Noise

Electrical noise is unwanted electrical signals that alter the shape of the signals transmitted on a LAN cable.  Signals that are severely distorted by noise can cause communication errors in a LAN.

Electrical noise is generated by any device that uses or generates voltages that vary over time.  Varying voltage generates a varying electromagnetic field, which transmits noise to nearby devices in the same way that a radio transmitter transmits signals to your radio.  For example, fluorescent lights, which use 50 to 60 Hz ac (alternating current) power, continuously radiate a 50 to 60 Hz signal that can be received by nearby devices as electronic noise.

LAN cables act as antennas that can pick up noise from fluorescent lights, electric motors, electric heaters, photocopiers, refrigerators, elevators, and other electronic devices.  Coaxial cable is far less susceptible to noise than twisted pair cable because it is shielded by a conductive sheath.  The Sheath is electrically grounded to prevent noise from reaching the inner conductor.