Communication channels operate in one of three basic transmission modes: simplex, half-duplex, or full-duplex. The mode of transmission is dependent upon the application and the terminal equipment used. A simplex transmission is unidirectional, or one-way. A simplex modem can either send or receive data; it cannot do both. Half-duplex transmission permits data to flow in two directions but only one way at a time. Modems capable of half-duplex transmission are commonly used in telephone devices and networks. Full-duplex transmission permits data to flow in both directions simultaneously. A modem capable of full-duplex transmission is the most versatile type available. The figure below illustrates the channel transmission modes.
Last Updated Jan.7/99