1. Information of the LO Subject: Information Theory
Domain: Digital Communication
Sub domain: Lossless Data Compression
Topic: Lempel–Ziv–Welch Compression
Education Level: UG
The Lempel-Ziv algorithm is an adaptive type of algorithm. The basic idea is to replace recurring patterns with references to a dictionary.
Author: Mentor:
Jasveer Singh T. Jethra Prof. Sachin Patkar 1 1

2. Pre requisites Basics of Information coding. 22
Before using this LO, the user should know about the following:
Basics of Information coding. 2

3. Project OSCAR IDD Template 4.7
Learning objectives
After interacting with this LO the user should be able to:
Compress stream of data using Lempel–Ziv–Welch algorithm.
Decompress by decoding the Lempel–Ziv–Welch encoded data.
Project OSCAR IDD Template 4.7 3 3

4. Keywords in this LO Dictionary:44
Keywords in this LO
These are the keywords and their description in this LO
Dictionary:
The pre-defined dictionary are the ASCII characters from , because they are the displayable ones. Additional characters are added as set of 2 or more characters together.
2. Compression Ratio:
The term used to quantify the reduction in data-representation size produced by a data compression algorithm.
Compression Ratio = Compressed Size
Uncompressed Size 4

5. Element groups in this LO55
Element groups in this LO
Name of the element group and Abbreviation
Name of the elements in the group
Rank of element group (in terms of the most important element group on the screen that students should focus on) 1: most important, 2: next in importance, … and so on
Input
Textbox 1
Dictionary
Listbox
Output 5 5

6. Write the name of the element group 166
Write the name of the element group 1
The title of each column explains contains guidelines on the details required.)
Name of the element
Abbreviation
Reference image
Image for using in LO
Functional or interactivity details
Connected to
Input
Textbox
1. Here text will be entered by the user
2. After every character is entered by the user the character will be checked in the dictionary as explained in the algorithm and following steps. 6 6

7. Write the name of the element group 177
Write the name of the element group 1
The title of each column explains contains guidelines on the details required.)
Name of the element
Abbreviation
Reference image
Image for using in LO
Functional or interactivity details
Connected to
Dictionary
Listbox
Initialize dictionary with ASCII characters from by numbering them from 0-94.
Refer to the algorithm shown in the next slide and the further steps. 7 7

8. Search for the longest block W which has appeared in the dictionary.
Encoding Algorithm:
After having the dictionary initialized to contain the ASCII characters from redefined as 0-94, start taking input from the given string and name it as W.
Search for the longest block W which has appeared in the dictionary.
Encode W by its index in the dictionary.
Add W followed by the first symbol of the next block to the dictionary.
Go to Step 2. 8 8

9. Supportive image for step 1a
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Total no. of codes sent 9 9

10. Supportive image for step 1b
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11. Step 1 11111111 S. No. Description of the action / interactivity
Start Time of the action
End Time of the action
Audio narration
On screen text 1.
The dictionary is to be reset with ASCII characters from assigned as 0-94 respectively is a code for that particular character.
For eg., ASCII of a is 97, so after rearranging ASCII characters as 0-94, we have code for a as 65.
So 0-94 are the codes for the respective characters.
0 sec 2.
The figures shown in the next few slides is based on the example string: “abaabccffebacdabcf” taken serially 11 11

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13. Step 2 13131313 Description of the action / interactivity
Start Time of the action
End Time of the action
Audio narration
On screen text
When the 1st character is entered, as this character is already present in the dictionary, it waits for the next character to come, again checks if this character along with the previous character together as a block of characters is present in the dictionary. As the dictionary initially only has single length characters encoded in the dictionary, it will add this block of characters to its dictionary as well and code them as 95.
When user enters 1st character in the input textbox.
Now the output box will show the code for the 1st character. 13 13

14. Supportive image for step 3
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15. Step 3 15151515 Sr. no. Description of the action / interactivity
Start Time of the action
End Time of the action
Audio narration
On screen text 1.
if previous block (this block can be a single character as well) and the next character together as a block is already present in the dictionary then
the code for this block is shown in the output along with the previous output.
else
the code for previous character is shown in the output along with the previous output and will be sent to the output box and the previous block along with the next character together formed as a block is updated in the dictionary with a next new code assigned to it.
Wait till user inputs next character 2.
Jump to serial no. 2. and keep on repeating it until the dictionary is filled with 256 blocks. (already 95 (0-94) codes/blocks were present, so additional 161 blocks can be added )
After sr.no.1 is complete 15 15

16. Supportive image for step 3a
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17. Supportive image for step 3b
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20. Supportive image for step 3e
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21. Supportive image for step 3f
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29. Supportive image for step 3n
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30. Supportive image for step 3o
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31. Supportive image for step 3p
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Total no. of codes sent 31 31

32. Question 1: What type of data compression is LZW?
Answers:
1. Lossy 2. Lossless 3. None of above
Correct answer: 2
Feedback/Justification to be displayed:
If user clicks correct answer then display: “Correct! Make sure you can explain the reasoning!”
If user clicks incorrect answer then display “Have a look at the animation and Try again!” 32 32

33. Question 2: LZW compression is based on which method?
Answers:
1. Dictionary 2. Statistical 3. None of above
Correct answer: 1
Feedback/Justification to be displayed:
If user clicks correct answer then display “Correct! Make sure you can explain the reasoning!”
If user clicks incorrect answer then display “Have a look at the animation and Try again!” 33 33

34. References
Website links for further reading:
Books: The Data Compression Book by Mark Nelson.
Research papers:
Terry Welch, "A Technique for High-Performance Data Compression", IEEE Computer, June 1984, p. 8–19 34 34