The Fiber Distributed Data Interface

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

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to transmitting data, while ATM can transmit multimedia information. As a result, the packet format for FDDI is long and variable, while the packet format for ATM is short and fixed. ATM has no top speed, while FDDI is currently limited to 100 Mbps (although multiple FDDIs can be used to overcome this limitation). FDDI's primary advantages are that it has been tested and proven effective, and that no single point of failure can block its performance. ATM's primary advantages are that wide area ATM's are being developed, and that it is best suited for the rising interest in multimedia transmission. FDDI's primary disadvantages are that its bandwidth degrades and that there are distance limits to the rings themselves. ATM's primary disadvantages are that switches can block it and that the vendor interoperability, a key factor in network management, is currently untested. The prices for both protocols are falling (Kerr, 1993, p. 30; Hurwicz, 1993, p. 31). FDDI is popular among large corporations, including Coors Brewing Company, General Motors, Martin Marietta and Mitsubishi International. Applications for the protocol are varied across organizations, but the government agency in charge of Italy's national highway system pr

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