Understanding Wi-Fi channels

Now for a little talk about frequency bands used
by the various Wi-Fi standards. In 1985, the FCC
made changes to the radio spectrum regulation
and assigned three bands designated as the
industrial, scientific, and medical (ISM) bands.
These frequency bands are
902 MHz–928 MHz: A 26 MHz bandwidth
2.4 GHz–2.4835 GHz: An 83.5 MHz bandwidth
5.15–5.35 GHz and 5.725 GHz–5.825 GHz: A
300 MHz bandwidth
The FCC also opened some additional frequencies,
known as Unlicensed National Information
Infrastructure (U-NII), in the lower reaches of
the five GHz frequencies.
The purpose of the FCC change was to encourage
the development and use of wireless networking
technology. The new regulation
permits a user to operate, within certain guidelines,
radio equipment that transmits a signal
within each of these three ISM bands without
obtaining an FCC license.
Wireless networks use radio waves to send
data around the network. 802.11a uses part of
the U-NII frequencies, and IEEE 802.11b and g
use the ISM 2.4 GHz band. 802.11n can use
either band, though not all 802.11n systems do
(many use only the 2.4 GHz band).
An important concept when talking about frequencies
is the idea of overlapping and
nonoverlapping channels. As we discuss in
Chapter 18, signals from other APs can cause
interference and poor performance of your
wireless network. This happens specifically
when the APs’ signals are transmitting on the
same (or sometimes nearby) channels. Recall
that the standards call for a number of channels
within a specified frequency range.
The frequency range of 802.11g, for example, is
between 2.4 GHz and 2.4835 GHz, and it’s broken
up into fourteen equal-sized channels. (Only
eleven can be used in the United States — any
equipment sold for use here allows you to
access only these eleven channels.) The problem
is that these channels are defined in such a
way that many of the channels overlap with one
another — and with 802.11g, there are only
three nonoverlapping channels. Thus, you
wouldn’t want to have channels 10 and 11 operating
side by side because you would get signal
degradation. You want noninterfering, nonoverlapping
channels. So you find that people tend
to use Channels 1, 6, and 11, or something similar.
802.11a doesn’t have this problem because
its eight channels, in the 5 GHz frequency band,
don’t overlap; therefore, you can use contiguous
channels. As with 802.11b and g, however,
you don’t want to be on the same channel.