4.5.5 Information Coding Systems

Question 1

What is the purpose of information coding systems?

Answer 1

When computers need to represent a character, an information coding system si used to match characters to character codes.

Question 2

What is a character code?

Answer 2

A decimal digit used to represent a character.

Question 3

What are the two most common information coding systems?

Answer 3

ASCII and Unicode

Question 4

How many characters can ASCII represent?

Answer 4

ASCII makes use of 7 bits to represent 128 different characters.

Question 5

What is the advantage of unicode?

Answer 5

Represents a wide variety of alphabets by computers.

Question 6

How many bits does unicode use?

Answer 6

Unicode uses anywhere from 8 to 48 bits per character allowing it to represent a much wider range of different characters than ASCII.

Question 7

What is the purpose of error checking?

Answer 7

Reduce the chances of incorrect data being used.

Question 8

What is a parity bit?

Answer 8

• A single bit added to a transmission that can be used to check for errors in the transmitted data.
• Its value is calculated based on the transmitted data itself.

Question 9

What are the two kinds of parity bit?

Answer 9

Even parity

Odd parity

Question 10

How does even parity work?

Answer 10

The value of the parity bit is chosen so as to make the total number of 1s int he transmitted data even.

Question 11

How does odd parity work?

Answer 11

Adds a parity bit so that the total number of 1s in the transmitted data is odd.

Question 12

How does the parity check work?

Answer 12

• When data is received, a parity check is carried out.
• 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.
• Otherwise the computer will request that the sender re-transmits the data.

Question 13

What is the issue with parity bits?

Answer 13

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

Question 14

How does majority voting work?

Answer 14

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

Question 15

What is the advantage of majority voting over parity bits?

Answer 15

• When an error occurs and the value of a bit is changed:
Majority voting detects the error and
corrects the data, eliminating the need for
retransmission like when using a parity bit.

• Majority voting has the ability to correct errors when the value of multiple bits have changed

Question 16

What is the disadvantage of majority voting?

Answer 16

• The volume of data being transmitted is increased with the repetition of bits, this significantly increases the time taken to transmit data.

Question 17

What are checksums?

Answer 17

Checksums involve adding a value, determined by the data itself to the transmitted data.

Question 18

How does checksum function?

Answer 18

An algorithm is used to determine the value of a checksum based on the data being transmitted.

Question 19

How do checksums work?

Answer 19

once received, 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.
If the two do not match, the recipient cannot correct itself so must request that the sender re-transmits the data.

Question 20

What is a check digit?

Answer 20

A type of checksum in which only a single digit is added to the transmitted data.

Question 21

What is the disadvantage of check digits?

Answer 21

• 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 22

What error checks can be used to detect errors in transmission?

Answer 22

• Parity bit, but only if an odd number of bit are changed
• Majority vote
• Checksum
• Check digit

Question 23

What error checks can be used to correct errors in transmission?

Answer 23

• Majority vote ( as long as the majority of the bits remain unchanged )

Question 24

What is the efficiency of a parity bit?

Answer 24

Very efficient

Question 25

What is the efficiency of majority vote?

Answer 25

Inefficient, as each bit is sent multiple times

Question 26

What is the efficient of checksums?

Answer 26

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

Question 27

What is the efficiency of a check digit?

Answer 27

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