A mathematical way of representing power ratios, such as signal loss within a circuit or a portion of a network’s cabling system. A decibel is the ratio of two values that measure signal strength, such as voltage, current, or power.
This ratio is expressed logarithmically using base 10 logarithms. In mathematical terms, this means that the decibel (dB) is defined as follows, where P1 and P2 are the power (signal strength) measurements:
dB = 10 log10 (P1/P2)
Decibels are used in network cabling systems for measuring signal losses. In addition, quantities such as attenuation and near-end crosstalk (NEXT) for fiber-optic cabling are expressed in units that contain decibels. In this scenario, P1 is the strength of the signal when it enters the cabling system, and P2 is its strength at some later point, after it has traversed segments of cable, repeaters, connectors, and other cabling system components. The following table shows signal strength ratios expressed both as ratios and as decibels for conversion purposes.
| Signal Strength Ratio (P1:P2) | Decibels (dB) |
| 1:1 (no signal loss) | 0 dB |
| 2:1 (50 percent signal loss) | -3 dB |
| 4:1 (75 percent signal loss) | -6 dB |
| 10:1 (90 percent signal loss) | -10 dB |
| 100:1 (99 percent signal loss) | -20 dB |
| 1000:1 (99.9 percent signal loss) | -30 dB |
The category 5 cabling version of unshielded twisted-pair (UTP) cabling has an attenuation rating of 30 dB/1000 feet.
This means that after traveling 1000 feet along a UTP cable, the electrical strength of the signal typically diminishes by 99.9 percent and is only 0.1 percent of its original strength at the far end of the cable.