Computer Science · Topic 2 · Data transmission — error checking
Corrupt a byte. Catch the error.
Send a byte across a noisy channel with a parity bit attached. Flip one bit mid-flight and the receiver catches it. Flip two and parity is fooled — the exact limitation examiners love to ask about.
0478 Topic 2 — Error detection
parity · checksum · ARQ
Variables
1-bits in data
–
Parity bit sent
–
Receiver verdict
–
Transmission log
| # | Sent (P+data) | Received | Verdict |
|---|
Parity adds ONE extra bit so the count of 1s matches the agreed rule.
📋 Parity check (Cambridge)
- Sender and receiver agree on even or odd parity before transmission.
- The parity bit is set so the total number of 1s (including the parity bit) is even (or odd).
- The receiver counts the 1s — a wrong count means the byte was corrupted and a resend is requested (ARQ: acknowledgement + timeout + retransmission).
- Limitation: if TWO bits flip, the count is right again — the error goes undetected. Try it above.
➕ Checksum & check digit
- A checksum is a value calculated from a whole block of data and sent with it; the receiver recalculates and compares — a mismatch means an error.
- A check digit is the same idea for short identifiers typed by humans (ISBNs, barcodes) — it catches mistyped or transposed digits.
- An echo check sends the data back to the sender for comparison — simple, but you can't tell which copy was corrupted.
🎯 Syllabus reference (0478)
- Topic 2 — Data transmission: why errors occur (interference); parity checks (even/odd, parity byte), checksums, echo checks and check digits; how ARQ uses acknowledgements and timeouts to trigger retransmission.