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3.3 Bus topologies

In a bus system a single cable links all points of the site as in a ring system,
but the ends of the cable are not joined together. Fig. 3.5 shows how a bus
might address the need of the network shown in Figs. 3.1 and 3.2.

Fig. 3.5 Layout of bus system

The network is still subject to the physical restrictions imposed by the
building, and in terms of topology there is little practical advantage of
bus systems over ring systems.

3.3.1 Ethernet

For many years Ethernet has been the only bus topology LAN that is
commercially practical. It was developed from work done on the ALOHA
project at the University of Hawaii, and the first commercial work was
done by the Xerox Corporation and the Digital Equipment Corporation.
Whilst new bus systems are becoming available, Ethernet is still being
developed and is likely to be around for some time. The components of
the network are shown in Fig. 3.6.

The cable is traditionally a thick coaxial type that is defined to be
yellow. The cable must be handled with some care to avoid kinks and
other physical damage, and much be laid so that the bends have a radius
of not less than 250 mm. The cable is marked every 21⁄2 metres, and the
ends and any connections to the cable must be made at those points. All
of this is to prevent internal reflections of the signal and consequent data
corruption. The ends of the cable are terminated by a resistor which
matches the impedance of the cable and again reduces reflections. More
modern systems may use thin coaxial cable or even twisted-pair wiring
with consequent reductions in cost and difficulty of installation.

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