Communication between two devices can be simplex, half-duplex, or full-duplex.
SIMPLEX:
In simplex mode, the communication is unidirectional, as on a one-way street. Only one of the two devices on a link can transmit; the other can only receive. Keyboards and traditional monitors are examples of simplex devices. The keyboard can only introduce input; the monitor can only accept output. The simplex mode can use the entire capacity of the channel to send data in one direction.
HALF-DUPLEX:
In half-duplex mode, each station can both transmit and receive, but not at the same time. When one device is sending, the other can only receive, and vice versa The half-duplex mode is like a one-lane road with traffic allowed in both directions. When cars are traveling in one direction, cars going the other way must wait. In a half-duplex transmission, the entire capacity of a channel is taken over by whichever of the two devices is transmitting at the time. Walkie-talkies and CB (citizens band) radios are both half-duplex systems. The half-duplex mode is used in cases where there is no need for communication in both directions at the same time; the entire capacity of the channel can be utilized for each direction.
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| Figure. Data flow (simplex, half-duplex, and full-duplex) |
FULL-DUPLEX:
In full-duplex mode (also called duplex), both stations can
transmit and receive simultaneously. The full-duplex mode is like a two-way
street with traffic flowing in both directions at the same time. In full-duplex
mode, signals going in one direction share the capacity of the link with signals
going in the other direction. This sharing can occur in two ways: Either the
link must contain two physically separate transmission paths, one for sending
and the other for receiving; or the capacity of the channel is divided between
signals traveling in both directions. One common example of full-duplex communication
is the telephone network. When two people are communicating by a telephone
line, both can talk and listen at the same time. The full-duplex mode is used
when communication in both directions is required all the time. The capacity of
the channel, however, must be divided between the two directions.