Week 5 - Character Encoding & Error Detection

Question 1

State two limitations of ASCII

Answer 1

It only allows a small number of characters (≤128) to be represented.
It’s an AMERICAN standard and only allows communication in English.

Question 2

Explain why Unicode was introduced

Answer 2

• Introduced to store more characters so that more languages could be represented (ASCII only allows for English)
• Unicode can represent a wider range of characters

Question 3

State three methods of error detection when transmitting bits

Answer 3

Parity bits
Check digits
Majority voting

Question 4

State two limitations of parity bits in detecting errors

Answer 4

1. Not corrective – does not reveal which bit/s changed during transmission
2. If an even number of bits change (2,4…) it will not detect this

Question 5

Explain how the even parity system works. Include a description of the roles of the sending device and the receiving device during transmission. [4]

Answer 5

Sending Device
• Sender counts the number of 1s in the bit pattern
• and adds an extra bit to ensure even number of 1s

Receiving Device
• Receiver counts the number of 1s in the bit pattern received
• If there are an odd number of 1s it identifies that an error has occurred

Question 6

Explain how a check digit is created [1]

Answer 6

A check digit is calculated by applying a function to our value, until the result is a single number

Question 7

Explain how the check digit system works. Include a description of the sender and receiver. [3]

Answer 7

1. The sender generates the check digit using a function and appends it to the data being sent.
2. The receiver recalculates the check digit based on what it receives.
3. If the recalculated check digit is NOT EQUAL to the received check digit they assume an error has occurred.

Question 8

State the principle behind majority voting [1]

Answer 8

It copies each bit three (or an odd number) of times before sending.
If one bit changes during transmission, the other bits in the section allow determination of the original value.

Question 9

State the role of the sender and receiver in majority voting [2]

Answer 9

Sender triples each bit to make and sends this tripled binary word
Receiver determines the modal bit from each triplet and reconstructs the original binary word from them.

Question 10

State an advantage of majority voting vs other error detection methods [1]

Answer 10

It is corrective – if there is data corruption the original data can be determined through the modal bit

Question 11

State a disadvantage of majority voting [1]

Answer 11

Majority voting is space inefficient, as you are tripling the data that you are sending.
This makes transmission take longer.