Why data is checked after transmission

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Quick Questions

Shade in the A or B circle for each question.

1. Why does a receiver need to check data after it has been transmitted?

A The data may have been damaged on its way from the sender

B The data always arrives exactly as it was sent

2. Which statement about the receiver is true?

A It cannot see the original data, so it needs a method to check what arrived

B It can look at the original data and compare it with the received copy

3. A student opens a file they downloaded and it is full of strange symbols. What is the most likely cause?

A The file was made smaller by the sender on purpose

B The data was corrupted during transmission

4. What is the purpose of error detection?

A To stop interference from ever happening

B To find out whether the received data has been damaged

5. What is the first step to fixing a transmission error?

A Detecting that an error has happened

B Ignoring the data and using it anyway

Match the Pairs

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

Left column		Right column
1error detection		AData can be damaged by interference in transit, so arriving does not mean it is correct. It must be checked.
2corrupted file		BThe receiver cannot see the original data, so it cannot compare directly. It must use an error detection check instead.
3The receiver cannot see the original data, so it needs a check to tell whether the data was damaged		Cif error: request_resend()
4Detecting an error is the first step to fixing it, so a failed check leads to a request to send again		DA file whose data was changed or damaged in transit, so it no longer matches what was sent
5Data arrives, so it must be correct. No check needed.		EChecking received data to find out whether it was damaged on its way from the sender
6The receiver compares the data with the sender's original copy.		Fif check_failed: error found

Definitions

error detection

Fill in the Blanks

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

IF checkReceived __ checkSent THEN

______ "Error detected"

_______ message FROM sender errorFound ← FALSE

IF checkReceived <> checkSent THEN errorFound ← ____ ENDIF

__ errorFound THEN OUTPUT "Resend"

Spot the Error

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

Question 1

A OUTPUT 'Waiting for data'

B RECEIVE msg FROM receiver

C error ← CHECK(msg)

Question 2

A error_found ← CHECK(msg)

B IF error_found = FALSE THEN

C OUTPUT 'Data damaged'

D ENDIF

Question 3

A error ← CHECK(original)

B IF error = TRUE THEN

C OUTPUT 'Ask for resend'

D ENDIF

Question 4

A SEND file TO sender

B damaged ← CHECK(file)

C IF damaged = TRUE THEN

D OUTPUT 'Send it again'

E ENDIF

Question 5

A RECEIVE msg FROM sender

B error ← CHECK(msg)

C IF error = TRUE

D OUTPUT 'Damaged: resend'

E ENDIF

Answer Key

Why data is checked after transmission

Section A: Quick Questions

A — The data may have been damaged on its way from the sender

A — It cannot see the original data, so it needs a method to check what arrived

B — The data was corrupted during transmission

B — To find out whether the received data has been damaged

A — Detecting that an error has happened

Section B: Match the Pairs

error detection → Checking received data to find out whether it was damaged on its way from the sender

corrupted file → A file whose data was changed or damaged in transit, so it no longer matches what was sent

The receiver cannot see the original data, so it needs a check to tell whether the data was damaged → if check_failed: error found

Detecting an error is the first step to fixing it, so a failed check leads to a request to send again → if error: request_resend()

Data arrives, so it must be correct. No check needed. → Data can be damaged by interference in transit, so arriving does not mean it is correct. It must be checked.

The receiver compares the data with the sender's original copy. → The receiver cannot see the original data, so it cannot compare directly. It must use an error detection check instead.

Section C: Definitions

error detection — Checking received data, because it can be damaged in transit and the receiver cannot see the original. Spotting the error is the first step to fixing it.

Section D: Fill in the Blanks

IF checkReceived <> checkSent THEN

OUTPUT "Error detected"

RECEIVE message FROM sender
errorFound ← FALSE

IF checkReceived <> checkSent THEN
    errorFound ← TRUE
ENDIF

IF errorFound THEN OUTPUT "Resend"

Section E: Spot the Error

B — The receiver gets data from the sender, not from itself. Line 2 should be RECEIVE msg FROM sender so the arrived data can then be checked.

B — The damaged message should only be shown when an error WAS found. The condition must be error_found = TRUE, otherwise good data is reported as damaged.

A — The receiver cannot see the original data, only what arrived. The check must use the received data, so line 1 should be error ← CHECK(received).

A — The receiver must first receive the file before it can check it for damage. Line 1 should be RECEIVE file FROM sender, not SEND.

C — The IF statement is missing THEN. Line 3 should be IF error = TRUE THEN so the code runs when an error is detected.