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
The 900-MHz band was the first area for which spread-spectrum
WLANs were developed. A nearby neighbor of the 900-MHz band was the cellular
phone band. This helped the early development of the WLAN industry in the
900-MHz band because of the availability of inexpensive, small RF components
developed for use in that industry. Because the WLAN and cellular phone
frequencies were very close, many components could be "borrowed" from the
fast-growing cellular industry.
The 900-MHz band had a couple of major drawbacks, however. It
was limited in its use worldwide, with only North America, some parts of South
America, Australia, and a handful of other smaller countries permitting WLAN
usage in the 900-MHz band. Another disadvantage of the 900-MHz band was the
limited bandwidth. Data rates were limited to 1 and 2 Mbps maximum because of
the limited frequency span that was available. Figure 3-2 depicts the overall bandwidth requirement at
900 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
The desire for higher data rates, more
scalability, and greater global deployment drove the development in the 2.4-GHz
band. This band was generally available in almost every major country worldwide.
Although it initially provided for data rates of only up to 2 Mbps, it did offer
more channel capability. Development of 2.4-GHz devices was encouraged by the
fact that the 2.4-GHz band had neighbors in the Personal
Communication Services (PCS) wireless systems as well as some radar
systems. The close frequencies meant that some of the RF component and
development costs could be shared among the different technologies. As the
industry started to invest into the 2.4-GHz technology, the IEEE was developing
a specification to provide interoperability for the new WLAN market.
In 1997, the IEEE completed the 802.11 specification, defining
data rates up to 2 Mbps for the 2.4-GHz band and setting down a channel scheme
that provided three nonoverlapping and noninterfering channels. In the North
American domain, there was a need to limit the upper channels because of a very
tight restriction for RF signals that fell outside the band at the top end of
the band. Therefore, there were only 11 channels specified. For the ETSI domain,
the upper-band restriction was not an issue, and 13 channels were defined. In
Japan, a very strict regulation limited WLAN usage to only a narrow section and
limited the number of channels to 1, and that channel was incompatible with any
of the ETSI or North American channels. Several years later, the Japan TELEC
changed the regulations, permitting operation of the 13 ETSI channels plus the
old single Japan channel, thus providing for 14 channels under the Japan
domain.
Because of the demand for higher data rates, the IEEE added an
amendment in 1999 to increase the data rate for 2.4-GHz 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).
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