chapter 15: data transmission business data communications, 5e
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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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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 ()– Relative position in time, measured in degrees
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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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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 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