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Effectively, the hub is the
electronic center of the network for 10nBaseT networks. All signals
come to the center where they are repeated on all active (in-use) ports.
As you can see from the illustration below, multiple cables,
each leading to a device on the network, plug into a single hub,
which takes care of transmitting signals among all devices plugged into
the hub, allowing the devices to communicate with each other. You can
stack hubs to expand the network. In most cases, PCs
connect to the hub via patch cables using the RJ-45 connector
that is plugged in to the PCs NIC. In classic hubs, there
are limits to how many can be stacked, and performance degrades quickly
RJ means Registered Jack.
When stacking hubs, the hub counts as one repeater hop in the stack.
Figure 34: Hub
When you stack hubs
you send the signal received from one hub and it is
transmitted to the other, even if the intended receiver is
not on the second hub. All stations must listen anyway. This
means you have only one collision domain. To stack two hubs,
connect a cable from the specially-marked port (normally
called uplink, cascade, repeater or sometimes just X) on
one hub, to a standard port on the other. This allows the signals
to pass between both hubs.
In Fast Ethernet, (100Mbit 100BaseTX) you can only stack two hubs, total!
Note that all hubs are not created
equal. Different models of hubs have different capabilities in
terms of network error isolation, statistics reporting,
port management and expandability. In general, smart
hubs cost more, and have more functions built in to their
hardware, than do other types of hubs. Some smart hubs can even report
their status to monitoring software, and be controlled
by that software, for ease in detecting and correcting
network issues without physical access to the hardware.
Hubs function on Layer 1, have a single collision domain, and do not segment network traffic, as a switch does. Use a switch to improve performance and segment subnets. The switch is discussed in the section on Layer 2 hardware.
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