In telecom, signal levels span an enormous range — from 50 watts at the transmitter to 0.000000000001 watts at the receiver. Using linear numbers would be impractical. Decibels compress this huge range into manageable numbers. dB is a ratio: 10·log10(P1/P2) dBm is absolute power referenced to 1 milliwatt: 10·log10(P/1mW)