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