Cisco WIRELESS ACCESS POINT WAP4410N - Manual (Page 57)

Troubleshooting
WAP4410N Wireless-N Access Point with Power Over Internet Administration Guide 57
A
Q. What is the ISM band?
The FCC and their counterparts outside of the U.S. have set aside
bandwidth for unlicensed use in the ISM (Industrial, Scientific and Medical)
band.
Spectrum in the vicinity of 2.4 GHz, in particular, is being made available
worldwide. This presents a truly revolutionary opportunity to place
convenient high speed wireless capabilities in the hands of users around
the globe.
Q. What is Spread Spectrum?
Spread Spectrum technology is a wideband radio frequency technique
developed by the military for use in reliable, secure, mission-critical
communications systems. It 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 trade-off 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 main alternatives, Direct Sequence Spread Spectrum (DSSS)
and Frequency Hopping Spread Spectrum (FHSS).
Q. What is DSSS? What is FHSS? And what are their differences?
Frequency Hopping Spread Spectrum (FHSS) uses a narrowband carrier
that changes frequency in a pattern that is 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 (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. 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.