← IGCSE Computer Science Paper 1

Describing the packet switching process step by step

Data is split into numbered packets, routers pick each packet's route, packets may arrive out of order, then they are reordered to rebuild the data once the last packet arrives.

Part of: IGCSE Computer Science 0478 — 2.1 Data Transmission Types and Methods — Module 21, ACARA Digital Technologies Years 7–8 — Wired and Wireless Networks — Module 52

Understanding Describing the packet switching process step by step in IGCSE Computer Science Paper 1

What it does: The arrival order 3, 1, 4, 2 is not the order they were sent in (1, 2, 3, 4), which shows packets arriving out of order. The destination uses the packet numbers to sort the packets into 1, 2, 3, 4, which rebuilds the original message. Because each packet may take a different route, a later packet can arrive before an earlier one. The data must be split and numbered before it is sent. It is reordered only after the packets have arrived. Splitting and numbering is always the first step, because the numbers are needed later to rebuild the data in order.

Common mistakes: The first step of packet switching is to split the data into packets, not join it. The line should be SPLIT message INTO packets. Joining (reordering) happens last, at the destination. A router chooses the route for each packet separately, so packets can take different routes. The line should be ROUTER picks route per packet.

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This module's key terms are also in a printable crossword (Modules 121–130) and word search (Modules 121–141).

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Learning
Duo
What is the FIRST step of packet switching?
The packets are reordered at the destinationThe data is split into packets and each packet is numbered
Matches
Step 1: Send the whole message as one packet with no numbers
↓
Packet switching starts by splitting the data into several packets and numbering them. Without numbers the destination cannot reorder them.
Applying
Meanings
Term
packet switching steps
Meaning
Data is split into numbered packets, routers pick each packet's route, packets may arrive out of order, then they are reordered to rebuild the data once the last packet arrives.
Practice
1. Split the message into packets
Testing
Gaps
Step 1: Split the data into
Check Answer
Spots
1JOIN message INTO packets
2NUMBER each packet 1,2,3

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Quick Questions

1. What is the FIRST step of packet switching?
A. The data is split into packets and each packet is numbered ✓
B. The packets are reordered at the destination
Packet switching starts at the sender: the data is broken into packets, and each one gets a packet number in its header so it can be put back in order later.
2. Who chooses the route each packet takes?
A. A router, for each packet separately ✓
B. The sender picks one route for all packets
Each router looks at the destination address in the packet header and decides which route to send that packet on next.
3. Packets from the same message are sent across a network. What can happen?
A. They can take different routes and arrive out of order ✓
B. They must always arrive in the order they were sent
Because each packet may take a different route, a later packet can arrive before an earlier one.
4. When is the data put back into order at the destination?
A. As soon as the first packet arrives
B. Once the last packet has arrived ✓
The destination waits until all packets, including the last one, have arrived. Then it uses the packet numbers to reorder them and rebuild the data.
5. The message HELLO is split into packets 1:HE, 2:LL, 3:O. They arrive as 2, 3, 1. What does the destination do?
A. Joins them in the order they arrived to get LLOHE
B. Uses the packet numbers to reorder them as 1, 2, 3 to get HELLO ✓
The packet numbers let the destination reorder the packets, so 1 (HE), 2 (LL), 3 (O) rebuild HELLO.

Match the Pairs

Step 1: Split the message into packets and number them → The sender breaks the data into small packets and gives each one a packet number in its header
Step 2: Each router chooses the route for each packet → Routers on the network decide which path every packet takes, so packets from one message can travel different routes
Packets can arrive at the destination out of order because they took different routes of different lengths → Packets arrive: 3, 1, 4, 2
When the last packet arrives, the packets are put back in order using their packet numbers to rebuild the message → Reorder 3, 1, 4, 2 into 1, 2, 3, 4 = HELLO
Step 1: Send the whole message as one packet with no numbers → Packet switching starts by splitting the data into several packets and numbering them. Without numbers the destination cannot reorder them.
Step 4: Reorder the packets as soon as the first packet arrives → The packets are reordered once the last packet has arrived, not the first. Packets may still be missing until then.

Key Term

packet switching steps
Data is split into numbered packets, routers pick each packet's route, packets may arrive out of order, then they are reordered to rebuild the data once the last packet arrives.

Fill in the Blanks

1. Step 1: Split the data into packets Step 2: Number each packet
Hint: Type the name for the small pieces the data is split into.
2. Step 1: Number each packet Step 2: A router picks each packet's route
Hint: Type the network device that chooses the route for each packet.
3. Step 3: Packets may arrive out of order
Hint: Type the word describing the sequence packets may not arrive in.
4. Step 4: Wait for the last packet to arrive
Hint: Type the word for which packet must arrive before reordering is complete.
5. Packets arrive as 2, 1, 3 Reordered by packet number: 1, 2, 3
Hint: Type the header detail used to put the packets back in sequence.

Spot the Error

Question 1
A JOIN message INTO packets
B NUMBER each packet 1,2,3
C ROUTER picks route per packet
D REORDER at destination
Line A — The first step of packet switching is to split the data into packets, not join it. The line should be SPLIT message INTO packets. Joining (reordering) happens last, at the destination.
Question 2
A SPLIT data INTO packets
B NUMBER each packet
C ROUTER uses one route for all
D REORDER at destination
Line C — A router chooses the route for each packet separately, so packets can take different routes. The line should be ROUTER picks route per packet.
Question 3
A SPLIT data INTO packets
B NUMBER each packet
C ROUTERS pick each route
D PACKETS always arrive in order
E REORDER by packet number
Line D — Packets can take different routes with different delays, so they may arrive out of order. The line should say they may arrive unordered, which is why reordering is needed.
Question 4
A message = HELLO
B split into HE, LL, O
C number them 1, 1, 1
D router picks route for each
Line C — Each packet needs its own number so the destination can put them back in order. The numbers should be 1, 2, 3, not the same number for every packet.
Question 5
A packet 3 arrives first
B packet 1 arrives second
C packet 2 arrives third
D reorder by arrival time
E message rebuilt: 1, 2, 3
Line D — Packets are reordered using the packet number in the header, not by when they arrived. Arrival order may be wrong because of different routes. It should be reorder by packet number.

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