Signal to Noise Ratio (SNR) is defined as the ratio of the transmitted power from the AP to the ambient (noise floor) energy present. To calculate the SNR value, we add the Signal Value to the Noise Value to get the SNR ratio. A positive value of the SNR ratio is always better.
Here is an example to tie together this information to come up with a very simple RF plan calculator for a single AP and a single client.
+ Access Point Power = 20 dBm
+ 50 foot antenna cable = - 3.35 dB Loss
+ Signal attenuation due to glass wall with metal frame = -6 dB
+ External Access Point Antenna = + 5.5 dBi gain
+ RSSI at WLAN Client = -75 dBm at 100ft from the AP
+ Noise level detected by WLAN Client = -85 dBm at 100ft from the AP
Based on the above, we can calculate the following information.
+ EIRP of the AP at source = 20 - 3.35 + 5.5 = 22.15 dBm
+ Transmit power as signal passes through glass wall = 22.15 - 6 = 16.15 dBm
+ SNR at Client = -75 + -85 = 10 dBm (difference between Signal and Noise) Reference:
https://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/Borderless_Networks/Unified_Access/ CMX/CMX_RFFund.html Receive Signal Strength Indicator (RSSI) is a measurement of how well your device can hear a signal from an access point or router. It's a value that is useful for determining if you have enough signal to get a good wireless connection.
EIRP tells you what's the actual transmit power of the antenna in milliwatts.
dBm is an abbreviation for "decibels relative to one milliwatt," where one milliwatt (1 mW) equals
1/1000 of a watt. It follows the same scale as dB. Therefore 0 dBm = 1 mW, 30 dBm = 1 W, and -
20 dBm = 0.01 mW