Electromagnetic flux is a potential problem that can disrupt network communications wherever there are active electrical and electronic devices. The selection of the right cabling and its network routing design is important to reduce communications interference problems. All network components, including the connectors and patch panels, must be designed to satisfactorily perform in the presence of external noise. Cable routing should conform to the manufacturer’s recommendations and always avoid potential interference sources. Likely office building sources of EMI are lift motors (elevators), automatic doors, and air-conditioning units. The older the equipment, the more likely it will produce EMI. Closed metal conduits and ducting for the cabling system will provide extra protection against EMI sources that cannot be corrected or avoided. Balanced transmission over UTP cable offers strong protection against external noise. In EMI-sensitive or hostile environments, the only solution may be optical fiber cable that is immune to external noise.
There are regulations specified by the FCC (part 68 and part 15 subpart) that cover telecommunications network electromagnetic compatibility (EMC) with other electronic devices. Network system installers and users are responsible for conforming to EMC guidelines. Installers must ensure that cable specifications for routing and ducting eliminate interference problems. Some manufacturers provide warranties on the EMC performance of certified installations using their cabling.
In addition to the potential for interference from external electrical and electronic source devices, the active pairs in a multipair cable can interfere with each other. Interference between cable pairs is known as crosstalk. Crosstalk measurements may be performed with two methods: pair-to-pair and PowerSum. The pair-to-pair method measures only the maximum interference caused by any other single active cable pair. Near end cross talk (NEXT), the pair-to-pair measurement metric, is defined as the signal coupled from one pair to another in a UTP cable. It is called NEXT because it measures the crosstalk at the end where one pair is transmitting (and the transmitted signal is largest and, hence, causes the most crosstalk). Crosstalk is minimized by the twists in the cable, with different twist rates causing each pair to act as antennas sensitive to different frequencies so that signals are not picked up from neighboring pairs. Keeping the twists as close as possible to the terminations minimizes crosstalk. Far end crosstalk (FEXT) measures the effect of signal coupling from one pair to another over the entire length of the cable, and it is measured at the far end.
Another frequently cited measurement associated with crosstalk is the attenuation to crosstalk (ACR) ratio. Attenuation is the reduction in signal strength due to loss in the cable. ACR measures how much “headroom” the signal should have at the receiver. It is important that the signal strength at the receiver end be high enough for reception by the network hub/switch to pass through to workstation nodes or other hubs/switches. Ethernet LANs send very high-speed signals through the cable, and the attenuation varies with the frequency of the signal. Attenuation tests are performed at several wavelengths, as specified in the 568 standards. The test requires a tester at each cable end, one to send and one to receive. The loss between the ends is calculated, recorded, and compared with pass/fail criteria for UTP cable at Category 3, 4, and 5 frequencies.
Performance losses can be greater than indicated by pair-to-pair measurement if there are several active pairs in a multipair cable strand. For this reason, the preferred method of measuring crosstalk is known as PowerSum. It is based on the measurements taken when all pairs in a multipair cable are active. This is the more realistic crosstalk measurement for Fast Ethernet and Gigabit Ethernet LANs, where all pairs are used to carry signals, often simultaneously. PowerSum is the recommended method to use for cables with more than four wires.
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