Originally Ethernet was a half-duplex technology. Using half-duplex, a host could either transmit or receive at one time, but not both. Each Ethernet host checks the network to see whether data is being transmitted before it transmits additional data. If the network is already in use, the transmission is delayed. Despite transmission deferral, two or more Ethernet hosts could transmit at the same time. This results in a collision. When a collision occurs, the host that first detects the collision will send out a jam signal to the other hosts. Upon receiving the jam signal, each host will stop sending data, then wait for a random period of time before attempting to retransmit. The back-off algorithm generates this random delay. As more hosts are added to the network and begin transmitting, collisions are more likely to occur.

Ethernet LANs become saturated because users run network intensive software, such as client/server applications, which cause hosts to transmit more often and for longer periods of time. The network interface card (NIC), used by LAN devices, provides several circuits so that communication among devices can occur.