Error Detection and Correction What is an error
- Slides: 7
Error Detection and Correction
What is an error and when do errors occur? Corrupted files Attachments that won’t open Files that won’t download Videos that won’t play Errors occur when there is a problem with the data, it’s time consuming to fix. Parity checks are used to check the data is OK before it is sent.
Parity Bit Error detection Suppose you are sending a stream of data to a server. By adding a parity bit, you enable the server to detect some basic transmission errors. A parity bit is a bit that is added as the leftmost bit of a bit string to ensure that the number of bits that are ‘ 1’ in the bit string are even or odd. In an even parity scheme the eighth bit, the parity bit, is set to 1 if the number of 1 s in the 7 data bits is odd. It is set to 0 if the number of 1 s in the data is even. In an odd parity scheme the eighth bit, the parity bit, is set to 1 if the number of 1 s in the 7 data bits is even. It is set to 0 if the number of 1 s in the data is odd.
Even Parity - what does it look like? Parity bits have been added to the left hand side of the data This row has an odd number of ‘ 1’ so a ‘ 1’ is added This row is already even – a ‘ 0’ added to the left hand side This column has an odd number of ‘ 1’ so a ‘ 1’ is added 1 1 1 0 1 This column had an even number of ‘ 1’ so a ‘ 0’ is added 0 0 1 0 1 1 1 0 0 0 1 A string of data that is ready to send The number of bits in a row must always add up to an even number
Odd and even Parity Here, you always want to get an even number of 1 s Here, you always want to get an odd number of 1 s
Odd Parity – how would it appear? 1 0 0 1 1 1 0 0 1 0 1 A string of data that is ready to send The number of bits in a row must always add up to an even number
Odd and even Parity NOTE: Either even or odd parity systems can be used to check data, but you will usually find that most people tend to use the even parity scheme.
