8.3.Frequencies and Channels

• There are some country-specific variations, certain frequency ranges (or frequency bands ) have been reserved internationally for industrial, scientific, and medical purposes.

• These frequency bands are called the ISM bands , where ISM derives from industrial, scientific, and medical.

• Two of these bands are commonly used for WLANs.

  1. WLANs can use the range of frequencies in the 2.4-GHz to 2.5-GHz range (commonly referred to as the 2.4-GHz band )

  2. The 5.725-GHz to 5.875-GHz range (commonly referred

     to as the 5-GHz band ).

• Within each band are specific frequencies (or channels ) at which wireless devices operate.

2.4-GHz band

• Channel frequencies are separated

  by 5 MHz (with the exception of channel 14, which has 12 MHz of separation from

  channel 13).

• A single channel’s transmission can spread over a frequency range of 22 MHz.

• Channels must have five channels of separation (5 * 5 MHz = 25 MHz, which is greater than 22 MHz).

• Even though some countries use channel 14 as a nonoverlapping channel, it is not supported in the United States.

Channel	Frequency (GHz)	Recommended as a Nonoverlapping Channel
1	2.412	Yes
2	2.417	No
3	2.422	No
4	2.427	No
5	2.432	No
6	2.437	Yes
7	2.442	No
8	2.447	No
9	2.452	No
10	2.457	No
11	2.462	Yes
12	2.467	No
13	2.472	No
14	2.484	Yes (not supported in the United States)

5-GHz band

• The 5-GHz band has a higher number of channels, as compared to the 2.4-GHz band.

• Table below lists the recommended nonoverlapping channels for the 5-GHz band in the United States.

• Note that additional channels are supported in some countries.

Channel	Frequency (GHz)
36	5.180
40	5.200
44	5.220
48	5.240
52	5.260
56	5.280
60	5.300
64	5.320
100	5.500
104	5.520
108	5.540
112	5.560
116	5.580
136	5.680
140	5.700
149	5.745
153	5.765
157	5.785
161	5.805
165	5.825