What do they do?

A hub is essentially a multi-port repeater, (a repeater receives digital data, regenerates the signal and then re-transmits the data.) Therefore, hubs are a subset of repeaters - they receive data and then repeat it out of all ports. Hubs can be purchased in many different configurations with a fixed number of ports. Hubs are utilised in star physical topologies, hence have one PC per port.

How do they do it?

Hubs can support different media types - ethernet can operate over unshielded twisted pair, shielded twisted pair, thick and thin co-axial cable and fibre optic. This ability to support multiple media types enables the integration of network segments, having different cabling, with relative ease.

It is possible to "daisy-chain" hubs with a cross-over cable or via a special port that allows data to be repeated to an adjacent hub. Should a workgroup require a quantity of computers beyond the number of ports supported on its hub, another hub could be daisy-chained to the existing one.  Daisy-chaining is sometimes referred to as "cascading" or "stacking" and makes such ethernet installations easily extensible.

All traffic must pass through the hub, therefore it is an ideal piece of equipment to interrogate for network traffic information. Some are marketed as "managed hubs." These devices can be supplied with software that allows a local console to monitor and manage its performance. If a hub is part of a massive network installation, it may be advantageous if it shared information with network management systems such as Hewlett- Packard's OpenView or Sun's SunNet Manager. Managed hubs can share information via Simple Network Management Protocol - SNMP operates at the application layer of the TCP/IP suite.

As a signal travels down a line, its amplitude decreases. This is known as signal attenuation, it increases as a function of transmission frequency. Attenuation has to be reversed by repeating (amplifying) the signal. Since a signal comprises multiple frequencies, attenuation results in distortion. Hubs have to be designed to amplify different frequencies by different amounts.

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