Hubs are the simplest network devices, and their simplicity is reflected in their low cost. Small hubs with four or five ports (often referred to as workgroup hubs) cost less than $50; with the requisite cables, they provide everything needed to create a small network. Hubs with more ports are available for networks that require greater capacity. Figure 3.1 shows an example of a workgroup hub, and Figure 3.2 shows an example of the type of hub you might see on a corporate network.
Figure 3.1 A workgroup hub.
Figure 3.2 A high-capacity, or high-density, hub.
Computers connect to a hub via a length of twisted-pair cabling. In addition to ports for connecting computers, even a very inexpensive hub generally has a port designated as an uplink port that enables the hub to be connected to another hub to create larger networks. The " Working with Hubs and Switches" section later in this chapter presents a detailed discussion of this feature.
Token Ring and MSAUs
Both hubs and switches are used in Ethernet networks. Token Ring networks, which are few and far between, use special devices called multistation access units (MSAUs) to create the network. In some cases, MSAUs are referred to as Token Ring switches; but because of the way Token Ring operates, these devices perform a very different function from the hubs and switches discussed in this section.
Most hubs are referred to as active because they regenerate a signal before forwarding it to all the ports on the device. In order to do this, the hub needs a power supply; small workgroup hubs normally use an external power adapter, but on larger units the power supply is built in. Passive hubs (which are rare) do not need power because they don't regenerate the signal.
Regeneration of the signal aside, the basic function of a hub is to take data from one of the connected devices and forward it to all the other ports on the hub. This method of operation is very inefficient because, in most cases, the data is intended for only one of the connected devices. You can see a representation of how a hub works in Figure 3.3.
Figure 3.3 How a hub works.
Due to the inefficiencies of the hub system and the constantly increasing demand for more bandwidth, hubs are slowly but surely being replaced with switches. As you will see in the next section, switches offer distinct advantages over hubs.