SNR
Signal-to-Noise Ratio
How far your signal rises above the background noise. A loud signal in a quiet room beats a louder one in a noisy crowd — which is why SNR often predicts real speed better than signal strength alone.
SNR — signal-to-noise ratio — measures how far a desired signal stands above the background noise floor, expressed in decibels (dB). Think of a conversation: it is not just how loudly the other person speaks (signal strength) but how loud they are relative to the surrounding din. A clear voice in a quiet room is easy to follow; the same voice in a noisy bar is not. SNR captures exactly that relationship for radio and broadband signals.
How it is calculated
SNR is simply the gap between the signal level and the noise floor, both measured in dBm:
SNR (dB) = signal (dBm) − noise floor (dBm)
Example:
signal = -55 dBm
noise floor = -90 dBm
SNR = -55 − (-90) = 35 dBThat 35 dB is the margin your signal has over the noise. The larger the margin, the more reliably and quickly data can be decoded.
What counts as good
| SNR | Quality | Expect |
|---|---|---|
| 40 dB and up | Excellent | Maximum data rates |
| 25–40 dB | Good | Fast, reliable for everything |
| 15–25 dB | Marginal | Usable; speeds reduced |
| Below 15 dB | Poor | Slow, unstable, frequent retries |
Why it beats raw signal strength
A device can show a strong RSSI yet still perform poorly if the airwaves are full of interference. Imagine -55 dBm of signal sitting on top of a -65 dBm noise floor: that is only 10 dB of headroom, and the link will struggle despite the "strong" signal. Move to a spot with the same -55 dBm signal but a quiet -90 dBm noise floor and you have 35 dB of headroom and excellent throughput. Because SNR accounts for both the signal and the noise, it more directly reflects the data rate a link can sustain — which is why Wi-Fi adapters effectively pick their speed based on it.
How to improve SNR
There are only two levers, and both raise the margin:
- Increase the signal: move closer to the router, improve placement, add an access point, or use beamforming-capable gear.
- Reduce the noise: switch to a less congested channel or the cleaner 5 GHz band, and move away from interference sources like microwaves, Bluetooth devices, and crowded neighbouring networks.
SNR beyond Wi-Fi
The concept is universal. DSL connections report an SNR margin that determines the maximum sync speed a copper line can hold — a low margin causes the line to drop to a slower, more stable rate. Cable and fiber links have their own signal-quality measures rooted in the same idea. Wherever a signal must be pulled out of noise, SNR is the number that decides how fast and how reliably it can be done.