CIS 267 Data Transmission Lecture Chapter 6

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CIS 267 Data Transmission Lecture Chapter 6

• Vaughn L. Lucas

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A Model of Communication

• Any form of communication requires four things
• A source or sender
• A message
• A channel or communication medium
• A receiver

Coding Message Decoding
Sender
Receiver
Channel or Communication Medium
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Purpose of Text

• Concerned with one particular means of
  communicating information
• The transmission of electromagnetic waves
• VIViD information can be represented by
• Electromagnetic signals and transmitted over a
  suitable medium
• See Figure 6.1 p.136

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Information Transmission
Figure 6.1 from p.136
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Electromagnetic Signals

• Function of time
• Analog -continuous (varies smoothly over time)
• Digital -discrete (constant level for some time,
  followed by a change to another level)
• Function of frequency
• Spectrum (range of frequencies)
• Bandwidth (width of the spectrum)

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Periodic Signal Characteristics

• Amplitude (A) signal value, measured in volts
• Frequency (f) repetition rate, cycles per second
  or Hertz
• Period (T) amount of time it takes for one
  repetition, T1/f
• Phase (F) relative position in time, measured in
  degrees

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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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Why Study Analog in Data Comm.?

• Much of our data begins in analog form must
  understand it in order to properly convert it
• Telephone system is primarily analog rather than
  digital (designed to carry voice signals)
• Low-cost, ubiquitous transmission medium
• If we can convert digital information (1s and 0s)
  to analog form (audible tone), it can be
  transmitted inexpensively

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Data vs Signals

• Analog data
• Voice
• Images
• Digital data
• Text
• Digitized voice or images

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Analog Signaling

• represented by sine waves

phase difference
1 cycle
amplitude (volts)
time
(sec)
frequency (hertz)
cycles per second
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Signals add to form a composite
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An infinite number of analog signals makes a
digital signal
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Signaling Concepts

• Spectrum
• range of frequencies contained in a signal
• Bandwidth
• Width of a spectrum
• Data Rate
• information carrying capacity, related to
  bandwidth but not the same

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Voice/Audio Analog Signals

• Easily converted from sound frequencies (measured
  in loudness or decibels/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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Video Signals

• TV Video Signals are typically analog
• TV Picture Production

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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

• represented by square waves or pulses

1 cycle
amplitude (volts)
time
(sec)
frequency (hertz)
cycles per second
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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 form ASCII

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ASCII Character Codes

• Use 8 bits of data (1 byte) to transmit one
  character
• 8 binary bits has 256 possible outcomes (0 to
  255)
• Represents alphanumeric characters, punctuation,
  and special characters

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Digital Image Signals

• Analog facsimile
• similar to video scanning
• Digital facsimile, bitmapped graphics
• uses pixelization
• Object-oriented graphics
• image represented using library of objects
• e.g. Postscript, TIFF

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Digital Image Signals

• Pixelization and binary representation
• Used in digital fax, bitmapped graphics

1-bit code 00000000 00111100 01110110 0111
1110 01111000 01111110 00111100 00000000
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Transmission Media

• the 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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Using Repeaters
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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
• Noise

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Types of Noise

• Thermal (white noise) - electron agitation
• Uniformly distributed, cannot be eliminated
• Intermodulation
• when different frequencies collide (harmonics)
• Crosstalk - from nearby cables
• Overlap of signals
• Impulse noise - e.g. atmospheric changes
• Irregular spikes, less predictable

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Effects of Noise on a Digital Signal
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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