Published by: Research & Development Department, Technologie Optic.ca Inc., December 2025
Accurate interpretation of signal power and signal loss is fundamental in optical fiber and wireless communication systems. Two units are commonly encountered in technical documentation and field measurements: dB (decibel) and dBm (decibel-milliwatt). Although they are closely related and often used together, they describe fundamentally different physical concepts. Confusing these units can lead to incorrect link budgets, misinterpreted measurements, and flawed system designs. This article provides a clear and rigorous explanation of the difference between dB and dBm, followed by a discussion of watt-based power units and metric prefixes.
The Decibel (dB): A Relative Measure
The decibel (dB) is a dimensionless logarithmic unit that expresses the ratio between two power levels. It does not represent an absolute value of power. Instead, it quantifies how much a signal has increased or decreased relative to another signal. In fiber-optic systems, dB is most commonly used to describe loss, gain, or attenuation. Mathematically, the decibel is defined as:
dB = 10log₁₀(P2 / P1)
where P1 is the reference power and P2 is the measured power. For example, if an optical transmitter launches a signal at –20 dBm and the receiver measures –21 dBm, the link loss is:
-20- (–21) = 1 dB
This means the signal power has been reduced by 1 dB during transmission. Because the decibel scale is logarithmic, it efficiently represents large variations in power. A 3 dB change corresponds approximately to a factor of two in power. A 10 dB loss corresponds to a tenfold reduction, while 20 dB corresponds to a hundredfold reduction. This logarithmic behavior makes dB particularly useful in long-distance fiber links and RF systems where power levels span many orders of magnitude.
The Decibel-Milliwatt (dBm): An Absolute Measure
Unlike dB, dBm is an absolute unit of power. It represents power referenced to 1 milliwatt (1 mW). By definition:
0 dBm=1 mW
Positive dBm values correspond to powers greater than 1 mW, while negative dBm values correspond to powers less than 1 mW. The conversion between dBm and watts is given by:
P(dBm)=10log10 (P(mW)/1 mW)
In optical communications, typical values are strongly negative. For instance, an LED source may output around –20 dBm, while laser or VCSEL-based test sources may operate near –10 dBm. In wireless systems, received signal strength indicators (RSSI) are often expressed in dBm, with values around –70 dBm considered strong and values below –100 dBm considered weak.
It is important to emphasize that dBm and dB are not directly convertible. dB measures a difference, while dBm measures an absolute level. dB can be used to compare two dBm values, but it cannot be converted into watts on its own.
Why Logarithmic Scales Matter
Both dB and dBm rely on logarithmic scaling. This means that small numerical changes correspond to large physical differences. For example, an increase from –80 dBm to –77 dBm may appear modest numerically, but it represents a doubling of received power. This property explains why signal coverage plots—such as distance versus received dBm from an antenna—show rapid performance degradation over distance, even when numerical changes appear small.
Power in Watts and Metric Prefixes
Although dBm is widely used for expressing signal levels in optical and wireless measurements, all power quantities ultimately derive from the SI base unit watt (W). Modern communication systems operate over an exceptionally wide dynamic range, spanning many orders of magnitude in power. For this reason, SI metric prefixes are essential for expressing both transmitted and received signal levels in a compact and interpretable form.
Positive metric prefixes denote power levels greater than one watt, while negative prefixes indicate fractional power levels below one watt. In practical communication systems, particularly in optical fiber and RF receivers, the detected power is typically extremely small. Received signals commonly fall within the milliwatt (mW), microwatt (µW), nanowatt (nW), or picowatt (pW) ranges, which correspond to strongly negative values when expressed in dBm.
It is important to emphasize that negative dBm values do not represent negative physical power. Instead, they indicate very small but strictly positive power levels referenced logarithmically to 1 mW, which is defined as 0 dBm. The use of metric prefixes in conjunction with logarithmic units such as dBm provides a consistent and scalable framework for describing signal power across the full operating range of modern communication systems.
| Multiples metric prefixes | Submultiple metric prefixes | ||||
|---|---|---|---|---|---|
| Power of 10 | Prefix | Symbol | Power of 10 | Prefix | Symbol |
| 10¹ | deca | da | 10⁻¹ | deci | d |
| 10² | hecto | h | 10⁻² | centi | c |
| 10³ | kilo | k | 10⁻³ | milli | m |
| 10⁶ | mega | M | 10⁻⁶ | micro | µ |
| 10⁹ | giga | G | 10⁻⁹ | nano | n |
| 10¹² | tera | T | 10⁻¹² | pico | p |
Conclusion
In summary, dB and dBm serve distinct but complementary roles in communication engineering. dB quantifies relative changes such as gain and loss, while dBm specifies absolute power levels referenced to 1 mW. Understanding their difference is essential for interpreting link budgets, analyzing attenuation, and designing reliable optical and wireless systems. When combined with watt-based units and metric prefixes, these logarithmic measures provide a compact and powerful framework for describing signal behavior across vast dynamic ranges.
Technologie Optic.ca Inc.