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A111 Wireless Card Adaptor 37
Chapter 5
Chapter 5 - Glossary
Glossary
COFDM (for 802.11a or 802.11g)
Signal power alone is not enough to maintain 802.11b-like distances in
an 802.11a/g environment. To compensate, a new physical-layer encoding
technology was designed that departs from the traditional direct-sequence
technology being deployed today. This technology is called COFDM
(coded OFDM). COFDM was developed specifically for indoor wireless
use and offers performance much superior to that of spread-spectrum
solutions. COFDM works by breaking one high-speed data carrier into
several lower-speed subcarriers, which are then transmitted in parallel.
Each high-speed carrier is 20 MHz wide and is broken up into 52
subchannels, each approximately 300 KHz wide. COFDM uses 48 of
these subchannels for data, while the remaining four are used for error
correction. COFDM delivers higher data rates and a high degree of
multipath reflection recovery, thanks to its encoding scheme and error
correction.
Each subchannel in the COFDM implementation is about 300 KHz wide.
At the low end of the speed gradient, BPSK (binary phase shift keying) is
used to encode 125 Kbps of data per channel, resulting in a 6,000-Kbps,
or 6 Mbps, data rate. Using quadrature phase shift keying, you can double
the amount of data encoded to 250 Kbps per channel, yielding a 12-
Mbps data rate. And by using 16-level quadrature amplitude modulation
encoding 4 bits per hertz, you can achieve a data rate of 24 Mbps. The
802.11a/g standard specifies that all 802.11a/g-compliant products must
support these basic data rates. The standard also lets the vendor extend
the modulation scheme beyond 24 Mbps. Remember, the more bits per
cycle (hertz) that are encoded, the more susceptible the signal will be to
interference and fading, and ultimately, the shorter the range, unless
power output is increased.
Default Key
This option allows you to select the default WEP key. This option allows you
to use WEP keys without having to remember or write them down. The WEP
keys generated using the Pass Phrase is compatible with other WLAN products.
The Passphrase option is not as secure as manual assignment.
Device Name
Also known as DHCP client ID or network name. Sometimes provided by an
ISP when using DHCP to assign addresses.