In computer networking, fiber-distributed data interface (FDDI) is a standard for data transmission in a local area network that can extend in range up to 200 km (124 miles). The FDDI protocol is based on the token ring protocol . In addition to being large geographically, an FDDI local area network can support thousands of users. The underlying medium is optical fibre (though it can be copper cable, in which case it may be called CDDI ) and the topology is a dual-attached, counter-rotating token ring.

FDDI is a product of American National Standards Institute X3-T9 and conforms to the open system interconnect (OSI) model of functional layering. LANs using other protocols. FDDI-II is a version of FDDI that adds the capability to add circuit-switched service to the network so that voice and video signals can also be handled. Work is underway to connect FDDI networks to the developing Synchronous Optical Network SONET.

An 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. If the secondary ring is not needed for 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 rings are normally constructed 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, are connected to both rings - these are referred to as "dual-attached". Host computers are then connected as single-attached devices to the routers or concentrators. The dual ring in its most degenerate form is simply collapsed into a single device. The whole dual ring is typically contained within a computer room although FDDI was also deployed as a Metropolitan area network as well.

This network topology is required 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 these are considered to be unreliable and error-prone). Devices such as workstations and minicomputers that may not be under the control of the network managers are not suitable for connection to the dual ring.

As an alternative to a dual-attached connection, the same degree of resilience is available to a workstation through a dual-homed connection which is 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.

FDDI has been largely made redundant by fast Ethernet and more recently Gigabit Ethernet due to their speed, cost and ubiquity.

The four standards are:

• ANSI X3T9.5, containing Physical Media Dependent (PMD) specifications
• ANSI X3T9.5, containing the Physical (PHY) specifications
• ANSI X3.139, containing Media Access Control (MAC) specifications
• ANSI X39.5, containing the Station Management (SMT) specifications.

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