Content

3.1

Basic LAN Devices

3.1.6

Hubs

Instructor Note

The purpose of this target indicator is to allow the student to start recognizing hubs on a logical topology, to describe how they really appear, and to briefly describe their function.

A good class activity is to make flash cards -- with the device symbol on the front, and the device name, layer, and function on the back. Hubs are multiport repeaters, hence Layer 1 devices, and they regenerate and retime signals while providing inexpensive connectivity for numbers of networking devices.

The purpose of a hub is to regenerate and retime network signals. This is done at the bit level to a large number of hosts (e.g. 4, 8, or even 24) using a process known as concentration.

You will notice that this definition is very similar to the repeater's, which is why a hub is also known as a multi-port repeater. The difference is the number of cables that connect to the device. Two reasons for using hubs are to create a central connection point for the wiring media, and increase the reliability of the network. The reliability of the network is increased by allowing any single cable to fail without disrupting the entire network. This differs from the bus topology where having one cable fail will disrupt the entire network. Hubs are considered Layer 1 devices because they only regenerate the signal and broadcast it out all of their ports (network connections).

There are different classifications of hubs in networking. The first classification is active or passive hubs. Most modern hubs are active; they take energy from a power supply to regenerate network signals. Some hubs are called passive devices because they merely split the signal for multiple users, like using a "Y" cord on a CD player to use more than one set of headphones. Passive hubs do not regenerate bits, so they do not extend a cable's length, they only allow two or more hosts to connect to the same cable segment.

Another classification of hubs is intelligent or dumb. Intelligent hubs have console ports, which means they can be programmed to manage network traffic. Dumb hubs simply take an incoming networking signal and repeat it to every port without the ability to do any management.

The hub's role in a token-ring network is played by a Media Access Unit (MAU). Physically it resembles a hub, but token-ring technology is very different, as you will learn later. In FDDIs, the MAU is called a concentrator. MAUs are also Layer 1 devices.

The symbol for a hub is not standardized. You will use the symbol shown here throughout this curriculum.

For more information about commercially available hubs, visit:

	Web Links
	Blackbox Network Services
	Micro Warehouse
	Hubs