FDDI uses counter-rotating primary and secondary rings. A new LAN media was needed that could easily support these workstations and their new distributed applications. At the same time, network reliability had become an increasingly important issue as system managers migrated mission-critical applications from large computers to networks. FDDI was developed to fill these needs. FDDI Transmission Media FDDI uses optical fiber as the primary transmission medium, but it also can run over copper cabling.
Optical fiber has several advantages over copper media.
In particular, security, reliability, and performance all are enhanced with optical fiber media because fiber does not emit electrical signals. Finally, FDDI allows two kilometers between stations using multi-mode fiber, and even longer distances using a single mode. A mode is a ray of light that enters the fiber at a particular angle. Multi-mode fiber allows multiple modes of light to propagate through the fiber. Because these modes of light enter the fiber at different angles, they will arrive at the end of the fiber at different times.