Network Card User Manual

11. 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 is, 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.

12. What is Spread Spectrum?

Spread Spectrum technology is a wideband radio frequency technique

developed by the military for use in reliable, secure, mission-critical

communication 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).