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. A repeater is a device that boosts a network's signal as it passes through. The repeater does this by electrically amplifying the signal it receives and rebroadcasting 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 regenerates all the signals that pass through it allowing for the total length of cable on the network to exceed the 100 meter limit. Switches Switches allow you to avoid the congestion of a shared Ethernet network by permitting you to create individual segments.