← IGCSE Computer Science Paper 1

Positive acknowledgement

A message sent by the receiver to tell the sender that data arrived without error. Often written ACK. It is sent only after the error check passes, so the sender can send the next data.

Part of: IGCSE Computer Science 0478 — 2.2 Methods of Error Detection — Module 40, ACARA Digital Technologies Years 7–8 — Wired and Wireless Networks — Module 70, ACARA Digital Technologies Years 9–10 — Networked Systems and Access Control — Module 68

Understanding Positive acknowledgement in IGCSE Computer Science Paper 1

What it does: The IF condition shows that the ACK is only sent when the error check has passed. Receiving the ACK tells the sender the last data arrived without error, so it moves on to the next packet. The receiver sends it back to the sender after checking the data it received. ACK is short for acknowledgement. NAK is used for a negative acknowledgement, which is a different message. A matching checksum means no error was found, so the receiver sends ACK. The broken code sends the ACK too early. The fix is to check the data first and send the ACK only if the check passes.

Common mistakes: The receiver checks the packet for errors, not for an ACK. The ACK is what the receiver sends after the check passes. It should be CHECK packet for errors. The sender may send the next packet only when a positive acknowledgement (ACK) arrives. A NAK means an error was found. It should be IF reply = ACK THEN.

Class: _____________________  ·  Name: _____________________  ·  Date: _________

Free printable worksheet with full answer key — or add this module to a class and your students practise it interactively.

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This module's key terms are also in a printable crossword (Modules 179–188) and word search (Modules 168–188).

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Learning
Duo
What does a positive acknowledgement tell the sender?
The data arrived with an error and must be resentThe data arrived without error
Matches
Receiver sends ACK before checking the data for errors
↓
An ACK must only be sent after the error check passes. Sending it before the check means the receiver cannot know the data arrived without error.
Applying
Meanings
Term
positive acknowledgement
Meaning
A message sent by the receiver to tell the sender that data arrived without error. Often written ACK. It is sent only after the error check passes, so the sender can send the next data.
Practice
RECEIVE packet
Testing
Gaps
SEND
Check Answer
Spots
1RECEIVE packet
2CHECK packet for ACK

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Quick Questions

1. What does a positive acknowledgement tell the sender?
A. The data arrived with an error and must be resent
B. The data arrived without error ✓
A positive acknowledgement (ACK) is sent by the receiver to confirm that the data arrived without error.
2. Who sends a positive acknowledgement?
A. The receiver ✓
B. The sender
The receiver sends it back to the sender after checking the data it received.
3. How is a positive acknowledgement often written?
A. NAK
B. ACK ✓
ACK is short for acknowledgement. NAK is used for a negative acknowledgement, which is a different message.
4. When is a positive acknowledgement sent?
A. Only after the error check passes ✓
B. Before the error check is done
The receiver must check the data first, for example with a parity check or checksum. ACK is sent only if the check passes.
5. The receiver gets a packet and its checksum matches. What should it send back?
A. A positive acknowledgement (ACK) ✓
B. A negative acknowledgement
A matching checksum means no error was found, so the receiver sends ACK.

Match the Pairs

ACK → Short for positive acknowledgement: a message from the receiver telling the sender the data arrived without error
receiver sends ACK to sender → The receiver tells the sender that the data passed the error check and the sender can send the next data
A positive acknowledgement is sent only after the error check passes → IF errorCheck = PASS THEN SEND ACK
When the sender receives an ACK, it can send the next data → IF ACK received THEN send next packet
Receiver sends ACK before checking the data for errors → An ACK must only be sent after the error check passes. Sending it before the check means the receiver cannot know the data arrived without error.
Sender gets ACK so it sends the same data again → An ACK means the data arrived without error, so the sender should send the next data. Resending the same data is what happens after an error, not after an ACK.

Key Term

positive acknowledgement
A message sent by the receiver to tell the sender that data arrived without error. Often written ACK. It is sent only after the error check passes, so the sender can send the next data.

Fill in the Blanks

1. IF errorCheck = PASS THEN SEND ACK ENDIF
Hint: Type the short name for the positive acknowledgement message the receiver sends after the check passes.
2. IF errorCheck = PASS THEN SEND ACK ENDIF
Hint: Type the keyword that starts the code to run when the error check passes.
3. RECEIVE reply IF reply = ACK THEN SEND nextData
Hint: Type the operator that compares the reply with the acknowledgement message.
4. RECEIVE reply IF reply = ACK THEN SEND nextData
Hint: Type the command the sender uses to transmit the next data once the acknowledgement arrives.
5. IF errorFound = FALSE THEN SEND ACK ENDIF
Hint: Type the Boolean value that means no error was found, so the receiver can acknowledge.

Spot the Error

Question 1
A RECEIVE packet
B CHECK packet for ACK
C IF no error THEN
D SEND ACK to sender
E ENDIF
Line B — The receiver checks the packet for errors, not for an ACK. The ACK is what the receiver sends after the check passes. It should be CHECK packet for errors.
Question 2
A SEND packet 1
B WAIT for reply
C IF reply = NAK THEN
D SEND packet 2
E ENDIF
Line C — The sender may send the next packet only when a positive acknowledgement (ACK) arrives. A NAK means an error was found. It should be IF reply = ACK THEN.
Question 3
A RECEIVE packet
B CHECK parity of packet
C IF parity is wrong THEN
D SEND ACK
Line C — An ACK is sent only when the error check passes. The condition must test that the parity is correct, not wrong.
Question 4
A RECEIVE byte
B CHECK parity bit
C IF parity is correct THEN
D SEND NAK to sender
E ENDIF
Line D — When the check passes, the receiver sends a positive acknowledgement (ACK). NAK is the negative acknowledgement used when an error is found. It should be SEND ACK to sender.
Question 5
A RECEIVE packet 3
B IF check fails THEN
C SEND ACK for packet 3
D ENDIF
Line B — The ACK means the data arrived without error, so it is sent only when the check passes. It should be IF check passes THEN.

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