Business data communications 5e by stallings chapter 15

Chapter 15:
Data Transmission
Business Data Communications, 5e


Electromagnetic Signals
• Analog Signal
– signal intensity varies in a smooth fashion
over time. In other words, there are no breaks
or discontinuities in the signal

• Digital Signal
– signal intensity maintains a constant level for
some period of time and then changes to
another constant level
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Analog Sine Wave

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Digital Square Wave

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Periodic Signal Characteristics
• Peak Amplitude (A)
– Maximum signal value, measured in volts

• Frequency (f)
– Repetition rate
– Measured in cycles per second or Hertz (Hz)

• Period (T)
– Amount of time it takes for one repetition, T=1/f

• Phase (φ)
– Relative position in time, measured in degrees
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s(t) = (4/π) × (sin (2πft) + (1/3) sin (2π(3f)t))

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Frequency Domain Concepts
• Spectrum of a signal is the range of frequencies
that it contains
• Absolute bandwidth of a signal is the width of
the spectrum
• Effective bandwidth contained in a relatively
narrow band of frequencies, where most of
signal’s energy is found
• The greater the bandwidth, the higher the
information-carrying capacity of the signal
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Bandwidth
• Width of the spectrum of frequencies that
can be transmitted
– if spectrum=300 to 3400Hz,
bandwidth=3100Hz

• Greater bandwidth leads to greater costs
• Limited bandwidth leads to distortion

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Analog Signaling

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Voice/Audio Analog Signals
• Easily converted from sound frequencies
(measured in loudness/db) to electromagnetic
frequencies, measured in voltage
• Human voice has frequency components ranging
from 20Hz to 20kHz
• For practical purposes, the telephone system has
a narrower bandwidth than human voice, from
300 to 3400Hz
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Image/Video: Analog Data to
Analog Signals
• Image is scanned in lines; each line is

displayed with varying levels of intensity
• Requires approximately 4Mhz of analog
bandwidth
• Since multiple signals can be sent via the
same channel, guardbands are necessary,
raising bandwidth requirements to 6Mhz
per signal
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Digital Signaling

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Digital Text Signals
• Transmission of electronic pulses
representing the binary digits 1 and 0
• How do we represent letters, numbers,
characters in binary form?
• Earliest example: Morse code (dots and
dashes)
• Most common current forms: ASCII, UTF
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Transmission Media
• Physical path between transmitter and
receiver (“channel”)
• Design factors affecting data rate
–
–
–
–

bandwidth
physical environment
number of receivers
impairments
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Impairments and Capacity
• Impairments exist in all forms of data
transmission
• Analog signal impairments result in
random modifications that impair signal
quality

• Digital signal impairments result in bit
errors (1s and 0s transposed)
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Transmission Impairments:
Guided Media
• Attenuation
– loss of signal strength over distance

• Attenuation Distortion
– different losses at different frequencies

• Delay Distortion
– different speeds for different frequencies

• Noise
– distortions of signal caused by interference
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Transmission Impairments:
Unguided (Wireless) Media
• Free-Space Loss

– Signals disperse with distance

• Atmospheric Absorption
– Water vapor and oxygen contribute to signal loss

• Multipath
– Obstacles reflect signal creating multiple copies

• Refraction
• Thermal Noise
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Types of Noise
• Thermal (aka “white noise”)
– Uniformly distributed, cannot be eliminated

• Intermodulation
– When different frequencies collide (creating
“harmonics”)

• Crosstalk
– Overlap of signals

• Impulse noise
– Irregular spikes, less predictable
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Channel Capacity
• The rate at which data can be transmitted
over a given path, under given conditions
• Four concepts
–
–
–
–

Data rate
Bandwidth
Noise
Error rate
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Shannon Equation
• C = B log2 (1 + SNR)
– B = Bandwidth
– C= Channel
– SNR = Signal-to-noise ratio

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