Chapter 7
Data Coding
Agenda
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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
• Binary digit (bit)
Machine Codes - I
• Characteristics
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Two-state code
Same number of bits
Perfectly formed
Same transmission duration
• Character Assignment: unique sequence of
bits
Machine Codes - II
• Types of characters
– Alphanumeric
– Format effector for terminal screen or paper
– Control (device & transmission)
Parity checking
• Parity bit
• Even or odd
Escape mechanisms
• Escape or ESC Character
• Pro:
– Increase codes
• Con:
– Code and decode the data
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
Baudot Code
• 5 bits (32 code points)
• ESC mechanism
– Figure shift (uppercase)
– Letter shift (lowercase)
• Teletypewriters before 1965
• No error checking
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
Extended binary Coded Decimal
Interchange Code (EBCDIC)
• 8-bit code or 256 characters
• IBM mainframe computers
• 01234567
Unicode
• 16-bit or 65,536 characters
• By Unicode Consortium for international
languages
• Used by Windows NT
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
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
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
Code Conversion
• Harder from larger no. of bits code to
smaller no. of bits code
• Use ESC mechanism
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
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
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
Example of Encryption - II
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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
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
Example of Decryption - II
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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
Points to Remember
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Coding
Code efficiency and conversion
Compression/compaction
Code encryption/decryption
Discussion
• Design the efficient and secure coding
system for an international company
Assignment
• Review chapters 1-7
• Read chapters 8-9