Methods of Error Detection

Question 1

Why do errors occur?

Answer 1

Errors can occur using wired or wireless technology due to interferences/crosstalk like:

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

Question 2

Parity Check

Answer 2

• A parity bit is added (to the parity byte)
• Counts / checks number of 1’s
• Can be even or odd including the parity bit
• If the parity is incorrect, an error is detected

Question 3

Parity bytes and block check

Answer 3

• A parity block consists of a block of data with the number of 1’s totaled horizontally and vertically
• A parity byte is also sent with the data which contains the parity bits from the vertical parity calculation

Question 4

Checksum

Answer 4

• Checksums are calculated using an algorithm and the value is added to the transmission
• The receiving device re-calculates the checksum and compares to the original
• If the checksums do not match, it is assumed an error has occurred

Question 5

Echo Check

Answer 5

• An 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
• This method isn’t reliable as an error could have occurred when the sender transmits the data or when the receiver transmits the data
• If an error does occur the sender will retransmit the data

Question 6

Where are check digits used?

Answer 6

• International standard book numbers (ISBN)
• Barcodes

Question 7

Describe how check digits work

Answer 7

• Data is Input with check digit
• A calculation is performed on the (inputted) data
• The calculated digit is compared to a stored value
• If it matches, the data entered is correct
• If it does not match, the data entered is incorrect

Question 8

Describe how Automatic Repeat reQuests(ARQ) works

Answer 8

• Uses acknowledgement / request and time-out
• Error control protocol
• Check performed on receiving data // error is detected by e.g. parity check, check sum
• negative acknowledgement
• Resend request is repeated till data is sent correctly / requests timeout / limit is reached
• positive acknowledgement
• If acknowledgement not received in a set amount of time data is resent

Question 9

Similarities between check digit and checksum (2)

Answer 9

• They both calculate a value from the data
• They both append the calculated value to the data
• They both recalculate the value
• They both report an error if they don’t match