What is RSSI?
According

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to 802.11-1999, Received Strength Signal Indicator (RSSI) is a PHY layer measure of the energy observed by the antenna used to receive the current PPDU - in other words, the relative strength of the physical RF carrier signal, as communicated to the MAC layer. To learn more about how RSSI is used, visit the IEEE website and follow links to the wireless zone and 802.11 standards.

This was first published in June 2003