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WLAN Frequencies of Operation

As mentioned previously, unlicensed WLANs fall into three basic frequency bands: 900 MHz, 2.4 GHz, and 5 GHz. Each has its own advantages and disadvantages, and each is broken down into channels or channel groups. The different regulatory domains have defined which frequencies and channels may be used, and these channels are in an ever-changing state. These bands are often referred to as the Industrial, Scientific, and Medical (ISM) bands. Figure 3-1 shows where these bands fall in the overall frequency spectrum.

900-MHz Frequency Band

MHz when running the various data rates. As you can see, running the higher data rates limits the number of channels to one that incorporates the entire band and severely limits scalability.

As the IEEE 802.11 specification was being developed, the IEEE recognized the deficiencies of this band and chose not to include it in the standard. For the same reason, this book concentrates on the 2.4- and 5-GHz bands, using the 900-MHz band only as a reference for historical information.

2.4-GHz Frequency Band

direct sequence (DS) systems to include 5.5 Mbps and 11 Mbps, which is known as the 802.11b specification. The number of channels did not change, and the new specification required that products be backward compatible to the older 1-Mbps and 2-Mbps 802.11 products.

Likewise, in 2003, the IEEE added another part to the 802.11 specifications. The 802.11g standard is yet another, even higher data-rate scheme in the 2.4-GHz band yielding rates as high as 54 Mbps, and again, requiring backward compatibility to the 802.11b specification.

Because the frequency scheme is identical between the initial 2.4-GHz 802.11, the 802.11b, and the 802.11g specifications, most countries that permitted operation for the early 2.4-GHz 802.11 devices also permitted the 802.11b and 802.11g products.

The 802.11 specification defines the channel scheme as being 22 MHz wide, starting with the center frequency of the first channel at 2.412 GHz. The center frequencies for the channels are spaced at 5-MHz intervals; this channel scheme results in two overlapping channels, as shown in Figure 3-3.

The 2.4-GHz channel overlap results in much confusion for many users. To many, the fact that there are 11 (or 13 or 14) channels available logically indicates that you can use a WLAN system on one channel in the same vicinity as another system on a different channel. Although this is true, the design engineer must be certain to use channels that are not overlapping.

Based on the defined channel scheme for both ETSI and North America, three nonoverlapping channels can be used in the same area with no interference between them. Although you may see papers written on the ability to use four or even five separate channels in the same area, by using channels that are slightly overlapping, the WLAN industry in general recommends the use of the three nonoverlapping channel scheme (see Figure 3-4).

Using the three nonoverlapping channels, you can reuse the channels in a rotating scheme and carefully define adjacent cells on channels that are noninterfering (see Figure 3-5).

5-GHz Frequency Band

The 5-GHz band was initially used in Europe for the ETSI HiperLAN specification, but traction for this technology never seemed to take a good foothold, and it was overtaken by the development of a competing 802.11 standard from the IEEE. The 802.11a specification, which was completed in 1999, defined several different channel groups within the 5-GHz band. Because of many varying regulations around the world with 5 GHz, the channel groups and area of permitted operation must be reviewed carefully.

There has been a lot of activity in the regulatory bodies concerning the 5-GHz WLAN bands recently. In 2003, there was a meeting of the world's regulatory bodies that discussed reworking many of these regulations and opening up new frequencies.

As mentioned, the 5-GHz band is broken down into several different channel groups. In the United States, these are referred to as the Unlicensed National Information Infrastructure (UNII) bands. The three bands or groupsUNII1, UNII2, and UNII3permit operation in the 5.215- to 5.225-GHz, 5.225- to 5.235-GHz, and 5.725- to 5.825-GHz frequency ranges, respectively. After the recent changes in regulations, a new band of frequencies are now available ranging from 5.470 to 5.725 GHz (see Figure 3-6).

When compared to 2.4 GHz, the 5 GHz offers at minimum eight channels. Although there is a slight overlap in the sidebands, the channels are typically referred to as nonoverlapping. Some installers believe it is fine to use adjacent channels in adjacent cells; however, it is recommended that when possible (and with the number of channels available, it is usually possible) to avoid adjacent channels in adjacent cells (see Figure 3-7).