WIFI RF BASICS
WiFi RSSI, SNR and Noise Floor Explained
Why signal strength alone does not tell you whether a wireless client has a usable RF link.
RSSI
RSSI is the received signal level reported by the client or survey system. More negative dBm values represent weaker received power. The correct design threshold depends on the client and application.
Noise floor
Noise floor is the background energy seen in the channel. A client can have apparently strong signal but poor performance if the noise floor is also high.
SNR
SNR is the difference between the received signal and the noise level. For example, a signal at -67 dBm with a noise floor at -92 dBm gives 25 dB SNR. Cisco uses that combination as an example target for a referenced voice deployment. It should not be treated as a universal target for every WiFi application.
Channel utilisation
Channel utilisation measures airtime occupancy. It can remain high even when your AP is quiet because neighbouring APs and interference share the medium. Add channel utilisation to RSSI/SNR when diagnosing slow wireless performance.
Survey rule
Define the client/application requirement first, then measure RSSI, SNR, channel utilisation and real application behaviour across the required areas and roaming paths.
Technical references
These sources support the standards, test-method and manufacturer-specific statements on this page. Project specifications and current manufacturer documentation take precedence for a real installation.