Channel sensing VS RSSI

Channel sensing is used in MAC protocols to optimize the Energy Efficiency vs Throughput when RSSI (Read Signal Strength Indicator) is used in any other type of receivers to indicate the level of the signal (usually just before the detector).

Thanks Vfone.
I want to know how does MAC perform Channel sensing measurement. Is it done by reading RSSI reading or is it done by some other method ?

To understand the difference from a standard level reading there are a few factors of channel sensing that should take into consideration: carrier sensing (coherently sensing), energy detection (non-coherently sensing) and preamble detection (using a matched filter).