Chapter 15: Data Transmission

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Chapter 15Data Transmission

• Business Data Communications, 5e

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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, T1/f
• Phase (?)
• Relative position in time, measured in degrees

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s(t) (4/?) ? (sin (2?ft) (1/3) sin (2?(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
  signals 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 spectrum300 to 3400Hz, bandwidth3100Hz
• 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 ImpairmentsGuided 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 ImpairmentsUnguided (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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