ETHERNET is a CSMA/CD (Carrier Sense Multiple Access/Collision Detection) system. In this type of network, all equipment is connected to a single cable - the ETHERNET. When one piece of equipment wants to communicate with another it sends a message, which contains the destination address, over the ETHERNET. Before doing this it ``listens'' to the carrier on the ETHERNET (Carrier Sense) to make sure that nothing is already using it. If in use, it waits for the broadcast to end. It then broadcasts its own message which will reach all other devices but will only be accepted by the one that was addressed. It is possible that two devices may want to broadcast simultaneously. They will both wait for silence and then start to transmit. In this case the messages will overlap in parts of the ETHERNET and will be garbled there. To recover from this each device also listens to its own message to make sure that it is not corrupt (Collision Detection). If a collision is detected then the device waits a short time and tries again. For this to be totally reliable every message must last long enough that it fills the ETHERNET; two remote devices might be able to send short messages which would be error free locally but would overlap half way between them. The message length requirement places an upper limit of about 2Km on the size of the network.