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23. 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.
24. 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).
25. 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.
