Fiber Distributed Data Interface (FDDI)

Fiber Distributed Data Interface (FDDI) is a 100-Mb/sec fiber optic local area network standard standard developed by the American National Standards Institute (ANSI) for transmitting data over local area networks (LAN). The X3T9.5 ANSI Task Group is responsible for the standards. The FDDI physical and media access sub-layers operate under the standard IEEE Logical Link Control / 802.2 interface.

An operating system utilizes FDDI in exactly the same way as it would 802.3 Ethernet or 802.5 Token-Ring. You can learn more about 802.2 by reading the Logical Link Control articles in the compendium. In 1987 the first FDDI adapter cards began appearing on the market and the market for FDDI connectivity grew from there.

A FDDI network can have a maximum of 500 stations. There are two types of optical fiber that can be used to interconnect the stations: Single-Mode and Multimode. When Single-Mode fiber is used the stations can be as much as 60 kilometers apart. With Multimode the stations must be no more than 2 kilometers apart.

The stations are connected in a physical ring; with the transmitter port on one station generating a light signal that is carried on the optical fiber to the receiver port on the next station. The stations are interconnected into a star configuration using a device called a Concentrator.

The fundamental ring network architecture is implemented by connecting a group of concentrators together in a circle. To provide a redundant optical path (in the event of a failure on the primary path) a second set of fiber-optic cables is connected so that each concentrator is actually attached to two concentric rings. These are referred to as Dual Attached Concentrators, or DACs (pronounced, “dak”, rhymes with “black”). This dual attached ring is referred to as the Primary Ring. Hanging off this primary ring there can be additional devices that are not dual-attached. These use an interconnect device called a Single Attached Concentrator, or SAC (pronounced, “sak”).

The challenges for the engineer responsible for a FDDI network include those for any network and, in addition, there are some additional things to understand:

• The optical engineering principles that underlie FDDI engineering
• The various components that can be used to interconnect devices

This section of the Compendium discusses the FDDI standards and sets out the engineering information that is critical to the analysis and troubleshooting of FDDI networks.