Wi-Fi MAC is based on “listen before talking”, which is also a good leadership quote. While listening, the threshold to treat the medium as occupied differs for Wi-Fi signal (Rx sensitivity = -82 dBm) and non-Wi-Fi signal (Rx sensitivity + 20 dB =-62 dBm).
A Wi-Fi device infers whether a received signal is Wi-Fi or non-Wi-Fi based on detecting the Wi-Fi PHY preamble and decoding the L-SIG field. If it cannot detect valid L-SIG, it will assume non-Wi-Fi, and go back to listening/random backoff count down. This implies if a device misses the preamble portion of Wi-Fi frame for whatever reason, it will treat the rest of the frame as a non-Wi-Fi signal and apply the non-Wi-Fi power based threshold of -62 dBm for deciding whether medium is busy.
Here is a detailed diagram with time axis labelled every 0.1ms.
Assume 2ms frame from STA1 collides with a 0.5ms frame from STA2.
Both STA1 and STA2 can hear each other at -65 dBm.
At 0.516ms, STA2 expects to hear ACK, but it experiences an ACK timeout.
This causes STA2 to double its contention window (CW) and attempt retransmission.
But, STA2 begins a new random backoff timer at around 0.534ms itself while STA1 is still transmitting, because STA2 has missed STA1 frame’s preamble and STA1 frame power is below -62 dBm.
Assume STA2 backoff timer is 20 slots. During the backoff timer countdown period (0.534ms to 0.75ms), STA2 does not hear a Wi-Fi preamble or any signal above -62 dBm.
Therefore, STA2 completes backoff at 0.75ms and begins transmitting, resulting in a collision again.
Please note from the diagram, STA1 and STA2 start a new backoff timer at completely different time instants and have become kind of asynchronous.
They will re-synchronize when either STA1 or STA2 manages to detect 1 Wi-Fi preamble from the other.
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