MODEMS

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 1-8