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[16. Apendices
16.1 Datacommunication and modem
Data communication means information in the form of binary digits
transferred between for instance two CPU's or from a terminal to a
CPU. Data can be transferred in parallel or in serial form, but only
serial form will be considered in this chapter.
The simplest form of serial data transmission is a direct cable
between the terminal and the CPU, where the binary digits are
transferred one after another, represented as high or low voltage
levels.
This is possible only for short distances. For longer distances, other
solutions must be found.
The telephone network is able to transfer information all over the
world in the form of human speech. It is therefore natural to consider
it used for data transmission as well.
The challenge is then to convert the electrical data signals to
analogue signals within the frequency band that will be passed
through by the telephone network. This frequency band ranges from
300 Hz to app. 3400 Hz.
This problem is solved by a modem. The word modem is a
contraction for Modulator and Demodulator.
The modulator part converts the digital signals to analogue signals
that can be transferred through the telephone network. The principle
is mainly to modify a single tone called carrier, in such ways that
these modifications can be recognised by the receiver.
The demodulator part interprets the modifications in the analogue
signals, and convert them back to digital signals.
LZTR104006 Rt.00
16-1
Zat 14.4/14.4-7 and
19.2/19.2-7
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