2.3 Error Detection
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2.3 Error Detection Trainer
Master parity checks, checksums, echo checks, check digits and ARQ — the five error detection methods Cambridge tests.
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§1 · Topic overview
§2 · Learning objectives
By the end of this lesson you should be able to:
- Explain why errors occur in transmission, using the word interference
- Describe how a parity check uses a parity bit to make an odd or even total
- Explain what a parity block check catches that a single parity check misses
- Describe how a checksum is calculated, sent, recalculated and compared
- Describe how an echo check sends the data back for comparison
- Describe ARQ using acknowledgement, timeout and resend — and say it responds to errors
- State that a check digit checks data entry, not transmission
- Say that these methods detect errors — they do not correct them
§3 · Key terminology
| Term | Definition |
|---|---|
| Interference | Disruption during transmission that can cause data to be lost, gained or changed. |
| Parity check | An error detection method that adds an additional bit to each byte to create an odd or even sum. |
| Parity bit | The extra bit added to a byte — normally the first bit — set to make the total number of 1s odd or even. |
| Parity block check | Parity applied across both rows and columns of a block of bytes, so the position of an error can be pinpointed and errors a single check would miss are caught. |
| Checksum | An error detection method that performs a calculation on the data to create a checksum value. The values are compared after transmission to see if they match. |
| Echo check | An error detection method that sends a copy of the transmitted data back to the sender to be compared with the original data sent. |
| ARQ | Automatic Repeat reQuest — a method that uses acknowledgement and timeout to see if data has arrived correctly after transmission. |
| Acknowledgement | A message sent back to the sender saying whether the data arrived error free (positive) or with errors (negative). |
| Timeout | What happens when the sender does not receive a positive acknowledgement within a set timeframe. |
| Check digit | An extra digit added to data such as a barcode or ISBN to check it was entered correctly. Not a transmission method. |
§4 · Core theory
The methods at a glance
| Method | How it spots the error | Used for |
|---|---|---|
| Parity check | Counts the 1s in each byte; a total of the wrong parity means an error. | Transmission |
| Parity block check | Parity on rows and columns, so the faulty bit can be located. | Transmission |
| Checksum | A value calculated before and after; mismatched values mean an error. | Transmission |
| Echo check | Data is sent back and compared with the original. | Transmission |
| ARQ | Acknowledgements and timeouts trigger a resend. Responds — does not detect. | Transmission |
| Check digit | A calculated digit checks the value was keyed in correctly. | Input |
Parity check, step by step
| Step | What happens |
|---|---|
| 1 | Before transmission, the system is set to odd or even parity. |
| 2 | Each byte carries 7 bits of data plus 1 parity bit. |
| 3 | The number of 1s in the 7 data bits is totalled. |
| 4 | The parity bit is set to 1 or 0 so the total number of 1s matches the agreed parity. |
| 5 | The data is transmitted. |
| 6 | The receiving device counts the 1s in each byte. Wrong parity means an error is detected. |
ARQ — the two acknowledgement methods
| Positive acknowledgement | Negative acknowledgement |
|---|---|
| Receiver checks the packet. If error free, it sends a positive acknowledgement. | Receiver checks the packet. If errors are found, it sends a negative acknowledgement. |
| Sender receives it and sends the next packet. | Sender receives it and resends that packet. |
| No acknowledgement inside the set timeframe → timeout → resend. | If no errors are found, no further action is taken. |
§5 · Worked examples
Example 1 — How a parity check detects an error [4 marks]
Question: Describe how a parity check is used to detect an error in transmitted data.
Answer: Before transmission the system is set to odd or even parity [1]. A parity bit is added to each byte so that the total number of 1s in the byte matches the agreed parity [1]. After transmission the receiving device counts the number of 1s in each byte [1]. If the total does not match the agreed parity, an error has been detected [1].
Why it earns the marks: four distinct stages — agree parity, set the bit, recount, compare. Note the answer says detected, never "corrected".
Example 2 — How a checksum works [3 marks]
Question: Describe how a checksum is used to detect errors in data transmission.
Answer: A value is calculated from the data before transmission and sent along with it [1]. The receiving device performs the same calculation on the data it received [1]. If the two values do not match, an error has occurred during transmission [1].
Why it earns the marks: calculate → send with data → recalculate → compare. The comparison is the mark most often dropped.
Example 3 — ARQ [3 marks]
Question: Describe how automatic repeat request (ARQ) is used when transmitting data.
Answer: The receiving device checks each packet for errors and sends an acknowledgement back to the sender [1]. If the sender does not receive a positive acknowledgement within a set timeframe, a timeout occurs [1]. The sender then resends the data packet, repeating this until it receives a positive acknowledgement or a resend limit is reached [1].
Why it earns the marks: the three ARQ keywords — acknowledgement, timeout, resend — all appear. Miss "timeout" and you cap yourself.
§6 · Common misconceptions
It detects. None of these methods corrects data — they flag that something is wrong so a resend can be requested.
No. A check digit checks data was entered correctly — barcodes, ISBNs, account numbers. If the question says transmission, do not offer it.
They sound alike and are not alike. Checksum = transmission. Check digit = input. One word apart, both marks gone.
ARQ is how the system responds once an error has been detected — acknowledgement, timeout, resend.
It does not. If two bits in the same byte change, the total number of 1s can still match the agreed parity and the error slips through. That is exactly why the parity block check exists.
Not necessarily. The corruption could have happened on the return journey. All you know is that an error occurred somewhere.
§7 · Cambridge exam focus
§8 · Quick knowledge check
Tap each card to reveal the answer. Say it out loud first — that's the whole trick.
🔒 Nearly there
Work through the Quick Knowledge Check above. Once you've reviewed all six, the Activities unlock recommendation appears here.
Five error detection methods
Parity check
An extra parity bit is added to make the total number of 1s either even or odd. After transmission, the 1s are counted again — if the count doesn't match, an error is detected.
Checksum
A calculated value is sent with the data. The receiver recalculates and compares. If the values differ, there's an error.
Echo check
The receiver sends the data back to the sender. The sender compares both versions. If different, an error occurred.
Check digit
An extra digit added to input data (barcodes, ISBN). Used to check data was entered correctly — NOT used during transmission.
ARQ (response, not detection)
Automatic Repeat reQuery. If an error is detected → negative acknowledgement → data resent. If no error → positive acknowledgement. ARQ responds to errors, it does not detect them.
How parity works — step by step
Even parity example
1. Sender and receiver agree on even parity.
2. Data: 1 0 1 1 0 0 1 → count of 1s = 4 (already even).
3. Parity bit = 0 (no change needed — 4 is even).
4. Sent: 1 0 1 1 0 0 1 [0]
5. Receiver counts 1s. If not even → error detected.
Interactive parity machine
See how parity bits are calculated and watch error detection + ARQ in action.
Parity block check
A parity byte check checks each byte individually — but if two bits in the same byte flip, the error is invisible (even number of errors cancels out).
A parity block check adds parity to rows AND columns. This can detect the specific bit that changed — even when a parity byte check would miss it.
Quick check
Check digit is used during:
Activity 1 — Name that Method
Which error detection method is being described?
Press New Question.
Activity 2 — Spot the Mistake
Press New Question.
Activity 3 — Transmission or Input?
Is this method used during data transmission or data input?
Press New Question.
Adaptive practice
Press New Question.
Cambridge-style questions
Press New Question.
Review quiz
Press New.
Exam traps
Memory triggers
🧠 "Parity = count the 1s"
Even parity: total 1s must be even. Odd parity: total 1s must be odd. The parity bit makes it so.
🧠 "Echo = echo back"
The clue is in the name — echo check sends data back like an echo for comparison.
🧠 "ARQ = Ask, Resend, Query"
Error found → ask for resend. No error → say it's fine. It responds, it doesn't detect.