methods of error detection 2.2

Question 1

How do errors occur?

Answer 1

Data loss - data is lost in transmission
Data gain - additional data is received
Data change - some bits have been changed or flipped

Question 2

Error Detection Methods =5

Answer 2

• Parity check
• Checksums
• Echo check
• Check Digits
• Automatic Repeat Request (ARQ)

Question 3

how parity bits are used to detect errors

Answer 3

1. the parity type is agreed between sender and reciever
2. sender calculates the number of 1’s to decide which parity to use
3. parity byte and data will be sent to the receiver
4. receiver will calculate the number of 1’s
5. if the number of 1 bit dont match the parity, an error has occured

Question 4

what is the issue with parity check

Answer 4

Parity checks only check that an error has occurred, they do not reveal where the error(s) occurred, so cant identify if two bits have transposed (swaped)

Question 5

how checksum detects error

Answer 5

1. a value is calculated from the data using an agreed algorithm
2. The checksum value is transmitted with the data
3. The receiver re-calculates the checksum value using the received data
4. The re-calculated checksum is compared to the transmitted checksum
5. If they are the same then there is no error otherwise an error has occurred
6. and an requet to resend the data will be made

Question 6

how does Echo check work?

Answer 6

Echo checks involve transmitting the received data back to the sender. The sender then checks the data to see if any errors occurred during transmission

Question 7

issue with echo check

Answer 7

This method isn’t reliable as an error could have occurred when the sender transmits the data or when the receiver transmits the data. Neither will know when the error occurred.

Question 8

how is check digit used

Answer 8

1. A digit that is calculated from the data
2. It is added to the data
3. Digit is recalculated when data is entered
4. Digits are compared to check for error

Question 9

how does Automatic Repeat Request (ARQ) work

Answer 9

1. timer is started when sending device transmits a data packet to the receiver
   Check performed on receiving data
2. If no error is detected the receiver sends a positive acknowledgement
3. If an error is detected the receiver sends a negative acknowledgement
4. if sender receives no acknowledgement transmission then the sender waits untill timer ends ans a time-out occurs then the data is resent
5. This process is repeated until all data has been received and acknowledged