Products conforming to the 802.11a standard will let WLANs achieve data
rates as high as 54 Mbps. IEEE 802.11a devices operate in the 5 GHz frequency
range.
It is from this
higher frequency that the standard gains some of its performance. The rest
comes from the combination of the coding and modulation techniques used.
802.11a moved to a wider frequency (5 GHz) in part to get the higher speeds,
but also to avoid interference issues in the more crowded 2.4 GHz band. In
addition to 802.11b WLANs, HomeRF LANs, Bluetooth devices, cordless phones, and
even microwave ovens all operate in the 2.4 GHz band.
The benefits of
using the 5 GHz spectrum are offset somewhat by the lack of backwards
compatibility with the current generation of 802.11b LANs, because the
frequencies do not match. Many vendors are addressing this problem by producing
dual-mode products that actually contain two radios, one that operates in the
2.4 GHz range, and one that operates in the 5 GHz range.
Orthogonal
Frequency Division Multiplexing (OFDM)
The IEEE 802.11a standard uses
orthogonal frequency division multiplexing, a technique that divides a
communications channel into a number of equally spaced frequency bands. OFDM
uses multiple subcarriers, of which there are 52, spaced 312.5 KHz apart. Data
is sent on 48 carriers simultaneously, with each subcarrier carrying a portion
of the user data. Four subcarriers are used as pilots. Subcarriers are
orthogonal (independent) of each other.
The time to transmit each bit
increases in proportion to the number of carriers. This makes the system less
sensitive to multipath interference, a major source of distortion. Figure
shows
the different coding and modulation schemes that are used by 802.11a, along
with the corresponding data rates.