Most hubs are active hubs, which are also called multiport repeaters. A repeater is a network device that is used to regenerate or replicate signals that are weakened or distorted by transmission over long distances and through areas with high levels of electromagnetic interference (EMI). An active hub functions in the same way as a repeater, except that it must also broadcast the regenerated signals to all connected network segments rather than forwarding them to just one segment.
Passive hubs, which are also called concentrators, serve simply as a conduit for data and broadcast incoming data to every port. They provide no signal regeneration. In contrast to active hubs, they require no external power supply. They also have the advantage of costing less than active hubs.
Ports on a hub are jacks (i.e., female connectors) that are built into either the front or back side. In the case of a hub for twisted pair cable, they are standard RJ-45 jacks. Hubs range in size from four ports up to several hundred and are specific to the network type.
In addition to ports for connecting to computers, even very inexpensive hubs generally have an uplink port that enables the hub to be connected directly to another hub using a standard patch cable to create larger networks.
If a signal comes into two ports at the same time, a collision occurs, and every device directly connected to the hub shares the same collision domain. In addition, hubs only support half duplex (i.e., one direction at a time) communications, and provide bandwidth that is shared among all the connected devices.
Sometimes hubs are advertised as managed hubs, switched hubs, and even intelligent hubs. Each of these provides additional features beyond those of a basic hub, such as enabling an administrator to monitor the traffic passing through the hub and to configure each port in the hub.
On the outside, hubs and network switches have a very similar appearance; inside, however, they work very differently. As is the case with repeaters, hubs operate at the physical layer of the seven layer OSI (open systems interconnect) reference model, which provides no filtering of packets. Switches, however, operate at the data link layer, which allows a greater degree of control, including the filtering of packets by their MAC addresses to specific network segments rather than just broadcasting them to all segments and thus helps conserve network bandwidth. This, together with a narrow price gap, has resulted in the increasing replacement of hubs by switches.
Created October 28, 2005.