Fundamentals of computer networks: bus.

The idea

In a bus topology all devices share a single backbone cable, often called the bus, with terminators at each end. Every device taps into this shared cable, so a message travels along the backbone and all devices can see it; addressing decides which device accepts the data. Because there is one shared cable, a bus needs less cabling than a star, which keeps cost down and makes it simple to set up for a small network. The weakness is that the backbone is a single point of failure: if the main cable breaks, the whole network can stop working. Performance also falls as more devices share the same cable and collisions become more likely. Adding devices is easy but increases contention. When answering, describe the shared cable, the terminators and the consequences for reliability and cost.

What earns the marks

• A bus topology uses a single shared backbone cable to which all devices connect.
• Terminators at each end of the backbone absorb signals to prevent reflection.
• A break in the main cable can disable the whole network, which is the main weakness.
• A bus uses less cabling than a star, reducing cost and simplifying installation.
• All devices share the same cable, so performance can fall as more devices are added.

Common mistakes and how to put them right

• Thinking each device has its own cable to a central switch.

  Put it right by stressing the single shared backbone.

• Forgetting that a fault in the backbone can affect all devices.

  Put it right by explaining the shared-cable risk.

• Assuming a bus always performs well as devices are added.

  Put it right by noting increased contention on the shared cable.

Exam tips

• State clearly that all devices share one backbone cable.
• Give a disadvantage linked to the shared cable, such as a break disabling the network.
• Mention terminators only if the question asks about bus components or signal behaviour.