2.2 Error checking

Errors can occur during data transmission due to interference eg. Data loss, data gain and data change

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

Types of error checking methods:

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 is occured