There are three general types of noise cause of network disruptions:

• Impulse noise that is more commonly referred to as voltage or current spikes induced on the cabling.
• Random (white) noise distributed over the frequency spectrum.
• Alien crosstalk (crosstalk from one cable to another adjacent cable).

Of the three, impulse noise is most likely to cause network disruptions. Most cable analyzers have impulse noise test capabilities. The 802.3 standard set the default threshold level for the detection of impulse noise at 264 mV in Clause 14.4.4.

For higher-speed network applications such as 1000BASE-T, the threshold value for impulse noise detection is 40 mV in Clause 40.7.6. If there are very few pulses at this threshold level (less than 1 in 100 seconds), the cabling will be able to deliver very good support.

Impulse and random noise sources include nearby electric cables and devices, usually with high current loads. These may include large electric motors, elevators, photocopiers, coffee makers, fans, heaters, welders, compressors, and so on.

Another less obvious source is radiated emissions from transmitters, including: TV, radio, microwave, cell phone towers, hand-held radios, building security systems, avionics, and anything else that includes a transmitter more powerful than a cell phone.