Ethernet was developed in the early '70s by Xerox Corporation in their Palo Alto Research Centre in California US of A. Ethernet uses a protocol called CSMA/CD or Carrier Sense Multiple Access with Collision Detection.
Carrier Sense means that when a device connected to an Ethernet network wants to send data it first checks to make sure it has a carrier on which to send its data (usually a piece of copper cable connected to a hub or another machine).
Multiple Access means that all machines on the network are free to use the network whenever they like so long as no one else is transmitting.
Collision Detection means of ensuring that when two machines start to transmit data simultaneously, that the resultant corrupted data is discarded, and re-transmissions are generated at differing time intervals.
This means that CSMA/CD is in half-duplex mode, it uses a single signal path for sending and receiving data. But no one or only few knows this phrase until the birth of full-duplex.
The phrase "full duplex" means that there are separate signal paths for sending and receiving data. When ethernet is in full-duplex mode, CSMA/CD is not used.
So to answer your question, we used to use half-duplex during the birth of ethernet (10Base5, 10Base2, and early 10BaseT). We have a taste of full-duplex when twisted ethernet is further developed.
Original Ethernet only used a shared media. When a station transmits all the other stations on the same media see the transmission. (Similar to walkie talkies.) If more than one station transmits at the same time, the signal is corrupted.
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