The ratio of the voltage at a point in a transmission system to an arbitrary voltage reference. The arbitrary reference point is specified by the CCITT (see standardization) as a point of zero level, and expressed in dBm0, i.e. decibels measured with reference to the zero relative level. Absolute measurements of received signal magnitudes or noise levels at various receivers may be compared by converting them to the zero relative values in order to assess the performance of the receiving circuits.