FDDI is an optical fiber-based local area network (LAN) that uses the American National Standards Institute (ANSI) 3T9.5 standard for a token-passing ring media access control (MAC) protocol. Stations, concentrators, and bridges in a FDDI network are physically connected to one of the counter-rotating rings or both of the counter-rotating rings. The rings operate at 100 Mbps.
FDDI networks allow devices to be attached to one or both of the rings. Typically only the primary ring in a FDDI network is active. The secondary ring is used to maintain the network when a dual-access station or a concentrator becomes inactive.