Local Electromagnetic Interference (EMI) is commonly known as noise. There
are four types of noise that are most significant to data networks:

- Impulse noise – voltage fluctuations or current spikes induced on the
cabling
- Random (white) noise distributed over the frequency spectrum
- Alien cross talk
- Near End Cross Talk (NEXT)
Random Noise may be generated by many sources ranging from wireless
communications such as FM radio stations, police radio, building security,
avionics for automated landing, and many more. A Spectrum Analyzer is needed to
determine the frequency characteristics and the magnitude of the noise signals
picked up by the cabling links. This laboratory type device can identify the
frequencies of the signals (noise) on the link as well as the relative
magnitude of the signals at the different frequencies.
Alien cross talk
is noise induced by other cables in the same pathway. The detection is the same
as those mentioned above. Anytime a UTP link is tested in a bundle of cables in
which some links are active, the chances are good that the tester will detect
alien cross talk, especially when the traffic is 100BASE-T. The tester will
report the "external noise detected" message. Typically, this alien
cross talk will not impact the reliability of the network traffic.
Excessive cross talk is usually reported in the Near End Cross Talk (NEXT)
test results on cable testing devices. NEXT can originate either inside or
outside the link. Crosstalk originating nearest the transmission source is
usually the loudest and has the greatest amplitude or leaks into the measured
pair. To minimize cross talk, ensure that the cable, at each termination point,
is left untwisted for no more than the allowed 13 mm (0.5 inch). Crosstalk
becomes correspondingly worse as more cable is untwisted.
The cross talk
occurring outside the link is actually cross talk from other adjacent cables,
or noise from a variety of sources. 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. This would include TV, radio, microwave, cell
phone towers, hand-held radios, and anything else that includes a transmitter
more powerful than a cell phone.