← IGCSE Computer Science Paper 1

Why data is broken down into packets

A small piece of a larger file. Data is split into numbered packets so they share the network and a lost packet can be resent instead of the whole file.

Part of: IGCSE Computer Science 0478 — 2.1 Data Transmission Types and Methods — Module 6, ACARA Digital Technologies Years 7–8 — Wired and Wireless Networks — Module 39, ACARA Digital Technologies Years 9–10 — Networked Systems and Access Control — Module 35, QCE Digital Solutions Unit 4 — Digital Methods for Exchanging Data — Module 81, WA Computer Science ATAR Year 11 — Network Communications — Module 63

Understanding Why data is broken down into packets in IGCSE Computer Science Paper 1

What it does: The file is divided into smaller numbered packets, so it travels across the network in manageable pieces. Each packet is numbered, so the destination can tell which one is missing and ask for just that packet again. A packet is one small, numbered piece of the data being sent.

Common mistakes: The number of packets is the file size divided by the packet size, so it needs DIV (1200 DIV 100 = 12). Multiplying gives a meaningless number. Each loop should send one small numbered packet, not the whole file. The line should be SEND packet.

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This module's key terms are also in a printable crossword (Modules 86–109) and word search (Modules 86–120).

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Learning
Duo
Why is a large file split into packets before it is sent across a network?
So it can be sent as one single blockSo it can be sent in small, manageable pieces
Matches
packet 5 lost -> resend whole file
↓
Only the lost packet needs to be resent. Resending the whole file wastes time and network capacity.
Applying
Meanings
Term
packet
Meaning
A small piece of a larger file. Data is split into numbered packets so they share the network and a lost packet can be resent instead of the whole file.
Practice
FOR n ← 1 TO 5
Testing
Gaps
packets ← (file, 100)
Check Answer
Spots
2size ← 100
3packets ← total * size

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Quick Questions

1. Why is a large file split into packets before it is sent across a network?
A. So it can be sent in small, manageable pieces ✓
B. So it can be sent as one single block
Large data is broken into many small packets so that it can be sent across the network in manageable pieces.
2. A 20 MB video is sent over a network. What happens to the video?
A. It is sent whole as a single packet
B. It is split into many numbered packets ✓
The file is divided into numbered pieces (packets). The numbers let the destination put the pieces back in the right order.
3. Which statement about packets and the network is correct?
A. Small packets from many users can share the network at the same time ✓
B. One large file must use the whole network until it has finished
Because packets are small, packets from different users can take turns on the network instead of one big file blocking everyone.
4. One packet of a file is lost on the way. What needs to be resent?
A. Only the lost packet ✓
B. The whole file from the start
Each packet is numbered, so the destination can tell which one is missing and ask for just that packet again.
5. Why is each packet given a number?
A. So the packet travels faster than the others
B. So the pieces can be put back in order and a missing one can be spotted ✓
Packet numbers do not affect speed. They show the order of the pieces and help find any that are missing.

Match the Pairs

packet → A small piece of a larger file, sent across the network on its own
packet_number = 3 → Numbers a piece of the file so the pieces can be put back in order at the destination
A large file is split into numbered pieces before it is sent → file (12 MB) -> packet 1, packet 2, packet 3 ... packet 12
If one packet is lost, only that packet is resent, not the whole file → packet 7 lost -> resend packet 7
packet 5 lost -> resend whole file → Only the lost packet needs to be resent. Resending the whole file wastes time and network capacity.
One 500 MB file sent as one piece, so it takes over the network until it is done → A large file sent as a single piece blocks other users. Packets let many users share the network.

Key Term

packet
A small piece of a larger file. Data is split into numbered packets so they share the network and a lost packet can be resent instead of the whole file.

Fill in the Blanks

1. packets ← SPLIT(file, 100)
Hint: Type the function name that breaks a large file into small pieces.
2. FOR number ← 1 TO 5 SEND(packets[number]) NEXT number
Hint: Type the loop variable that gives each numbered packet its position.
3. IF packetLost = TRUE THEN RESEND(packetNumber) ENDIF
Hint: Type the function that sends only the missing packet again, not the whole file.
4. totalPackets ← fileSize DIV packetSize
Hint: Type the whole-number division operator used to work out how many packets are needed.
5. WHILE packetNumber <= totalPackets SEND(packets[packetNumber]) packetNumber ← packetNumber + 1
Hint: Type the arithmetic operator that moves on to the next packet number.

Spot the Error

Question 1
A total ← 1200
B size ← 100
C packets ← total * size
D OUTPUT packets
Line C — The number of packets is the file size divided by the packet size, so it needs DIV (1200 DIV 100 = 12). Multiplying gives a meaningless number.
Question 2
A FOR n ← 1 TO 5
B packet.number ← n
C packet.data ← piece[n]
D SEND file
E NEXT n
Line D — Each loop should send one small numbered packet, not the whole file. The line should be SEND packet.
Question 3
A FOR n ← 1 TO 10
B IF ack[n] = FALSE THEN
C lostCount ← lostCount + 1
D RESEND file
E ENDIF
F NEXT n
Line D — Only the lost packet needs to be sent again, which is the benefit of using packets. It should be RESEND packet[n], not the whole file.
Question 4
A IF fileSize < packetSize THEN
B SPLIT file INTO packets
C SEND each packet
D ENDIF
Line A — A file only needs splitting when it is larger than the packet size, so the comparison should be >, not <.
Question 5
A FOR n ← 1 TO count
B packet[n].number ← 1
C SEND packet[n]
D NEXT n
Line B — Each packet needs its own number so the pieces can be put back in order. The number should be n, not 1 for every packet.

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