Choosing an error detection method for a scenario

Name:

Date:

Class:

Quick Questions

Shade in the A or B circle for each question.

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

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

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

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

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

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1Sending a file over a network		ACheck digit: ISBN-13 weights 1 and 3, total divisible by 10
2Scanning a bar code at a till		BA 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.
3A shop worker types a 13-digit ISBN by hand, so the error is in data entry and a check digit is the right fit.		CThe 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.
4Combining a check with ARQ: the receiver checks the data, then sends an acknowledgement or asks for a resend.		DParity check fails -> send NAK -> data is sent again
5Answer: Use a check digit to detect interference on a file being sent.		EThe 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.
6Answer: Use a parity bit because it is a parity bit.		FThe 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.

Definitions

Choosing a method

Fill in the Blanks

Complete each line of code by filling in the missing parts.

IF stage = "transmission" THEN check ← ________ ENDIF

item ← "bar code" IF item = "bar code" ____ check ← checkdigit

entry ← "ISBN typed by hand" IF entry _ "ISBN typed by hand" THEN check ← checkdigit

IF checkok = _____ THEN SEND "NAK" ENDIF

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

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A scenario = 'send a file'

B error_type = 'data entry'

C method = 'checksum'

Question 2

A scenario = 'scan a bar code'

B error_type = 'data entry'

C method = 'check digit'

D limit = 'detects every error'

Question 3

A scenario = 'type an ISBN'

B method = 'echo check'

C valid = isbn_ok(code)

Question 4

A got = checksum(packet.data)

B if got != packet.check:

C send('ACK')

D else:

E send('ACK')

Question 5

A method = 'parity bit'

B why = 'corrects bit errors'

C limit = 'misses two bit flips'

Answer Key

Choosing an error detection method for a scenario

Section A: Quick Questions

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

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

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

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

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

Section B: 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.

Section C: Definitions

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.

Section D: Fill in the Blanks

IF stage = "transmission" THEN
  check ← checksum
ENDIF

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

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

IF checkok = FALSE THEN
  SEND "NAK"
ENDIF

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

Section E: Spot the Error

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'.

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'.

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'.

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.

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