Basic Concept Of Hub And Bridge


What is a Hub?

As its name implies, a hub is a center of activity. In more specific network terms, a hub, or concentrator, is a common wiring point for networks that are based around a star topology. Arcnet, 10base-T,and 10base-F,as well as many other proprietary network topologies, all rely on the use of hubs to connect different cable runs and to distribute data across the various segments of a network. Hubs basically act as a signal splitter. They take all of the signals they receive in thought one port and redistribute it out through all ports.

Some hubs actually regenerate weak signals before re-transmitting them. Other hubs retime the signal to provide true synchronous data communication between al ports.

Hubs with multiple 10base-F connectors actually use mirrors to split the beam of light among the various ports.

A diagram of a more complex 10base-T network. Notice the way that this 10base-T network connects each hub using a backbone. Typically, a backbone can be a higher-speed type of connection such as FDDI, and your server can be connected directly to the backbone.

The maximum distance of cable runs for different types of Ethernet, as determined by the IEEE.

Ethernet Type Distance
10base-2 185 meters/607 feet
10base-5 500 meters/1,640 feet
10base-T 100 meters/330 feet
10base-F 2 kilometers
10broad-36 3600meters/2.25miles

Types of Hubs

Passive Hubs

Passive Hubs, as the name suggest, are rather quiescent creatures. They do not 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.

Most passive hubs are excellent entry-level devices that you can use as your starting points in the world of star topology Ethernet. Most eight-port passive hubs cost less than $200, and if you are upgrading from 10base-2, even the most inexpensive 10base-Tsetup will deliver a whole new world of performance.

Active Hubs

Active hubs actually do something other than simply rebroadcasting 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& 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 packet 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 networks 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-phone connections. Of course, retiming packet delivery slows down overall network performance for all devices connected to that particular hub, but sometimes that is preferable to data loss-especially since the retiming can actually lower the number of collisions seen on your LAN. If data does not have to be broadcast over and over again, the LAN is available for use for new requests more frequently. Again, it is important to point out that active hubs can help you diagnose bad cable runs by showing which port on your hub warrants the retransmission or retiming.

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. Intelligent hubs into your network infrastructure give 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. Of course, intelligent hubs have additional ports for connecting high-speed backbones—just like other types of hubs.However, the intelligent hubs support standard transmission rates of 10,26 and 100Mbps to desktop systems using standard topologies such as Ethernet, Token Ring or FDDI. That means that you can gradually upgrade your systems from 10Mbps connections to 100Mbps connections, or simply deliver faster transmission speeds to devices that need faster services.

In addition, to boost the flexibility in configuration and management of networks of mixed media and mixed levels of technology, intelligent hubs have incorporated support for other technologies such as terminal servers, bridges, routers, and switches. Additionally, modern intelligent hubs provide more comprehensive and easier-to-use network management software, which make them a crucial component of most comprehensive network management systems.

Advanced Features

There are many additional features supported by some of the more high-end hubs from various manufactures. Some hubs feature redundant AC power supplies. If one should fail, the other takes over—and is fully capable of powering the entire unit. Other hubs have built-in DC power supplies to function in the event of a power outage. Redundant fans in some hubs provide cooling of the hub in the event that either fan fails. Other more commonly encountered advanced features in some intelligent hubs include automatic termination for coaxial connections, full hot –swap capabilities for connector modules, as well as the ability to automatically reverse the polarity of improperly wired 10base-T connections. More advanced intelligent hubs have features such as redundant configuration storage and redundant clocks. The redundant clocks allow any hub with an on broad clock on the network to act as a master to help in timing packet delivery. The redundant configuration storage feature available on some intelligent hubs is used when assigning various ports. Each similar hub on the network stores the configuration of one other hub, allowing configuration information to be restored by way of the intelligent link between hubs. Additionally, some manufactures deliver modules for routing and bridging services that can live inside of the same chassis as your larger enterprise-level hubs.

 What Is a Bridge?

Bridges, which operate at the data link layer, connect two LANs (local area networks) together, and forward frames according to their MAC (media access control) address. Often the concept of a router is more familiar than that of a; it may help to think of a bridge as a “low -level router”(routers operate at the network layer, forwarding by addresses such as an IP address).

A remote bridge connects two remote LANs over a link that is normally slow (for example telephone line), while a local bridge connects two locally adjacent LANs together.



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