Physical carrier sense (CSMA/CA)

Physical carrier sense (CSMA/CA)
. . . .

Physical carrier sense (CSMA/CA)

The first step that an 802.11 CSMA/ CA device needs to do to begin transmitting is to perform a carrier sense.
This is a check to see whether the medium is busy. Think of it like listening for a busy signal when you call someone on the phone.
There are two ways that a carrier sense is performed:

It is possible that a station did not hear the other radio transmitting and was therefore unable to read the Duration/ ID field and set its NAV timer. There could be numerous reasons why.
CSMA/ CA utilizes another line of defense to ensure that a station does not transmit while another is already transmitting: The 802.11-2012 standard also defines a physical carrier sense mechanism to determine if the medium is busy.
Physical carrier sensing is performed constantly by all stations that are not transmitting or receiving. When a station performs a physical carrier sense, it is actually listening to the channel to see whether any other transmitters are taking up the channel.
Physical carrier sense has two purposes:

to determine whether a frame transmission is inbound for a station to receive.
If the medium is busy, the radio will attempt to synchronize with the transmission.
to determine whether the medium is busy before transmitting.
This is known as the clear channel assessment (CCA)

The CCA involves listening for RF transmissions at the Physical layer.
The medium must be clear before a station can transmit.
It is important to understand that both virtual carrier sense and physical carrier sense are always happening at the same time.
Virtual carrier sense is a layer 2 line of defense, while physical carrier sense is a layer 1 line of defense.

Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.