There are two modes in network communication, one is parallel communication, the other is serial communication. In serial communication, there are simplex, half duplex, full duplex communications. Please see the attachment!
Simplex, the signal is transferred in a fixed direction from A to B, such as the broadcast.
Half duplex, the signal is transferred in a direction per time, first time from A to B, next time from B to A, such as the interphone, 100BASE-T4, 1000 BASE-TX.
Full duplex, the signal is transferred in two directions simultaneously, from A to B and from B to A, such as the telephone, 100BASE-TX, 1000BASE-T, 10G BASE-T.
