FDDI is a set of LAN standards developed in the 1980’s; it is recognized by the ISO and is governed by the ANSI X3T9.5 standards committee. FDDI is not actually a single standard, but a collection of four standards that will be defined shortly. A FDDI network consists of a 100 Mbps dual-ring topology that runs over fiber optic cabling (copper is possible over shorter distances, and is referred to as CDDI). Because of the long distances that FDDI networks can span, it is often used for the purpose of creating and connecting a Metropolitan Area Network (MAN). The IEEE has defined MANs in their 802.6 standard.
FDDI is comprised of four standards specifications that exist at the Physical and Data Link layers of the OSI model. These include:
Physical Layer Protocol (PHY). Defines FDDI data encoding, clocking and framing.
Media Access Control (MAC). Defines frame formatting, token management, CRC calculations, and addressing functions.
Physical Medium Dependent (PMD). Defines elements of the physical media in use including bit rates, connectors, and power levels.
Station Management (SMT). Defines station configuration (including ring insertion and removal) as well as network management and fault tolerance features.