Hub
In a star topology every cable runs to one box in the middle. The simplest box that can do that job is a hub. It is also the least clever, and seeing why is the best way to understand the switch that replaced it.
1What a hub is
A hub does not look at who a message is for. It cannot: it does not know which computer is on which port. So every message goes to every computer, and each computer checks whether the message is addressed to it. Sending to everyone like this is called a broadcast.
Send a message and watch who receives it. Then try the other two buttons.
Think of a school announcement
2Three problems with a hub
Every message travels down every cable, even the cables of computers it is not for. The more computers, the slower it gets.
If two computers send at the same moment, their signals meet in the hub and both are garbled. Both have to send again.
Every computer receives every message. A computer that chose not to ignore messages for others could read them.
A hub also passes on damaged data without noticing, as the third button in the lab shows. What a hub has going for it is that it is simple and cheap. Today switches cost so little that hubs are hardly sold any more — but they are still in the syllabus, because they make the switch easy to understand.
3Check yourself
PC A sends a message to PC C through a hub with four computers. Which computers receive it?
Why is a hub called a non-intelligent device?
Two computers on a hub send data at exactly the same time. What happens?