Principles and technologies of local area networks - Intelligent Buildings and Building Automation - page 46

Hub Hub

Figure 4.3 Star topology LAN.

Figure 4.4 Bus topology LAN.

use a physical star topology to connect devices such as concentrators, but
logically they operate as a ring.
Bus topology: The bus topology is most commonly found in Ethernet/
IEEE 802.3 (ISO/IEC 8802-3) environments, although it can also be found
in other LANs such as ARCnet that uses the Token Passing Bus standard
of ANSI 878.1, which is close to IEEE 802.4 (ISO/IEC 8802-4). Typically
implemented in coaxial cable, and terminated at each end, this topology
is simple and inexpensive to install, and equally simple and inexpensive to
expand. These advantages may become its major disadvantage as well, since
bus topology networks are often expanded without thought or planning, and
they grow beyond their design limitations. In addition, any break in the cable
renders the entire network inoperable. Figure 4.4 shows a simple example
of a bus topology LAN in which either Ethernet/IEEE 802.3 or IEEE 802.4
could be implemented.
Ring topology: Figure 4.5 shows a LAN using a ring topology. This is
normally associated with Token Ring and FDDI environments although,
as said before, these LANs are often presented as star topology networks
physically nowadays. Certainly, when Token Ring and FDDI environments
are chosen, there is a relatively complex access protocol using an ordered
method of node access. That means, with ring topologies, one has a robust