Ralink 150N wireless adapter User Manual page 84

12. 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.
13. 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).
14. What is WMM?
Wi-Fi Multimedia (WMM), a group of features for wireless
networks that improve the user experience for audio, video and
voice applications. WMM is based on a subset of the IEEE 802.11e
WLAN QoS draft standard. WMM adds prioritized capabilities to
Wi-Fi networks and optimizes their performance when multiple
concurring applications, each with different latency and throughput
requirements, compete for network resources. By using WMM,
end-user satisfaction is maintained in a wider variety of
environments and traffic conditions. WMM makes it possible for
home network users and enterprise network managers to decide
which data streams are most important and assign them a higher
traffic priority.
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