Cisco Systems LINKSYS WAP54G Guide D'installation Rapide page 102

Wireless-G Access Point
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 as the Access Point of the 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).
Appendix A: Troubleshooting
Frequently Asked Questions
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