A Milliwatt test is a test method or test facility used in telecommunications to measure line quality and transmission loss between stations or points in an analog telephone system.

The test consists of transmitting an analog sinusoidal signal at the frequency of 1004 Hz

Hertz

The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

 with the power level of 0 (zero) dBm

DBm

dBm is an abbreviation for the power ratio in decibels of the measured power referenced to one milliwatt . It is used in radio, microwave and fiber optic 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...

. By definition, this is the equivalent of a continuous power dissipation of 1 mW (milliWatt

Watt

The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

), i.e., the power consumed if a voltage of 0.775 V

Volt

The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...

(RMS

Root mean square

In mathematics, the root mean square , also known as the quadratic mean, is a statistical measure of the magnitude of a varying quantity. It is especially useful when variates are positive and negative, e.g., sinusoids...

) is applied to a telephone line with 600 Ohm

Ohm

The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...

 nominal impedance.

In the Bell System

Bell System

The Bell System was the American Bell Telephone Company and then, subsequently, AT&T led system which provided telephone services to much of the United States and Canada from 1877 to 1984, at various times as a monopoly. In 1984, the company was broken up into separate companies, by a U.S...

, central offices provided this type of service on a dedicated telephone number (102 type Milliwatt line) for remote subscriber line testing. In conjunction, a second line (type 100 line) provided quiet termination (cf. loop around

Loop around

A loop line or loop around is a telephone company test circuit. The circuit has two associated phone numbers. When one side of the loop is called , the caller receives a test tone of approximately 1000 Hz...

). Various types of test lines were called "100", "102", "104" etc because these numbers accessed the test line in tandem offices in lieu of an area code.

In digital central office installations, the Milliwatt test facility was implemented using a synthesized version of the 1004 Hz signal, see digital milliwatt

Digital milliwatt

In digital telephony, the digital milliwatt is a standard test signal consisting of eight 8-bit words corresponding to one pulse-code modulated cycle of a sinusoidal signal approximately 1 kHz in frequency and one milliwatt in power....

.