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 (see Figure a). 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.
Full-Duplex:
In full-duplex both stations can transmit and receive simultaneously (see Figure c). The
full-duplex mode is like a tW<D-way street with traffic flowing in both directions at the same
time. In full-duplex mode, si~nals going in one direction share the capacity of the link: with
signals going in the other din~c~on. This sharing can occur in two ways: Either the link must
contain two physically separate t:nmsmissiIDn paths, one for sending and the other for receiving;
or the capacity of the ch:arillilel 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.
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