📡 Paper 1 · Topic 2: Data Transmission
2.1c Network Protocols & Packet Switching
Cambridge IGCSE Computer Science 0478 · ~15 min read · ⭐ Pro

What is a Protocol?

A protocol is a set of rules that govern how data is transmitted between devices. Without agreed protocols, different devices couldn't communicate — like trying to have a conversation in different languages.

Protocols define: data format, speed of transmission, error checking methods, and how connections are established.

The TCP/IP Model

TCP/IP is the foundation of the internet. It uses a layered model where each layer has a specific role. Cambridge requires you to know 4 layers:

LayerNameRoleProtocols
4ApplicationProvides services directly to users (email, web browsing)HTTP, HTTPS, FTP, SMTP, DNS
3TransportBreaks data into packets; reassembles; error checkingTCP, UDP
2InternetLogical addressing (IP addresses); routing between networksIP
1Link (Network Access)Physical transmission of data over hardwareEthernet, Wi-Fi

How the Layers Work

When you send data, it travels down the layers (adding headers at each step — called encapsulation). When received, it travels up the layers (stripping headers — decapsulation). This ensures each layer only needs to worry about its own job.

Packet Switching

When large amounts of data are sent over the internet, they are broken into small units called packets. Each packet travels independently and may take a different route to the destination.

Packet Structure

Each packet contains:

  • Header: source IP address, destination IP address, packet number, total number of packets, TTL (time to live)
  • Payload: the actual data being sent (portion of the original message)
  • Trailer: error-checking data (e.g. checksum)

Why Use Packets?

  • Multiple packets can take different routes — avoids congestion
  • If one packet is lost, only that packet needs to be resent (not the whole message)
  • Many users can share the same network — efficient use of bandwidth
  • Packets are reassembled in the correct order at the destination using packet numbers

Key Application Layer Protocols

ProtocolFull nameUse
HTTPHyperText Transfer ProtocolWeb pages (unsecured)
HTTPSHTTP SecureWeb pages (encrypted with SSL/TLS)
FTPFile Transfer ProtocolTransferring files between computers
SMTPSimple Mail Transfer ProtocolSending email
DNSDomain Name SystemConverts domain names to IP addresses
Exam tip: A common question asks you to name TCP/IP layers and what happens at each. Remember the order: Application → Transport → Internet → Link. TCP handles breaking data into packets and reassembly; IP handles routing with IP addresses.
⚠️ Common Mistakes
  • Saying TCP and IP are the same thing — TCP handles packets/reassembly; IP handles addressing/routing
  • Forgetting that packets may arrive out of order — the transport layer uses packet numbers to reassemble correctly
  • Saying HTTP is secure — HTTP is unencrypted; HTTPS adds encryption via SSL/TLS
▶
Video coming soon
Click slide or press arrow keys to navigate
✍️

Worksheet — Protocols & Packet Switching

5 questions · 12 marks

Q1Describe what is meant by a 'protocol' in the context of computer networking.[2]
✅ Mark scheme
A set of agreed rules [1] that govern how data is transmitted/communicated between devices on a network [1]
Q2Name the four layers of the TCP/IP model and state one role of each layer.[4]
✅ Mark scheme
Application: provides services to users, e.g. web browsing [1]; Transport: breaks data into packets and reassembles [1]; Internet: logical addressing and routing via IP addresses [1]; Link/Network Access: physical transmission over hardware [1]
Q3Describe two advantages of packet switching over sending data as one continuous stream.[2]
✅ Mark scheme
Packets can take different routes to avoid congestion [1]; if a packet is lost only that packet is resent [1]; multiple users can share the network simultaneously [1] — any 2
Q4State the difference between HTTP and HTTPS.[2]
✅ Mark scheme
HTTP transmits data in plain text (unencrypted) [1]; HTTPS encrypts the data using SSL/TLS so it is secure [1]
Q5A packet has a header, payload and trailer. State the information held in a packet's header.[2]
✅ Mark scheme
Any 2 from: source IP address; destination IP address; packet number; total number of packets; TTL (time to live) [1+1]
Quiz — Protocols & Packets
Q 1 of 8
Score
/ 8
Click to reveal
TermDefinition
🎯

Mini Test — Protocols & Packets

10 minutes · mixed questions

← 2.1b Network Types Topic 2: Data Transmission Next: 2.1d Client-Server →
🔒
Pro Content
Upgrade to Pro to access all lessons.
£7.99/month
or £59/year
Subscribe now →