Wireless LAN Technology
Manufacturers of wireless LANs have a range of technologies to choose
from when designing a wireless LAN solution. Each technology comes with its own set of
advantages and limitations.
Narrowband Technology
A narrowband radio system transmits and receives user information on a
specific radio frequency. Narrowband radio keeps the radio signal frequency as narrow as
possible just to pass the information. Undesirable crosstalk between communications
channels is avoided by carefully coordinating different users on different channel
frequencies.A private telephone line is much like a radio frequency. When each home in a
neighborhood has its own private telephone line, people in one home cannot listen to calls
made to other homes. In a radio system, privacy and noninterference are accomplished by
the use of separate radio frequencies. The radio receiver filters out all radio signals
except the ones on its designated frequency. From a customer standpoint, one drawback of
narrowband technology is that the end-user must obtain an FCC license for each site where
it is employed.
Spread Spectrum Technology
Most wireless LAN systems use spread-spectrum technology, a wideband
radio frequency technique developed by the military for use in reliable, secure,
mission-critical communications systems. Spread-spectrum is designed to trade off
bandwidth efficiency for reliability, integrity, and security. In other words, more
bandwidth is consumed than in the case of narrowband transmission, but the tradeoff
produces a signal that is, in effect, louder and thus easier to detect, provided that the
receiver knows the parameters of the spread-spectrum signal being broadcast. If a receiver
is not tuned to the right frequency, a spread-spectrum signal looks like background noise.
There are two types of spread spectrum radio: frequency hopping and direct sequence.
Frequency-Hopping Spread Spectrum Technology
Frequency-hopping spread-spectrum (FHSS) uses a narrowband carrier that
changes frequency in a pattern known to both transmitter and receiver. Properly
synchronized, the net effect is to maintain a single logical channel. To an unintended
receiver, FHSS appears to be short-duration impulse noise.Direct-Sequence Spread Spectrum
TechnologyDirect-sequence spread-spectrum (DSSS) generates a redundant bit pattern for
each bit to be transmitted. This bit pattern is called a chip (or chipping code). The
longer the chip, the greater the probability that the original data can be recovered (and,
of course, the more bandwidth required). Even if one or more bits in the chip are damaged
during transmission, statistical techniques embedded in the radio can recover the original
data without the need for retransmission. To an unintended receiver, DSSS appears as
low-power wideband noise and is rejected (ignored) by most narrowband receivers.
Infrared Technology
See Free Space Optics (FSO)
