Error detection

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.

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.

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.

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)

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