P2 T1 L9 - Error checking and correction

Question 1

Why is error checking and correction needed?

2 points

Answer 1

1.Problems may occur when data is transmitted so a system is needed to check for errors:
E.g. Electrical interference

2.This may happen during processing or transmission

Question 2

Define parity checks

2 points

Answer 2

1. A system to detect errors and request retransmission if there is an error
2. If the ‘wrong’ number of 1s turn up then the character is sent again

Question 3

What are the 2 of the parity checking types?

Each of these work the same way

It just depends on whether the system is checking for __s or __s

Answer 3

1. Even parity
2. Odd parity

0s or 1s

Question 4

Describe the process of even parity

4 steps

Answer 4

1. Sender examines bits to be sent
2. The point is to maintain an even number of 1s
3. If it has an even number of 1s a 0 is added at the left side
4. If it has an odd number of 1s a 1 is added

e.g.
A wants to transmit: 1001
A computes parity bit value: 1001 = 1
A adds parity bit and sends: 11001

B receives: 01001
B computes overall parity: 1001 = 1
B reports incorrect transmission after observing result.
Requests retransmission

Question 5

Describe the process of odd parity

4 steps

Answer 5

1. Sender examines bits to be sent
2. The point is to maintain an odd number of 1s
3. If it has an even number of 1s a 1 is added
4. If it has an odd number of 1s a 0 is added

e.g.
A wants to transmit: 1001 
A computes parity bit value: 1001 = 1 
A adds parity bit and sends: 11001 
B receives: 11001
B computes overall parity: 1001 = 1 
B reports correct transmission after observing expected odd result.

Question 6

Explain majority voting

3 points

Answer 6

1. Another method used to identify errors in transmitted data
2. Each bit is sent 3 times e.g. 1001 = 111000000111
3. The idea is that when you receive the data, you expect the same value 3 times, where there is a discrepancy, you can see which bit occurs the most frequently.

Question 7

What is a check digit?

1 point

Answer 7

1. It is a value which is added to the end of a number to try and ensure the number has not been corrupted.