In an Ethernet network, every host connected to the network can transmit at any time. This may result in two hosts transmitting at the same time. If a collision between packets is sensed by both hosts, their NICs (Network Interface Cards) perform a Back Off - a transmission time-out. Since the length of the back-off is randomly determined, in most cases when the time-out has elapsed both NICs will be able to re-transmit without interfering with each other. There is always a change that another host will try to send a packet, causing the back-off procedure to start again