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1-16
Chapter 1
Planning a Network Topology
Hub
Hub
Hub
Hub
F01pm03
Figure 1-3 A four-hub Ethernet network
Off the Record
When designing an Ethernet network, most network engineers use a basic
set of cabling guidelines specified by the Ethernet standards. For example,
the 10 Mbps
Ethernet standard uses the 5-4-3 rule, which says that a network can consist
of no more than
five network segments, connected by four repeaters, with no more than three
of those seg-
ments being populated with devices such as workstations and servers. There
are, however,
more exacting formulae in the Ethernet standards that add the lengths of the
individual cable
segments and a coefficient for each hub to arrive at a more precise
configuration for the net-
work. In other words, using the more complex formula, you might discover
that you are able to
exceed the number of hubs specified in the basic Ethernet guidelines if the
lengths of your
cables are short enough. When designing a complex Ethernet network, you
should consult
the Ethernet standards and use the more precise formula to ensure that your
design falls
within the specified requirements.
Fiber Optic Although it can use the same topology and many of the same
data-link
layer protocols support it, fiber-optic cable operates on a different
principle than UTP
and all other copper-based cables. The actual network medium in a
fiber-optic cable is
a strand of plastic or glass that carries signals in the form of light
pulses. Because the
signals are not electric, they are immune to electromagnetic interference.
In an envi-
ronment where interference levels are high, such as a factory floor,
fiber-optic cable
can eliminate the performance degradation that the interference causes on
copper
cable. Even in a normal office environment, you can install fiber-optic
cable near fluo-
rescent light fixtures or electric motors without any difficulties. When
using UTP, your
network design should keep the copper cables a safe distance away from these
possi-
ble sources of interference.
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