Checksum, echo check and check digit
Checksum: a value is calculated from a block of data; the receiver recalculates and compares to detect a transmission error. Echo check: the receiver sends the data back to compare with the original; weakness — corruption on the return trip cannot be told from an original error. Check digit: an extra digit at the end of a number, detecting data-entry errors (mistyped/transposed digits) on ISBNs and bar codes.
ARQ: acknowledgement plus timeout
Automatic repeat query (ARQ) uses acknowledgement and a timeout. The sender transmits a block and starts a timer; the receiver checks it. Error-free -> a positive acknowledgement; error found -> a negative acknowledgement and the block is re-transmitted. If no acknowledgement arrives before the timeout, the sender assumes loss and automatically re-sends. Describe one consistent scheme, not both mixed.
Why errors occur; the parity check
Interference causes data loss, data gain or data change (bits flip). In a parity check, both ends agree even or odd parity, and a parity bit is set so the byte total of 1s matches. Example: even parity, data 1011001 has four 1s, so the parity bit is 0. The receiver then re-counts the 1s after transmission; a mismatch = error. Two most-missed marks: the parity bit is added to each byte, and the data is re-checked after transmission.
Drawn from real examiner reports.
Incomplete parity description
Setting the parity value is only half the answer. Two details earn the marks and are routinely dropped: (a) the parity bit is added to each byte before sending, and (b) after transmission the receiver re-counts the 1s in each byte and compares to the agreed parity. Without both, the description is not technical for full marks.
November 2023 Paper (parity question b): "The most common details missed were adding the parity bit to each byte and stating the data was checked again after transmission."
Parity misses a two-bit flip
A single parity bit only detects an odd number of bit flips. If two bits in the same byte flip, the count of 1s is unchanged, the parity still matches, and the error goes undetected. This is why a parity byte / block (parity bits across rows and columns) is stronger — it can locate the flipped bit. Do not claim parity always detects errors.
Checksum vs check digit
A checksum is sent with a block of transmitted data and detects errors during transmission. A check digit is the last digit of a number (ISBN, bar code) and detects data-entry errors — mistyped or transposed digits. Anchor each to a stage: checksum -> a transmitted block; check digit -> an entered number.
Parity bit vs check digit
Both a parity bit and a check digit are appended to guard data, but at different scales. A parity bit is a single bit protecting one byte during transmission. A check digit is a whole digit protecting one number during data entry. Do not describe a check digit as a parity bit for an ISBN — the mechanism and the stage differ.
ARQ: mixing schemes / no timeout
Two recurring ARQ errors. First, mixing positive and negative acknowledgement — pick the scheme the question implies and stay consistent. Second, forgetting the timeout: the defining feature is that if no acknowledgement arrives in a set time, the sender assumes loss and automatically re-transmits. Omit the timeout and ARQ is just a plain error check.
November 2023 Paper (ARQ question c): "Many candidates described both the positive and negative methods, often mixing them together." November 2022 Paper Q9: ARQ marks commonly limited to "acknowledgment and timeout".
Odd vs even parity confusion
Even parity makes the whole byte hold an even number of 1s; odd parity makes it odd. Candidates often compute the parity bit for the wrong scheme. State the rule precisely: the parity bit is chosen so the data bits and the parity bit together have the agreed parity. Re-read which parity the question specifies.
Set a parity bit in four steps
Setting a parity bit is a fixed routine: (1) count the 1s in the data bits; (2) even parity -> the bit is 1 if that count is odd, else 0; (3) odd parity -> 1 if the count is even, else 0. Place it in the agreed MSB/LSB position, then re-count on receipt to check.
ISBN-13 check-digit routine
For an ISBN-13 check digit: (1) multiply the first 12 digits alternately by 1 and 3; (2) add the products for the weighted sum S; (3) the check digit is 10 minus the last digit of S (use 0 if that gives 10). Show the working — method marks are awarded.
Structure describe-process answers
For "describe the process" questions (echo check, ARQ, full parity), use a Send -> Check -> Respond chain: state what is sent, what comparison the receiver makes, and what happens on error (re-send, acknowledgement or timeout).
When data is transmitted, interference can corrupt it — causing data loss, data gain or data change (bits flipping). Error-detection methods check, after transmission (or after data entry), whether the data is still correct. This topic covers five methods: parity check, checksum, echo check, check digit, and the automatic repeat query (ARQ).
| Term | Mark-scheme definition (two elements) |
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