In the world of telecommunications, we commonly encounter the terms "dB" and "dBm." But what do these terms mean?
These concepts are defined below.
What is a dB?
The decibel (symbol: dB) is a relative unit of measurement equal to one-tenth of a bel: 1 dB = 0.1 B. The definition of the decibel originated with the measurement of transmission loss and power in telephony in the early 20th century with the Bell System in the United States. The bel was named in honor of Alexander Graham Bell.
When referring to power measurements, decibels express the ratio of two power values on a logarithmic scale. This ratio can be expressed as ten times the base-10 logarithm of the ratio between the measured quantity and the reference value. Thus, the ratio of P (measured power) and P_0 (reference power) is represented by L_p, this ratio expressed in decibels, which is calculated using the formula:
This unit can express a relative change or an absolute value. In the latter case, the numerical value expresses the ratio of a value to a fixed reference value; when used in this way, the unit symbol usually has letter codes as suffixes indicating the reference value. For example, for the reference value of 1 watt, a common suffix is "W" (e.g., "30 dBW").
What is a dBm?
dBm or dBmW (decibel-milliwatts) is a level unit used to indicate that an (absolute) power level is expressed in decibels (dB) with reference to one milliwatt (mW). dBm is used in radio, microwave, and fiber optic communication networks as a convenient measure of absolute power due to its ability to express very large or small values in abbreviated form. Below is a table of dBm to mW conversions.
The decibel (dB) is used for a wide variety of measurements in science and engineering, most prominently in acoustics, electronics, and control theory. In electronics, amplifier gains, signal attenuation, and signal-to-noise ratios are often expressed in decibels.
In the case of fiber optic cable, dB is the comparison of the power injected at one end of the cable with the power received at the other end. For example, if the injected optical power was 1 dBm and the power received at the other end was -21 dBm, then the optical loss of the link would be = 1 - (-21) = 22 dB. The terms dB and dBm can also be found on test and measurement equipment such as power meters, OTDRs, light sources, among others.
