Hubs are multislot concentrators into which can be plugged a number of multi-port cards to provide additional access as the network grows in size. Some concentrators are passive, that is they allow the signal to pass from one computer to another without any change. Most concentrators are active, that is they electrically amplify the signal as it moves from one device to another. Active concentrators are used like repeaters to extend the length of a network. The repeater electrically amplifies the signal it receives and rebroadcasts it.
Repeaters can be separate devices or they can be incorporated into a concentrator.
They are used when the total length of your network cable exceeds the standards set for the type of cable being used. A good example of the use of repeaters would be in a local area network using a star topology with unshielded twisted-pair cabling. The length limit for unshielded twisted-pair cable is 100 meters. The concentrator amplifies all the signals that pass through it allowing for the total length of cable on the network to exceed the 100 meter limit. Bridges A bridge is a device that allows you to segment a large network into two smaller, more efficient networks.