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| 3.2 |  | Characteristics of Physical Layer Optimization Problems |
| 3.2.3 |  | Noise |

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)

The default threshold level for the detection and registration of impulse noise is 270 mV (determined by 10BASE-T specification in the IEEE 802.3i standard). For high-speed network applications such as 1000BASE-T (Gigabit Ethernet), the recommended threshold value for impulse noise detection is 30 or 40 mV.

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 very 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.