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Computer Science by Ian Sinclair

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Computer Science
it possible to pause now and again without any loss of data. A serial link to a
printer can make use of hardware handshaking meaning that the hand-
shaking can be implemented by using electrical signals over another set of
lines,
but this option is not open to most communications applications
because extra telephone lines are not available for this use.
The handshaking is therefore implemented in software by using the
XON/XOFF system. This uses the ASCII code numbers 17 and 19, which
are not used for characters, sent between one computer and the other. Data
can be sent out from a computer following the ASCII 17 code, and disabled
following the ASCII 19. Since these codes are sent over the normal data
lines,
no additional electrical connections are needed. The rate of data
transfer is slower because of the time that is needed to send the XON/XOFF
signals, so that if a system is organized in such a way that the least use of
handshaking occurs, both transmission and reception will be faster. The use
of XON/XOFF is by far the most common method of software handshaking
that is used in communications (another system is called EIA).
The modem
Two computers that are reasonably close to each other can be linked by a
serial cable of the non-modem type, but a much more common requirement
is to link computers for which the only connecting link is a telephone line.
This calls for the use of a modem. The telephone system is intended to
transmit the electrical signals that are obtained from a microphone operated
by the human voice. Like any other waves, the waves of sound of the voice
cover a range of frequencies, meaning the number of vibrations per second.
In honour of the pioneer of radio, Heinrich Hertz, the unit of one vibration
per second is called the hertz, abbreviated to Hz. The range of frequencies
that is needed for intelligible (as distinct from high-quality) speech trans-
mission is quite small, of the order of 300 to 3000 vibrations per second,
usually written as 300-3000 Hz. By contrast, the transmission of music of the
quality that we get on a good recording demands (but seldom gets) a range of
frequencies from about 30 Hz to 18,000 Hz, and television pictures require a
range from 0 Hz to 5.5 million Hz because of their complicated pattern. The
equally spiky signals from a computer also, even if they are slowed down to a
low speed of 300 per second or lower, simply cannot be transmitted through a
telephone circuit.
What we need is a device that detects a computer on signal and turns it into
one tone and will similarly turn an off signal into another tone, or a tone that
is different from the first in some way, like being out of step. The action of
converting the computer signals into tones is called modulation, and we
must be able also to carry out the opposite action which is called demodul-
ation. The combining of the words modulation and demodulation gives us
the word modem, which is the name of the device that carries out this
transformation. The modem is connected to the computer by a serial link,
using normal hardware handshaking, but the lead out from the modem to the
outside world is a single lead, suitable for telephone connection, using
XON/XOFF handshaking.
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