Rev. 1 - YML669
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NP5430 11g Wireless PC Card User Guide
Page 17
strength and quality, as well as the message load currently being carried by each access point and
the distance of each access point to the wired backbone. Based on that information, the node
next selects the right access point and registers its address. Communications between end node
and host computer can then be transmitted up and down the backbone.
As the user moves on, the end node's RF transmitter regularly checks the system to determine
whether it is in touch with the original access point or whether it should seek a new one. When a
node no longer receives acknowledgment from its original access point, it undertakes a new
search. Upon finding a new access point, it then re-registers, and the communication process
continues.
What is 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.
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).
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 nar-
rowband receivers.
Would the information be intercepted while transmitting on air?
WLAN features two-fold protection in security. On the hardware side, as with Direct Sequence
Spread Spectrum technology, it has the inherent security feature of scrambling. On the software
side, WLAN offers the encryption function (WEP) to enhance security and access control.
What is WEP?
WEP is Wired Equivalent Privacy, a data privacy mechanism based on a shared key algorithm, as
described in the IEEE 802.11 standard.
