The Fiber Distributed Data Interface (FDDI) is a 100-Mbps token ring local area network (LAN) protocol. Standards for FDDI developed by the American National Standards Institute (ANSI), which focus on high-speed (more than 50 megabits per second [Mbps]) LANs, cover the physical and data-link layers for the reference model. There are no absolute limitations on the length of the network fiber or the number of nodes, but an assumed system has approximately 1000 nodes spaced at distances of up to two kilometers over a total fiber length of 200 kilometers. These are the figures used to calculate the default values in the ANSI standards (Fortier, 1992, p. 693).
A bit error rate of 2.5E-10 is specified for a ring implementation. In order to provide increased fault tolerance, dual counterrotating ring configurations are possible. When this configuration is used, each ring (one primary and one secondary) operates at 100 Mbps over separate paths. In this dual ring implementation, there are two dual-fiber cable connections attached to each node. The first dual fiber cable connection carries the incoming signal for the primary ring and the outgoing signal for the secondary ring. The second connection carries the outgoing signal for the primary ring and the incoming signal for the secondary ring. Normal traffic uses the primary ring; the secondary ring is redundant and is used for fault recovery.
Two types of network stations (nodes) can be used under FDDI: class A and class B. Class A nodes can connect to both the primary and redundant rings simultaneously. Class B nodes can connect to only one ring at a time. Class A stations can use the dual connection configuration, while class B network stations use only a single connection to carry both the incoming and outgoing lines of a ring. Class A and class B stations can only interconnect through a wiring concentrator.
There are three distinct levels of failure from which the sys...