> Cover Story> Full Story
IN THE SKY
The invisible technology options
There is a wide range of diverse, often incompatible,
technology options available for WLAN manufacturers to choose
from. This creates hindrances in interoperability. Here's
a look at the advantages and disadvantages of each.
Narrowband technology: A narrowband radio system transmits
and receives user information on a specific radio frequency.
The radio signal frequency is kept as narrow as possible just
to help pass the information efficiently. Undesirable crosstalk
between communication channels is avoided by carefully coordinating
different users on different channel frequencies.
In a radio system, privacy and non-interference are accomplished
by the use of separate radio frequencies. The radio receiver
filters out all radio signals except the ones on its designated
Spread spectrum technology: Most WLAN vendors build systems
that function on spread spectrum technology. It is a wideband
radio frequency technique originally developed by the military
for use in reliable and secure mission-critical communication
systems. Spread spectrum is designed to provide reliability,
integrity, and security, but at the cost of bandwidth efficiency.
It consumes more bandwidth than any narrowband transmission,
but the signal generated is louder and easier to detect. This
is only possible if the receiver knows the parameters of the
spread spectrum signal being broadcast. If the receiver is
not tuned to the right frequency, a spread spectrum signal
appears like background noise. There are two types of spread
spectrum radio technology. They are FHSS (Frequency Hopping
Spread Spectrum) and DSSS (Direct Sequence Spread Spectrum).
FHSS uses a narrowband carrier that changes frequency in a
pattern known to both transmitter and receiver. When the changing
pattern is properly synchronized across both ends, the net
effect is to maintain a single logical channel. To an unintended
receiver, FHSS appears to be short-duration impulse noise.
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.
But a long chip also needs more bandwidth.
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
ignored by most narrowband receivers.
Infrared Technology: Infrared (IR) systems use very high frequency-light
waves to carry data. The waves are just below visible light
in the electromagnetic spectrum. IR cannot penetrate opaque
objects and is either direct (line-of-sight) or diffuse technology.
Inexpensive direct systems provide a very limited range up
to three feet, and are typically used for PANs (Personal Area
Networks). High performance direct technology is impractical
for mobile users and is only implemented on fixed sub networks.
Das Gupta can be reached at email@example.com