Fiber Distributed Data Interface, also known by the acronym FDDI, is an optical fiber-based network technology. Usually organized in a ring, star or tree network topology, an FDDI network can span many miles or kilometers. FDDI is often used in university campus networks due to its relatively high speeds and long-distance coverage. It is also used in some metropolitan area network backbones for the same reasons.
The American National Standards Institute (ANSI) created the Fiber Distributed Data Interface specifications in the mid 1980s. FDDI was primarily designed to provide a faster and more reliable network backbone. Copper-based 10 Megabit per second (Mbps) Ethernet and 4/16 Mbps Token Ring were no longer adequate for many networks. In contrast, FDDI can offer 100 Mbps data transmission over secure, interference-free, long-distance optical fiber. Its dual-ring architecture generally provides redundancy and fault tolerance as well.
The traffic on a Fiber Distributed Data Interface network flows in opposite directions on the two rings. In typical operation, the main ring carries all of the data while the other ring remains available in case of a hardware fault. Individual nodes can be attached to both rings simultaneously or to just the main ring. A node attached to the main ring alone is connected through a concentrator. In this case, the concentrator can use the secondary ring to provide a path around a problem on the primary ring.
A node attached to both rings of a Fiber Distributed Data Interface network does not need to be connected to a concentrator. The dual-ring attachment by itself allows an alternate path if part of the primary ring fails. This configuration can only tolerate a single point of failure on the main ring, however. If more than one node is disconnected, powered off or otherwise fails, portions of the ring will be unable to communicate. Concentrators can be used with nodes attached to both rings in order to provide another layer of fault tolerance.
Some networks consist of a Fiber Distributed Data Interface backbone along with an Ethernet or Token Ring local area network. This keeps the overall cost down by minimizing the amount of FDDI technology needed. If shorter distances are involved and interference is not an issue, the same network protocols can be implemented with copper. This method, called the Copper Distributed Data Interface, is very similar to FDDI, but utilizes copper instead of fiber-optic cables. FDDI networks can also be configured to run data in both directions, doubling the throughput while losing fault tolerance.
The high cost and complexity of Fiber Distributed Data Interface networks has made them less popular since the 1990s. Fast Ethernet, Gigabit Ethernet and Fiber Channel technologies offer greater speed at a much lower cost, for example. They also continue to function when connected devices are powered down or removed from the network.
