Error detection
Introduced| English | Meaning |
|---|---|
| interference/ˌɪntəˈfɪərəns/ | Unwanted signals or disturbances that disrupt communication or alter the signal being received. |
| check digit/tʃek ˈdɪdʒɪt/ | An extra digit at the end of a number, worked out from the others — used for numbers that are typed in (barcodes, ISBNs). |
| parity check/ˈpærɪti tʃek/ | An error-detection method that checks whether the number of set bits matches an agreed odd or even parity. |
| parity bit/ˈpærɪti bɪt/ | An additional bit chosen so a specified set of bits has an even or odd total number of ones, enabling some transmission errors to be detected. |
| checksum/ˈtʃeksəm/ | Recalculate and compare. |
| echo check/ˈekəʊ tʃek/ | An error check in which received data are sent back and compared with the sender's original data. |
| acknowledgement/əkˈnɒlɪdʒmənt/ | acknowledgement |
| timeout/ˈtaɪmaʊt/ | The expiry of an allowed waiting period, after which a system treats an expected event or response as missing. |
| transposition/trænspəˈzɪʃn/ | transposition |
Checking for errors
- While data travels, interference 干扰 can change it — bits lost, gained or flipped.
- So the receiver checks the data arrived correctly.
- Several methods do this, plus a check digit 校验码 for typed-in numbers.
Computing concept lab
Classify concrete examples by the computing idea they demonstrate.
Parity check 奇偶校验
- A parity check adds one parity bit 奇偶校验位 to each byte.
- Even parity: the total number of 1s must be even; odd parity: it must be odd.
- The receiver counts the 1s; if the parity is wrong, an error occurred.
- It can't catch every error — if two bits flip, the count may still look correct.
A few of the USB connector shapes. All use the same USB standard, so devices are detected automatically.
In an even-parity system, the parity bit is set so that:
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Even parity makes the count of 1s even; the receiver re-counts to detect a single-bit error.
A parity check catches a single flipped bit, but if TWO bits flip the 1-count's parity can still look correct, hiding the error.
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That blind spot is why stronger checks (checksums, CRC) are used when reliability really matters.
Checksum 校验和, echo check 回送校验 and ARQ
- Checksum — a value calculated from the data before sending; the receiver recalculates it and compares. A mismatch → resend.
- Echo check — the receiver sends the data back; the sender compares it with the original (slow — data sent twice).
- ARQ (Automatic Repeat reQuest) — the receiver sends a positive/negative acknowledgement 确认; if no positive one arrives before a timeout 超时, the sender resends.
Serial sends one bit at a time down one wire; parallel sends several bits at once down several wires.
A checksum detects errors by:
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The receiver recomputes the checksum; if it differs from the sent one, the data is resent.
In ARQ, the sender resends a packet when:
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ARQ uses acknowledgements and a timeout timer; no positive ack in time → resend.
Check digit
- A check digit is an extra digit at the end of a number, worked out from the others — used for numbers that are typed in (barcodes, ISBNs).
- When entered, it is recalculated and compared.
- It catches an incorrect digit, a transposition 换位 (two digits swapped, 21→12), a missing/extra digit, and phonetic errors.
A parity bit is added to make the number of 1s even (or odd)
Match each error-detection method to how it works.
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Parity and checksums detect errors in transit; a check digit guards typed-in numbers; ARQ triggers the resend.
You've got it
- parity check = even/odd count of 1s via a parity bit; misses some 2-bit errors
- checksum = recalculate and compare; echo check = send the data back
- ARQ = acknowledgements + timeout resend
- a check digit catches typing mistakes (incorrect digit, transposition) in barcodes/ISBNs