Introduction to the Technology
802.11 is a standards
group under the IEEE that develops standards related to wireless and
wired Ethernet transmission. This includes the actual physical layer,
including 802.11a and 802.11b modulation schemes.
802.11b is a Direct
Sequence Spread Spectrum technology that, in the United States,
occupies 11 channels centered on frequencies in the Industrial,
Scientific, and Medical (ISM) band from 2.412 to 2.462GHz, in 5MHz
steps. The spectrum used by a single 802.11b signal is 22MHz wide.
Because the channels are smaller than the occupied bandwidth, only
three channels (1, 6, and 11) can be used in a small area without
running into interference.
802.11a
doesn't use Direct Sequence. Instead it uses a
modulation scheme called Orthogonal
Frequency Division Multiplexing (OFDM). OFDM uses 52 300KHz-wide
carriers grouped into one 20MHz-wide channel. The slower symbol speed
of OFDM and the forward error correction incorporated into 802.11a
make it more resilient to multi-path and interference. However,
because 802.11a is broadcast at more than double the frequency of
802.11b, there is greater free space loss. An 802.11a installation
with gain antennas and powered transmitters has a signal strength
that is about 18 percent weaker than that of a similar 802.11b setup.
While 802.11b occupies the portion of the ISM band at 2.4GHz, 802.11a
can occupy either the ISM band at 5.8GHz (5.725 to 5.850GHz), or a
section of spectrum known as the Unlicensed National Information
Infrastructure (U-NII) band. This band was approved in 1997 and
promoted by the group WINForum, which was made up of individuals and
companies such as Apple Computer.
The band takes up 300MHz of spectrum and is divided into three 100MHz
sections. The first two are next to each other, and the third is
375MHz up from the top of the second band. The
"low" band runs from 5.15GHz to
5.25GHz, the "middle" band runs
from 5.25GHz to 5.35GHz, and the
"high" band runs from 5.725 GHz to
5.825 GHz.
Sections
