RSSI in 802.11 implementations
RSSI can help determine when the level of radio energy in the channel is lower than a certain point so that the network card is clear to send (CTS). At that time the packet of data can be sent.
The network monitoring tool like NetSpot allows an end user to observe an RSSI value while measuring the signal strength of a WiFi network. Just for reference, Cisco Systems cards have an RSSI_Max value of 100 and will report back 101 power levels.
The 802.11 standard does not give any definition of how RSSI value and power level in mW or dBm relate. Chipset makers and vendors give their own accuracy, granularity, and range for the actual power (mW or dBm). Also the range of RSSI values is individual, ranging from 0 to RSSI_Max. RSSI value is obtained during the preamble stage of getting an 802.11 frame, not from the full frame.
To understand and use your signal strength measurements for the most efficient channel planning decisions, NetSpot shows signal strength in two ways.