Fiber distributed data interface (FDDI), which is an optical data communication standard used for long distance networks provides communication with fiber optic lines up to 20…0 kilometers at a speed of 100 megabit per second (Mbps). It should be noted that relatively recently, a related copper specification, called Copper Distributed Data Interface (CDDI) has emerged to provide 100-Mbps service over copper. CDDI is the implementation of FDDI protocols over twisted-pair copper wire. FDDI uses a dual-ring architecture with traffic on each ring flowing in opposite directions (calledcounter-rotating). The dual-rings consist of a primary and a secondary ring. During normal operation, the primary ring is used for data transmission, and the secondary ring remains idle and available for backup. The primary purpose of the dual rings, as will be discussed in detail later in this chapter, is to provide superior reliability and robustness. FDDI was later extended to FDDI-2 for long distance voice and multimedia communication.