Star Topology
Look behind the computers in your school lab. The cables do not run from one desk to the next — each one heads off to the same box. That is a star, and it is the most common topology you will see.
1What a star topology is
Try the hub, then the switch. Then cut PC C's cable, or make the central device fail.
2How data travels: hub or switch?
PC A always sends its message down its own cable to the central device. What happens next depends on which device it is. Switch between them in the lab above and send the same message.
A hub is not intelligent. It does not know which computer is on which cable, so it copies the message out to every computer. Each one checks the address; only PC D keeps it. Sending to everyone like this is called a broadcast.
A switch is intelligent. It remembers which computer is on which cable, and sends the message down PC D’s cable only. Sending to exactly one computer is called unicast.
A switch keeps traffic off cables that do not need it, so the network is faster and other computers cannot see messages meant for someone else. That is why switches have almost completely replaced hubs. You will meet both again in Network Devices.
3Two very different faults
Only that computer loses the network. The other five carry on as if nothing happened. This is the star’s biggest strength.
Every cable meets at the hub or switch, so every computer loses the network at once. The central device is a single point of failure.
4Advantages and disadvantages
- A broken cable affects only one computer.
- Faults are easy to find — every cable ends at the central device.
- Computers can be added or removed without disturbing others.
- With a switch, data goes only where it is sent.
- Needs more cable than a bus, so it costs more.
- If the central hub or switch fails, the whole network fails.
- The central device has a fixed number of ports, which limits growth.
5Check yourself
In a star network, the cable to one computer breaks. What happens?
Which is the main disadvantage of a star topology?