Data Modems
Data
Modems
Data modems are devices that convert signals between analog
and digital formats for transfer to other types of communication lines. Data
modems are used to transfer data signals over conventional analog telephone
lines. The term modem also may refer to a device or circuit that converts analog
signals from one frequency band to another.
A single connection (point-to-point) analog data circuit requires
a modem at each end to transfer digital signals. The type of modems used on each
end must be compatible due to encoding and decoding processes. Analog
communication lines are restricted to audio bandwidth of 300 Hz to 3300 HZ. To
communicate digital data and control signals, the modems vary the frequency of
the carrier in each direction based on an agreed to algorithm for encoding bits.
Figure 8 shows a
modem with its functional responsibilities listed. From the DTE (serial
interface RS 232-C) to the line the modem performs a digital-toanalog conversion
and from the line to the DTE an analog-to-digital conversion.
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