(1) Fiber Distributed Data Interface (FDDI) is a 100 megabit per second optical networking standard that can connect local area networks over distances up to 200 kilometers. It uses a ring-based token network topology derived from token bus protocols. (2) FDDI networks can support thousands of users over large geographic areas using optical fiber as the underlying medium. It offers dual-attached station and single-attached station topologies. (3) An FDDI network contains two counter-rotating rings, with one as a backup, to provide redundancy. The dual rings can extend the maximum distance to 100 km while a single ring provides up to 200 megabits per second capacity.

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2. Hot Tip What’s FDDI?(1) Fiber Distributed Data Interface ( FDDI ) provides a 100 Mbit/s optical standard for data transmission in a local area network that can extend in range up to 200 kilometers (124 miles). Although FDDI logical topology is a ring-based token 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.

3. Hot Tip What’s FDDI?(2) 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 , although it can use copper cable, in which case it may be refer to as CDDI . FDDI offers both a Dual-Attached Station (DAS), counter-rotating token ring topology and a Single-Attached Station (SAS), token bus passing ring topology.

4. What’s FDDI?(3) A FDDI network contains two rings, one as a secondary 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 (4,352 bytes) than standard 100 Mbit/s Ethernet which only supports a maximum-frame size of 1,500 bytes, allowing better throughput.

5. Mitigating failure (1) 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 might not come under the control of the network managers are not suitable for connection to the dual ring.

6. Mitigating failure (2) 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.

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