Lecturer: Tamanna Haque Nipa Data Communication Chapter 5: Analog Transmission DIGITAL-TO-ANALOG CONVERSION Digital-to-analog conversion is the process of changing one of the characteristics of an analog signal based on the information in digital data ( Digital-to-analog conversion is the process of changing one of the characteristics of an analog signal based on the information in digital data (Public telephone system 300Hz to 3400Hz, modem (modulator-demodulator)) Types of digital-to-analog conversion Bit rate is the number of bits per second. Baud rate is the number of signal elements per second. In the analog transmission of digital data, the baud rate is less than or equal to the bit rate. In amplitude shift keying, the amplitude of the carrier signal is varied to create signal elements. Both frequency and phase remain constant while the amplitude changes. Values represented by different amplitudes of carrier low bandwidth requirements very susceptible to interference Used over optical fiber 5 Figure 5.3 Binary amplitude shift keying One binary digit represented by presence of carrier, at constant amplitude Other binary digit represented by absence of carrier where the carrier signal is s(t)= Acos(2fct) for 1 binary 0 for 0 binary Values represented by different frequencies (near carrier) Less susceptible to error than ASK change frequency with each symbol needs larger bandwidth Even higher frequency on LANs 7 Two binary digits represented by two different frequencies near the carrier frequency where f1 and f2 are offset from carrier frequency fc s(t)= Acos(2f1t) for 1 binary Acos(2f2t) for 0 binary For MFSK S(t)=Acos(2fit), 1