|  | 4.1 |
| Background for Studying Frequency-Based Cable Testing
|  | 4.1.7 |
| Noise in time and frequency
Noise is an important concept in communications systems, including LANS. While noise usually refers to undesirable sounds, noise related to communications refers to undesirable signals. Noise can originate from natural and technological sources, and is added to the data signals in communications systems. All communications systems have some amount of noise. Even though noise cannot be eliminated, its effects can be minimized if the sources of the noise are understood. There are many possible sources of noise: Nearby cables which carry data signals Radio frequency interference (RFI), which is noise from other signals being transmitted nearby Electromagnetic interference (EMI), which is noise from nearby sources such as motors and lights Laser noise at the transmitter or receiver of an optical signal Noise that affects all transmission frequencies equally is called white noise. Noise that only affects small ranges of frequencies is called narrowband interference. When detected on a radio receiver, white noise would interfere with all radio stations. Narrowband interference would affect only a few stations whose frequencies are close together. When detected on a LAN, white noise would affect all data transmissions, but narrowband interference might disrupt only certain signals. If the band of frequencies affected by the narrowband interference included all frequencies transmitted on the LAN, then the performance of the entire LAN would be compromised.
|  | Interactive Media Activity
Interactivity: Noise - White and Narrowband This activity allows the user to enter a value for amplitude and a value for frequency and then generate the white noise and narrowband noise. Interactive Media Activity 4.1.7 is located in the CCNA 3.0 Bridge 1 Activities and Quizzes file. The Activities and Quizzes file can be downloaded in the Course Material section of your class homepage.