SNR and decibel: understanding signal strength

In short: Decibels (dB) describe ratios logarithmically. In networks dBm (absolute power) and SNR (the gap between signal and noise) are the most important values.

In more detail: The logarithmic scale makes huge differences manageable: every 10 dB means a factor of 10 in power, 3 dB roughly a doubling.

In Depth

• dB: Ratio of two powers: 10 · log₁₀(P₁/P₂). +3 dB ≈ double, −3 dB ≈ half, +10 dB = tenfold.
• dBm: Power relative to 1 milliwatt. 0 dBm = 1 mW, 20 dBm = 100 mW (typical Wi-Fi transmit power), −70 dBm = weak but usable reception.
• Wi-Fi rules of thumb: −50 dBm very good, −67 dBm good for video, −75 dBm borderline, −85 dBm barely usable.
• SNR: Signal level minus noise level in dB. A high SNR allows dense modulation and thus high data rates. For DSL and cable it is a key quality value.
• Attenuation: On cables given in dB per length, in radio it grows with distance and obstacles.
• Antenna gain: In dBi, describes focusing, creates no extra energy.

See also: Wi-Fi channels and frequency bands