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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.