/mcpAs is common in most electrical systems, power is measured in units of watts, abbreviated W. Since the power encountered in transmission systems is relatively small (compared to the power of a light bulb), power is usually expressed in milliwatts, abbreviated mW. 1 mW = 1 W = 0.001W = 10›W 1000 In transmission, the common interest is in power ratios rather than in absolute power. In addition, transmission is concerned with an extremely wide range of absolute power values. For these reasons, a convenient mathematical expression of relative power, the decibel (dB), is commonly used. In order to describe relative power in terms of decibels, you must define the reference point from which you measure. Based upon the transmission parameter that is measured, you can use different forms of decibel measurement. Each form of measurement has a specifically defined reference point. When you use the appropriate units of power related to specific references, you can measure absolute power, relative power, and power gains and losses. Milliwatt and Hertz Since the power in telephone circuits is small, the milliwatt is used as the basic power measurement unit, just as the foot is used as the basic measurement of length. Most measurements of absolute power in transmission are made in milliwatts or in units that are directly related to milliwatts. The frequencies that are used in testing usually fall within the voice frequency band. Commonly used pure (sine wave) test tones are 404 Hz, 1004 Hz, and 2804 Hz. (The 4−Hz offset is not always stated. However, actual test frequencies should be offset by 4 Hz in order to compensate for effects that some carrier facilities have on test tones.) A measurement of 1004 Hz is near the voice−band frequencies that carry much of voice power, 404 Hz is near the low end of the spectrum, and 2804 Hz is in the range of higher−frequency components of the voice spectrum that are important to the intelligibility of speech. In addition to pure test tones, "white noise" within specific frequency ranges is used for certain tests. White noise test tones are complex waveforms that have their power evenly distributed over the frequency range of