Topologies: bus, star, ring, mesh, hybrid

Ten laptops in a lab, and one loose cable. Whether that kills one laptop or the entire lab depends only on how the cables were arranged.

Why & what

Why shape matters. The devices are the same in every case. What changes is the wiring pattern, and that pattern decides three practical things.

• How much cable you have to buy.
• What stops working when one cable or one device fails.
• How easy it is to plug in a new device later.

What a topology is. A topology is the layout — the shape in which devices are connected. The five you must know:

• Bus — everyone taps into one shared cable. Cheapest. One break kills the whole line.
• Star — every device has its own cable to one middle device. One broken cable kills only that device.
• Ring — each device connects to the next, forming a loop. Data travels round in one direction.
• Mesh — every device links directly to every other. Extremely reliable, extremely expensive.
• Hybrid — a mix. Real networks are almost always this.

For mesh, the cable count is n(n−1)/2. For 10 devices that is 45 links. That is why nobody wires a lab this way.

How it works

To judge any topology, run these four checks.

1. Cable cost — how many links for n devices? Bus needs 1 shared line, star needs n, mesh needs n(n−1)/2.
2. Single point of failure — what one break takes everything down? Bus: the main cable. Star: the middle device.
3. Adding a device — star and mesh let you add without touching anyone else. Bus and ring need the line opened.
4. Traffic — on a bus, only one device can talk at a time. On a star with a switch, many pairs talk at once.

Common confusion

Students think star topology has no single point of failure, because only bus has that problem. Actually, the middle device in a star is a single point of failure too.

The real difference is what a cable break does.

• Bus: one cable break stops everyone.
• Star: one cable break stops one device.

But if the middle switch dies in a star, the whole star is down. Star wins because that middle box is one cheap thing you can swap, not because the weakness disappeared.

Interview angle

Asked as: "Which topology would you choose and why?" or "How many cables does a mesh with n nodes need?"

Model answer:

Star, in almost every real case. Each device gets its own cable to a central switch, so one bad cable affects only one device, and adding a new device does not disturb the rest. Its weakness is that the central device is a single point of failure, so in critical setups you add a second switch. Bus is cheapest but one cable break stops everyone, and only one device can transmit at a time. Mesh is the most reliable but needs n(n−1)/2 links, so it is used only for small critical backbones. Most real networks are hybrid — stars joined together.