Simplex, Half Duplex, and Full Duplex
Simplex, half duplex, and full duplex are referred to as dialog modes, and they determine the direction in which data can flow through the network media.
Simplex allows for one-way communication of data through the network, with the full bandwidth of the cable being used for the transmitting signal. One-way communication is of little use on LANs, making it unusual at best for network implementations. Far more common is the half-duplex mode, which accommodates transmitting and receiving on the network but not at the same time. Many networks are configured for half-duplex communication.
The preferred dialog mode for network communication is the full-duplex mode. To use full duplex, both the network card and the hub or switch must support full duplexing. Devices configured for full duplexing are capable of transmitting and receiving simultaneously. This means that 100Mbps network cards are capable of transmitting at 200Mbps using full-duplex mode.