The physical topology of a network refers to the physical layout of the devices and cabling. You must match the appropriate physical topology to the type of cabling (twisted pair, coaxial, fiber, and so on) that will be installed. Therefore, understanding the type of cabling used is important in understanding each type of physical topology. Here are the three primary categories of physical topologies:
• Bus: In early bus topologies, computers and other network devices were cabled together in a line using coaxial cable. Modern bus topologies establish the bus in a hardware device
and connect the host devices to the bus using twisted-pair wiring.
•Ring: Computers and other network devices are cabled together, with the last device connected to the first to form a circle, or ring. This category includes both ring and dual- ring topologies. The physical connection can be made using either coaxial or fiber.
• Star: A central cabling device connects the computers and other network devices. This category includes both star and extended-star topologies. The physical connection is
commonly made using twisted-pair wiring.
The logical topology of a network reters to the logical paths that the signals use to travel from one point on the network to another; that is, the way in which data accesses the network media and transmits packets across it.
The physical and logical topologies of a network can be the same. For example, in a network physically shaped as a linear bus, the data travels along the length of the cable. Therefore, the network has both a physical bus topology and a logical bus topology.
On the other hand, a network can have physical and logical topologies that are quite different. For example, a physical topology in the shape of a star, in which cable segments connect all computers to a central hub, can have a logical ring topology. Remember that in a ring, the data travels from one computer to the next, and inside the hub, the wiring connections are such that the signal actually travels around in a circle from one port to the next, creating a logical ring. Therefore, it is not always possible to predict how data travels in a network simply by observing its physical layout.
Star topology is by far the most common implementation of LANs today. Ethernet uses a logical bus topology in either a physical bus or a physical star. An Ethernet hub is an example of a physical star topology with a logical bus topology.