Carrier Sense/Clear Channel Assessment (CS/CCA)
If the station is not currently transmitting or receiving, it listens and senses the channel either to detect the beginning of a network signal that can be received (carrier sense) or to identify whether the channel is unused and available prior to transmitting a packet (clear channel assessment).
Essentially, either a station is transmitting or it is idle. If it is idle, it could be listening and waiting for the channel to become available so that it can transmit a frame. It could also be listening, waiting to sense the beginning of a transmission from another station, or it could be listening and receiving a frame that is being transmitted by another station
18.104.22.168 CCA requirements
The PHY shall indicate a medium busy condition by issuing a PHY-CCA.indication primitive when the
carrier sense/clear channel assessment (CS/CCA) mechanism detects a channel busy condition.
For the operating classes requiring CCA-Energy Detect (CCA-ED), the PHY shall also indicate a medium
busy condition when CCA-ED detects a channel busy condition.
The start of a valid OFDM transmission at a receive level greater than or equal to the minimum modulation
and coding rate sensitivity (–82 dBm for 20 MHz channel spacing, –85 dBm for 10 MHz channel spacing,and –88 dBm for 5 MHz channel spacing) shall cause CS/CCA to detect a channel busy condition with a probability > 90% within 4 us for 20 MHz channel spacing, 8 s for 10 MHz channel spacing, and 16 us for 5 MHz channel spacing.
NOTE—The requirement to detect a channel busy condition for any signal 20 dB above the minimum modulation and
coding rate sensitivity (minimum modulation and coding rate sensitivity + 20 dB resulting in –62 dBm for 20 MHz
channel spacing, –65 dBm for 10 MHz channel spacing, and –68 dBm for 5 MHz channel spacing) is a mandatory energy detect requirement on all Clause 17 receivers.
reference: IEEE 802.11-2016