Network Hardware - Hubs and Repeaters (Dumb Devices)


Hub

Hub

As described earlier, the hub is the center of a star topology network. It operates at the first layer (physical) of the OSI 7-layer model (see the TCP/IP section). Put simply, this means that a hub is (usually) a dumb device: it forwards all data it receives at a port onto all other ports. It does not know what devices are attached to it, does not filter and has no packet-inspection abilities.

If you go around your mate's house to play Unreal Tournament with him and his friends, you'll probably all be connected to each other using a hub. Hubs are convenient for small networks and are very cheap.

Repeater

Signals degrade with transmission distance, and this is why each Ethernet type has a maximum cable or, in the case of Base-T Ethernet, segment length. To increase the distance a LAN can encompass, a device called a repeater (not to confused with a rapid-fire weapon from Jedi Knight) can be used which essentially amplifies the signals it receives.

It should be noted that a hub is also a form of repeater, but has multiple ports rather than just 'in' and 'out'. Hence a repeater is also a 'dumb' device. So, adding a repeater effectively doubles the segment length of an Ethernet LAN.

So, if you wanted your LAN to spread over a significant distance, say, a couple of miles, can you do this by simply chaining segments with repeaters? Well, not really. Because the CSMA/CD mechanism of Ethernet relies on short transmission delays, Ethernets become less performant if the segment length becomes too long. Hence, when using repeaters or hubs, the maximum number of segments is five.

(If you want to extend a LAN over a longer distance than is possible using repeaters, another option is fibre optic extensions. I won't discuss this any further here.)

What's next

Now move on to look at some more intelligent network devices: bridges and switches.