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4.2 | |
Signals and Noise | |
| 4.2.3 | |
Sources of noise on copper media |
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Noise is any electrical energy on the
transmission cable that makes it difficult for a receiver to interpret
the data sent from the transmitter. TIA/EIA-568-B certification of a
cable now requires testing for a variety of types of noise.
Crosstalk involves the transmission of signals from one wire to a nearby wire. When voltages change on a wire, electromagnetic energy is generated. This energy radiates outward from the transmitting wire like a radio signal from a transmitter. Adjacent wires in the cable act like antennas, receiving the transmitted energy, which interferes with data on those wires. Crosstalk can also be caused by signals on separate, nearby cables. When crosstalk is caused by a signal on another cable, it is called alien crosstalk. Crosstalk is more destructive at higher transmission frequencies. Cable testing instruments measure crosstalk by applying a test signal to one wire pair. The cable tester then measures the amplitude of the unwanted crosstalk signals induced on the other wire pairs in the cable. Twisted-pair cable is designed to take advantage of the effects of crosstalk in order to minimize noise. In twisted-pair cable, a pair of wires is used to transmit one signal. The wire pair is twisted so that each wire experiences similar crosstalk. Because a noise signal on one wire will appear identically on the other wire, this noise be easily detected and filtered at the receiver. Twisting one pair of wires in a cable also helps to reduce crosstalk of data or noise signals from an adjacent wire pair. Higher categories of UTP require more twists on each wire pair in the cable to minimize crosstalk at high transmission frequencies. When attaching connectors to the ends of UTP cable, untwisting of wire pairs must be kept to an absolute minimum to ensure reliable LAN communications.
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