Information Technology Reference
In-Depth Information
The unlicensed nature of wireless networking technology also means that the
user must tolerate legal interference sources. This means that you must carefully
design and site the network to avoid or minimize interference problems.
The term
spread spectrum
comes from a transmission method whereby the trans-
mitted signal is spread out among many frequencies, rather than based at just one
carrier frequency, as with conventional AM and FM transmission. Spread spectrum
operation was originally developed for military use, where radio and radar trans-
missions needed to be difficult to detect and relatively immune to detection or jam-
ming. This method can put the transmitted signal level just above the level of
ambient noise and uses a mathematical algorithm to allow the receiver to know
where to look for each signal increment.
Spread spectrum has many advantages in wireless networking applications. It
is inherently low power, and the transmission technique rejects most random inter-
ference. Multiple devices can share the same set of frequencies, simply by hopping
to them at different times. The current level of electronic technology makes the com-
plex RF and digital circuitry small and affordable.
Frequency-Spreading Variants: DSSS, QFDM, and FHSS
Several types of spread spectrum standards are available. The two most common are
direct sequence spread spectrum (DSSS), quadrature frequency-division multiplexing
(QFDM), and frequency-hopping spread spectrum (FHSS). Direct sequence (DSSS) is
the method used for IEEE 802.11b (WiFi) wireless networking at 2.4 GHz and yields
from 2 to 11 Mbps throughput. QFDM is used for 802.11a and 802.11g at 5 GHz
and 2.4 GHz, respectively, boosting the maximum data rate to 54 Mbps. FHSS is
used for Bluetooth and some older 802.11 equipment, both in the 2.4 GHz band.
In order to receive the spread-spectrum transmissions, the discrete transmit fre-
quencies and their exact sequence must be known. Also, a method of synchronizing
the beginning of the sequence must be allowed. Any system that meets the interna-
tional standards will use compatible methods of generating the sequence and recov-
ering the transmitted data.
Resistance to Interference
Spread spectrum transmission is inherently tolerant of random interference. If, at
any one instant an interfering signal is present on one of the hopping frequencies,
