3.5.5.3 Error Checking and Correction.

Question 1

Define a parity bit.

Answer 1

A parity bit is a single bit added to transmission that can be used to check for errors in the transmitted data.

Question 2

How is the value of the parity bit calculated?

Answer 2

Based on the transmitted data itself.

Question 3

Outline the two types of parity bits.

Answer 3

Even parity.

Odd parity.

Question 4

Define an even parity bit.

Answer 4

The value of the parity bit is chosen as to make the total number of 1’s in the transmitted data even.

Question 5

Example of an even parity bit:

Answer 5

If the data 01101110 were to be transmitted, the parity bit would be set to 1, so that the total number of one’s is even.

Question 6

Define an odd parity bit

Answer 6

The value of the parity is chosen as to make the total number of 1’s in the transmitted data odd.

Question 7

Example of an odd parity bit

Answer 7

If the data 011011101 were to be transmitted, the parity bit would be set to 1, so that the total number of one’s is odd.

Question 8

Outline a parity check.

Answer 8

Occurs when data is received. If the value of the received parity bit conforms to the type of parity (odd or even) in use, then the received data is treated as correct.

Question 9

What if the parity check comes back with an error?

Answer 9

Then the received parity bit does not conform to the type of parity in use and the computer will request that the user re-transmits the data.

Question 10

What is an issue with parity bits?

Answer 10

Whether using an odd or even parity, if an even number of bits are changed during transmission, the error is not detected.

Question 11

Outline majority voting.

Answer 11

When using majority voting, each bit of the data is transmitted multiple times. When the data is received, the most commonly occurring value is to be taken as correct.

Question 12

Example of majority voting?

Answer 12

Data to send: 0110
Each bit is transmitted five times:

00000

111111

11111

00000

Data received majority vote
01000 0

011111 1

11111 1

01010 0

Data received after majority vote: 0110

Question 13

What is the primary disadvantage of majority voting?

Answer 13

The volume of data being transmitted is increased with the repetition of bits. In the example, the data transmitted is five times the larger than the original data. This would significantly increase the time taken to transmit data.

Question 14

Define a checksum.

Answer 14

As with parity bits, checksums involve adding a value, determined by the data itself, to the transmitted data.

Question 15

How is a checksum value determined?

Answer 15

By an algorithm, based on the data being transmitted. There is no agreed algorithm and different systems will use their own solutions.

Question 16

Provide an example algorithm which could be used to determine a checksums value.

Answer 16

The modulo function, which returns the remainder after a division. The value of the checksum will be appended to the original data in binary before being transmitted.

Question 17

Example of the modulo checksum algorithm:

Answer 17

Data to send:

46 = 10111

Calculate value of checksum:

46 MOD 8 = 6 = 110

Data transmitted:

101110110

Question 18

Outline what happens after the checksum has being applied and data has been sent and received.

Answer 18

The recipient can remove the checksum and apply the same algorithm as was used when sending the data to ensure that the checksum matches the transmitted data.

Question 19

What happens if the checksum does not match the transmitted data?

Answer 19

Then the recipient cannot correct the error itself so it must request that the sender re-transmits the data.

Question 20

Define a check digit.

Answer 20

A type of checksum in which only a single digit is added to the transmitted data. This reduces the number of different algorithms that could be used to calculate the value of the check digit and so reduces the variety of errors that the method can detect.

Question 21

Can a parity bit detect errors in transmission?

Answer 21

Yes but only if an odd number of bits are changed.

Question 22

Can a parity bit correct errors in transmission?

Answer 22

No.

Question 23

Is a parity bit efficient?

Answer 23

Very efficient.

Question 24

Can a majority vote detect errors in transmission?

Answer 24

Yes.

Question 25

Can a majority vote correct errors in transmission?

Answer 25

Yes as long as the majority of bits remain unchanged.

Question 26

Is a majority vote efficient?

Answer 26

It is inefficient as each bit is sent multiple times.

Question 27

Can a checksum detect errors in transmission?

Answer 27

Yes.

Question 28

Can a checksum correct errors in transmission?

Answer 28

No.

Question 29

Is a checksum efficient?

Answer 29

Mostly efficient as a complex algorithm could make the process less efficient.

Question 30

Can a check digit detect errors in transmission?

Answer 30

Yes.

Question 31

Can a check digit correct errors in transmission?

Answer 31

No.

Question 32

Is a check digit efficient?

Answer 32

Efficient as the algorithms used to calculate the check digit are limited in complexity.