← QCE Digital Solutions Unit 4

Timely delivery

Data arrives quickly enough to be useful, even if some packets are lost. Live calls and games favour speed over guaranteed delivery, so late or missing packets are skipped, not resent.

Part of: QCE Digital Solutions Unit 4 — Digital Methods for Exchanging Data — Module 89

Understanding Timely delivery in QCE Digital Solutions Unit 4

What it does: Discarding late packets keeps the game current, which is timely delivery in action. Turning off resending removes the delay it causes, which is the trade-off against guaranteed delivery. In a live call, old data is useless once the moment has passed. It is better to skip a lost packet than to freeze the call waiting for it. Timely delivery puts speed first. Guaranteed delivery puts completeness first. Late data is out of date. Games favour speed over resending, so old packets are dropped. The mistake is putting completeness ahead of speed in a live situation.

Common mistakes: The comparison is the wrong way round. It would skip packets that arrive on time. A packet is skipped when its delay is greater than the deadline: if delay_ms > deadline_ms:. FTP is for transferring files and guarantees delivery. Live calls need timely delivery, so they use a fast protocol such as UDP. It should be protocol = "UDP".

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This module's key terms are also in a printable crossword (Modules 89–125) and word search (Modules 17–125).

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Learning

Duo

A live video call is running. Which approach suits it better?

Timely delivery: skip lost packets and keep goingGuaranteed delivery: pause until every lost packet is resent

Matches

Live call: pause audio until every lost packet is resent so nothing is missed

↓

This applies guaranteed delivery to a live call. Waiting for resends makes the audio late and useless. Timely delivery skips lost packets to keep up.

Applying

Meanings

Term

timely delivery

Meaning

Data arrives quickly enough to be useful, even if some packets are lost. Live calls and games favour speed over guaranteed delivery, so late or missing packets are skipped, not resent.

Practice

FOR EACH packet IN stream

Testing

Gaps

callActive:

Check Answer

Spots

2delay_ms = 200

3if delay_ms < deadline_ms:

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Quick Questions

1. A live video call is running. Which approach suits it better?

A. Timely delivery: skip lost packets and keep going ✓

B. Guaranteed delivery: pause until every lost packet is resent

In a live call, old data is useless once the moment has passed. It is better to skip a lost packet than to freeze the call waiting for it.

2. What does timely delivery mean?

A. Data arrives quickly enough to be useful, even if some packets are lost ✓

B. Every packet is guaranteed to arrive, no matter how long it takes

Timely delivery puts speed first. Guaranteed delivery puts completeness first.

3. In an online game, a packet showing where a player was 2 seconds ago arrives late. What should happen?

A. The game freezes until the packet is processed

B. It is skipped, because the game has moved on ✓

Late data is out of date. Games favour speed over resending, so old packets are dropped.

4. What is the trade-off when you choose timely delivery?

A. You gain speed but may lose some packets ✓

B. You gain a guarantee that every packet arrives but lose speed

The second option describes guaranteed delivery. Timely delivery accepts some loss in return for low delay.

5. Which activity needs guaranteed delivery more than timely delivery?

A. Talking to a friend on a live voice call

B. Downloading a file where every byte must be correct ✓

A file with missing parts is corrupted, so completeness matters more than speed. A live call needs timely delivery.

Match the Pairs

timely delivery → Data arrives quickly enough to still be useful, even if some packets are lost along the way

UDP for a live video call → Late or missing packets are skipped rather than resent, so the call keeps running in real time

In an online game, a packet arriving late is worthless, so it is better to skip it than wait for a resend → if packet.arrivedLate: discard(packet)

Guaranteed delivery resends lost packets, which adds delay, so timely delivery trades it away for speed → resend_lost_packets = False

Live call: pause audio until every lost packet is resent so nothing is missed → This applies guaranteed delivery to a live call. Waiting for resends makes the audio late and useless. Timely delivery skips lost packets to keep up.

Timely delivery means every single packet must arrive, however long it takes → This describes guaranteed delivery. Timely delivery means data arrives quickly enough to be useful, even if some packets are lost.

Key Term

timely delivery

Data arrives quickly enough to be useful, even if some packets are lost. Live calls and games favour speed over guaranteed delivery, so late or missing packets are skipped, not resent.

Fill in the Blanks

1. while callActive: packet = receive() play(packet)

Hint: Type the keyword that repeats the loop for as long as the live call continues.

2. for packet in callStream: if packet.late: continue

Hint: Type the keyword that skips a late packet and moves on to the next one without resending it.

3. gameMode = "live" resendLost = False

Hint: Type the Boolean value a live game uses for resending lost packets, since waiting would make the data too late.

4. if mode == "live": protocol = "UDP"

Hint: Type the short name of the fast protocol with no guaranteed delivery that live calls and games favour.

5. if arrivalTime > deadline: drop(packet)

Hint: Type the comparison operator that shows a packet arrived after its deadline and is too late to be useful.

Spot the Error

Question 1

A deadline_ms = 150

B delay_ms = 200

C if delay_ms < deadline_ms:

D skip_packet()

Line C — The comparison is the wrong way round. It would skip packets that arrive on time. A packet is skipped when its delay is greater than the deadline: if delay_ms > deadline_ms:

Question 2

A protocol = "FTP"

B call_type = "live"

C if call_type == "live":

D use(protocol)

Line A — FTP is for transferring files and guarantees delivery. Live calls need timely delivery, so they use a fast protocol such as UDP. It should be protocol = "UDP".

Question 3

A for packet in stream:

B if packet.on_time:

C continue

D play(packet)

Line B — The condition checks for on-time packets, so the loop would skip good packets and play late ones. It should check packet.late so that late packets are skipped.

Question 4

A lost = 3

B total = 100

C loss_pct = total / lost * 100

D print(loss_pct)

Line C — The division is upside down. Packet loss is lost divided by total, then multiplied by 100. It should be loss_pct = lost / total * 100.

Question 5

A def handle(packet):

B if packet.late:

C resend(packet)

D else:

E play(packet)

Line C — In timely delivery a late packet is skipped, not resent. A resent packet would arrive even later and be useless in a live call. It should be drop(packet).

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