A parity bit is added to each byte so the total number of 1s is even (even parity) or odd (odd parity). The receiver checks the parity — if it doesn't match, an error is detected.
Limitation: parity can only detect an odd number of errors — two bit errors can go undetected
Checksum & Check Digit
Block-Level Error Detection
Checksum: calculated from the data being sent (e.g. sum all bytes mod 256). Sent with the data. Receiver recalculates — if different, error occurred. Used in TCP/IP.
Check digit: extra digit added to a code (e.g. ISBN, barcode). Calculated from the other digits. Detects errors in data entry. Used on barcodes and bank card numbers.
Check digits catch common human errors like transposing digits (1234 vs 1324)
Echo Check
Confirming Correct Receipt
In an echo check, the receiver sends the received data back to the sender. The sender compares the original and returned data. If they match, the transmission was error-free.
Simple but doubles the data transmitted — inefficient for large transfers
Used in some keyboard-to-screen connections and simple serial protocols
Cambridge 0478: know all three methods and be able to describe how they detect errors
Exam Practice
Have a go at this question
Cambridge IGCSE 0478 style
A byte 1011010 uses even parity. What is the parity bit that should be added, and what is the complete byte sent?
2 marks
1011010 has four 1s — already even, so parity bit = 0 [1]. Complete byte sent: 10110100 [1].
Key Takeaways
What to Remember
Parity bit: ensures total 1s in a byte is even/odd — detects single-bit errors
Checksum: mathematical total checked against recalculated value at receiver
Check digit: extra digit on codes (ISBN, barcodes) to catch data entry errors
Echo check: receiver sends data back; sender compares — error-free if identical