Error Checking Methods

When and why do errors occur?

• Errors can occur during data transmission due to interference eg. data loss, data gain and data change
• Problems during packet switching (can lead to data loss or even data gain)
• Skewing of data (can occur during parallel transmission, causing data to arrive out of synchronisation)

Explain how data might have errors after transmission

• Data could be lost
• Data could be gained
• Data could be changed
• Bits could be reassembled in the wrong order
• Interference could occur
• Crosstalk could occur
• Data collisions could occur
• Data packets could time out/reach their hop count
• Network could be infected with malware

Parity Check

• Scenario with odd parity:
• The number of 1s are counted
• A parity bit is added to each byte before transmission to make the sum of the bits in each byte odd
• After transmission if the number is odd no error is detected
• After transmission if the number is even an error is detected

Problem with Parity Check

• Cannot detect transposition errors
• Cannot detect if bits have flipped

Parity Block

• Instead of sending 1 byte of data at a time an entire block is sent. This means that the 1 bits are checked both horizontally and vertically.
• Using parity blocks not only identifies that an error has occurred but where the error occurred

Checksum

• A value is calculated from the data // Valid description of calculation
• It is transmitted with the data
• Value is recalculated after transmission
• Values are compared after transmission to check for error

Check Digit

• A digit that is calculated from the data // uses modulo to calculate digit // valid description of modulo
• It is appended/added to the data
• Digit is recalculated when data is entered
• Digits are compared to check for error
• (Used to identify books --- ISBN Number)

Automatic Repeat Request

• Timer is started when sending device transmits a data packet to receiver
• Receiving device checks the data packet for errors
• Once the receiving device knows the packet is error free it sends a positive acknowledgement back to the sending device and the next packet is sent
• If the sending device does not receive an acknowledgement before the timer ends a timeout occurs the data packet is resent
• until acknowledgement received

Echo Check

• When data is sent to another device, this data is sent back again to the sender
• The sender compares the two sets to check if any errors occurred
• If the original data and copy do not match an error has occurred