← IGCSE Computer Science Paper 1

Choosing an error detection method for a scenario

Decide if the error happens in transmission or data entry, pick the check that fits, then justify it by what it can and cannot detect.

Part of: IGCSE Computer Science 0478 — 2.2 Methods of Error Detection — Module 46

Understanding Choosing an error detection method for a scenario in IGCSE Computer Science Paper 1

What it does: Typing errors in an ISBN are caught by recalculating the check digit with the weights and seeing if the result matches. The check detects the error and ARQ fixes it by resending, so together they detect and correct, unlike a check alone. A check digit is calculated from the other digits of the number. The system recalculates it to catch a misread or mistyped digit. A justified answer says why the method fits and states what it can and cannot detect. 'It is the best' gives no reason. Detection methods only find errors. ARQ uses acknowledgements and timeouts so the sender resends the data.

Common mistakes: Sending a file over a network is a transmission scenario, so errors come from interference in transit. error_type should be 'transmission', not 'data entry'. A check digit cannot catch every possible error, because some different mistakes give the same check digit. A justified answer must state this limit, so it should be 'misses some errors'.

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Learning

Duo

A 2 MB file is sent over the internet. Which check suits this scenario best?

A checksum calculated on the data and sent with it, because the error happens in transmissionA check digit, because the error happens in transmission

Matches

Answer: Use a check digit to detect interference on a file being sent.

↓

A check digit is for data entry errors, not transmission errors. A parity check, checksum or echo check should be chosen for a file being sent.

Applying

Meanings

Term

Choosing a method

Meaning

Decide if the error happens in transmission or data entry, pick the check that fits, then justify it by what it can and cannot detect.

Practice

IF scenario = "sending a file" THEN

Testing

Gaps

check ←

Check Answer

Spots

1scenario = 'send a file'

2error_type = 'data entry'

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Quick Questions

1. A 2 MB file is sent over the internet. Which check suits this scenario best?

A. A check digit, because the error happens in transmission

B. A checksum calculated on the data and sent with it, because the error happens in transmission ✓

Check digits are for data entry errors such as typing. Transmission errors are found with parity, checksum or echo check, often with ARQ.

2. A shop scanner reads a bar code and the number is checked on entry. Which method fits?

A. A check digit, because the number is read or entered into a system ✓

B. A parity bit, because bar codes are sent as bytes

A check digit is calculated from the other digits of the number. The system recalculates it to catch a misread or mistyped digit.

3. A student types the 13 digits of an ISBN into a library system. Which justification is better?

A. A check digit can catch a single wrong digit or swapped digits in the typed number ✓

B. An echo check is best because it proves the typed number is correct

An echo check only shows the data matches what was sent, so it cannot catch a typing mistake made before sending. A check digit is built for data entry.

4. Which answer is a justified choice for a file sent between two computers?

A. Use a checksum, because it is the best method

B. Use a checksum, because it detects changed data, but it cannot detect every error ✓

A justified answer says why the method fits and states what it can and cannot detect. 'It is the best' gives no reason.

5. You want errors in transmission to be detected AND fixed without manual help. Which combination fits?

A. An error check with ARQ, so the receiver asks for the data to be sent again ✓

B. A check digit alone, because it corrects the data automatically

Detection methods only find errors. ARQ uses acknowledgements and timeouts so the sender resends the data.

Match the Pairs

Sending a file over a network → The error would happen in transmission, so use a transmission check such as a parity check, checksum or echo check, with ARQ to resend bad data.

Scanning a bar code at a till → The error would happen in data entry, so use a check digit, which can spot a misread or mistyped digit, but it cannot prove the number is the right product.

A shop worker types a 13-digit ISBN by hand, so the error is in data entry and a check digit is the right fit. → Check digit: ISBN-13 weights 1 and 3, total divisible by 10

Combining a check with ARQ: the receiver checks the data, then sends an acknowledgement or asks for a resend. → Parity check fails -> send NAK -> data is sent again

Answer: Use a check digit to detect interference on a file being sent. → A check digit is for data entry errors, not transmission errors. A parity check, checksum or echo check should be chosen for a file being sent.

Answer: Use a parity bit because it is a parity bit. → The answer gives no justification. It should say what the parity bit can detect, such as a single changed bit, and what it cannot, such as two bits changing.

Key Term

Choosing a method

Decide if the error happens in transmission or data entry, pick the check that fits, then justify it by what it can and cannot detect.

Fill in the Blanks

1. IF stage = "transmission" THEN check ← checksum ENDIF

Hint: Type the method suited to checking a whole file sent over a network.

2. item ← "bar code" IF item = "bar code" THEN check ← checkdigit

Hint: Type the keyword that starts the block run when the condition is true.

3. entry ← "ISBN typed by hand" IF entry = "ISBN typed by hand" THEN check ← checkdigit

Hint: Type the comparison operator that tests whether two values are equal.

4. IF checkok = FALSE THEN SEND "NAK" ENDIF

Hint: Type the Boolean value that means the check failed, so a resend is requested.

5. method ← "check digit" reason ← "detects typing errors" OUTPUT method + " because " + reason

Hint: Type the operator that joins the method and the reason into one sentence.

Spot the Error

Question 1

A scenario = 'send a file'

B error_type = 'data entry'

C method = 'checksum'

Line B — Sending a file over a network is a transmission scenario, so errors come from interference in transit. error_type should be 'transmission', not 'data entry'.

Question 2

A scenario = 'scan a bar code'

B error_type = 'data entry'

C method = 'check digit'

D limit = 'detects every error'

Line D — A check digit cannot catch every possible error, because some different mistakes give the same check digit. A justified answer must state this limit, so it should be 'misses some errors'.

Question 3

A scenario = 'type an ISBN'

B method = 'echo check'

C valid = isbn_ok(code)

Line B — Typing an ISBN is a data entry situation, so a check digit is the fitting method. An echo check is for data transmission, so method should be 'check digit'.

Question 4

A got = checksum(packet.data)

B if got != packet.check:

C send('ACK')

D else:

E send('ACK')

Line C — When the checksum does not match, an error has been detected. The receiver must send a negative acknowledgement, send('NAK'), so ARQ makes the sender resend the packet.

Question 5

A method = 'parity bit'

B why = 'corrects bit errors'

C limit = 'misses two bit flips'

Line B — A parity bit only detects that an error has happened. It cannot correct it, so the justification should say it 'detects bit errors'.

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