Data Flow

Data Flow
Communication between two devices can be simplex, half-duplex, or full-duplex as shown in Figure 1.2.
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 1.2a).
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 (see Figure 1.2b).
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 m.,lle (als@ called duplex), both stations can transmit and receive simultaneously
(see Figure 1.2c).
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.