Description of Network HUB

Description of Network HUB


v  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.

v  Arc net, 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.

v  Hubs basically act as a signal splitter. They take all of the signals they receive in through one port and redistribute it out through all ports.

v  In the most basic sense, a hub is really nothing more than a multiport repeater. As signals are sent between systems connected to a hub, they are not only regenerated, but also forwarded out each port.

v  In this way, all devices plugged into a hub are exposed to all traffic passing between systems, regardless of whether that traffic is actually relevant to them.

v  Like repeaters, hubs are also considered to exist at the Physical Layer of the OSI model – they neither modify the data stream nor make any decisions. Instead, they act as a conduit by which systems can interconnect and communicate.

v  Hubs are a crucial element to all star topology LANs. Hubs serve as a central device through which data bound for a workstation travels. The data may be distributed, amplified, regenerated, screened or cut off.

v  Hubs classify as Physical Layer devices in the OSI model. At the physical layer, hubs can support little in the way of sophisticated networking. Hubs do not read any of the data passing through them and are not aware of their source or destination. Essentially, a hub simply receives incoming packets, possibly amplifies the electrical signal, and broadcasts these packets out to all devices on the network – including the one that originally sent the packet!


v       Hubs have different names depending on the type of LAN.

*          In token ring LANs they are referred to as Multi station Access Units or Controlled Access Units (MAUs or CAUs).

*          In 10BASE-T Ethernet, they are referred to as concentrators.

*          In ARC net they are simply called hubs.

Types of HUB

1) Passive hubs

v  A passive hub simply combines the signals of network segments.

v  There is no signal processing or regeneration. Because it does not boost the signals and in fact, absorbs some of the signal, a passive hub reduces by half the maximum cabling distance permitted.

v  For example, if a segment normally allows a reliable transmission distance of 200 meters, the distance between a passive hub and a device can be only 100 meters.

v  Passive hub merely acts as a connection point and does not amplify or regenerate the signal.

v  Passive hubs do not require electrical power to run.

2) Active hubs

v  Active hubs are like passive hubs expect that they have electronic components that regenerate or amplify signals (depending of type of hub). Because of this, the distance between devices can be increased.

v  The hub that regenerates the electrical signal and sends it to all the computers connected to it is often called a multiport repeater.

v  Active hub requires electrical power to run.

v  Some people use the terms concentrator when referring to a passive hub and multiport repeater when referring to an active hub.

3) Intelligent hubs

v  In addition to signal regeneration, intelligent hubs perform some network management and intelligent path selection.

v  A switching hub chooses only the port of the device where the signal needs to go, rather than sending the signal along all paths.

v  The limitations of a hub become evident as a network grows. While providing basic connectivity, a hub does nothing to prevent network collisions or broadcasts from reaching all connected systems. For this reason, all devices connected to a hub are considered to be part of the same broadcast domain and the same collision domain – concepts we’ll look at shortly. In order to be able to control traffic to a greater degree, you would need to implement devices capable of making forwarding decisions based on source or destination addresses. Examples of such devices include switches, bridges, and routers.

v  There are a variety of hubs that exist for different network technologies, including Token Ring. In a Token Ring environment, systems are connected to something that looks similar to a hub, but is properly referred to as a Multi-Station Access Unit (MSAU).

The advantages of hubs are,

v  Each port has exclusive access to its bandwidth (no CSMA/CD)

v  Hubs may be cascaded to add additional ports

v  SNMP managed hubs offer good management tools and statistics

v  Utilize existing cabling and other network components

v  Becoming a low cost solution 


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