cs441-lecture-20240924

Lecture Notes - CS441

Tuesday, September 24th 2024

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• Compression

• Lossless vs Lossy

• Compression is another technique to squeeze more data over a communications line.
• If you can compress a data file down to one half of it’s original size, file will obviously transfer in less time
• Lossless: when data is uncompressed, original data returns
• Lossy: when data is uncompressed, you do not have the original data
• Lossy example: gray object on gray background, speed is optimized in video when the missing data is generated after video is uncompressed, doesn’t have to deal with every single pixel
• Preserves the bandwidth.
• Financial file: lossless is preferrable
• Video, image, movie, or audio: lossy is okay
• Lossless compression examples
  • Huffman codes
  • Run-length compression
  • Lempel-Ziv compression
• Lossy compression examples:
  • MPEG
  • JPEG
  • MP3

• Error Correction

• (On midterm)
• Noise is always present
• If you had goggles to see electromagnetic waves you’d be able to see a lot of it
• If a communications line experiences too much noise, the signal will be lost or corrupted
  • Enemy of electrical engineers is noise
  • Mechanical engineers is vibrations
• Communications systems should check for transmission errors
• Once an error is detected, a system may perform some action
• Some systems perform no error control, but simply let the data in the error be discarded

• Types of Noise

• White noise

• Aka thermal or Gaussian noise
• Relatively constant and can be reduced
• If white noise gets too strong, it can be completely disrupt the signal

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• Impulse noise

• One of the most disruptive forms of noise
• Random spikes of power can destroy one or more bits of information
• Difficult to remove from an analog signal because it may be hard to distinguish from the original signal
• Can damage more bits if the bits are closer together transmitted at a faster rate)

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• Crosstalk

• Unwanted coupling between two different signal paths
  • Ex: hearing another conversation while talking on the telephone
• Relatively constant and can be reduced with proper measures

• Echo

• The reflective feedback of a transmitted signal as the signal moves through a medium
• Most often occurs on a coaxial cable – relates to bus topology, without terminators echo would occur at the end of the cable

• Jitter

• The result of small timing irregularities during the transmission of digital signals
• Occurs when a digital signal is repeated over and over
• If serious enough, jitter forces systems to slow down their transmission
• Steps can be taken to reduce jitter

• Delay Distortion

• Occurs because the velocity of propagation of a signal through a medium varies with the frequency of the signal
• Can be reduced

• Attenuation

• The continuous loss of a signal’s strength as it travels through a medium
• Why repeaters and boosters exist

• Error prevention

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