Understanding FDDI

Although we have been primarily looking at network architectures that you may see on small and intermediate- sized networks (as well as some large networks), we should discuss an architecture that you would typically only see on networks of greater size. The Fiber Distributed Data Interface (FDDI) is an architecture that provides high-speed network backbones that can be used to connect and extend the range of LANs. FDDI uses fiber- optic cable and is wired in a ring topology. FDDI uses token passing as its media-access method and can operate at high speeds (most implementations are 100Mbps, but faster data-transfer rates are possible).

FDDI does not have an IEEE specification but rather has a designation of ANSI X3T9.5, which has been formulated by the American National Standards Institute (ANSI). Because FDDI uses a token-passing media-access strategy, it is considered reliable and provides equal access to all the computers on a network. With FDDI, however, you can set priority levels, which means you can designate that servers be allowed to send data more often over the network than client computers.

FDDI is expensive to set up because the computers require a special network card and fiber-optic cabling is also much more expensive than copper twisted-pair wire. Because most FDDI installations use a redundant second ring, twice as much cabling is required.

Note

Because FDDI uses a true ring topology, there is always the potential problem of a break in the ring. To combat this problem, FDDI installations consist of two rings. Each ring passes the token in a different direction, and certain key computers are connected to both rings. Then, if there is a break in one of the rings, the data can be rerouted in the opposite direction on the other ring. Since FDDI networks are typically used as high-speed backbones connecting a wide variety of LAN types, it is extremely important that they supply a consistent connection between the LANs. This is why the redundant rings are used.