5.725–5.825 GHz: UNII-3, upper
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The IEEE 802.11 standard addresses the 2.4 GHz ISM band and the 5 GHz UNII bands. In the United States, the 2.4 GHz ISM band allows for 11 of 14 channels to be used for wireless LAN communications. The 5 GHz UNII band consists of three bands utilizing four frequency ranges. The three bands are UNII-1, the lower band; UNII-2 and UNII-2e, the middle bands; and UNII-3, the upper band. Table 4.3 shows unlicensed frequency bands and channels used by IEEE 802.11 wireless LAN technology.
TA b L e 4 . 3 IEEE 802.11 Frequency and Channel Allocations
Band Frequency Number of ChannelsISM 2.400–2.4835 GHz 11UNII-1 5.150–5.250 GHz 4UNII-2 5.250–5.350 GHz 4UNII-2e 5.470–5.725 GHz 11UNII-3 5.725–5.825 GHz 4ISM 5.725–5.850 1A chart of the United States Frequency Allocation is available from the National Telecommunications and Information Administration. To view this Understanding Radio Frequency (RF)
103Channels
As we have seen, radio frequency is divided into bands. These bands can be further sepa-rated into channels. One familiar application in which this is accomplished is television. Until over the air television became available in digital format, television was allocated certain frequency ranges. Common television channels operated in the very high frequency (VHF) band—for example, channels 2 through 13 operated from 54 through 216 MHz. This frequency range was divided into 12 channels, allowing optimal use of the frequency range for the application, in this case television signals. A viewer can change channels on a television to watch different programs running simultaneously. However, only one program can be viewed at any one time depending on which channel is currently selected. (Picture-in-picture televisions can show two channels at once on the screen, but each picture is still being received on a different channel.)Wireless LANs use channels in the same way. Certain unlicensed frequency ranges are allocated for wireless networking and those frequency ranges are subdivided into channels. In order for a transmitter and receiver to communicate with one another they must be on the same channel. The 2.4 GHz ISM band has a total of 14 channels available for wireless networking. The locale where they are used will determine which of the 14 channels can be legally used for wireless networking. In the United States, IEEE 802.11b/g/n wireless networks use 11 of the 14 channels available in the 2.4 GHz ISM band. Each of these 11 channels for DSSS are 22 MHz wide and channels for OFDM are 20 MHz wide. Simple mathematics show there will be overlap in order to accommodate all of the 20 MHz or 22 MHz wide channels in this frequency range. Figure 4.8 shows the 14 channels in the 2.4 GHz range.
F i g u R e 4 . 8 Channels in the 2.4 GHz ISM band
