A very important concept in communications systems, including WLANs, is
noise. The word noise has a general meaning of undesirable sounds. However, in
the context of telecommunications, noise can be better defined as undesirable
voltages from both natural and technological sources. Since noise is just
another signal that produces waves, the noise will be added to other signals,
as previously discussed. If the affected signal is representing information in
a communications system, the noise can change the information. Clearly, this is
not acceptable.
Relevant to a WLAN, sources of noise in a WLAN include
the electronics in the WLAN system, plus radio frequency interference (RFI),
and electromagnetic interference (EMI) found in the WLAN environment. By
studying noise, network designers can find ways to reduce the effects of noise
on the WLAN system.
One form of noise is called Gaussian, or white noise.
The spectrum analyzer graph of white noise is a straight line across all
frequencies. Theoretically, Gaussian noise affects all different frequencies
equally. In actuality, white noise does not follow such a simple pattern.
However, this is still a very useful concept when studying communications
systems. Because white noise would affect all of the frequencies in a radio
signal equally, there are implications for both the transmitter and receiver
circuitry.
Another form of noise is called narrowband interference. The
term band refers to a grouping of frequencies. A narrowband has a relatively
smaller range of frequencies. FM radio is an example of narrowband
interference. While white noise would disturb all radio stations equally,
narrowband interference would only interfere with some radio stations.
Both forms of noise are important in understanding WLANs. Since white noise
would degrade the various channels equally, the various components of FHSS and
DSSS would be equally affected. Narrowband interference might disrupt only
certain channels or spread spectrum components. It might even be possible to
use a different channel to avoid the interference entirely.