← IGCSE Computer Science Paper 1

Comparing the error detection methods

Parity, parity block, checksum and echo check find errors in transmission; a check digit finds them at data entry; ARQ asks for a resend. Each sends different extra data.

Part of: IGCSE Computer Science 0478 — 2.2 Methods of Error Detection — Module 45, ACARA Digital Technologies Years 7–8 — Wired and Wireless Networks — Module 75

Understanding Comparing the error detection methods in IGCSE Computer Science Paper 1

What it does: A check digit is the one method in the comparison that is used for data entry, such as ISBNs and bar codes. Automatic repeat query uses acknowledgements and a timeout to request a resend, rather than adding check data to the data. In an echo check the receiver returns the data it got. A parity bit just adds one extra bit to each byte. A parity block uses row and column parity. Where the failing row and column cross, that is the wrong bit. A single parity bit only shows that a byte has an error.

Common mistakes: A parity bit checks data during transmission, not at data entry. Line 1 should read 'parity: used in transmission'. A checksum sends a calculated value with the data block. It does not send one bit per byte (that is a parity bit). Line 2 should be 'extra data: calculated value'.

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Learning
Duo
A librarian types an ISBN into a computer. Which method checks it for typing errors?
ChecksumCheck digit
Matches
A check digit is the best method for finding errors in data sent across a network.
↓
A check digit is used for data entry, such as typing in an ISBN. Transmission uses parity, parity blocks, checksums, echo checks or ARQ.
Applying
Meanings
Term
error detection method
Meaning
Parity, parity block, checksum and echo check find errors in transmission; a check digit finds them at data entry; ARQ asks for a resend. Each sends different extra data.
Practice
parity bit: 1 extra bit per byte, transmission
Testing
Gaps
OUTPUT "Parity bit adds extra bit"
Check Answer
Spots
1parity: used in data entry
2extra data: 1 bit per byte

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Quick Questions

1. A librarian types an ISBN into a computer. Which method checks it for typing errors?
A. Check digit ✓
B. Checksum
A check digit is calculated from the other digits and is used at data entry. A checksum is used for transmitted blocks of data.
2. Which method sends a copy of the data back to the sender so the two can be compared?
A. Parity bit
B. Echo check ✓
In an echo check the receiver returns the data it got. A parity bit just adds one extra bit to each byte.
3. What extra data does a single parity check add to each byte?
A. A whole extra byte
B. One extra bit ✓
A parity bit is one bit per byte. A parity byte is a whole extra byte added after a block of bytes.
4. Which method can find the exact bit that changed in a block of data?
A. Parity block check ✓
B. Single parity bit check
A parity block uses row and column parity. Where the failing row and column cross, that is the wrong bit. A single parity bit only shows that a byte has an error.
5. Which method asks the sender to send the data again if there is a problem?
A. ARQ ✓
B. Checksum
ARQ uses acknowledgements and a timeout to request a resend. A checksum only detects an error and does not ask for anything to be resent.

Match the Pairs

Parity bit → One extra bit added to each byte so the number of 1s is even or odd. It spots a single changed bit but cannot show which bit is wrong.
Echo check → The receiver sends the data back to the sender, who compares it with the original. No extra check data is added, but a mismatch does not show whether the error happened on the way there or the way back.
A method used at data entry rather than transmission: an extra digit is calculated from the other digits, so a mistyped number can be detected. → Check digit
A method that does not add check data but asks for the data to be sent again when an error is found or no acknowledgement arrives. → ARQ
A check digit is the best method for finding errors in data sent across a network. → A check digit is used for data entry, such as typing in an ISBN. Transmission uses parity, parity blocks, checksums, echo checks or ARQ.
A checksum is a single extra bit added to each byte so that the number of 1s is even. → That describes a parity bit. A checksum is a value calculated from a whole block of data and sent with it.

Key Term

error detection method
Parity, parity block, checksum and echo check find errors in transmission; a check digit finds them at data entry; ARQ asks for a resend. Each sends different extra data.

Fill in the Blanks

1. OUTPUT "Parity bit adds 1 extra bit"
Hint: Type the number of extra bits a single parity bit adds to each byte.
2. OUTPUT "Check digit is used at data entry"
Hint: Type the word for where a check digit is used, as opposed to during transmission.
3. OUTPUT "Echo check sends the data back"
Hint: Type the word for the direction the receiver sends the data in an echo check.
4. OUTPUT "ARQ asks the sender to resend data"
Hint: Type the action ARQ requests from the sender when an error is found.
5. IF where = "transmission" THEN OUTPUT "parity, checksum or echo check" ENDIF
Hint: Type the keyword that closes the selection block.

Spot the Error

Question 1
A parity: used in data entry
B extra data: 1 bit per byte
C detects: a change in one bit
D misses: two bits changed
Line A — A parity bit checks data during transmission, not at data entry. Line 1 should read 'parity: used in transmission'.
Question 2
A method: checksum
B extra data: 1 bit per byte
C receiver recalculates it
D used in: transmission
Line B — A checksum sends a calculated value with the data block. It does not send one bit per byte (that is a parity bit). Line 2 should be 'extra data: calculated value'.
Question 3
A method: check digit
B extra data: 1 digit on number
C detects: typing mistakes
D used in: data transmission
Line D — A check digit is used when data is entered, for example typing an ISBN or reading a bar code. It is not a transmission check. Line 4 should be 'used in: data entry'.
Question 4
A method: echo check
B receiver sends data back
C receiver compares both copies
Line C — In an echo check the sender compares the original with the copy echoed back, because only the sender has the original. Line 3 should say the sender compares both copies.
Question 5
A method: ARQ
B receiver sends ACK or NAK
C on NAK the data is corrected
D no reply: timeout, resend
Line C — ARQ does not correct data. A negative acknowledgement (NAK) makes the sender send the data again. Line 3 should be 'on NAK the data is resent'.

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