Spec & Goals 3 min
AQA Spec 3.5.1.1 — Star and bus network topologies
By the end of this lesson you can:
- Describe the star and bus topologies and draw each.
- Give the advantages and disadvantages of each topology.
- Choose and justify a suitable topology for a scenario.
Warm-Up 5 min
You can connect the same computers in different shapes. The shape — the topology — changes the cost, the speed and what happens when something breaks.
Quick starter
If every computer in a room plugged into one central box, what single failure would knock out the whole network?
Reveal the idea
The central box. In a star topology that box is a switch — fast and reliable, but a single point of failure for the whole network.
Key Concept — two layouts 14 min
A topology is the layout — the way devices are connected together on a network. AQA examines two: star and bus.
Star topology
Every device connects to a central switch by its own cable. Data is sent through the switch to its destination.
| Star — advantages | Star — disadvantages |
|---|---|
| Fast — each device has its own dedicated cable, so few collisions. | Needs lots of cable (one per device) — more expensive. |
| Reliable — if one cable fails, only that device drops; the rest keep working. | If the central switch fails, the whole network goes down. |
| Easy to add a device — just plug into the switch. | Depends on the switch having enough free ports. |
Bus topology
All devices share a single backbone cable, with a terminator at each end to stop signals reflecting.
| Bus — advantages | Bus — disadvantages |
|---|---|
| Cheap — uses the least cable; no central device needed. | The shared cable causes data collisions, so it's slow with many users. |
| Easy and quick to set up for a small network. | If the backbone cable fails, the whole network goes down. |
| — | Less secure — every device sees all the data on the cable. |
Worked Example — single point of failure 12 min
Problem: In each topology, what happens if (a) one device's own cable is cut, and (b) the central / backbone cable fails?
| Star | Bus | |
|---|---|---|
| (a) One device's cable cut | Only that device drops — the rest keep working. | Only that device drops (its drop cable) — the backbone still works. |
| (b) Central / backbone fails | Switch fails → whole network down. | Backbone fails → whole network down. |
Answer: both have a single point of failure (the switch / the backbone). The star is more fault-tolerant for individual devices, because each has its own dedicated cable.
Try It Yourself 12 min
Goal: Draw a star topology connecting four computers.
Hint: one central switch, four separate cables.
Goal: Give two advantages of a star topology over a bus topology.
Goal: A small startup with four PCs and almost no budget asks which topology to use. Recommend one and justify it, then state its main weakness.
Hint: "almost no budget" is the clue.
📝 Exam Practice 10 min
Describe how devices are connected in a star topology.
Mark scheme
- Each device has its own cable (1).
- Connecting to a central switch / hub (1).
Compare the star and bus topologies in terms of cost and reliability.
Mark scheme
- Cost — bus uses less cable / no switch, so cheaper than star (1); star needs a cable per device + a switch, so more expensive (1).
- Reliability — in star, one cable failing affects only one device (1); in bus, a backbone failure / collisions affect the whole network (1).
Explain why a bus network slows down as more devices are added.
Mark scheme
- All devices share one backbone cable (1).
- So data collisions increase / only one device can transmit at a time, slowing the network (1).
Recap & Key Terms 3 min
A topology is the layout of a network. A star gives each device its own cable to a central switch — fast and reliable but more cable. A bus shares one backbone cable — cheap but prone to collisions, and the whole network fails if the backbone breaks.
- Network topology
- The arrangement (layout) of the devices and connections that make up a network.
- Star topology
- Each device has its own cable to a central switch; fast and reliable, but uses more cable.
- Bus topology
- All devices share a single backbone cable (terminated at both ends); cheap, but collisions slow it and a backbone fault stops everything.
- Switch
- The central device in a star topology that directs data to the correct destination.
Homework 1 min
Task (≤ 15 min): A computer lab in a Penang school has 24 PCs that must stay fast and reliable. Recommend a topology, draw a simplified diagram, and give one reason and one drawback.
Model answer
Use a star topology: 24 PCs each cabled to a central switch. Reason: each PC has a dedicated cable, so the lab is fast and one faulty cable only drops one PC. Drawback: it needs a lot of cable and a switch (more expensive), and if the switch fails the whole lab goes down.
Award marks for: star recommended with valid reason (1); correct diagram — central switch, separate cables (1); valid drawback (1).