Information Technology Reference
In-Depth Information
5
CHAPTER
Troubleshooting Fiber Distributed Data Interface
The Fiber Distributed Data Interface (FDDI) standard was produced by the ANSI X3T9.5 standards
committee in the mid-1980s. During this period, high-speed engineering workstations were beginning
to tax the capabilities of existing local-area networks (LANs)—primarily Ethernet and Token Ring. A
new LAN was needed that could easily support these workstations and their new distributed applications.
At the same time, network reliability was becoming an increasingly important issue as system managers
began to migrate mission-critical applications from large computers to networks. FDDI was developed
to fill these needs.
After completing the FDDI specification, ANSI submitted FDDI to the International Organization for
Standardization (ISO). ISO has created an international version of FDDI that is completely compatible
with the ANSI standard version.
Although FDDI implementations are not as common as Ethernet or Token Ring, FDDI has gained a
substantial following that continues to increase as the cost of FDDI interfaces diminishes. FDDI is
frequently used as a backbone technology as well as a means to connect high-speed computers in a local
area.
FDDI Technology Basics
FDDI specifies a 100-Mbps, token-passing, dual-ring LAN using a fiber-optic transmission medium. It
defines the physical layer and media-access portion of the link layer, and is roughly analogous to IEEE
802.3 and IEEE 802.5 in its relationship to the Open System Interconnection (OSI) reference model.
Although it operates at faster speeds, FDDI is similar in many ways to Token Ring. The two types of
networks share many features, including topology (ring), media-access technique (token passing), and
reliability features (redundant rings, for example). For more information on Token Ring and related
technologies, refer to Chapter 6, “Troubleshooting Token Ring.”
One of the most important characteristics of FDDI is its use of optical fiber as a transmission medium.
Optical fiber offers several advantages over traditional copper wiring, including security (fiber does not
emit electrical signals that can be tapped), reliability (fiber is immune to electrical interference), and
speed (optical fiber has much higher throughput potential than copper cable).
FDDI defines use of two types of fiber: single mode (sometimes called monomode) and multimode.
Modes can be thought of as bundles of light rays entering the fiber at a particular angle.
Single-mode
fiber
allows only one mode of light to propagate through the fiber, whereas
multimode fiber
allows
multiple modes of light to propagate through the fiber. Because multiple modes of light propagating
through the fiber may travel different distances (depending on the entry angles), causing them to arrive
