The decibel (dB) is a relative scale used to represent the increase or decrease of signal strength. Because the decibel is logarithmic and therefore a dimensionless unit, we apply the unit dBm as an absolute point of reference.
This means that signal strength is measured in reference to one milliwatt, and can be calculated as ten times the log of the signal strength in milliwatts. Unit is expressed as a negative number. The dB simply quantifies the ratio between two values, and those values are measured by dBm. So once the dBm has been established, only then can you measure any change in decibels (dB).
Its capability to express values both large and small in short form makes dBm the measurement of choice in radio, microwave, and fiber optic networks. In the world of cellular signal boosters, dBm measurements represent the strength of a signal at any given location, as well as the amount of power an antenna is capable of amplifying.