The test consists of transmitting an analog sinusoidal signal at the frequency of 1004 Hz with the power level of 0 (zero) dBm.
By definition, this is the equivalent of a continuous power dissipation of 1 mW (milliWatt), i.e., the power consumed if a voltage of 0.775 V(RMS) is applied to a telephone line with 600 Ohm nominal impedance.
[1] In the Bell System, central offices provided this type of service on a dedicated telephone number (102 type Milliwatt line) for remote subscriber line testing.
Various types of test lines were known as "100", "102", "104" etc., because these numbers accessed the test line in tandem offices in lieu of an area code.
[citation needed] In digital central office installations, the Milliwatt test facility was implemented using a synthesized version of the 1004 Hz signal, often known as the digital milliwatt.