Signal-to-noise ratio (SNR) is just what it sounds like - a comparison between the strengths of a radio's transmission signal and noise. For example, when signal strength is -37 dBm and noise is -100 dBm, SNR is 63. According to the CWNA Study Guide, an SNR of at least 22 is considered viable if absolute signal strength is larger than the recipient's receive threshold and conditions are relatively stable.
Many 802.11 cards come with client utilities that let you view a nearby AP's signal, noise, and SNR. Some can write these numbers to a log at regular intervals. Freeware discovery tools like NetStumbler usually display and log these values for all discovered APs. WLAN analyzers like AiroPeek and AirMagnet can also monitor signal and noise, plotting these values on charts over time and analyzing results to help you spot problems or changes in comparison to baseline measurements. In some cases, a noise source can be identified rather easily - for example, co-channel interference from another AP operating nearby. More difficult cases may require true RF spectrum analysis tools like Yellowjacket.
This was first published in November 2003