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C H A P T E R 8
Chapter Goals
• Provide background information about FDDI technology.
• Explain how FDDI works.
• Describe the differences between FDDI and Copper Distributed Data
Interface (CDDI).
• Describe how CDDI works.
Fiber Distributed Data Interface
Introduction
The Fiber Distributed Data Interface (FDDI) specifies a 100-Mbps token-passing, dual-ring LAN using
fiber-optic cable. FDDI is frequently used as high-speed backbone technology because of its support for
high bandwidth and greater distances than copper. 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. This chapter focuses mainly on FDDI specifications
and operations, but it also provides a high-level overview of CDDI.
FDDI uses dual-ring architecture with traffic on each ring flowing in opposite directions (called
counter-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. As will be discussed
in detail later in this chapter, the primary purpose of the dual rings is to provide superior reliability and
robustness. Figure 8-1 shows the counter-rotating primary and secondary FDDI rings.
8-1
rnetworking Technologies Handbook
Chapter 8
Fiber Distributed Data Interface
FDDI Transmission Media
Figure 8-1
FDDI Uses Counter-Rotating Primary and Secondary Rings
Standards
FDDI was developed by the American National Standards Institute (ANSI) X3T9.5 standards committee
in the mid-1980s. At the time, high-speed engineering workstations were beginning to tax the bandwidth
of existing local-area networks (LANs) based on Ethernet and Token Ring. A new LAN media was
needed that could easily support these workstations and their new distributed applications. At the same
time, network reliability had becom