Transcript Data Communication and Computer Networks 1303330

Data
Communication
Lecturer: Tamanna Haque Nipa
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 (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.
Amplitude Shift Keying
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

Figure 5.3 Binary amplitude shift keying
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BASK
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(2пfct) for 1 binary
0 for 0 binary

Frequency Shift Keying
 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
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Binary Frequency-Shift Keying
(BFSK)
 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(2пf1t) for 1 binary
Acos(2пf2t) for 0 binary
 For MFSK
S(t)=Acos(2пfit) , 1<=i<=M
Phase Shift Keying
 Phase
of carrier signal is shifted to represent
data
 Change phase with each symbol
 More complex
 robust against interference
Figure 5.9 Binary phase shift keying
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Two-level PSK (BPSK)

Uses two phases to represent binary digits
BPSK Constellation
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Quadrature PSK
 More
efficient use by each signal element
representing more than one bit


shifts of /2 (90o)
Each element represents two bits
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4 PSK Characteristic
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4 PSK
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8 PSK Characteristic
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4-QAM and 8-QAM Constellations
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Time Domain for 8-QAM Signal
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ANALOG to ANALOG
Analog-to-analog conversion is the representation of analog
information by an analog signal. One may ask why we need to
modulate an analog signal; it is already analog. Modulation is
needed if the medium is bandpass in nature or if only a
bandpass channel is available to us.
Types of analog-to-analog modulation
Figure 5.16 Amplitude modulation
The total bandwidth required for AM can be determined from the
bandwidth of the audio signal: BAM = 2B.
Standard Bandwidth Allocation for AM Radio
The bandwidth of an audio signal (speech and music) is usually 5 kHz.
Therefore, an AM radio station needs a bandwidth of 10 kHz. In fact, the
Federal Communications Commission (FCC) allows 10 kHz for each AM
station. AM stations are allowed carrier frequencies anywhere between 530 and
1700 kHz (1.7 MHz). However, each station's carrier frequency must be
separated from those on either side of it by at least 10 kHz (one AM bandwidth)
to avoid interference. If one station uses a carrier frequency of 1100 kHz, the
next station's carder frequency cannot be lower than 1110 kHz (see Figure
5.17).
Figure 5.18 Frequency modulation
The total bandwidth required for FM can be determined from the bandwidth
of the audio signal: BFM = 2(1 + β)B.
Figure 5.19 FM band allocation