Hawking 802.11b, User Manual

14 

9.  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, sta tistical 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. 

10. 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 b eing 

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).