3.2 Layer 2 Protocol Overview - LAN Protocols
3.2.8 Fiber distributed data interface
FDDI is an acronym for Fiber Distributed Data Interface. In fact, most people don't even spell out F-D-D-I; they run the letters together and pronounce it "fiddy." FDDI is a robust and reliable LAN technology that dates back to the mid-1980s. FDDI features a 100-Mbps data rate and dual counter-rotating rings. These rings can span up to 200 kilometers, using fiber-optic cables. This maximum distance means that FDDI can be used effectively in both a local and a metropolitan-area network (MAN). Despite these appealing features, like Token Ring, FDDI is losing market share as Fast Ethernet and Gigabit Ethernet become more and more common due to their relatively inexpensive implementation.

Access to FDDI transmission media is regulated through a token-passing scheme that is similar to Token Ring. An example of a FDDI frame is shown in the figure to the left. Only the device that holds the token may transmit. Also, similar to Token Ring, the token can pass in only one direction on a ring.

FDDI has three different physical-layer specifications, which are called physical-layer medium, or PMD, specifications. Originally, FDDI was limited to a single PMD: 62.5/125-micron diameter multimode fiber-optic cabling. It remained a glass-only technology until the 1990s. Then, the high cost of fiber-optic cable started cutting into its market share. The answer seemed obvious: develop a copper-based PMD that could support the FDDI protocols.

In June 1990, ANSI formed a working committee to build the specification for a twisted-pair PMD (TP-PMD). Originally, the TP-PMD was a proprietary product that grafted the FDDI Layer 2 on to a Category 5 unshielded twisted-pair (UTP) physical layer. The end result was marketed as CDDI, for Copper Distributed Data Interface. This specification became an ANSI standard in 1994. Today, CDDI is obsolete. Its biggest benefit was a 100-Mbps data rate over copper wires. With the arrival of Fast Ethernet, the market had a 100-Mbps LAN with copper wires and a familiar data-link protocol that easily integrated with their existing networks. Consequently, support for CDDI has been dropped by Cisco Systems, the original owner of the technology.

A single-mode fiber-optic version (SMF-PMD) has also been developed. Based on 8.3-micron diameter fiber-optic cabling, and driven by a laser rather than an LED, this PMD is more expensive than its multimode counterpart. It also consumes more energy and dissipates more heat than the other two FDDI PMDs. In its favor, the SMF-PMD can maintain the integrity of transmitted signals for much greater drive distances: up to 60 kilometers versus the paltry 2 kilometers of the multimode fiber, or 100 meters of twisted pair.

Note: A drive distance is the distance between two devices. It is not the maximum diameter of the network.