Signal-to-Noise Ratio Monitor
Monitoring Signal-to-Noise Ratio
Signal-to-noise ratio (SNR) is a measure of the level of a desired signal against the level of background noise, measured in decibels (dB). You can imagine this as a person trying to be heard in a noisy restaurant, where his voice is the signal and the background chatter blocks his voice. The SNR charts display the ratio between signal and background noise, the individual signal level, and the individual background noise level.
If you are viewing data for a time period longer than one hour, each data point on the graph represents data consolidated from more than one polling period. In this case, the graph shows multiple lines, which are labeled in the legend:
maxSnr—The largest value sampled during the consolidated polling periods.
snr—The average of the values sampled during the consolidated polling periods.
minSnr—The smallest value sampled during the consolidated polling periods.
The area between the maxSnr and minSnr lines is shaded to indicate the range of values.
You can perform the following actions on the SNR chart:
Change the time period over which to display the SNR by selecting a time period from the list in the upper right corner.
Display a numeric value by placing the cursor where a vertical grid line bisects a data line.
Highlight a line in the graph by mousing over the line’s legend.
Remove or restore a line by clicking its legend.
Click Details to view these three charts for a radio, one above the other:
Received signal strength indicator (RSSI), which measures the power of a radio signal
Signal to noise ratio (SNR) which measures a signal against the current RF background noise level
Noise floor which is the sum of all the noise interference for the radio—the current RF background noise level
Higher numbers on this chart indicate that a radio has more signal than noise, which is desirable. If the chart has more noise than signal (indicated by values less than 40 on the chart), the signal becomes more unreadable, because the noise level severely competes with it. A reading of 0-20 on this chart would indicate an unacceptable level of noise or a really low signal. This can cause a reduction in data speed because of frequent errors that require the source transmitter to resend data packets—see Monitoring the Percentage of RF Packet Retransmissions.
SNR is computed only for individual radios.
Signal-to-Noise Ratio Details
There are three charts in the Signal-to-Noise Ratio (SNR) Details window: RSSI, SNR, and Noise Floor. Table 1 briefly describes how these charts can be interpreted.
Table 1: Interpreting Signal-to-Noise Ratio Values
Signal-to-noise ratio is the ratio of a signal’s strength to the sum of all interference. (Signal-to-noise Ratio = RSSI/Noise Floor).
RSSI is signal strength, the first value used in the signal-to-noise ratio.
Noise is any signal (interference) that is not Wi-Fi traffic such as cordless phones, microwaves, radar, etc. This is the second value in the signal-to-noise ratio.
How is it measured?
SNR is the ratio of signal to background noise, measured as a positive value between 0 dB and 80 dB. You want the signal to be high and the background noise to be low. This produces a higher ratio, which is better.
RSSI is measured in decibels from -20 through -100.
Noise floor is measured in decibels from -90 through -120.
What does the chart mean?
If the chart has more noise than a signal (indicated by values less than 40 on the chart), the signal becomes more unreadable, because the noise level severely competes with it. A reading of 0-20 on this chart would indicate an unacceptable level of noise.
A louder signal is better, so the higher the RSSI is, the better. Typically voice networks require a better signal level than a data network does. Normal signal strength in a network would be around -45 dB through -87 dB.
A quiet noise floor is better, so the closer to -120 the noise floor is, the better because that means there is little to no interference. Typical environment noise floors are about 95 dB.
Deal with a low SNR reading by either increasing the signal (RSSI) or reducing the background noise. To get an idea what an acceptable SNR reading is for your network, check the values when the network is operating optimally—you might be able to do this by changing the time period on the chart.