Wireless LAN Technology Options
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.
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
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.
Figure 7. Frequency Hopping Spread Spectrum
Direct-Sequence Spread Spectrum Technology
Direct-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.
Figure 8. Direct Sequence Spread Spectrum
Infrared (IR) systems use very high frequencies, just below visible light
in the electromagnetic spectrum, to carry data. Like light, IR cannot penetrate
opaque objects; it is either directed (line-of-sight) or diffuse technology.
Inexpensive directed systems provide very limited range (3 ft) and typically
are used for PANs but occasionally are used in specific WLAN applications.
High performance directed IR is impractical for mobile users and is therefore
used only to implement fixed subnetworks. Diffuse (or reflective) IR WLAN
systems do not require line-of-sight, but cells are limited to individual