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Chapter 7

Chapter 7. Data Coding. Agenda. Coding Code efficiency and conversion Compression/compaction Code encryption/decryption. Coding. Definition A predetermined set of symbols having specific meanings Types Human code Morse code (dot and dash for telegraph) Machine code Binary states

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Chapter 7

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  1. Chapter 7 Data Coding

  2. Agenda • Coding • Code efficiency and conversion • Compression/compaction • Code encryption/decryption

  3. Coding • Definition • A predetermined set of symbols having specific meanings • Types • Human code • Morse code (dot and dash for telegraph) • Machine code • Binary states • Binary digit (bit)

  4. Machine Codes - I • Characteristics • Two-state code • Same number of bits • Perfectly formed • Same transmission duration • Character Assignment: unique sequence of bits

  5. Machine Codes - II • Types of characters • Alphanumeric • Format effector for terminal screen or paper • Control (device & transmission)

  6. Parity checking • Parity bit • Even or odd

  7. Escape mechanisms • Escape or ESC Character • Pro: • Increase codes • Con: • Code and decode the data

  8. Specific Codes • Baudot code • American Standard Code for Information Interchange (ASCII) • Extended Binary Coded Decimal Interchange Code (EBCDIC) • Unicode • Binary Coded Decimal • N-out-of-M code

  9. Baudot Code • 5 bits (32 code points) • ESC mechanism • Figure shift (uppercase) • Letter shift (lowercase) • Teletypewriters before 1965 • No error checking

  10. American Standard Code for Information Interchange (ASCII) • By American National Standards Institute (ANSI) • 7654321 (7-bit or 128 characters) • Pros: • Easy sorting by computers • Used by microcomputers

  11. Extended binary Coded Decimal Interchange Code (EBCDIC) • 8-bit code or 256 characters • IBM mainframe computers • 01234567

  12. Unicode • 16-bit or 65,536 characters • By Unicode Consortium for international languages • Used by Windows NT

  13. Other Codes • Binary Coded Decimal • 6-bit code Hollerith code • No standard • N-Out-Of-M Codes • M bits to transmit each character, n must be 1s • Error Detection • IBM’s 4-out-of-8

  14. Control Characters • Transmission control characters • SOH, STX, ETX, EOT, ACK, NAK, NUL • Device control characters • BEL, DC1 (X-ON), DC3 (X-OFF) • Format effect control characters • CR, LF, HT, VT

  15. Code Efficiency • Types of bits in a character • Information bits • Noninformation bits (parity bit) • Definition: No. of information bits divided by the total no. of bits in a character

  16. Code Conversion • Harder from larger no. of bits code to smaller no. of bits code • Use ESC mechanism

  17. Data Compression/Compaction • Types • Character compression/Huffman coding or adaptive Huffman coding (bits assignment) • Run length coding (repetitive characters) • Character stripping (heading & trailing characters) • Combination of the above three • Consideration • throughput • Storage and transmission cost • Hardware cost & software cost

  18. Code Encryption • Voice • Scramble and descramble • Data • Symmetric key • Data encryption standard (DES) by National Institute of Standard and Technology (2 to 56) • Triple DES (2 to 112) • Key security • Asymmetric key or RSA encryption • Public key & private key • Consideration • Hardware and/or software cost • Time delay • Security management cost

  19. Example of Encryption - I • Divide text into groups of 8 characters. Pad with blank at end as necessary • Select an 8-characters key • Rearrange text by interchanging adjacent characters • Translate each character into an ordinal number with blank as 0, A as 1, B as 2… • Add the ordinal number of the key to the results • Divide the total by 27 and retain the remainder • Translate the remainder back into a character to yield the cipher text

  20. Example of Encryption - II • Message: DATA COM • Key: PROTOCOL • A D A T C M O • 01 04 01 20 03 00 13 15 • 01 04 01 20 03 00 13 15 • 16 18 15 20 15 03 15 12 • 17 22 16 40 18 03 28 27 • 17 22 16 13 18 03 01 00 • Q V P M R C A SPACE

  21. Example of Decryption - I • Divide cipher text into groups of eight characters. Pad with blanks at end as necessary • Translate each cipher text alphabetic character and the encryption key into an ordinal number • For each group, subtract the ordinal number of the key value from the ordinal number of the cipher text • Add 27 to any negative number • Translate the number back to alphabetic equivalents • Rearrange the text by interchanging adjacent characters

  22. Example of Decryption - II • Q V P M R C A SPACE • 17 22 16 13 18 03 01 00 • 17 22 16 13 18 03 01 00 • 16 18 15 20 15 03 15 12 • 01 04 01 -7 01 00 -14 -12 • plus 27 27 27 27 • 01 04 01 20 01 00 13 15 • A D A T C M O • D A T A C O M

  23. Points to Remember • Coding • Code efficiency and conversion • Compression/compaction • Code encryption/decryption

  24. Discussion • Design the efficient and secure coding system for an international company

  25. Assignment • Review chapters 1-7 • Read chapters 8-9

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