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All about Hubs

In data communications, a hub is the pivot of convergence where data arrives from one or more directions and is forwarded out in one or more directions. A hub usually includes a switch (in telecommunications, a switch is a network device that selects a path or circuit for sending a unit of data to its next destination) of some kind. The distinction seems to be that the hub is the point where data comes together and the switch is what determines how and where data is forwarded from the place where data comes together. A hub is a hardware device that acts as a central connecting point and joins lines in a star network configuration.

Who Needs Hubs?
There is a simple way to determine whether you need a hub on your LAN. If you are building a network with a star topology and you have two or more machines, you need a hub. There is, however, an exception to this rule. If you are building a 10base-T network and you have only two machines, you can connect them to each other without using a hub.

Types of Hubs
As you may have already guessed, hubs perform a crucial function on networks with a star topology. There are many different types of hubs, each offering specific features that allow you to provide varying levels of service.

Now lets talk about some of the standard features of hubs, the differences between passive, active, and intelligent hubs, as well as some of the additional features found in today's more high-performance hubs.

Passive Hubs
Passive hubs, as the name suggests, are rather quiescent creatures. They do not do very much to enhance the performance of your LAN, nor do they do anything to assist you in troubleshooting faulty hardware or finding performance bottlenecks. They simply take all of the packets they receive on a single port and rebroadcast them across all ports--the simplest thing that a hub can do.

Passive hubs commonly have one 10base-2 port in addition to the RJ-45 connectors that connect each LAN device. As you have already read, 10base-5 is 10Mbps Ethernet that is run over thick-coax. This 10base-2 connector can be used as your network backbone.

Other, more advanced passive hubs have AUI ports that can be connected to the transceiver of your choice to form a backbone that you may find more advantageous.

Active Hubs
Active hubs actually do something other than simply rebroadcast data. Generally, they have all of the features of passive hubs, with the added bonus of actually watching the data being sent out. Active hubs take a larger role in Ethernet communications by implementing a technology called store and forward where the hubs actually look at the data they are transmitting before sending it. This is not to say that the hub prioritizes certain packets of data; it does, however, repair certain "damaged" packets and will retime the distribution of other packets.

If a signal received by an active hub is weak but still readable, the active hub restores the signal to a stronger state before rebroadcasting it. This feature allows certain devices that are not operating within optimal parameters to still be used on your network. If a device is not broadcasting a signal strong enough to be seen by other devices on a network that uses passive hubs, the signal amplification provided by an active hub may allow that device to continue to function on your LAN. Additionally, some active hubs will report devices on your network that are not fully functional. In this way, active hubs also provide certain diagnostic capabilities for your network.

Active hubs will also retime and resynchronize certain packets when they are being transmitted. Certain cable runs may experience electromagnetic (EM) disturbances that prevent packets from reaching the hub or the device at the end of the cable run in timely fashion. In other situations, the packets may not reach the destination at all.

Active hubs can compensate for packet loss by retransmitting packets on individual ports as they are called for and retiming packet delivery for slower, more error-prone connections.

Active hubs provide certain performance benefits and, sometimes, additional diagnostic capabilities. Active hubs are more expensive than simple, passive hubs and can be purchased in many configurations with various numbers and types of ports.

Intelligent Hubs
Intelligent hubs offer many advantages over passive and active hubs. Organizations looking to expand their networking capabilities so users can share resources more efficiently and function more quickly can benefit greatly from intelligent hubs. The technology behind intelligent hubs has only become available in recent years and many organizations may not have had the chance to benefit from them; nevertheless intelligent hubs are a proven technology that can deliver unparalleled performance for your LAN.

In addition to all the features found in active hubs, incorporating intelligent hubs into your network infrastructure gives you the ability to manage your network from one central location. If a problem develops with any device on a network that is connected to an intelligent hub, you can easily identify, diagnose, and remedy the problem using the management information provided by each intelligent hub--that is, in the event it is a problem that cannot be remedied by the hub itself.

This is a significant improvement over standard active hubs. Troubleshooting a large enterprise-scale network without a centralized management tool that can help you visualize your network infrastructure usually leaves you running from wiring closet to wiring closet trying to find poorly functioning devices.

Another significant and often overlooked feature of intelligent hubs is their ability to offer flexible transmission rates to various devices. Intelligent hubs support standard transmission rates of 10, 16 and 100Mbps to desktop systems using standard topologies such as Ethernet, Token Ring or FDDI.

Advantages of hubs

  • Hubs need almost no configuration.
  • Active hubs can extend maximum network media distance.
  • No processing is done at the hub to slow down performance.

Disadvantages of hubs

  • Passive hubs can greatly limit maximum media distance.
  • Hubs have no intelligence to filter traffic, so all data is sent out all ports whether it is needed or not.
  • Since hubs can act as repeaters, networks using them must follow the same rules as repeaters.


That's it for now. Next month we will be talking about switches.

NM - For more information contact netmagindia@vsnl.com

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