📚 Knowledge Library — Topic 2.1 — Data Transmission

Data Packet Structure

Understand how large messages are split into smaller packets, and how each packet uses a header, payload and trailer to travel across a network.

Start LearningWorked Example
1. Invitation

A large file does not travel in one piece.

When data is sent across a network, it is usually broken into smaller chunks.

These chunks are called packets. Each packet carries part of the data and enough extra information to help it reach the correct destination.

💡 Remember: a packet is one small part of a larger message or file.
Figure 1.1
File to Packets
Large file
↓
Packet 1
Packet 2
Packet 3
The file is split before transmission.
2. Big Idea

Every packet has three sections.

A packet is not just data. It has a structure that helps the network handle it correctly.

The three main sections are the header, the payload and the trailer.

💡 Packet structure = header + payload + trailer.
Figure 2.1
Packet Structure
Header
Payload
Trailer
Three parts. Three jobs.
3. FutureLogic Bridge

Think of numbered envelopes.

Imagine a long letter that is too large for one envelope. You split it into numbered pages and place each page in a separate envelope.

Each envelope needs a to address, a from address and a page number. The content inside is the message. A final marker helps show where the packet ends.

💡 Bridge: header = envelope labels, payload = letter page, trailer = end/check information.
Figure 3.1
Numbered Envelope
TO: Receiver
FROM: Sender
PAGE: 2 of 5
↓
Message inside
4. Header

The header tells the packet where to go.

The header contains control information used to deliver and rebuild the packet correctly.

It can include the sender's IP address, the receiver's IP address, the packet number, the total number of packets and a hop number or TTL.

🎯 Exam Tip: do not just write “IP address”. Say sender's IP address or receiver's IP address.
Figure 4.1
Header Contents
Sender IP
Receiver IP
Packet number
TTL
The header is the packet's label.
5. Payload and Trailer

The payload carries the actual data.

The payload is the actual data being transmitted. It is the content of the message.

The trailer marks the end of the packet and may contain error checking data, such as a checksum value.

💡 Payload = the data itself. Trailer = end marker and error check.
Figure 5.1
Packet Jobs
Payload
= content

Trailer
= end + check
6. Worked Example

Label a packet correctly.

When an exam asks for the structure of a packet, name the sections clearly.

Example packet

Header
Receiver IP
Packet 3
Payload
Part of image file
Trailer
Checksum
End marker
Structure: Header → Payload → Trailer
Figure 6.1
Correct Order
Header
→
Payload
→
Trailer
Use the exact section names.
7. Exam Tip

Structure and content are different questions.

If the question asks for packet structure, name the sections: header, payload and trailer.

If the question asks what is stored in each section, give examples such as sender IP, receiver IP, actual data, error checking data or end-of-packet marker.

🎯 Exam Tip: structure = section names. Content = what each section stores.
Figure 7.1
Structure vs Content
Structure
Header • Payload • Trailer

Content
IP • Data • Checksum
8. Common Mistake

Do not call the payload the “main section”.

The payload is the data section of the packet. In exams, accurate terminology matters.

⚠️ Common Mistake: writing “main section” instead of payload, or writing “IP address” without saying sender or receiver.
Figure 8.1
Use Precise Terms
Payload ✓
Main section ✗

Sender IP ✓
IP address ✗
9. Summary

Packet structure in one screen.

Data sent across a network is split into packets.

Each packet has three sections: header, payload and trailer.

The header contains control information. The payload contains the actual data. The trailer marks the end and may contain error checking data.

💡 Key idea: packets are small, labelled chunks of data that can be reassembled at the destination.
Figure 9.1
Final Model
Header
addresses
Payload
actual data
Trailer
check/end