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Methods of Error Detection | O Level Computer Science 2210 & IGCSE Computer Science 0478 | Detailed Free Notes To Score An A Star (A*)

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  • Data transferring from one device to another may not be perfect.
  • Here, interference can occur
  • Data loss
  • Data gained
  • Data changed
  • Password not transmitted correctly, it won’t work
  • Address not transmitted correctly, things won’t be received
  • Accuracy of data is extremely important
  • Types of error detection
    • Parity Check
      • Here, an odd or even methods is used
      • Each byte of data has 7 bits and 1 extra bit called the parity bit
      • It is the left most bit of the data.
        • Can be the first or last bit
      • Before transmission starts, it is set to either odd or even parity
      • Then, we total how many 1s are in the data. If they do not meet that odd or even number, we add an extra one for this purpose
      • Once the data has been transmitted, the bits are recounted.
        • If the count is the same, then we know there has been no error in transmission
        • If the count is not the same, we know there has been a problem in the transmission.
    • Checksum
      • A calculated value is used to check for errors
      • Usually, the value is calculated from the transmitted data.
      • Modulus 11, for example, can be used.
      • Checksum is added to the data before transmission
      • After transmission, the same checksum is calculate again.
      • If both checksums are the same, it means data bas been transmitted correctly
      • If the values do not match, then an error has occurred
    • Echo Check
      • Simple comparison of data
      • The entire data is sent back from the receiving to the sending device
      • Both devices check if the data is the same
      • If yes, then data transmission was correct
      • Otherwise, the data is sent again.
  • Automatic Repeat Request (ARQ)
    • Once the error has been detected, retransmission of the data is reqired
    • Sending device asks receiving device if the data bas been received correctly
    • If not, the receiving device will often use ARQ
    • Here, 2 main wys
      • Positive or negative acknowledgement/ time out
        • In positive acknowledgement
          • Sending device sends the first packet of data
          • Receiving device checks it for errors
          • Once data is error free, positive acknowledgement sent,
          • Here, the next packet of data is then transmitted
          • If no acknowledgement received for the set time limit, a timeout occurs and the data is resent
          • If the limit on the number of times is reached, data will no longer be sent again.
      • Negative acknowledgement uses the opposite method of telling when the data was not correct.
  • Check Digit
    • Errors can occur
    • They can occur due to interference
    • Errors can occur when data entry is performed
    • Manual entry error
    • Scanning error
    • Check digit can be a method to check for errors with data entry
      • Previously calculated from the data entered
      • For example a barcode or ISBN number
    • When data is entered, the check digit is recalculated from the data
    • Stored check digit must match the new calculated check digit
    • If no match, then data is incorrect.
    • Checksum and check digit are different things
  • Encryption
    • Data before encryption is called plain data
    • Once the encryption algorithm has been used on the data, it makes it meaningless and difficult to understand
    • It is called cipher text once encrypted
    • The encryption algorithm is called encryption key
    • The key is then used by the receiving device to decrypt the data.
    • Hacker will not be stopped by encryption from stealing the data.
    • However, the data is no longer meaningful for the hacker even if he steals it
    • Two types
    • Symmetric
      • Plain text is converted to cipher text using encryption key
      • Both the cipher text and the encryption key are sent separately to the receiving device
      • Same key is then used for decryption of the cipher text to the plain text
      • Simplest method of encryption
      • Not very safe
    • Asymmetric
      • Plain text converted to cipher text using public key
      • Then, cipher text sent to receiving device
      • Cipher text can only be decrypted using a private key
      • Person sending the data has pubic key
      • The person receiving the data has the private key
      • They both act as pairs
      • Private key is not sharded with anyone.
Hunain Zia

Hunain Zia has previously achieved considerable success in the high-school educational stream, gaining a massive 154 Total A Grades and 7 Major Distinctions, a process in which he broke/ set 11 different world records, including the most A grades ever achieved, most subjects ever appeared in, most distinctions, gaining distinctions across two separate boards (Pearson Edexcel and CAIE) within the same year/ ever, and even 19 hours of constant examination. All records stand intact today. Hunain pursued his Honors Accounting Degree from the prestigious Bentley University, and then completed an LLB (in Honors) from University of London. Currently, he manages multiple social and commercial projects, has founded digital streams and works tirelessly in the education sector.

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