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This includes parity bit, majority voting and check digit which are all explained with their rules and more information.

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- 1. Computer Science (A Level) Error Checking and Correction
- 2. Data can be corrupted at any point either in being processed or transmission. Therefore, there are methods of detecting errors in data. Error Checking and Correction There are 3 types of methods: oParity Bit oMajority Voting oCheck Bits
- 3. A method of checking binary codes by counting the numbers of 0s and 1s A bit is added to a block of data for error detection purposes (Definitions)
- 4. If the data has an error and there is 7 bits (ASCII) you can use 8th bit to detect and correct error. Parity Bit Rules: (even parity) 1. If there is an odd number of 1’s, then the parity bit (8th bit) is 1 to make it even. 2. If there is an even number of 1’s, then the parity bit (8th bit) is 0 to make/keep it even.
- 5. (Even) Parity Bit 1.1011010 = 10110100 2.1110111 = 11101110 3.1001001 = 10010011 4.0010000 = 00100001 5.1010101 = 10101010 Examples of using Parity bit checker
- 6. Parity Bit Disadvantages • Cannot detect all errors • Only detect an odd number of errors • Parity bit might change • Increases transmission length • Doesn’t show where the error is, it just says that an error has occurred
- 7. Each bit is transmitted 3 times to make it easier to detect errors (for computers) (Definitions)
- 8. Unlike parity bit, majority voting is able to repair errors. Majority Voting Rules: 1. Each bit is transmitted 3 times 2. If a set of 3 (1 bit) doesn’t have the same three values, majority voting will show and fix errors. e.g. 010 – ‘1’ is the error therefore the transmission should be 000 according to a majority vote
- 9. Majority Voting Original binary 8 bit code 11001010 Transmission (with errors) 101,111,001,010,110,100,011,001 Correction 111,111,000,000,111,000,111,000 Original code executed 11001010 Example of using Majority Voting
- 10. Majority Voting Disadvantages • If there is more than one error in one bit, it will not be detected and the computer will correct it incorrectly assuming that it is right. • The transmission is 3 times longer than what you want to send • Increased processing time
- 11. A digit is added to the end of the binary data to check if the data is accurate (Definitions)
- 12. Usually, the modulo-11 is used to find the check digit. Check Digit Rules (Modulo-11): Example : 23045 Number 2 3 0 4 5 Weighting 6 5 4 3 2 1) The weighting always starts from 2 from the right hand side. Place the numbers in this form/ position 2) Multiply Number by each weighting number Result 12 15 0 12 10 3) Add up results Total = 49 4) Total divided by 11 49 / 11 = 4 rem. 5 5) Subtract remainder from 11 11 – 5 = 6 Check Digit (put this digit on the end of the number)
- 13. 1) 73409 Another Check Digit Example Number 7 3 4 0 9 Weighting 6 5 4 3 2 Result 42 15 16 0 18 Total = 91 91 / 11 = 8 Rem. 3 11 – 3 = 8 Check Digit

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