MITSUBISHI ELECTRIC A111 - Manual (Page 40)

40 A111 Wireless Card Adaptor
Chapter 5
Chapter 5 - Glossary
Glossary
IEEE 802.11
IEEE 802.xx is a set of specifications for LANs from the Institute of Electrical
and Electronic Engineers (IEEE). Most wired networks conform to 802.3, the
specification for CSMA/CD based Ethernet networks or 802.5, the specification
for token ring networks. 802.11 defines the standard for wireless LANs
encompassing three incompatible (non-interoperable) technologies:
Frequency Hopping Spread Spectrum (FHSS), Direct Sequence Spread
Spectrum (DSSS), and Infrared. 802.11 specifies a carrier sense media
access control and physical layer specifications for 1 and 2 Mbps wireless
LANs.
IEEE 802.11a (54Mbits/sec)
Compared with 802.11b: The 802.11b standard was designed to operate in
the 2.4-GHz ISM (Industrial, Scientific and Medical) band using direct-
sequence spread-spectrum technology. The 802.11a standard, on the other
hand, was designed to operate in the more recently allocated 5-GHz UNII
(Unlicensed National Information Infrastructure) band. And unlike 802.11b,
the 802.11a standard departs from the traditional spread-spectrum
technology, instead using a frequency division multiplexing scheme that is
intended to be friendlier to office environments.
The 802.11a standard, which supports data rates of up to 54 Mbps, is the Fast Ethernet
analog to 802.11b, which supports data rates of up to 11 Mbps. Like Ethernet and Fast
Ethernet, 802.11b and 802.11a use an identical MAC (Media Access Control). However,
while Fast Ethernet uses the same physical-layer encoding scheme as Ethernet (only
faster), 802.11a uses an entirely different encoding scheme, called OFDM (orthogonal
frequency division multiplexing).
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.