The physical layout of a network, referred to as its topology, is a function of the practical constraints imposed by the environment, the protocols that must be supported, and the cost of installation. The most common protocols used with LANs, Ethernet and token ring, assume a bus and ring topology, respectively. The star topology is often used as a hub to connect several networks and in wireless networks, where multiple devices connect via radio frequency, it links to a central wireless access point or wireless hub.
The three pure topologiesring, bus, and starillustrated in Figure 3-8, rarely exist alone. More likely, they are part of a hybrid network such as a small workgroup connected by bus to a star network in another workgroup, perhaps supporting computers running under a different operating system as well.
Figure 3-8 Network Topologies.
The practical implications of network topologies are material and labor costs associated with running cable and purchasing and installing the new network electronics. For example, in establishing a laboratory with a new network, running cables from one workstation position to the next to support a bus topology is usually cheaper and less labor-intensive than running cables from each workstation and device position to a central closet to support a star topology. However, although wiring a laboratory to support a star topology is much more expensive in terms of cable required and the labor involved in pulling all cables to a central closet, the cables pulled for a star topology can be easily reconfigured in a bus to support Ethernet or token ring protocols.
Similarly, a network wired in a ring topology can easily be converted to support an Ethernet bus by breaking the ring and installing the appropriate electronics. These modifications are illustrated in Figure 3-9. This conversion of topologies is most difficult going from ring or bus topology to a star topology, because the electronics in the hub or center of the star typically controls each spoke of the hub individually, normally requiring a separate cable from the hub to each device.
Figure 3-9 Network Topology Conversion. Network topologies initially configured to support one protocol can be modified to support others. For example, a star topology can be converted to a bus or ring topology, and a ring can be converted to a bus topology.