A level is an absolute or relative value of a physical quantity in a transmission system that is used to determine the voltages, currents or powers
. Absolute levels
represent the actual voltage, current or power present and are given the appropriate unit (V, A, W). Thelevel is often expressed as a logarithmic ratio in units of decibels
for 1 mW of power, the absolute power level can also be referred to this reference value. Thus, 10 mW corresponds to a dBm value of 10 dBm, 100 mW corresponds to 20 dBm, and 1 W corresponds to 30 dBm. The relative level is the ratio of two equal physical quantities to each other, without reference to the reference value. For voltages and currents it is 20 x log U1/U2, for power-related levels 10 x log P1/P2.