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4.2 | |
Signals and Noise | |
| 4.2.2 | |
Attenuation and insertion loss on copper media |
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Attenuation is the decrease in signal
amplitude over the length of a link. Long cable lengths and high
signal frequencies contribute to greater signal attenuation. For this
reason, attenuation on a cable is measured by a cable tester using the
highest frequencies that the cable is rated to support. Attenuation is
expressed in decibels (dB) using negative numbers. Smaller negative dB
values are an indication of better link performance.
There are several factors that contribute to attenuation. The resistance of the copper cable converts some of the electrical energy of the signal to heat. Signal energy is also lost when it leaks through the insulation of the cable and by impedance caused by defective connectors. Impedance is a measurement of the resistance of the cable to alternating current (AC) and is measured in ohms. The normal, or characteristic, impedance of a Cat5 cable is 100 ohms. If a connector is improperly installed on Cat5, it will have a different impedance value than the cable. This is called an impedance discontinuity or an impedance mismatch. Impedance discontinuities cause attenuation because a portion of a transmitted signal will be reflected back to the transmitting device rather than continuing to the receiver, much like an echo. This effect is compounded if there are multiple discontinuities causing additional portions of the remaining signal to be reflected back to the transmitter. When this returning reflection strikes the first discontinuity, some of the signal rebounds in the direction of the original signal, creating multiple echo effects. The echoes strike the receiver at different intervals making it difficult for the receiver to accurately detect data values on the signal. This is called jitter and results in data errors. The combination of the effects of signal attenuation and impedance discontinuities on a communications link is called insertion loss. Proper network operation depends on constant characteristic impedance in all cables and connectors, with no impedance discontinuities in the entire cable system.
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