Cisco Systems Linksys WPC54GX4 Mode D'emploi page 41

Wireless-G Notebook Adapter with SRX400
What is roaming?
Roaming is the ability of a portable computer user to communicate continuously while moving freely throughout
an area greater than that covered by a single access point. Before using the roaming function, the workstation
must make sure that it is the same channel number with the access point of dedicated coverage area.
To achieve true seamless connectivity, the wireless LAN must incorporate a number of different functions. Each
node and access point, for example, must always acknowledge receipt of each message. Each node must
maintain contact with the wireless network even when not actually transmitting data. Achieving these functions
simultaneously requires a dynamic RF networking technology that links access points and nodes. In such a
system, the user's end node undertakes a search for the best possible access to the system. First, it evaluates
such factors as signal 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
Appendix A: Troubleshooting 36
Frequently Asked Questions