CSMA/CD

When a device on a network want  to transmit to another device.  The first device  to check to see if the line is clear and then transmit.  The only problem is that two devices perform this action at the same time.  They will both line is clear and so will both attempt to transmit.

Ethernet can accept only one signal at a time.  So the two transmission with collide with each other.  These collisions damage the data packet on the network.  To avoid another collision each system will wait a period of time, known as the back off, before retransmitting the data.  If  data collide again the delay before retransmitting in increase and data is sent again.

The more device that are connected o an Ethernet network the more likely it is there will be collisions on the network this decrease in  performance has driven  improvements in the structures of Ethernet network, improvement include substation of older hubs with new high performance, Ethernet switches.