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Fiber distributed data interface (FDDI) provides a standard for data transmission in a local area network that can extend in range up to 200 kilometers (124 miles). Although FDDI protocol is a token ring network, it does not use the IEEE 802.5 token ring protocol as its basis; instead, its protocol is derived from the IEEE 802.4 token bus timed token protocol. In addition to covering large geographical areas, FDDI local area networks can support thousands of users. As a standard underlying medium it uses optical fiber (though it can use copper cable, in which case one can refer to CDDI). FDDI uses a dual-attached, counter-rotating token ring topology.

FDDI, as a product of American National Standards Institute X3T9.5 (now X3T12), conforms to the Open Systems Interconnection (OSI) model of functional layering of LANs using other protocols. FDDI-II, a version of FDDI, adds the capability to add circuit-switched service to the network so that it can also handle voice and video signals. Work has started to connect FDDI networks to the developing Synchronous Optical Network SONET.

A FDDI network contains two token rings, one for possible backup in case the primary ring fails. The primary ring offers up to 100 Mbit/s capacity. When a network has no requirement for the secondary ring to do backup, it can also carry data, extending capacity to 200 Mbit/s. The single ring can extend the maximum distance; a dual ring can extend 100 km (62 miles). FDDI has a larger maximum-frame size than standard 100 Mbit/s ethernet, allowing better throughput.

Designers normally construct FDDI rings in the form of a "dual ring of trees" (see network topology). A small number of devices (typically infrastructure devices such as routers and concentrators rather than host computers) connect to both rings - hence the term "dual-attached". Host computers then connect as single-attached devices to the routers or concentrators. The dual ring in its most degenerate form simply collapses into a single device. Typically, a computer-room contains the whole dual ring, although some implementations have deployed FDDI as a Metropolitan area network.

FDDI requires this network topology because the dual ring actually passes through each connected device and requires each such device to remain continuously operational (the standard actually allows for optical bypasses, but network engineers consider these unreliable and error-prone). Devices such as workstations and minicomputers that may not come under the control of the network managers are not suitable for connection to the dual ring.

As an alternative to using a dual-attached connection, a workstation can obtain the same degree of resilience through a dual-homed connection made simultaneously to two separate devices in the same FDDI ring. One of the connections becomes active while the other one is automatically blocked. If the first connection fails, the backup link takes over with no perceptible delay.

Due to their speed, cost and ubiquity, fast Ethernet and (since 1998) Gigabit Ethernet have largely made FDDI redundant.

FDDI standards include:

• ANSI X3.166-1989, Physical Medium Dependent (PMD) -- also ISO 9314-3
• ANSI X3.148-1988, Physical Layer Protocol (PHY) -- also ISO 9314-1
• ANSI X3.139-1987, Media Access Control (MAC) -- also ISO 9314-2
• ANSI X3.229-1994, Station Management (SMT) -- also ISO 9314-6
• ANSI X3.184-1993, Single Mode Fiber Physical Medium Dependent (SMF-PMD) -- also ISO 9314-4

Source: from Federal Standard 1037C and used with permission from http://www.Foldoc.org

• List of device bandwidths