Ethernet CSMA/CD
Is used when the link is half-duplex. For full-duplex links CDMA/CD isn’t used.
This standard provides for two modes of operation of the MAC sublayer:
a) In half duplex mode, stations contend for the use of the physical medium, using the CSMA/CD algorithms specified.
Bidirectional communication is accomplished by rapid exchange of frames, rather than full duplex operation.
Half duplex operation is possible on all supported media; it is required on those media that are incapable of supporting simultaneous transmission and reception without interference, for example, 10BASE2 and 100BASE-T4.
b) The full duplex mode of operation can be used when all of the following are true:
The physical medium is capable of supporting simultaneous transmission and reception without interference (e.g., 10BASE-T, 10BASE-FL, and 100BASE-TX/FX).
There are exactly two stations on the LAN. This allows the physical medium to be treated as a full duplex point-to-point link between the stations. Since there is no contention for use of a shared medium, the multiple access (i.e., CSMA/CD) algorithms are unnecessary.
Both stations on the LAN are capable of and have been configured to use full duplex operation.
Source - IEEE 802.3-2022 - Section 4.1.1 Media Access Control, Functional model of the MAC method, Overview