Chapter 3 • Telecommunications and Networking 71MeshTreeRingBusStarFIGURE 3.4 Network Topologies
RING Thering topologyis similar to the bus except that
the two ends of the cable are connected. In this case, a
single cable runs through every network device, including
(usually) a file server. The wiring for the ring is slightly
more complicated than for the bus, but the ring is not as
susceptible to failure. In particular, a single failure in the
ring still permits each network device to communicate
with every other device.
STAR Thestar topologyhas a mainframe or midrange
computer, a file server (usually a microcomputer), or a
networking device at its center, with cables (or media of
some type) radiating from the central device to all the other
network devices. This design is representative of many
small-to-medium computer configurations, with all
workstations and peripherals attached to the single
midrange computer. Advantages of the star include ease of
identifying cable failure, because each device has its own
cable; ease of installation for each device, which must only
be connected to the central device; and low cost for small
networks where all the devices are close together. The
star’s primary disadvantage is that if the central device
fails, the whole network fails. A cost disadvantage might
also be encountered if the network grows, for a separate
cable must be run to each individual device, even if several
devices are close together but far from the central device.
TREE The fourth basic topology is the tree, or hierarchical.
This topology is sometimes called a hierarchical star
because with some rearrangement (spreading the branches
out around the central device), it looks like an extension of
the star. The configuration of most large and very large
computer networks is a tree, with the mainframe at the top
of the tree connected to controllerssuch as a multiplexer^3
and perhaps to other smaller computers. Then these
controllers, or smaller computers, are, in turn, connected to
other devices such as terminals, microcomputers, and
printers. Thus, the tree gets “bushy” as one traverses it
from top to bottom.
The tree has the same primary disadvantage as the
star. If the central device fails, the entire network goes
down. On the other hand, the tree arrangement possesses a
great deal of flexibility. The cost disadvantage of the star
might not appear when devices are added to the network,
for the use of intermediate devices (e.g., multiplexers,
small computers) removes the necessity of connecting
every device directly to the center.MESH In a mesh topology, most devices are connected
to two, three, or more other devices in a seemingly
irregular pattern that resembles a woven net or a mesh. A
complete mesh would have every device connected to
every other device, but this is seldom done because of the
cost. The public telephone network is an example of a
mesh topology; another example is the system of networks
that makes up the Internet.
The ramifications of a failure in the mesh depend
upon the alternative paths or routes available in the vicin-
ity of the failure. In a complex mesh, like the telephone
network, a failure is likely to have little impact, except on
the devices directly involved.MORE COMPLEX NETWORKS Now the fun begins,
because the previous five network topologies can be
combined and modified in a bewildering assortment of
networks. For example, it is quite common to attach
multiple bus or ring LANs to the tree mainframe computer
network. Multiple bus and ring LANs might be attached to
a high-speed backbone cable, which is in effect a bus
network with the LANs as nodes on the network.
National and international networks—such as the
telephone network and the Internet—are much more
complex than those we have considered thus far, because
the designers have intentionally built in a significant
amount of redundancy. In this way, if one transmission line(^3) A multiplexer is a device, usually located at a site remote from the
mainframe or central device, whose function is to merge (“multiplex”) the
data streams from multiple low-speed input devices, such as terminals
and microcomputers, so that the full capacity of the transmission line to
the central device is utilized.