Unformatted text preview: rcuits are seldom used because a return path is generally needed
to send acknowledgement, control, or error signals. Half-Duplex
A half-duplex system can transmit data in both directions, but only in one
direction at a time. Thus, a half-duplex line can alternately send and receive data.
It requires two wires. This is the most common type of transmission for voice
communication because only one person is supposed to speak at a time. It is also
used to connect a terminal with a computer. The terminal might transmit data and
then the computer responds with an acknowledgement.
In a half-duplex system, the line must be "turned-around" each time the direction
is reversed. This involves a special switching circuit and requires a small amount
of time (approximately 150 milliseconds). With high-speed capabilities of the
computer, this turn-around time is unacceptable in many instances. Also, some
applications require simultaneous transmission in both directions. In such cases, a
full-duplex system is used that allows information to flow simultaneously in both
directions on the transmission path. Use of a full-duplex line improves efficiency
as the line turn-around time required in a half-duplex arrangement is eliminated. It
requires four wires.
DATA TRANSMISSION SPEED
A term used to describe the data-handling capacity of a communication system is
bandwidth. Bandwidth is the range of frequencies that is available for the
transmission of data. A narrow range of frequencies in a communication system is
analogous to a narrow road. The flow of information in such a system - its data
transfer rate - is limited, just as is the flow of traffic in a narrow road. Wider
bandwidths permit more rapid information flow. Thus, the wider the bandwidth of
a communication system, the more data it can transmit in a given period of time.
When dealing with computer input/output devices, we think in terms of characters
per second. However, in case of data transmission, we talk in terms of bits per
second. The ASCII code uses 7 data bits per character plus a parity bit. For data
communication, additional bits are added to control the process. Althou...
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- Spring '14