Coding Systems And Errors

Question 1

What is ASCII(American standard code for information Interchange)

Answer 1

Based on 7 bits of data for each character
128 different characters can be represented

Question 2

What is extended ASCII

Answer 2

8 Bits of data per character
256 different characters represented
For English language characters, most significant bit will be zero

Question 3

What is UNICODE

Answer 3

Industry standard code, 16 bits of data, 65,536 characters possible
ASCII and Extended ASCII incorporated
Extended ASCII characters have upper byte set to all zeros

Question 4

What are the common reasons for errors to happen when communicating between computers

Answer 4

Faulty hardware
Electromagnetic interference

Question 5

What are the 3 types of error detection systems

Answer 5

• Checksums
  -Parity
  -Majority voting

Question 6

What is checksum

Answer 6

A string of digits added to the end of the data to check the data is accurate. Calculated from the data itself using an algorithm

Question 7

What is Check digit

Answer 7

A special case of a checksum that is only one digit long, appropriate for small blocks of data

Question 8

What are examples of algorithms for creating check digits and checksums

Answer 8

-ISBN/EAN-13 barcodes (10(pre 2007) - 12 digits with 1-digit checksum)
-Credit card numbers (16 digits with a 2-digit checksum)
-Checking integrity of file downloads

Question 9

What is the ISBN/EAN-13 algorithm to calculate check digits

Answer 9

1- Add all digits in odd positions
2- Add all digits in even positions and triple the answer
3- Add answers to 1 and 2 together
4- Calculate remainder using mod(10)
5- Check digit = 10 - remainder. (Check digit is 0 if this gives 10)

Question 10

What is parity system

Answer 10

With even parity an extra bit is added at the end which will make the number of ‘1’s in the sequence even

With Odd parity an extra bit is added which will make the number of ‘1’s in the sequence odd

Question 11

What is majority voting

Answer 11

Where for each bit in a sequence a fixed number of that bit will be sent, then the receiver will go with the most common value

For example 5 bits will be sent per bit so if one is wrong the majority will be right and the receiver will go with the common value

Question 12

Name an advantage and disadvantage of using Checksum

Answer 12

Adv
- Can identify one or more errors

Dis
- Needs more processing than other systems
- Can’t correct errors found

Question 13

Name an advantage and disadvantage of using parity

Answer 13

Adv
- Simple to implement

Dis
- Can only detect an odd number of errors
- Can’t correct errors found

Question 14

Name an advantage and disadvantage of using majority voting

Answer 14

Adv
- Can correct errors found when receiver takes the common value

Dis
- Requires a lot more data to be sent, due to the bits sent to create the majority