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.
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.
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
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, 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.
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.
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 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.
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.
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
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
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 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.
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.
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
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.
need almost no configuration.
hubs can extend maximum network media distance.
processing is done at the hub to slow down performance.
hubs can greatly limit maximum media distance.
have no intelligence to filter traffic, so all data
is sent out all ports whether it is needed or not.
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
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