Bus Topologies

Bus Topologies

A bus physical topology is one in which all devices connect to a common, shared cable (sometimes called the backbone). A bus physical topology is shown in Figure 4.4.

If you think the bus topology seems ideally suited for the networks that use contention-based access methods such as CSMA/CD, you are correct. Ethernet, the most common contention-based network architecture, typically uses bus as a physical topology. 10BASE-T Ethernet networks (described later in this chapter) use bus as a logical topology but are configured in a star physical topology.

Most bus networks broadcast signals in both directions on the backbone cable, enabling all devices to directly receive the signal. Some buses, however, are unidirectional: signals travel in only one direction and can reach only downstream devices. Recall from Chapter 3, “Transmission Media,” that a special connector called a terminator must be placed at the end of the backbone cable to prevent signals from reflecting back on the cable and causing interference. In the case of a unidirectional bus, the cable must be terminated in such a way that signals can reflect back on the cable and reach other devices without causing disruption.