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Computer Networks
Bhushan Trivedi, Director, MCA
Programme, at the GLS Institute of
Computer Technology, Ahmadabad
© Oxford University Press 2011
Chapter 3
Data Communication
Fundamentals
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The prerequisites to communication
•
•
•
•
Bandwidth and Data Rate
The frequency and the band
The media effect on the bandwidth of signal
The composite signals and Fourier
components
• Filters
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Other factors affecting the data rate
• Analog or digital signaling.
• Analog
– Type of modulation
– Amount of constellation points
• Digital
•
number of levels
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Other factors affecting the data rate
• Sensitivity of the receiver
• Resistance of the media and thus its
temperature
• How the wires are structured and shielded
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Harmonics
and
composite
wave
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Properties of a channel
• More the bandwidth of the media, more the
number of harmonics that can pass through
the media.
• Higher the data rate, less will be the number
of harmonics that can pass through the media.
• MDR of a channel = 2 × Bandwidth × log2
(signal levels)
• MDR of a channel = Bandwidth × log2 (1 + S/N)
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Signal to noise ratio
• S/N =10 log (Ps/Pn)
• When Ps/Pn becomes 1
• MDR = Bandwidth × log2 (1+10 × log e 1)
= Bandwidth × log 2 (1+0)
= Bandwidth × log 2 1
=0
(as log21 = 0)
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Filtering a specific frequency
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Differentiating Analog and Digital
• Analog and Digital Signaling
– Analog Signaling
– Digital signaling
– Difference between Analog and Digital Signaling
• Analog and Digital Transmission
– Introduction to Transmission
– Analog Transmission
– Digital transmission
• Digital signaling and errors
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Digital and analog wave
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Periodic waves, digital and analog
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A periodic wave
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Digital signaling and bit rate
If we use 2n different voltage signals, then
bit rate = baud rate × n
If we use n voltage signals, then
bit rate = baud rate × log2 n
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Characteristics of Digital Signaling
•
•
•
•
Square waveforms
Limited discrete values of voltage levels
Need more bandwidth
Noise can be completely removed
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Repeater reshapes, amplifier amplifies
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Noise addition to waves
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Signaling is different than
transmission
• Analog and digital transmission
• We can use digital signaling irrespective of
whether the transmitted data is analog (voice
or video) or digital (zeros and ones)
• Similarly, analog signaling can be used for
both analog and digital transmission
• Coding mechanisms
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Modulation
•
•
•
•
•
•
Amplitude modulation
Frequency modulation
Phase Modulation
Modulation in Practice
Modulation in cable TV
Modulation in ADSL
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Amplitude modulation
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Amplitude modulation
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A square wave to represent bits
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A distorted square signal
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Frequency modulation
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Phase
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Phase change to indicate bit
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Constellation pattern for QPSK
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QAM-16
constellation
pattern
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Frequency
bands used in
cable TV
transmission
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Multiplexing
• Multiplexing and de-multiplexing
– Time Division Multiplexing
– Frequency division multiplexing
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Switching
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Routing
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Transmission Errors
• Attenuation
• Distortion
• Noise
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