Figure 1-9.—Full-duplex modem.
MODEMS
Transmitter Section
Modems come in a variety of configurations.
Their design depends
including the following:
Asynchronous
transmissions
on a number of factors,
or
synchronous
data
Simplex,
half-duplex, or
full-duplex
communications
Type of communications channel
Type of modulation/demodulation used
Modems may be stand-alone devices with their
own power supplies and indicators. They may also be
integrated into the design of larger equipments in
which the modulations or demodulations are only one
of the functions performed by the device.
A functional block diagram of a modem is shown
in figure 1-9. A full-duplex modem consists of two
sections: the transmitter and the receiver sections.
These two sections are functionally separate from
each other.
The transmitter section consists of a data encoder,
the modulator, the band-pass filter, and the transmit
control logic. The data encoder takes the digital data
signal to be transmitted, and when necessary, converts
it into the bit pattern acceptable to the modulator
circuit. The modulator converts the data into the
carrier signal. The most popular forms of modulation
are frequency-shift keying (FSK), phase-shift keying
(PSK), and quadrature phase-shift keying. After the
data signals are modulated, they are fed to the
band-pass filter circuitry. The band-pass filter then
allows only the desired frequency to pass through the
communications channel. The transmit control logic
provides the timing signals necessary for the
transmission of data to take place.
Receiver Section
The receiver section consists of a band-pass filter,
a demodulator, a data decoder, and the receiver
control circuit. The band-pass filter allows only the
desired carrier signal to be received from the
communications channel. The demodulator removes
the data from the carrier signal and feeds the data to
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