The 802.11b and 802.11g amendments require 25 MHz of separation between the center frequencies of HR-DSSS channels to be considered non-overlapping. The 802.11g amendment also requires 20 MHz of separation between the center frequencies of ERP-OFDM channels.
As pictured in Figure below, only channels 1, 6, and 11 can meet these IEEE requirements in the 2.4 GHz ISM band in the United States if 3 channels are needed.
Channels 2 and 7 are non-overlapping, as well as 3 and 8, 4 and 9, and 5 and 10. The important thing to remember is that there must be 5 channels of separation in adjacent coverage cells.
Some countries use all 14 channels in the 2.4 GHz ISM band, but due to positioning of the center frequencies, no more than 3 channels can be used while still avoiding frequency overlap.
Even if all 14 channels are available, most countries still choose to use channels 1, 6, and 11. When designing a wireless LAN, you need overlapping coverage cells in order to provide for roaming.
However, the overlapping cells should not have overlapping frequencies, and only channels 1, 6, and 11 should be used in the 2.4 GHz ISM band in the United States to get the most available, non-overlapping channels.
Overlapping coverage cells with overlapping frequencies causes what is known as co-channel interference (CCI), which causes a severe degradation in performance and throughput.
If overlapping coverage cells also have frequency overlap, frames will become corrupted, retransmissions will increase, and throughput will suffer significantly. As defined by the IEEE, there are currently 12 channels available in the 5 GHz UNII bands.
These 12 channels are technically considered non-overlapping channels because there is 20 MHz of separation between the center frequencies.
However, in reality there will also be some frequency overlap of the sidebands of each ERP-OFDM channel. The good news is that you are not limited to 3 channels and all 12 channels can be used in a channel reuse pattern.
As pictured in Figure below, the United States and other countries have designated more license-free frequency space in the 5 GHz range and 11 more channels have been approved for use.
In some countries, 802.11a radio cards will soon have the ability to transmit on a total of 23 channels.
Channel Reuse
One of the most common mistakes many businesses make when first deploying a WLAN is to configure multiple access points all on the same channel. This will of course cause co-channel interference and degrade performance significantly.
To avoid co-channel interference, a channel reuse design is necessary. Once again, overlapping RF coverage cells are needed for roaming but overlap frequencies must be avoided.
The only three channels that meet these criteria in the 2.4 GHz ISM band are channels 1, 6, and 11 in the United States. Overlapping coverage cells therefore should be placed in a channel reuse pattern similar to the one pictured in Figure below.
Channel reuse patterns should also be used in the 5 GHz UNII bands. All 12 802.11a channels can be used, as pictured in Figure below. Due to the frequency overlap of channel sidebands, there should always be at least 2 cells between access points on the same channel.
It is also a recommend practice that any adjacent cells use a frequency that is at least 2 channels apart and not use an adjacent frequency. It is necessary to always think three-dimensional when designing a channel reuse pattern.
If access points are deployed on multiple floors in the same building, a reuse pattern will be necessary, such as the one pictured in Figure below.
A common mistake is to deploy a cookie-cutter design by performing a site survey on only one floor and then placing the access points on the same channels and same locations on each floor.
A site survey must be performed on all floors, and the access points often need to be staggered to allow for a threedimensional reuse pattern.
Also, the coverage cells of each access point should not extend beyond more than one floor above and below the floor on which the access point is mounted. It is inappropriate to always assume that the coverage bleed over to other floors will provide sufficient signal strength and quality.
In some cases, the floors are concrete or steel and allow very little, if any, signal coverage through. As a result, a survey is absolutely required. Many enterprise access points currently have dual radio card capabilities, allowing for both 2.4 GHz and 5 GHz wireless networks to be deployed at the same.
The 802.11a radio in an access point transmits at 5 GHz, and the signal will attenuate faster than the signal that is being transmitted at 2.4 GHz from the 802.11b/g radio card.
Therefore, when performing a site survey for deploying dual frequency WLANs, it is a recommended practice to perform the 5 GHz site survey first and determine the placement of the access points.
Once those locations are identified, channel reuse patterns will have to be used for each respective frequency. In some cases, only the 802.11a radio will be active.
