Lan Topologies

There are three types of basic LAN topology1: bus, ring, and star. These are illustrated in Figure 11.1 along with the tree network, which is a simple derivative of the conventional bus topology.

A bus is a length of transmission medium from which users tap into, as shown in Figure 11.1a. Originally the medium was coaxial cable. Today coaxial cable is being phased out in favor of unshielded twisted pair (UTP) or fiber-optic cable.

A ring is simply a bus that is folded back onto itself. A ring topology is illustrated in Figure 11.1b. User traffic flows in one direction around the ring. In some other approaches a second transmission medium is added for flow in the opposite direction. Such a dual counterrotating ring concept improves reliability in case of a failed station or a cut in the ring.

1 Topology means the logical and/or physical arrangement of stations on a network. In other words, topology tells how these assets are connected together.

Figure 11.1a Atypical bus network.

Figure 11.1b Atypical ring network.

A star network is illustrated in Figure 11.1c. At the center of the star is a switching device. This could be a switching hub. Users can be paired, two at a time, three at a time, or all at a time, segmented into temporary families of users depending on the configuration of the switch at that moment in time. Such a concept lends itself particularly

Figure 11.1c A star network.

Figure 11.1d Atypical tree configuration.

well to ATM (asynchronous transfer mode). Each user is connected to the switch on a point-to-point basis.

A tree network is illustrated in Figure 11.1 d.

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