Transmission Efficiency: Data Compression

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Transmission Efficiency Data Compression

• Reduces the size of data files to move more
  information with fewer bits
• Used for transmission and for storage
• Combines w/ multiplexing to increase efficiency
• Works on the principle of eliminating redundancy

• Codes are substituted for compressed portions of
  data
• Lossless reconstituted data is identical to
  original (ZIP, GIF)
• Lossy reconstituted data is only perceptually
  equivalent (JPEG, MPEG)

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Run Length Encoding

• Replace long string of anything with flag,
  character, and count
• Used in GIF to compress long stretches of
  unchanged color, in fax transmissions to transmit
  blocks of white space

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Run-Length Encoding Example
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Huffman Encoding

• Length of each character code based on
  statistical frequency in text
• Tree-based dictionary of characters
• Encoding is the string of symbols on each branch
  followed. String Encoding TEA 10 00 010
  SEA 011 00 010 TEN 10 00 110

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Lempel-Ziv Encoding

• Used in V.42 bis, ZIP
• buffer strings at transmitter and receiver
• replace strings with pointer to location of
  previous occurrence
• algorithm creates a tree-based dictionary of
  character strings

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Lempel-Ziv Example