IEEE 802.11: Channels

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Channels

2.4 GHz Band

802.11b, 802.11g, and 802.11n-2.4 utilize the 2.400 - 2.500 GHz spectrum, one of the ISM bands. 802.11a and 802.11n use a more heavilty regulated band of 4.915 - 5.825 GHz. These two bands are referred to as 2.4 and 5 GHz bands respectively. The 2.4 GHz band is divided into 14 channels spaced 5 MHz apart. Figure 1 shows the drawing of the 2.4 GHz band.

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Figure 1: 2.4 GHz WiFi

Also of note, the channel numbering of the 5.725 - 5.875 GHz spectrum is less intuitive due to the differences in regulations between countries.

Channel	Frequency (MHz)	North America	Japan	Most of world
1	2412	Yes	Yes	Yes
2	2417	Yes	Yes	Yes
3	2422	Yes	Yes	Yes
4	2427	Yes	Yes	Yes
5	2432	Yes	Yes	Yes
6	2437	Yes	Yes	Yes
7	2442	Yes	Yes	Yes
8	2447	Yes	Yes	Yes
9	2452	Yes	Yes	Yes
10	2457	Yes	Yes	Yes
11	2462	Yes	Yes	Yes
12	2467	No	Yes	Yes
13	2472	No	Yes	Yes
14	2484	No	11b Only	No

The spectral mask for this band requires the signal to be attenuated a minimum of 20 dB from its peak amplitude at 11 MHz from the center frequency, the point at which a channel is effectively 22 MHz wide. One consequence is that stations can only use everyth fourth or fifth channel without overlap. Additionally the channels are regulated by counties, constrained in part by how each country allocates radio spectrum to various services. One extreme example is Japan which permits the use of all 14 channels for 802.11b and 1-13 for 802.11g/n-2.4. Other countries such as Span initially only allowed channels 10 and 11, and France only allowed 10-13. However, they both now allow channels 1-13. North America and some Central and South American countries allow only 1 through 11.

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Figure 2: 2.4 GHz WLAN

The spectral mask only defines power output restrictions up to 11 MHz from the center frequency to be attenuated by -50 dBr. It is often assumed that the energy does not extend past this limit. This assumption is to show that given the separation between channels, the overlapping signal on any channel would be sufficiently attenuated to minimally interfere with the transmitter on any other channel. The near-far problem of a transmitter allows it to negatively impact a "non-overlapping" channel when it is close to a victim receiver (within a meter) or operating above allowed power levels.

Additionally, there is concern with channel separation needed when there are multiple transmitting devices. FOr 802.11b the DSSS modulation utilizes a 22 MHz bandwidth which allows for three "non-overlapping" channels (1, 6, and 11). 802.11g utilizes OFDM modulation with a bandwidth of 20 MHz. This allows for the belief in four "non-overlapping" channels (1, 5, 9, and 13) although this is not actually the case according to the standard.

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Figure 3: Non-Overlapping Channels

It is interesting to note that if spectral analyzers are available, overlapping channels may be used under certain circumstances allowing more channels to be available for communication at the same time.

5 GHz Band

This is used with 802.11a, h, j, n, and ac. Countries apply their own regulations to the allowable channels, allowed users, and maximum power levels within these frequency ranges. The total frequency range of this band is 4.915 to 5.825 GHz. Note that not all of this spectrum may be used in the United States or any other country.

European standard EN 304 893 covers 5.15-5.725 GHz operation, and v1.7.1 is in force.

In 2007 the FCC (United States) began requiring that devices operating on 5.250-5.350 GHz and 5.470-5.725 GHz must employ dynamic frequency selection (DFS) and transmit power control (TPC) capabilities. This is to avoid interference with weather-radar and military applications. In 2010, the FCC further clarified the use of channels in the 5.470-5.725 GHz band to avoid interference with Terminal Doppler Weather Radar (TDWR) systems. This eliminated the use of channels 120, 124, and 128. Channels 116 and 132 may be used so long as they are separated by more than 30 MHz (center-to-center) from a TDWR located within 35 km of the device. There are now at least five relevant KDBs about operation in the 5 GHz radar bands.

Channel	Frequency (MHz)	United States	Europe	Japan		Singapore	China	Israel	Korea	Turkey	Australia	South Africa	Brazil	Taiwan	New Zealand
		40/20 MHz	40/20 MHz	40/20 MHz	10 MHz	40/20 MHz	40/20 MHz	20 MHz	20 MHz	40/20 MHz	40/20 MHz	40/20 MHz	40/20 MHz	40/20 MHz	40/20 MHz
183	4915	No	No	No	No	Yes	No	No	No	No	No	No	No	No	No
184	4920	No	No	Yes	Yes	No	No	No	No	No	No	No	No	No	No
185	4925	No	No	No	Yes	No	No	No	No	No	No	No	No	No	No
187	4935	No	No	No	Yes	No	No	No	No	No	No	No	No	No	No
188	4940	No	No	Yes	Yes	No	No	No	No	No	No	No	No	No	No
189	4945	No	No	No	Yes	No	No	No	No	No	No	No	No	No	No
192	4960	No	No	Yes	No	No	No	No	No	No	No	No	No	No	No
196	4980	No	No	Yes	No	No	No	No	No	No	No	No	No	No	No
7	5035	No	No	No	Yes	No	No	No	No	No	No	No	No	No	No
8	5040	No	No	No	Yes	No	No	No	No	No	No	No	No	No	No
9	5045	No	No	No	Yes	No	No	No	No	No	No	No	No	No	No
11	5055	No	No	No	Yes	No	No	No	No	No	No	No	No	No	No
12	5060	No	No	No	No	No	No	No	No	No	No	No	No	No	No
16	5080	No	No	No	No	No	No	No	No	No	No	No	No	No	No
34	5170	No	No	Client Only	No	Yes	No	Yes	Yes	Indoors	No	Indoors	Indoors	No	Indoors
36	5180	Yes	Yes	Yes	No	Yes	Yes	Yes	Yes	Indoors	Yes	Indoors	Indoors	No	Indoors
38	5190	No	No	Client Only	No	Yes	Yes	Yes	Yes	Indoors	No	Indoors	Indoors	No	Indoors
40	5200	Yes	Yes	Yes	No	Yes	Yes	Yes	Yes	Indoors	Yes	Indoors	Indoors	No	Indoors
42	5210	No	No	Client Only	No	Yes	Yes	Yes	Yes	Indoors	No	Indoors	Indoors	No	Indoors
44	5220	Yes	Yes	Yes	No	Yes	Yes	Yes	Yes	Indoors	Yes	Indoors	Indoors	No	Indoors
46	5230	No	No	Client Only	No	Yes	Yes	Yes	Yes	Indoors	No	Indoors	Indoors	No	Indoors
48	5240	Yes	Yes	Yes	No	Yes	Yes	Yes	Yes	Indoors	Yes	Indoors	Indoors	No	Indoors
52	5260	DFS	Indoors/DFS/TPC	DFS/TPC	No	Yes	DFS/TPC	Yes	Yes	Indoors	DFS/TPC	Indoors	Indoors	No	DFS/TPC
56	5280	DFS	Indoors/DFS/TPC	DFS/TPC	No	Yes	DFS/TPC	Yes	Yes	Indoors	DFS/TPC	Indoors	Indoors	Yes	DFS/TPC
60	5300	DFS	Indoors/DFS/TPC	DFS/TPC	No	Yes	DFS/TPC	Yes	Yes	Indoors	DFS/TPC	Indoors	Indoors	Yes	DFS/TPC
64	5320	DFS	Indoors/DFS/TPC	DFS/TPC	No	Yes	DFS/TPC	Yes	Yes	Indoors	DFS/TPC	Indoors	Indoors	Yes	DFS/TPC
100	5500	DFS	DFS/TPC	DFS/TPC	No	No	No	No	Yes	DFS/TPC	DFS/TPC	Yes	DFS	Yes	DFS/TPC
104	5520	DFS	DFS/TPC	DFS/TPC	No	No	No	No	Yes	DFS/TPC	DFS/TPC	Yes	DFS	Yes	DFS/TPC
108	5540	DFS	DFS/TPC	DFS/TPC	No	No	No	No	Yes	DFS/TPC	DFS/TPC	Yes	DFS	Yes	DFS/TPC
112	5560	DFS	DFS/TPC	DFS/TPC	No	No	No	No	Yes	DFS/TPC	DFS/TPC	Yes	DFS	Yes	DFS/TPC
116	5580	DFS	DFS/TPC	DFS/TPC	No	No	No	No	Yes	DFS/TPC	DFS/TPC	Yes	DFS	Yes	DFS/TPC
120	5600	No	DFS/TPC	DFS/TPC	No	No	No	No	Yes	DFS/TPC	No	Yes	DFS	Yes	DFS/TPC
124	5620	No	DFS/TPC	DFS/TPC	No	No	No	No	Yes	DFS/TPC	No	Yes	DFS	Yes	DFS/TPC
128	5640	No	DFS/TPC	DFS/TPC	No	No	No	No	Yes	DFS/TPC	No	Yes	DFS	Yes	DFS/TPC
132	5660	DFS	DFS/TPC	DFS/TPC	No	No	No	No	No	DFS/TPC	DFS/TPC	Yes	DFS	Yes	DFS/TPC
136	5680	DFS	DFS/TPC	DFS/TPC	No	No	No	No	No	DFS/TPC	DFS/TPC	Yes	DFS	Yes	DFS/TPC
140	5700	DFS	DFS/TPC	DFS/TPC	No	No	No	No	No	DFS/TPC	DFS/TPC	Yes	DFS	Yes	DFS/TPC
149	5745	Yes	No, but in study, SRD (25mW)	No	No	Yes	Yes	No	Yes	No	Yes	No	Yes	Yes	Yes
153	5765	Yes	No, but in study, SRD (25mW)	No	No	Yes	Yes	No	Yes	No	Yes	No	Yes	Yes	Yes
157	5785	Yes	No, but in study, SRD (25mW)	No	No	Yes	Yes	No	Yes	No	Yes	No	Yes	Yes	Yes
161	5805	Yes	No, but in study, SRD (25mW)	No	No	Yes	Yes	No	Yes	No	Yes	No	Yes	Yes	Yes
165	5825	Yes	No, but in study, SRD (25mW)	No	No	Yes	Yes	No	Yes	No	Yes	No	Yes	Yes	Yes

Regulatory Domains

IEEE uses the phase "regdomain" to refer to a legal regulatory region. Different countries define different levels of allowable transmitter power, time that a channel can be occupied, and different available channels. The following countries have domain codes: United States, Canada, ETSI (Europe), Spain, France, Japan, and China. Most Wi-Fi devices default to regdomain 0 or the least common denominator settings. In this case, the device will not transmit a a power above the allowable power in any nation nor will it use frequencies that are not permitted in any nation. The regdomain settings are often difficult or impossible to change so end users do not conflict with the local regulator agencies such as the United States' Federal Communications Commision.