Network Topologies
Discover how the physical arrangement of devices on a network affects performance, cost and fault tolerance. Compare star, bus, ring and mesh topologies and understand exactly which fails when a cable or device breaks.
What is a Network Topology?
A hospital IT manager needs to build a new network for 80 computers across three floors. One doctor insists every computer must stay online even if multiple cables fail. The finance director wants to keep costs as low as possible. The head of IT says the network must be easy to manage and expand. These three requirements point to three completely different topologies.
A network topology describes the arrangement of devices and the connections between them. There are two types: physical topology (how cables and hardware are actually arranged) and logical topology (how data flows through the network). At GCSE, questions focus on physical topology.
Topology determines three things that exam questions always ask about: fault tolerance (what breaks when something fails), cost (how much cable and hardware is needed) and performance (how efficiently data travels).
Star and Bus Topologies
The two most commonly tested topologies. Star is the modern standard; bus is historical but still appears in every exam paper.
- If one cable fails, only that device is affected - all others remain online
- Easy to add new devices without disrupting the network
- Performance is consistent - devices do not share bandwidth with each other
- Easy to identify and isolate faults
- If the central switch fails, the entire network goes down
- Requires more cable than bus topology
- Central switch is a single point of failure
Central switch fails: all 6 devices disconnect immediately.
- Uses less cable than star - cheaper to install
- Simple to set up for a small number of devices
- If the backbone cable breaks, the entire network fails
- All devices share bandwidth - performance drops as more devices are added
- Data collisions occur when two devices transmit simultaneously
- Difficult to diagnose faults and to add new devices
- Terminators are required at each end of the backbone
Ring and Mesh Topologies
Ring topology creates a closed loop; mesh topology creates multiple paths. Mesh is the basis of how the internet itself is structured.
- Data travels in one direction, reducing collisions compared to bus
- Equal access for all devices - no device dominates bandwidth
- Can handle heavy traffic better than bus topology
- A break in the ring can disrupt the entire network
- Adding or removing a device disrupts the network
- Fault diagnosis is more difficult - data must travel the whole ring
- Slower than star for large networks as data passes through every device
One device fails: the ring is broken at that point, disrupting the whole network.
- Extremely high fault tolerance - multiple paths exist between devices
- Data can be re-routed if a connection fails
- Very fast - data can take the most direct path
- No single point of failure (full mesh)
- Very expensive - a full mesh with n nodes needs n(n-1)/2 connections
- Complex to set up and manage
- Large amount of cabling required
A full mesh network with n nodes requires n(n-1) / 2 connections. A network with 10 devices needs 45 connections; with 20 devices, 190 connections. This is why full mesh is rare in practice - partial mesh is used instead, providing redundancy without the full cost.
Topology Comparison
Exam questions regularly ask you to compare topologies across specific criteria. This table covers the key dimensions.
| Topology | Fault Tolerance | Cost | Performance | Scalability | Used where |
|---|---|---|---|---|---|
| Star | Medium - switch failure = total failure; cable failure = 1 device | Medium - more cable than bus; switch required | High - dedicated paths, no collisions | Easy - add device without disruption | School networks, offices, homes |
| Bus | Low - backbone failure = total failure | Cheap - least cable, no central device | Low - shared bandwidth, collisions | Difficult - disrupts network to add devices | Legacy networks only |
| Ring | Low-Medium - one break can fail all | Medium - similar to bus | Medium - no collisions but sequential | Difficult - disrupts ring to add devices | Industrial networks, some WANs |
| Full Mesh | Very High - multiple paths always available | Very High - n(n-1)/2 connections | Very High - direct paths, no bottleneck | Very Hard - connections grow exponentially | The internet (partial), critical systems |
Topology Fault Simulator
The most common exam question on topology is: "What happens to the network if a cable or device fails?" Click on a cable or device below to simulate a failure and see exactly which devices lose connection.
Mesh Connection Calculator
Use the formula n(n-1) ÷ 2 to calculate how many connections a full mesh network needs. See how rapidly the number grows as you add more devices.
Choosing the Right Topology
Exam questions frequently describe a scenario and ask you to recommend and justify a topology choice. You need to link the requirements of the scenario directly to the characteristics of the topology.
- The network must be easy to manage and expand
- Individual device failures must not affect others
- Cost is reasonable but not the absolute priority
- Examples: school, office, home network
- The network absolutely cannot fail under any circumstances
- Multiple simultaneous failures must be tolerated
- Budget is not a constraint
- Examples: internet backbone, military, hospitals
When justifying a topology choice, always state the requirement then link it to the topology feature. Wrong: "Star topology is good." Correct: "Star topology is most suitable because individual cable failures only disconnect one device, meaning the other 29 computers in the office can continue working - the school cannot afford network downtime during lessons." Marks come from the link, not the label.
1. A small business with 8 computers is choosing between star and bus topology. Give two reasons to choose star and one reason why bus might still be considered.
1. If one cable fails, only that device loses connection - the other 7 computers continue working. With a bus, the entire network fails.
2. Easy to identify and fix faults - the faulty cable connects only one device to the switch, so the problem is isolated.
3. Easy to add new devices without disrupting the existing network.
Bus consideration:
Lower cost - bus uses less cable and requires no central switch, which could matter for a small business with a very limited budget.
Note: At GCSE, star topology is almost always the recommended choice for modern networks. Answers should link reasons to the scenario.
2. The internet is described as a "network of networks" using a partial mesh topology. Explain why full mesh is not used for the internet, and why partial mesh is preferred.
Why partial mesh works: Each router connects to several others, not all of them. This provides redundancy (multiple paths exist if one link fails) without the exponential cost of full mesh. Data is routed intelligently around failures. A packet from London to New York might travel via different routes each time, selecting whichever path is fastest and available.
Key point: partial mesh gives most of the fault tolerance benefit of full mesh at a fraction of the cost.
3. Evaluate star topology for use in a large secondary school with 500 computers across 20 classrooms. Consider performance, fault tolerance, cost and management.
Fault tolerance: A single cable failure only disconnects one device. However, if a classroom switch fails, that entire classroom loses connectivity. Hierarchical star (switches connecting to a core switch) mitigates this.
Cost: Requires significant cabling (each device to the nearest switch) and multiple switches across 20 classrooms. However, equipment costs have dropped significantly, making this acceptable.
Management: Centralised management - the network administrator can monitor and control all switches from one location. Adding a new device only requires one cable run to the nearest switch.
Conclusion: Star topology is the appropriate choice for this school. The advantages (fault isolation, performance, easy management) far outweigh the cost of additional cabling, which is standard practice in modern school networks.
Practice what you've learned
Three levels of worksheet. Download, print and complete offline.