The Simplest Form of Error Detection

Parity check is one of the oldest and simplest methods for detecting errors in data transmission. It doesn't correct errors, but it provides a basic way for the receiver to determine if a block of data has been corrupted during its journey through a communication channel. The core idea is to add one extra bit, the parity bit, to each block of data being sent.

This method adds a minimal amount of redundancy to achieve a basic level of integrity checking.

How Parity Check Works

The sender and receiver must first agree on which type of parity to use. There are two options: even parity or odd parity.

The Process in Action (Even Parity Example)

1. Sender's side: Imagine we want to send the 7-bit ASCII code for the letter 'K', which is $1001011$.
   • Count the '1's in $1001011$: There are four '1's.
   • Since four is an even number, the sender sets the parity bit to 0.
   • The transmitted 8-bit block is $1001011$ followed by $0$.
2. Receiver's side: The receiver gets the 8-bit block $10010110$.
   • It counts the total number of '1's: There are four '1's.
   • Since four is an even number, it matches the agreed-upon even parity. The receiver assumes the data is correct.

The Achilles' Heel: Detecting Errors

The crucial limitation of parity checking is its inability to detect an even number of bit errors. Let's see this in practice with our example (data $1001011$, even parity bit $0$, transmitted as $10010110$).

Conclusion: Parity check is guaranteed to detect any odd number of errors (1, 3, 5, etc.) but will fail to detect any even number of errors (2, 4, 6, etc.).

Interactive Parity Checker