← QCE Digital Solutions Unit 4

Guaranteed delivery

Every packet is confirmed as received (acknowledged). Missing or damaged packets are resent so no data is lost, but waiting and resending adds extra delay.

Part of: QCE Digital Solutions Unit 4 — Digital Methods for Exchanging Data — Module 88

Understanding Guaranteed delivery in QCE Digital Solutions Unit 4

What it does: This is the resending rule. A missing ACK means the packet is treated as lost, so it is sent again. Waiting for acknowledgements and resending lost packets both add time, so transmission takes longer than with no checks. Guaranteed delivery is about completeness. Every packet must be confirmed as received, not just sent quickly. Guaranteed delivery relies on acknowledgements. Silence is not treated as success. Missing or damaged packets are resent. That is how no data is lost. Files need all data intact, so delay is acceptable. Live video values speed over perfect delivery. TCP confirms packets and resends missing ones. UDP sends without confirmation.

Common mistakes: Guaranteed delivery needs the sender to wait for an acknowledgement (ACK) before moving on. Line 2 should be WAIT for ACK. With guaranteed delivery the sender cannot forget the packet. It must keep it, wait for an ACK and resend if needed. Line 1 should be SEND packet 5.

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This module's key terms are also in a printable crossword (Modules 17–88) and word search (Modules 17–125).

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Learning

Duo

What does guaranteed delivery promise about the packets of a message?

Every packet is confirmed as received, so no data is lostEvery packet arrives as fast as possible, even if some are lost

Matches

if ACK received: resend(packet)

↓

A received ACK means the packet arrived safely, so it should not be resent. Resending should happen when NO ACK arrives.

Applying

Meanings

Term

guaranteed delivery

Meaning

Every packet is confirmed as received (acknowledged). Missing or damaged packets are resent so no data is lost, but waiting and resending adds extra delay.

Practice

acknowledged ← FALSE

Testing

Gaps

WAIT FOR

Check Answer

Spots

1SEND packet 1

2SKIP waiting for ACK

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Quick Questions

1. What does guaranteed delivery promise about the packets of a message?

A. Every packet arrives as fast as possible, even if some are lost

B. Every packet is confirmed as received, so no data is lost ✓

Guaranteed delivery is about completeness. Every packet must be confirmed as received, not just sent quickly.

2. How does the sender find out that a packet arrived safely?

A. The sender assumes it arrived if no error message appears

B. The receiver sends back an acknowledgement (ACK) ✓

Guaranteed delivery relies on acknowledgements. Silence is not treated as success.

3. A packet goes missing and no acknowledgement arrives. What happens under guaranteed delivery?

A. The sender resends the packet ✓

B. The receiver skips it and carries on without it

Missing or damaged packets are resent. That is how no data is lost.

4. Which cost comes with guaranteed delivery?

A. Extra delay from waiting for acknowledgements and resending packets ✓

B. Packets are shortened so some data is removed

Waiting for ACKs and resending lost packets takes time. That is the price of reliability.

5. Which situation needs guaranteed delivery most?

A. Downloading a assignment file where every byte must be correct ✓

B. Watching a live video call where a late frame is not worth waiting for

Files need all data intact, so delay is acceptable. Live video values speed over perfect delivery.

Match the Pairs

ACK 1042 → The receiver confirms that packet 1042 arrived safely, so the sender can move on

resend(packet 7) → The sender sends packet 7 again because it was lost or damaged on the way

Guaranteed delivery: the sender waits for an acknowledgement and resends the packet if none arrives in time → if no ACK before timeout: resend(packet)

The cost of guaranteed delivery is extra delay, because the sender must wait for confirmations and resend packets → totalTime = sendTime + waitForACK + resendTime

if ACK received: resend(packet) → A received ACK means the packet arrived safely, so it should not be resent. Resending should happen when NO ACK arrives.

Guaranteed delivery = fastest possible transfer, no waiting → Guaranteed delivery is not faster. Waiting for acknowledgements and resending packets adds extra delay.

Key Term

guaranteed delivery

Every packet is confirmed as received (acknowledged). Missing or damaged packets are resent so no data is lost, but waiting and resending adds extra delay.

Fill in the Blanks

1. SEND packet WAIT FOR ACK IF ackReceived THEN SEND nextPacket

Hint: Type the short name for the confirmation message the receiver sends back after getting a packet.

2. IF NOT ackReceived THEN RESEND packet ENDIF

Hint: Type the logical operator that reverses a Boolean, so the packet is resent when no confirmation arrived.

3. IF timer > timeout THEN RESEND packet ENDIF

Hint: Type the action the sender takes when the wait runs out with no confirmation.

4. WHILE ackReceived = FALSE SEND packet ENDWHILE

Hint: Type the loop keyword that keeps sending until the packet is confirmed as received.

5. resends = 3 extraDelay = resends * roundTrip

Hint: Type the arithmetic operator that shows each resend adds another full round trip of waiting.

Spot the Error

Question 1

A SEND packet 1

B SKIP waiting for ACK

C IF ACK received THEN

D SEND packet 2

E ENDIF

Line B — Guaranteed delivery needs the sender to wait for an acknowledgement (ACK) before moving on. Line 2 should be WAIT for ACK.

Question 2

A SEND packet 5 then forget it

B WAIT for ACK up to 2 secs

C IF no ACK THEN

D RESEND packet 5

E ENDIF

Line A — With guaranteed delivery the sender cannot forget the packet. It must keep it, wait for an ACK and resend if needed. Line 1 should be SEND packet 5.

Question 3

A acked ← TRUE

B WHILE acked = FALSE

C SEND packet 3

D acked ← ACK received

E ENDWHILE

Line A — Starting with acked as TRUE means the loop never runs, so the packet is never sent or confirmed. It should start as FALSE until an ACK arrives.

Question 4

A RECEIVE packet 4

B CHECK checksum

C IF checksum differs THEN

D SEND ACK 4

E ELSE SEND resend request

Line C — The receiver should only send an ACK when the checksum matches, meaning the packet is undamaged. Line 3 should be IF checksum matches THEN.

Question 5

A SEND packets 1 to 3

B RECEIVE ACK 1, 2 and 3

C ACK 2 is missing

D RESEND packet 2

Line B — ACK 2 is missing, so only ACK 1 and ACK 3 arrived. Line 2 should be RECEIVE ACK 1 and ACK 3, which is why packet 2 is resent.

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