← IGCSE Computer Science Paper 1

The three parts of a packet

The three parts of a packet, in order: the header (start) holds addresses and packet number, the payload (middle) holds the data, the trailer (end) marks the packet's end.

Part of: IGCSE Computer Science 0478 — 2.1 Data Transmission Types and Methods — Module 7, ACARA Digital Technologies Years 7–8 — Wired and Wireless Networks — Module 40, ACARA Digital Technologies Years 9–10 — Networked Systems and Access Control — Module 36, QCE Digital Solutions Unit 4 — Digital Methods for Exchanging Data — Module 82, WA Computer Science ATAR Year 11 — Network Communications — Module 64

Understanding The three parts of a packet in IGCSE Computer Science Paper 1

What it does: The header comes at the start and stores the information needed to route and reorder the packet. The trailer sits at the end of the packet and tells the receiver that the packet has finished. A packet starts with the header, has the payload in the middle and ends with the trailer. The header comes first. The trailer is the part at the end. The payload is the part of a packet that holds the data itself, such as the text of a message.

Common mistakes: The packet number is stored in the header, not the trailer. The trailer at the end only marks the end of the packet, so it should read TRAILER: end marker. A packet is built in the order header, payload, trailer. The header must come first, so the line should be pkt = header+payload+trailer.

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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 shows the correct order of the three parts of a packet?
Payload, header, trailerHeader, payload, trailer
Matches
[ PAYLOAD | HEADER | TRAILER ]
↓
The parts are in the wrong order: the header must come first, then the payload, then the trailer
Applying
Meanings
Term
header, payload, trailer
Meaning
The three parts of a packet, in order: the header (start) holds addresses and packet number, the payload (middle) holds the data, the trailer (end) marks the packet's end.
Practice
HEADER: destination address, sender address, packet number
Testing
Gaps
packet ← + payload + trailer
Check Answer
Spots
3 PAYLOAD: data
4 TRAILER: packet number

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Quick Questions

1. Which shows the correct order of the three parts of a packet?
A. Payload, header, trailer
B. Header, payload, trailer ✓
A packet starts with the header, has the payload in the middle and ends with the trailer.
2. Where is the header found in a packet?
A. At the start of the packet ✓
B. At the end of the packet
The header comes first. The trailer is the part at the end.
3. Which part of a packet holds the actual data being sent, such as part of an email?
A. The payload ✓
B. The header
The payload in the middle carries the data. The header only holds control information such as addresses and the packet number.
4. Which part of a packet marks where the packet ends?
A. The payload
B. The trailer ✓
The trailer is at the end of the packet and shows the packet has finished.
5. A packet is drawn as three blocks side by side. Which block goes in the middle?
A. Payload ✓
B. Trailer
The payload sits between the header on the left and the trailer on the right.

Match the Pairs

[ HEADER | PAYLOAD | TRAILER ] → A packet drawn as three blocks in the correct order: header first, payload in the middle, trailer last
PAYLOAD = "Hello Sam" → The middle part of the packet that carries the actual data being sent
The header is the first part of a packet and holds the destination address, the originator's address and the packet number → HEADER = [dest, source, packet_no]
The trailer is the last part of a packet and marks where the packet ends → TRAILER = "END"
[ PAYLOAD | HEADER | TRAILER ] → The parts are in the wrong order: the header must come first, then the payload, then the trailer
[ HEADER | TRAILER | PAYLOAD ] → The trailer is placed in the middle, but it must be at the end after the payload

Key Term

header, payload, trailer
The three parts of a packet, in order: the header (start) holds addresses and packet number, the payload (middle) holds the data, the trailer (end) marks the packet's end.

Fill in the Blanks

1. packet ← header + payload + trailer
Hint: Type the name of the part that comes first, at the start of the packet.
2. packet ← header + payload + trailer
Hint: Type the name of the middle part, which carries the data being sent.
3. packet ← header + payload + trailer
Hint: Type the name of the last part, which marks the end of the packet.
4. blocks ← 3
Hint: Type the number of blocks you draw when you sketch a packet.
5. start ← header middle ← payload end ← trailer
Hint: Type the variable name for the position where the header sits in the packet.

Spot the Error

Question 1
A PACKET P1
B HEADER: dest, from, number
C PAYLOAD: data
D TRAILER: packet number
E END PACKET
Line D — The packet number is stored in the header, not the trailer. The trailer at the end only marks the end of the packet, so it should read TRAILER: end marker.
Question 2
A pkt = payload+header+trailer
B first = header
C middle = payload
D last = trailer
Line A — A packet is built in the order header, payload, trailer. The header must come first, so the line should be pkt = header+payload+trailer.
Question 3
A [ HEADER | TRAILER | PAYLOAD ]
B # left block: start of packet
C # middle block: the data
D # right block: end of packet
Line A — The drawing has the middle and last blocks swapped. The payload goes in the middle and the trailer goes at the end: [ HEADER | PAYLOAD | TRAILER ].
Question 4
A header = [dest, origin, num]
B payload = [dest, origin, num]
C trailer = 'END'
D pkt = [header,payload,trailer]
Line B — The payload holds the data being sent, such as the message. Addresses and the packet number belong in the header, so the payload should be a data value like 'Hi Sam!'.
Question 5
A pkt ← NEW Packet
B pkt.start ← header
C pkt.middle ← trailer
D pkt.end ← trailer
Line C — The middle of the packet holds the payload, not the trailer. The trailer goes at the end, so the line should be pkt.middle ← payload.

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