Stands for "Fiber Distributed Data Interface." FDDI is a group of networking specifications standardized by ANSI in the mid-1980s. An FDDI network supports data transfer speeds of 100 Mbps over a fiber-optic cable and uses a rotating token to define which system can send data at any given time.
FDDI networks are comprised of two physical paths, or "rings," that transfer data in opposite directions. The primary ring carries data between systems, while the secondary ring is used for redundancy. If a system on the network causes an interruption in the primary data path, the secondary ring is used until the primary ring is functional again. A variation of FDDI, called FDDI Full Duplex Technology (FFDT), uses the secondary ring as an additional primary channel. This type of FDDI network has no redundancy, but supports data transfer rates up to 200 Mbps.
FDDI was designed in the 1980s to provide faster networking than the 10 Mbps Ethernet and 16 Mbps token ring standards available at the time. Because of its high bandwidth, FDDI became a popular choice for high-speed backbones used by universities and businesses. While FDDI was the fastest LAN technology for several years, it was eventually superseded by Fast Ethernet, which offered 100 Mbps speeds at a much lower cost. Today, many networks use Gigabit Ethernet, which supports speeds up to 1,000 Mbps.