dBm or dBmW (decibel-milliwatts) is a unit of level used to indicate that a power level is expressed in decibels (dB) with reference to one milliwatt (mW). It is used in radio, microwave and fiber-optical communication networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form compared to dBW, which is referenced to one watt (1000 mW). In audio and telephony, dBm is typically referenced relative to a 600-ohm impedance, while in radio-frequency work dBm is typically referenced relative to a 50-ohm impedance.