Half-duplex Mode

Slow network card: what is half duplex mode? | TuxRadar Linux
Change NIC settings betwee Full / Half duplex
Going from full to half duplex on cable, any real speed difference on Net? - AnandTech Forums

In full duplex operation, you have, in essence, 10Mb/s guaranteed bandwidth up and down. When you go to full duplex, you move back to "regular" ethernet, which is a collision-based system. In a half duplex network, it's a first-come-first-serve operation. Your computer transmits when it sees the network is clear. It is very possible (and, in fact, very very likely under high traffic conditions) that another computer will have ALSO started transmitting at the same time you did. Both your signals go across the wire and interfere with each other, effectively cancelling out both transmissions. This is called a collision. When your computer sees a collision it stops transmitting and pauses for a random amount of time before trying to re-transmit.

What is Half Duplex and Full Duplex Ethernet Modes? | Technology Updates

Ethernet IEEE 802.3 standard defines the half duplex; Cisco describes, it uses a digital signal on a wire pair flowing in both tracks on the wire. Half Duplex always employs the Carrier Sense Multiple Access with Collision Domain (CSMA/CD), so that it can retransmit the transmission if collision occurs. Hub always works in half duplex mode. Half duplex Ethernet is not efficient because it has the limit up to 10BaseT, as Cisco describes 10BaseT is not more than 3 to 4Mbps.

The Difference Between Half and Full Duplex Explained