Half-duplex networks

Half-duplex networks
4.1.4 This page will explain how collisions occur on a half-duplex network.
Originally Ethernet was a half-duplex technology. Half-duplex allows hosts to either transmit or receive at one time, but not both. Each 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 hosts could transmit at the same time. This results in a collision. When a collision occurs, the host that detects the collision first, sends out a jam signal to the other hosts. When a jam signal is received, each host stops data transmission, then waits for a random period of time to re-transmit the data. The back-off algorithm generates this random delay. As more hosts are added to the network, 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.

The next page will discuss some other factors that cause network congestion.

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