Linksys WPC54GSV2 Benutzerhandbuch

32

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 
(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 narrowband receivers.