Data Coding

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
• IBMs 4-out-of-8

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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

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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

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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

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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

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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

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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

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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

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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