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