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Chapter 2
Protocol Attributes
Fault tolerance is another of FDDI's big improvements. Because FDDI uti-
lizes dual rings, the ring goes into a wrapped state and dynamically isolates
the failed section any time a failure occurs. When the ring wraps, the adja-
cent FDDI interfaces loop the ring. Thus, instead of transmitting data out the
primary ring on the outgoing port, it transmits the data out the outbound
port on the secondary ring. Figure 2.29 gives a better picture of what actually
happens.
F I G U R E 2 . 2 9
FDDI's self-healing ring
There are many different fiber/equipment failure scenarios, but FDDI's
fault-tolerant technology can resolve most of them. In addition to having the
dual ring, FDDI also uses beaconing to help isolate ring problems.
FDDI Specifications
Now that we have discussed what FDDI is and some of the features it has,
we need to understand how it is implemented with respect to the OSI model
and the frame type it uses.
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