Spotting a Mistake in Stored or Sent Data
Data gets corrupted on the way, so systems send extra bits that only fit when nothing has changed. A parity bit or a check digit catches an error without anybody reading the message.
What a learner can do afterwards
- Add a parity bit to a row, and find the flipped bit in a grid using row and column parity
- Work out the check digit of a short product code and show that one wrong digit fails it
- Say what error detection can and cannot do, using a case where two errors cancel out
1 · Read
Data rots on the way: a bit flips here, a digit smudges there. So systems send extra bits that only fit when nothing changed, catching an error without anybody reading the message.
Give each row an extra parity bit that makes the row hold an even count of ones. In a grid, the row and the column with broken parity cross exactly at the flipped bit.
Product codes do the same with a check digit. The digits 2, 4 and 1 add to 7, so the check digit is 3 to reach 10. Change one digit and the total misses 10, which fails the check.
Know the limit: two flipped bits can cancel and slip past parity. Detection tells you something changed, never that all is well.
Parity and check digits fit only when data is clean, and two cancelling errors can still slip through.
2 · Watch
Take it off screen
Where it sits
This opens up
Nothing builds on it yet.
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.