Networking Explained, 2nd Edition

Chapter 10: Fiber Distributed Data Interface (FDDI) 361

is typically done in a coarse way with a hand tool and completed with a polishing

machine. This allows a smooth, flawless end that will properly pass the light from the

source to the destination transceiver or repeater. This polishing effort may take up to 40

minutes per fiber strand (imagine this on a 48-fiber run!), which accounts for the high cost

of fiber installation. Improper polishing and connection of the termination points on a fiber

will at most render the fiber useless or will, at the very least, result in poor network perfor-

mance. After it is terminated, the fiber must be tested to ensure proper installation. This

normally involves using an optical time domain reflectometer (OTDR), an expensive test-

ing device. In summary, properly installing fiber for FDDI is painful, time-consuming, and

expensive.

21. So now I know how FDDI networks generally work. How would I use one in a

“real” environment?

Glad you asked. Typically, FDDI networks implement backbone configurations for

buildings and campus environments. In this design, a master FDDI “switch” connects sep-

arate FDDI networks on each floor in the building. This configuration is called a collapsed

backbone design, where the master backbone is the switch and all major connections have

been connected to the network switch (thus “collapsed” into a single box). This enables

expansion and traffic isolation between the networks and reduces network failure poten-

tial. If the switch is “intelligent,” it further isolates traffic and may include redundant facil-

ities to keep the switch operational in adverse conditions such as motherboard failure or

power supply failure.

The master “hub” switch of the building would be a very high speed, multinetwork

concentration facility. Some hubs can support up to 36 FDDI networks (in the extended

configuration) and can support T3 and asynchronous transfer mode (ATM) technologies in

the same box. (See Chapter 7 for information about T3 circuits and Chapter 14 for infor-

mation about ATM.) This tiered interconnection scheme using a very high-speed concen-

trating device is essential in developing high-speed, high-reliability networks.

22. What is an FDDI “switch?”

An FDDI “switch” interconnects many FDDIs through a common “box.” This “box”

or switch manages and sorts traffic between other switches so that it is sent only to where

it needs to go. On an FDDI, all connected systems “see” all frames on the network. In a

switched environment, the only traffic viewed by a specific FDDI is that which originates

on it or is destined to it. Thus, all traffic on all other FDDI segments is isolated to each

individual segment by the switch. This reduces overall traffic between the segments,

reduces unnecessary traffic between networks, and improves performance overall network

performance. (See Chapter 6 for additional information about switches.)

23. I have an Ethernet/802.3 LAN. How would I interconnect it to FDDI?

Interconnecting FDDI and Ethernet /802.3 is easier than ever today, especially given

the communications hub technologies available from vendors. What is normally required is

an Ethernet/802.3 to FDDI bridge unit. Such bridge units are available in many varieties,