels for 802.11n Used in the 5 GHz ISM Band
The yellow rectangles denote "double-wide" 40 MHz channels. U-NII-1 provides two 40 MHz channels for indoor use as does U-NII-3 for outdoor use. The question marks in the U-NII-2 channels refer to the fact that some manufacturers may not provide U-NII-2 support. U-NII-2 support is critical to get enough 40 MHz channels to avoid channel overlap.