← IGCSE Computer Science Paper 1

Packet switching

Data is split into packets that are sent separately, each routed on its own from router to router, then joined back together in the right order at the destination.

Part of: IGCSE Computer Science 0478 — 2.1 Data Transmission Types and Methods — Module 16, ACARA Digital Technologies Years 7–8 — Wired and Wireless Networks — Module 46, ACARA Digital Technologies Years 9–10 — Networked Systems and Access Control — Module 39, QCE Digital Solutions Unit 4 — Digital Methods for Exchanging Data — Module 83, WA Computer Science ATAR Year 11 — Network Communications — Module 66, WA Computer Science ATAR Year 11 — Cyber Security — Module 19

Understanding Packet switching in IGCSE Computer Science Paper 1

What it does: The arrow shows one message being split into several separate packets, which is the first step of packet switching. The packet moves across the network one router at a time, and its route is chosen independently of other packets. The destination computer uses the packet numbers to rebuild the original message in the correct order.

Common mistakes: In packet switching the loop must go through the packets, not the whole message. It should be FOR EACH p IN packets, so each packet is sent separately. Packets are sent separately, not stuck together. Line 3 should be SEND(p1, route1), and p2 is sent on its own in the next line.

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This module's key terms are also in a printable crossword (Modules 110–120) and word search (Modules 86–120).

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Learning

Duo

Which statement about packet switching is correct?

A message is split into packets that are sent separatelyA message is always sent as one single block

Matches

Packet switching sends the whole message as one single block along one fixed route.

↓

Packet switching splits the message into separate packets. It does not send one block, and each packet is routed on its own.

Applying

Meanings

Term

packet switching

Meaning

Data is split into packets that are sent separately, each routed on its own from router to router, then joined back together in the right order at the destination.

Practice

packets ← SPLIT(message)

Testing

Gaps

packets ← (message, 4)

Check Answer

Spots

2packets ← SPLIT(message, 4)

3FOR EACH p IN message

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Quick Questions

1. Which statement about packet switching is correct?

A. A message is always sent as one single block

B. A message is split into packets that are sent separately ✓

Packet switching breaks data into packets, and each packet is sent separately across the network.

2. How is each packet routed in packet switching?

A. All packets must follow the same fixed route

B. Each packet is routed on its own ✓

Each packet is routed independently, so different packets from the same message can take different routes.

3. What passes packets across the network from one point to the next?

A. Routers pass packets on from router to router ✓

B. The receiving computer picks every packet's route

Routers pass each packet along the network, from router to router, until it reaches its destination.

4. What happens to the packets when they reach the destination?

A. They are joined back together in the right order ✓

B. They are kept apart and read separately

The destination computer uses the packet numbers to rebuild the original message in the correct order.

5. Which is the correct order of the packet switching process?

A. Split into packets, send separately, rejoin at destination ✓

B. Send as one block, split at destination, rejoin at router

Data is first broken into packets, then the packets are sent separately, then they are put back together at the destination.

Match the Pairs

packet switching → Data is split into packets that are sent separately across a network, each routed on its own from router to router.

Packet 3 of 8 reaches the destination → One of the separate packets has arrived, and it will be joined back together with the others in the right order.

In packet switching, a message is broken into packets before it is sent → message -> [packet 1][packet 2][packet 3]

Each packet is passed from router to router and is routed on its own → Packet 2: Router A -> Router C -> Router D

Packet switching sends the whole message as one single block along one fixed route. → Packet switching splits the message into separate packets. It does not send one block, and each packet is routed on its own.

Packets arrive at the destination and stay in separate pieces, so the message is never rebuilt. → The packets are joined back together in the right order at the destination to rebuild the original message.

Key Term

packet switching

Data is split into packets that are sent separately, each routed on its own from router to router, then joined back together in the right order at the destination.

Fill in the Blanks

1. packets ← SPLIT(message, 4)

Hint: Type the function that breaks the message into separate packets.

2. FOR EACH p IN packets SEND(p, router)

Hint: Type the command that passes each packet on to the router.

3. ordered ← SORT(received, packetNumber)

Hint: Type the function that puts packets that arrived out of order back into the right order.

4. message ← JOIN(ordered)

Hint: Type the function that joins the packets back together at the destination.

5. total ← LENGTH(packets) IF count = total THEN message ← JOIN(ordered)

Hint: Type the variable that holds how many packets were sent, so we know when all have arrived.

Spot the Error

Question 1

A message ← "HELLO WORLD"

B packets ← SPLIT(message, 4)

C FOR EACH p IN message

D SEND(p)

Line C — In packet switching the loop must go through the packets, not the whole message. It should be FOR EACH p IN packets, so each packet is sent separately.

Question 2

A p1 ← "HELL"

B p2 ← "O WO"

C SEND(p1 + p2, route1)

D SEND(p2, route2)

Line C — Packets are sent separately, not stuck together. Line 3 should be SEND(p1, route1), and p2 is sent on its own in the next line.

Question 3

A packet ← RECEIVE()

B next ← BEST_ROUTE(packet)

C FORWARD(message, next)

Line C — A router passes on one packet at a time and never sees the whole message. It should forward the packet: FORWARD(packet, next).

Question 4

A received ← FIRST_PACKET()

B SORT(received, BY number)

C message ← JOIN(received)

D OUTPUT message

Line A — The destination needs all the packets to join the message back together. It should collect them all with ALL_PACKETS(), not only the first packet.

Question 5

A message ← "GOOD MORNING"

B packets ← JOIN(message, 4)

C OUTPUT LENGTH(packets)

Line B — At the sender the message is broken into packets, so it must use SPLIT, not JOIN. The correct line is packets ← SPLIT(message, 4).

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