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 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 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