OxfordAQA IGCSE Computer Science: Computer Networks — Revision Notes

Computer networks (topic 3.5 of OxfordAQA International GCSE Computer Science 9210) covers what a network is, LANs and WANs, wired and wireless connections, star and bus topologies, the common protocols, network security methods and the four-layer TCP/IP model. It is assessed on Paper 2, mostly through definitions, comparisons and “which protocol or layer” questions, so precise wording matters.

Condensed for the final weeks. For the full explanation, use the Computer networks study guide. Practise with the Computer networks practice questions.

Syllabus coverage

OxfordAQA 9210 specification (Version 3.4), section 3.5 Computer networks:

• define a computer network; benefits and risks of networks
• LAN and WAN
• wired (fibre, copper) and wireless networks; benefits and risks of wireless
• star and bus physical topologies
• protocols: Ethernet, Wi-Fi, TCP, UDP, IP, HTTP, HTTPS, FTP, SMTP, IMAP
• network security: authentication, encryption, firewall, MAC address filtering
• the 4-layer TCP/IP model and which protocols work at each layer

Networks: definitions and comparisons

A computer network is two or more computers (or devices) connected together so they can communicate and share resources.

Benefits	Risks
Share files, printers and an Internet connection	Malware can spread between connected computers
Communicate by email and messaging	Hackers may gain unauthorised access
Central backup and management of user accounts	A server or switch failure can stop many users working
Users can log in from any computer on the network	Setting up and managing a network has costs

	LAN (Local Area Network)	WAN (Wide Area Network)
Area	Relatively small geographical area, e.g. one building or site	Wide geographical area, e.g. between cities or countries
Ownership	Often owned and managed by a single person or organisation	Often under collective or distributed ownership
Example	A school network	The Internet – the biggest WAN

Wired and wireless

• Copper cable: cheap and easy to install; suitable for shorter distances, e.g. within a building.
• Fibre-optic cable: sends data as light; higher bandwidth, works over much longer distances and is not affected by electrical interference, but costs more – suits backbone links between buildings and long distances.
• Wireless benefits: no cables, so devices can move around; cheaper and quicker to add devices. Risks: signals can be intercepted by anyone in range, so encryption is essential; usually slower and less reliable than wired, affected by distance, walls and interference.
• Wi-Fi is a trademark; the generic term for this kind of network is WLAN (wireless LAN).

Topologies

STAR                                BUS
      PC   PC                       PC    PC    PC
        \  /                        |     |     |
  PC -- SWITCH -- PC            ====+=====+=====+==== backbone cable
        /  \                        |     |          (terminator at each end)
      PC   PC                       PC    PC

	Star	Bus
Layout	Each device has its own cable to a central switch	All devices share one main cable (backbone)
If one cable fails	Only that device is affected	A break in the backbone stops the whole network
Performance	Good – the switch sends data only to the destination	Slows with heavy traffic; data collisions on the shared cable
Cost	More cable and a switch	Less cable, cheaper to set up

Protocols

A protocol is a set of rules that govern how data is transmitted and received by devices.

Protocol	Used for	TCP/IP layer
HTTP	Requesting and sending web pages	Application
HTTPS	An encrypted version of HTTP for secure web transactions	Application
FTP	Transferring files between computers	Application
SMTP	Sending email (to and between mail servers)	Application
IMAP	Retrieving email; messages stay on the server so they sync across devices	Application
TCP	Reliable delivery: sets up a connection, numbers packets, checks they all arrive, resends lost ones	Transport
UDP	Fast delivery with no connection or checking – suits live streaming and online games	Transport
IP	Addressing packets and routing them across networks	Network
Ethernet	A family of protocols for wired LANs	Link
Wi-Fi	A family of protocols for wireless LANs	Link

You do not need to name the individual protocols in the Ethernet or Wi-Fi families. The specification states the layer for HTTP, HTTPS, SMTP, IMAP, FTP, TCP, UDP and IP; Ethernet and Wi-Fi are shown at the link layer because they govern the network hardware that sits there.

Network security

Method	What it does	Example of use
Authentication	Confirms a user’s identity before allowing access	Username and password; two-factor code sent to a phone
Encryption	Scrambles data so it cannot be understood without the key, if intercepted	WPA2 on Wi-Fi; HTTPS for online banking
Firewall	Hardware or software that monitors traffic and blocks it according to rules	Blocking incoming connections from unknown addresses
MAC address filtering	Allows or blocks devices based on the physical address in their network adapter	Only school-owned laptops may join the network

These methods work best together: a firewall blocks unwanted traffic, authentication stops unauthorised users, MAC filtering stops unknown devices, and encryption protects any data that is intercepted.

The four-layer TCP/IP model

Layer	Main function (specification wording)	Protocols
Application	Where network applications such as web browsers and email programs operate	HTTP, HTTPS, SMTP, IMAP, FTP
Transport	Sets up communication between the two hosts; they agree settings such as packet size	TCP, UDP
Network	Addresses and packages data for transmission; routes packets across the network	IP
Link	Where the network hardware such as the NIC is located; OS device drivers also sit here	Ethernet, Wi-Fi

Use these four names in answers; you are not expected to know alternative names for the layers.

Common mistakes / exam traps

• Calling the Internet a LAN, or saying a WAN is always owned by one company.
• Mixing up SMTP (sends email) and IMAP (retrieves email).
• Writing that HTTPS is “faster” – its point is that it is encrypted.
• Putting TCP at the network layer or IP at the transport layer.
• Saying a star network fails if one device’s cable breaks – only the central switch failing stops the network.
• Describing MAC filtering as blocking users: it allows or blocks devices by their physical address.

Frequently asked questions

What is the difference between TCP and UDP? TCP sets up a connection and checks that every packet arrives, resending any that are lost, so it is reliable. UDP sends packets without checking, so it is faster and suits streaming and games where speed matters more than every packet.

Is Wi-Fi a single protocol? No. The 9210 specification says Wi-Fi is a family of related protocols and a trademark; the generic term is WLAN. Ethernet is also a family of protocols.

Which layer does a network interface card belong to? The link layer, where the network hardware and the operating system’s device drivers are located.

Related resources

Computer networks study guide | Computer networks practice questions | Cyber security revision notes

Official syllabus

OxfordAQA International GCSE Computer Science (9210) specification, Version 3.4, section 3.5 – oxfordaqa.com/9210.