Mission 1 · Spec 3.5
What is a network?
Why we connect computers, the three types of network (PAN, LAN, WAN), and wired versus wireless.
- Starter 5 min
- Learn 10 min
- Lab 15 min
- Quiz 10 min
- Exam 10 min
Count your networks
List every network you have used today. Think about your earbuds, your home, school and your phone.
Reveal
Earbuds to phone is a PAN (personal area network). Home Wi-Fi and the school network are LANs. Mobile data and the internet are WANs. Most people use all three before lunch.
Key ideas
A computer network is two or more computers connected together so they can share data and resources.
Advantages
Share files, printers and internet access; communicate (email, messaging); central backups and security; users can log in from any computer.
Disadvantages
Cost of hardware and specialist staff; malware can spread between computers; if a central server or switch fails, many users are affected.
| Type | Covers | Example |
|---|---|---|
| PAN | A few metres around one person | Phone and smartwatch over Bluetooth |
| LAN | A small area such as one building or site; the owner usually owns the hardware | A school network |
| WAN | A large geographical area; uses hardware owned by others (e.g. telecoms companies) | The internet; a bank linking its branches |
Wired
✓ Faster and more reliable
✓ Harder to intercept
✗ Cables are costly to install, and limit movement
Wireless
✓ Devices can move around; no cables
✓ Easy to add new devices
✗ Slower, affected by walls and interference
✗ Signals can be intercepted, so encryption is vital
PAN, LAN or WAN?
Wired or wireless?
Which is the better choice for each situation?Exam-style questions
1. Define the term computer network.
[1 mark]Mark scheme
- Two or more computers connected together in order to share resources / data / communicate (1)
2. Explain two differences between a LAN and a WAN.
[4 marks]Mark scheme
- A LAN covers a small geographical area (1); a WAN covers a large geographical area (1)
- LAN hardware is usually owned by the organisation (1); a WAN uses infrastructure owned by third parties / telecoms companies (1)
3. A school is deciding whether its classrooms should use wired or wireless connections. Give one advantage of each.
[2 marks]Mark scheme
- Wired: faster / more reliable / more secure (1)
- Wireless: devices can be moved / no cables to install / easier to add devices (1)
TUTOR NOTES
- Misconception: LAN and WAN are defined by the number of computers. It's about geographical area and who owns the infrastructure.
- Misconception: the internet and the World Wide Web are the same thing.
- Exam habit: "explain" questions need a point plus a consequence.
- Extension: is a home network with 5G mobile broadband a LAN or a WAN, or both?
Mission 2 · Spec 3.5
Network topologies
The star and bus topologies: how they are laid out, and what happens when something breaks.
- Starter 5 min
- Learn 10 min
- Lab 15 min
- Quiz 10 min
- Exam 10 min
The Christmas lights
Some old strings of Christmas lights go completely dark when one bulb breaks. Others keep working. What's different about how they're wired?
Reveal
In the first kind, everything depends on one shared wire. In the second, each bulb has its own connection. Networks face exactly the same trade-off: a shared bus cable versus a star with a separate cable for each device.
Two topologies
Star
Every device has its own cable to a central switch.
✓ A broken cable only affects one device
✓ Fast: the switch sends data only to the device it's for
✓ Easy to add devices
✗ If the switch fails, the whole network fails
✗ More cable, so more expensive
Bus
All devices connect to one main cable (the backbone), with a terminator at each end.
✓ Cheap: little cable needed
✗ If the backbone breaks, the network fails
✗ Data collisions as traffic increases, so it slows down
✗ Everyone shares the same cable, so less secure
Break the network
Cut cables, fail devices and see who can still reach the server.Exam-style questions
1. Draw a star topology with four computers and a printer.
[2 marks]Mark scheme
- A central switch (or hub) (1)
- Each device connected to it by its own separate line, with no connections between devices (1)
2. Compare the star and bus topologies.
[4 marks]Mark scheme
- In a star, a cable failure only affects one device; in a bus, a backbone failure brings down the whole network (1)
- A star needs more cable so costs more to set up; a bus is cheaper (1)
- A star depends on the central switch; a bus has no central device (1)
- A bus suffers from data collisions / slows down with heavy traffic; a star performs better (1)
TUTOR NOTES
- Misconception: the star is "more reliable" full stop. The central switch is a single point of failure.
- Lab prompt: in the bus, cut the backbone between PC2 and the server. Which devices can still talk to each other?
- Exam habit: comparisons should mention both topologies in each point.
- Extension: why do almost all modern LANs use a star?
Mission 3 · Spec 3.5
Protocols and the TCP/IP model
The rules computers follow to communicate, and the four layers that every message passes through.
- Starter 5 min
- Learn 10 min
- Lab 20 min
- Quiz 10 min
- Exam 10 min
Sending a letter
To post a letter you write it, put it in an envelope, address it and hand it to the post office, who sort it and deliver it. Why is it useful that each stage is done by a different person?
Reveal
Each stage only has to do its own job and follow agreed rules. The writer doesn't need to know how the van is driven. Networks work the same way: each layer uses its own protocols (sets of rules), and can be changed without affecting the others.
Protocols you need to know
A protocol is a set of rules that allow devices to communicate.
| Protocol | Purpose | Layer |
|---|---|---|
| HTTP | Requesting and sending web pages | Application |
| HTTPS | HTTP with encryption, for secure web pages | Application |
| FTP | Transferring files between computers | Application |
| SMTP | Sending email to a mail server | Application |
| IMAP | Retrieving email; messages stay on the server | Application |
| TCP | Splits data into packets, checks they all arrive, reorders them | Transport |
| UDP | Sends packets quickly without checking they arrive (e.g. live streaming, games) | Transport |
| IP | Addresses packets and routes them between networks | Internet |
| Ethernet | Communication over wired LANs | Link |
| Wi-Fi | Communication over wireless LANs | Link |
Down and up the layers
Follow one message from a laptop to a web server.Protocol match
Which layer?
Exam-style questions
1. Name the four layers of the TCP/IP model, in order from the top.
[2 marks]Mark scheme
- Application, transport, internet, link (2)
- 1 mark if all four are named but in the wrong order
2. Explain why a website that takes card payments should use HTTPS instead of HTTP.
[2 marks]Mark scheme
- HTTPS encrypts the data sent between the browser and the server (1)
- So card details can't be read / understood if they are intercepted (1)
3. State the purpose of SMTP and IMAP.
[2 marks]Mark scheme
- SMTP: sending email / transferring it to and between mail servers (1)
- IMAP: retrieving / accessing email from a mail server (1)
TUTOR NOTES
- Misconception: HTTPS makes a website safe. It only encrypts the connection; the site itself could still be malicious.
- Misconception: mixing up SMTP (send) and IMAP (retrieve). "S for Send".
- Memory aid: the layers from the top: "All Tigers In Lines".
- Extension: why does a video call use UDP but a file download uses TCP?
Mission 4 · Spec 3.5
Network security
Four ways to keep a network secure: authentication, encryption, firewalls and MAC address filtering.
- Starter 5 min
- Learn 10 min
- Lab 15 min
- Quiz 10 min
- Exam 10 min
The nightclub
A nightclub has a bouncer at the door, a guest list, ID checks, and staff-only rooms. Match each one to a way of protecting a network.
Reveal
The bouncer is a firewall (checks everything coming in). The guest list is MAC address filtering (only listed devices get in). ID checks are authentication. Staff-only rooms are access levels.
Key ideas
Authentication
Confirms the identity of a user before allowing access, e.g. a username and password, or biometrics.
Encryption
Scrambles data using a key so it can't be understood if intercepted. Only someone with the right key can decrypt it.
Firewall
Hardware or software that monitors traffic in and out of a network and blocks anything that doesn't meet a set of rules.
MAC address filtering
Every network card has a unique MAC address. The network only allows devices whose MAC address is on an approved list.
Encryption in action
Be the firewall
Rule: allow web browsing and email from inside the school. Block everything else coming in.Match the method
Exam-style questions
1. Explain how MAC address filtering can improve the security of a wireless network.
[2 marks]Mark scheme
- Each device has a unique MAC address (1)
- Only devices whose MAC addresses are on an approved list are allowed to connect (1)
2. Describe the purpose of a firewall.
[2 marks]Mark scheme
- Monitors / checks incoming and outgoing network traffic (1)
- Blocks traffic that does not meet a set of security rules / prevents unauthorised access (1)
3. Explain why encryption is important on a wireless network.
[2 marks]Mark scheme
- Wireless signals can easily be intercepted by anyone in range (1)
- Encrypted data can't be understood without the key (1)
TUTOR NOTES
- Misconception: encryption stops data being intercepted. It stops it being understood.
- Misconception: MAC and IP addresses are the same. A MAC address is fixed to the hardware; an IP address can change.
- Links to 3.6: these methods reappear in cyber security.
- Extension: MAC addresses can be faked. What else should a network use as well?