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Chapter 5 - Glossary
The 802.11b spectrum is plagued by saturation from wireless phones, microwave
ovens and other emerging wireless technologies, such as Bluetooth. In contrast,
802.11a spectrum is relatively free of interference.
The 802.11a standard gains some of its performance from the higher frequencies
at which it operates. The laws of information theory tie frequency, radiated power
and distance together in an inverse relationship. Thus, moving up to the 5-GHz
spectrum from 2.4 GHz will lead to shorter distances, given the same radiated
power and encoding scheme.
Compared with 802.11g: 802.11a is a standard for access points and radio NICs
that is ahead of 802.11g in the market by about six months. 802.11a operates in the
5GHz frequency band with twelve separate non-overlapping channels. As a result,
you can have up to twelve access points set to different channels in the same
area without them interfering with each other. This makes access point channel
assignment much easier and significantly increases the throughput the wireless
LAN can deliver within a given area. In addition, RF interference is much less likely
because of the less-crowded 5 GHz band.
IEEE 802.11b (11Mbits/sec)
In 1997, the Institute of Electrical and Electronics Engineers (IEEE) adopted the
802.11 standard for wireless devices operating in the 2.4 GHz frequency band. This
standard includes provisions for three radio technologies: direct sequence spread
spectrum, frequency hopping spread spectrum, and infrared. Devices that comply
with the 802.11 standard operate at a data rate of either 1 or 2 Mbps.
In 1999, the IEEE created the 802.11b standard. 802.11b is essentially identical
to the 802.11 standard except 802.11b provides for data rates of up to 11 Mbps
for direct sequence spread spectrum devices. Under 802.11b, direct sequence
devices can operate at 11 Mbps, 5.5 Mbps, 2 Mbps, or 1 Mbps. This provides
interoperability with existing 802.11 direct sequence devices that operate only at 2
Mbps.
Direct sequence spread spectrum devices spread a radio signal over a range of
frequencies. The IEEE 802.11b specification allocates the 2.4 GHz frequency band
into 14 overlapping operating Channels. Each Channel corresponds to a different
set of frequencies.
IEEE 802.11g
802.11g is a new extension to 802.11b (used in majority of wireless LANs today)
that broadens 802.11b's data rates to 54 Mbps within the 2.4 GHz band using
OFDM (orthogonal frequency division multiplexing) technology. 802.11g allows
backward compatibility with 802.11b devices but only at 11 Mbps or lower,
depending on the range and presence of obstructions.
IEEE 802.11n
802.11n builds upon previous 802.11 standards by adding MIMO (multiple-input
multiple-output). MIMO usesmultiple transmitter and receiver antikennas to allow
for increased data throughout via sspacial multiplexing and increased range by
exploiting the spacial diversity. The real data throughout is estimated to reach a
theoretical 300 Mbps, and should be up to 30 times faster than 802.11b, and up to
6 times faster than 802.11g.
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