Tree Topology
One star is fine for one lab. But a school has a computer lab, a library and an office, each with its own star. Run one backbone cable past all three and plug each hub into it — and you get a tree.
1What a tree topology is
Picture a real tree: the trunk is the backbone, and each star is a branch with its leaves (the computers) growing from it. Because it branches out in levels — backbone, then hubs, then computers — it is also called a hierarchical topology.
Send A → C (same star), then A → D (the other star). Then make Hub 1 fail, and cut the backbone.
2The bus part and the star part
One backbone cable, closed with a terminator at each end. Every hub connects to it. Traffic between stars travels along it.
Each hub has its own group of computers, each on its own cable. Traffic inside a star never needs the backbone.
In the lab, a message from A to C stays inside the first star. A message from A to D goes up to Hub 1, along the backbone, and down through Hub 2.
3Faults stay in their branch
Try each fault in the lab and count the working pairs:
- PC C’s cable is cut — only PC C is lost.
- Hub 1 fails — A, B and C are cut off, even from each other. D, E and F keep working.
- The backbone is cut between the hubs — nobody is completely cut off, but the two stars can no longer reach each other. A can still talk to C; A cannot reach D.
4Advantages and disadvantages
- Easy to expand: connect a new star to the backbone.
- A fault in one branch does not affect the others.
- Suits organisations with departments, floors or blocks.
- Each branch can be managed on its own.
- If the backbone fails, the stars cannot reach each other.
- Needs more cable, hubs and planning than a single star.
- Harder to set up and look after as it grows.
5Check yourself
A tree topology is a combination of which two topologies?
In a tree network, the backbone is cut between two hubs. What happens?