The ANSI standard (X3T9-5) for a dual, counter-rotating, fiber optic, token-passing ring LAN.The specification pegs the signaling rate at 125 Mbps and the transmission rate (i.e., data rate) at 100 Mbps due to the 4B/5B line coding technique. FDDI is intended for backbone applications, interconnecting major computing resources such as high speed switches, routers, and servers. As the FDDI maximum frame size is 9000 symbols (1 symbol = 4 bytes), Ethernet and Token Ring frames can easily be encapsulated within FDDI frames for backbone transport. FDDI specifies devices separations of as much as 1.2 miles (2 kilometers) over multimode fiber (MMF) and 37.2 miles (62 kilometers) over single-mode fiber (SMF), with excellent error performance.The dual counter-rotating ring provides considerable redundancy, but requires that all directly connected devices be dual-attached, which adds to the cost and complexity. In consideration of the high cost and fragility of optical fiber, standards were developed to extend connectivity to workstations via unshielded twisted pairs.Those standards are known variously as CDDI (Copper Distributed Data Interface) and TPDDI (Twisted Pair Distributed Data Interface). FDDI is considered obsolete, having been overwhelmed by simpler, higher speed switched Ethernet technologies such as 1000Base-LX, 1000Base-SX, and 10GBase-LR, LW. See also 10GBase-LR, 1000Base-LX, 1000Base-SX, 4B/5B, ANSI, backbone, CDDI, Ethernet, fiber optics, frame, LAN, LW, MMF, signaling rate, SMF, symbol, token passing, Token Ring, and TPDDI.
(Fiber Distributed Data Interface) Often pronounced "fiddy," it was a LAN and MAN access method that had its heyday in the mid-1990s. FDDI was an ANSI standard token passing network that transmitted 100 Mbps over optical fiber up to 10 kilometers. It included its own network management system and could optionally run over copper wire (CDDI) with distance limitations. FDDI II added circuit-switched service to this normally packet-switched technology in order to support isochronous traffic such as real-time voice and video.Dual Rotating RingsFDDI provided an optional dual counter-rotating ring topology that contained primary and secondary rings with data flowing in opposite directions. If a line broke, the ends of the primary and secondary rings were bridged together at the closest node to create a single ring again.Single Attached and Dual AttachedNodes could be configured as Single Attached Stations (SAS) connected to concentrators or as Dual Attached Stations (DAS) connected to both rings. Groups of stations were typically wired to concentrators connected in a hierarchical tree to the main ring. Large networks could be configured as a "dual ring of trees," in which the dual ring provided the backbone to which multiple hierarchies of concentrators were attached.