Channels | RF Primer

One Wi-Fi radio does not occupy the entire unlicensed spectrum, unlike frequency-hopping technologies such as Bluetooth. 802.11 divides up the spectrum into a number of different channels. Channels are named with whole numbers, assigned by a formula to specific center frequencies for the channels. The idea behind small number of discreet channels is to carve up the spectrum, helping pack in as many devices as possible and avoiding requiring clients to have to tune in across a wide range of frequencies, the way that analog car radios must.

The channel numbers are somewhat arbitrary, and are arranged to let you know what band they occupy. Different 802.11 radio types allow for different channel selections.

The two key properties that define how the 802.11 radio uses the spectrum are its center frequency and bandwidth. The center frequency is the one the radio uses to determine where to look for the transmissions. This concept is similar to car radios: FM channel 97.3 means that the radio tunes its center frequency to 97.3MHz. Unfortunately, Wi-Fi channels do not convert as neatly to their center frequencies. Because of this, many people and tools will either interchangeably use the center frequency or the channel number to describe the channel. Wi-Fi uses center frequencies that are always in the gigahertz range. The bandwidth tells which other frequencies are occupied by a transmission. 802.11 radios used for mobility primarily have 20MHz bandwidth, except for 802.11n radios, which can also use 40MHz bandwidths. The channel and bandwidth together show which part of the spectrum the radio occupies. Although the different 802.11 radio types may fill the carved-out part of the spectrum differently, the amount that is carved out is roughly the same for the same bandwidth. Figure 1 sketches the general concept.

 
Figure 1: Shape of 802.11 Frequency Occupation

Table 1 lists the channels and what radio types can use them.

Table1: 802.11 Channels 

Channel	Frequency	US Band	11b, 11g	11a	11n	Notes
1	2.412GHz	ISM 2.4	✓		✓	Nonoverlapping	High power: 1 W maximum.
2	2.417GHz		✓		✓
3	2.422GHz		✓		✓
4	2.427GHz		✓		✓
5	2.432GHz		✓		✓
6	2.437GHz		✓		✓	Nonoverlapping
7	2.442GHz		✓		✓
8	2.447GHz		✓		✓
9	2.452GHz		✓		✓
10	2.457GHz		✓		✓
11	2.462GHz		✓		✓	Nonoverlapping
12	2.467GHz		✓		✓	Europe, Japan, Australia. No U.S. or Canada
13	2.472GHz		✓		✓
14	2.484GHz		11b only			Japan only. Channel 14 does not follow the channel to frequency formula.
36	5.18GHz	U-NII 2 Lower		✓	✓	Indoor use only. Low power: 40 mW maximum
40	5.20GHz			✓	✓
44	5.22GHz			✓	✓
48	5.24GHz			✓	✓
52	5.26GHz	U-NII 2 Upper		✓	✓	Non-DFS for equipment before July 2007	Radar detection and dynamic frequency selection (DFS) required
56	5.28GHz			✓	✓
60	5.30GHz			✓	✓
64	5.32GHz			✓	✓
100	5.50GHz	U-NII 2 Extended		✓	✓
104	5.52GHz			✓	✓
108	5.54GHz			✓	✓
112	5.56GHz			✓	✓
116	5.58GHz			✓	✓
120	5.60GHz			✓	✓	U.S., Europe, and Japan. No Canada, because of weather radar.
124	5.62GHz			✓	✓
128	5.64GHz			✓	✓
132	5.66GHz			✓	✓
136	5.68GHz			✓	✓
140	5.70GHz			✓	✓
149	5.745GHz	U-NII 3		✓	✓	U.S, Canada and Europe. No Japan	High power
153	5.765GHz			✓	✓
157	5.785GHz			✓	✓
161	5.805GHz			✓	✓
165	5.825GHz	ISM 5.8		✓	✓	U.S., Canada and Europe. No Japan.	High power

The formula for the channels to frequencies is 2.407GHz + 0.5GHz * channel for the 2.4GHz band, and the simpler to remember 5GHz + 0.5GHz * channel for the 5GHz band. The only channels that are in the 2.4GHz band are channels 1-14. Everything else is in the 5GHz band. Therefore, channel 36 is 5.18GHz, and channel 100 is 5.50GHz.

The total number of channels is large, but many factors reduce the number that can be practically used. First to note is that the 2.4GHz band, where 802.11b and 802.11g run, only has three nonoverlapping channels (four in Japan) to choose from. Unfortunately, the eleven channel numbers available in the United States gives the false impression of 11 independent channels, and to this day there exist some Wi-Fi deployments that mistakenly use all 11 channels, causing an RF nightmare. To avoid overlapping channels, adjacent channel selections need to be four channel numbers apart. Therefore, channels 1 and 5 do not overlap. In the 2.4GHz band, custom usually spreads the channels out even a bit further, and using only channels 1, 6, and 11 is recommended. The authors of the standard recognized the problem the overlap causes, and, for the 5GHz band, disallowed overlap by preventing devices from using the intermediate channels. Therefore, no channels in the 5GHz band overlap, when it comes to 20MHz channels.

The unlicensed spectrum was originally designed for, and still is allocated to, other uses besides Wi-Fi. The 2.4GHz band was created to allow, in part, for microwave ovens to emit radio noise as they operate, as it is impractical to completely block their radio emissions. Because that noise prevented being able to provide the protections from interference that licensed bands have, the regulatory agencies allowed inventors to experiment providing other services in this band. And so began 802.11. The 5GHz band is, in theory, more set aside from radiation. Except for the top 5.8GHz ISM band, the 5GHz range was designed for communications devices. However, interference still exists. One primary source, and the one important from a regulatory point of view, is radar. Radars operate in the same 5GHz band. Because the radars are given priority, Wi-Fi devices in much of the 5 GHz band are required to either be used indoors only, or to detect when a radar is present and shut down or change channels. This last ability is known asdynamic frequency selection (DFS). This is not a feature or benefit, per se, but a requirement from the various governments. DFS complicates the handoff process significantly