Open Communication
802.11b uses complementary code
keying (CCK) to get 11 Mbps. This
method transmits eight bits per modulation symbol (carrier phase).
If the range and environment
conditions are not ideal, the speed of
the radio automatically backs off a
predetermined lower speed to ensure
reliable data transmission. This lower
speed is 5. 5 Mbps, also using CCK. If
conditions worsen, the speed drops
further to 2 Mbps, using DQPSK (
differential quadrature phase shift keying) or a low of 1 Mbps, using plain
old ultra reliable DBPSK (differential
binary phase shift keying).
As for range, it really varies
depending upon local conditions.
Normally, microwave transmissions
are strictly line-of-sight (LOS), meaning that the receive antenna must
"see" the transmitting antenna.
However, if sufficient power is available, the signal will punch through
walls, floors, ceilings, office dividers,
and even trees and other buildings.
Obviously, the signal is greatly weakened as it passes through these
obstacles.
Range is also a function of trans-
mitter power and the antenna. With
most equipment, you can rely upon a
range of about 100 feet radius from
the gateway antenna. This assumes
an omnidirectional antenna and the
standard 100 mW of transmitter
power. A directional gain antenna will
boost the range significantly. By
using more power and a gain antenna high up and in the clear, you can
achieve a range up to several miles.
In some rural communities, 802.11b
Wi-Fi networks, with higher power
and high-mounted outdoor antennas,
have been set up to give high speed
broadband Internet access to those
without cable or DSL lines.
A newer standard 802.11a was
ratified in 2001. It uses the FCC designated U-NII or Unlicensed National
Information Infrastructure band in
the 5 GHz range. This band is far less
crowded than the already over subscribed 2.4 GHz band. Besides
microwave ovens, cordless phones,
Bluetooth wireless devices, HomeRF
wireless networks, and now jillions of
Wi-Fi transceivers, it is a miracle that
any reliable communications can be
obtained in the 2.4 GHz band.
If interference is a problem as it
is for some, the 802.11a equipment
gives you all the benefits of a WLAN
and with even higher speed and virtually no interference. Using orthogonal frequency division multiplexing
(OFDM) — a form of wide band modulation — speeds up to 54 Mbps are
possible. The range is a bit less at 5
GHz but is satisfactory for most applications. The speed backs off to 48,
36, 22, or 11 Mbps, as conditions
deteriorate.
The most recent version of the
standard is 802.11g, which was ratified last year. This version specifies
operation in the 2.4 GHz band, but, by
using OFDM, it can achieve an upper
speed of 54 Mbps. And it is fully backwards compatible with 802.11b. This
option, which is now becoming available in most products, gives you
802.11a speed over 802.11b range.
While most networks and hot spots
still support only the 802.11b standard, many will soon upgrade to the
g version. And the a version will also
grow, especially in companies where
interference is a problem.
Chip companies are recognizing
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FIGURE 3. Agere Systems
WaveLAN chipset for
multimode 802.11a/b/g access
points and interface cards. The
WL54040 is the dual band
(2.4 and 5 GHz) transceiver
with direct conversion (zero IF)
for CCK operation and a low
IF for OFDM. The WL54240 is
a dual band power amplifier
with 24 dBm for 802.11g and
20 dBm for 802.11a. The
WL64040 is the multimode
processor that handles the A
to D and D to A conversions
between the transceiver chip
and the baseband processor.
This chip also does the CCK
and OFDM modulation and
demodulation. The WL60040
is the MAC baseband with its
own embedded processor and
provides the encryption, as
well as the interfaces to the
PCMCIA or CardBus, USB port
or other PC connection.
Courtesy Agere Systems.
24
FEBRUARY 2004
