Recover transmitted data with ARQ
Trace automatic repeat query using acknowledgements, negative acknowledgements and timeouts.
Section 2 · Data transmission
This sequence follows Cambridge IGCSE Computer Science 0478, 2026–2028 syllabus Version 6. It covers subject content sections 2.1–2.3 only. The separately published 2029–2031 syllabus is not mapped here. Practice is original, self-checked and does not award an official Cambridge grade. Full timed papers and human subject-teacher review remain outstanding.
Before you begin
- No programming language is required. Be comfortable with place value and multiplication; each lesson introduces the computer-science notation it uses.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does ARQ do after a detected transmission error?
What you will learn
- Describe how ARQ checks whether data arrived without error
- Explain positive and negative acknowledgements
- Explain why a timeout causes retransmission
The receiver confirms or rejects a block
Automatic repeat query (ARQ) combines error detection with retransmission. After checking a received packet, the receiver sends a positive acknowledgement (ACK) if it is correct or a negative acknowledgement (NAK) if an error is found.
Apply and check the rule
If the sender receives a NAK, or receives no acknowledgement before a timeout, it retransmits the data. This helps establish that a correct block was received. The process adds messages and time but can recover from loss or corruption.
Pause and explain
What does a timeout indicate in ARQ?
Worked example
A sender transmits a packet and receives no reply before its timer expires. What should ARQ do?
Show the worked solution
- The sender cannot assume that the packet arrived correctly.
- The absence of an acknowledgement triggers the timeout rule.
- The sender transmits the packet again.
Answer Retransmit after the timeout because no positive acknowledgement arrived.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A receiver checks a packet and finds an error. Which acknowledgement should it send, and what happens next?
Give me a hint
Use the negative response for a failed check.
Compare my reasoning
- The receiver sends a NAK.
- The sender retransmits the packet.
- The receiver checks the new copy.
Send a NAK; the sender retransmits and the receiver checks again.
Look for these in your work
- NAK chosen
- Resend stated
- Recheck stated
A sender receives an ACK after transmitting a packet. What does this establish?
Give me a hint
Interpret the positive acknowledgement.
Compare my reasoning
- The receiver checked the data.
- It accepted the packet as correct.
- The sender can continue with the next data.
The receiver's check passed and it accepted the packet.
Look for these in your work
- Check occurred
- Acceptance stated
A mobile connection drops an acknowledgement even though the data arrived. Explain why ARQ may send the data twice and how packet numbers can help the receiver.
Give me a hint
Separate sender knowledge from what happened at the receiver.
Compare my reasoning
- The sender sees no ACK before timeout and assumes receipt is unconfirmed.
- It retransmits to avoid losing data, even if the first copy arrived.
- Packet numbers can help the receiver recognise order or a duplicate, depending on the protocol.
The sender cannot distinguish a lost acknowledgement from a lost packet, so it retransmits after timeout. Packet numbers can help the receiver identify ordering or duplicate copies when the protocol supports that handling.
Look for these in your work
- Sender uncertainty explained
- Timeout response
- Packet numbering role
Common mistakes
- Treating a missing acknowledgement as confirmation
- Confusing a NAK with a positive acknowledgement
What can you explain now?
What does a positive acknowledgement tell the sender?
Compare with the explanation
The received data passed the check
An ACK confirms that the receiver accepted the data as correct.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
What two events can trigger an ARQ retransmission?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.