HP 200 Series Services And Applications page 241

Access to the FDDI network is controlled by a token that circulates the
primary ring. When a station has data to transmit, it must capture the token
before it can transmit data onto the ring. Only one token may exist on the
ring at any one time. If the token is lost, the ring will enter a phase known as
beaconing (a state in which the beaconing station controls all access to the
ring). If more than one token has been generated, the ring is then scrubbed
(scrubbing is used to remove all data from the ring).
Ring Wrap
Ring wrap is one of the three techniques that FDDI uses in order to ensure
fault tolerance. If any station on the dual ring fails or if the cable is damaged,
the dual ring automatically wraps into a single ring as shown in figure 4.
Primary
Wrapped
Figure 4. Ring Wrap
Multiple failures within a single dual-ring network present other challenges.
The likelihood of multiple failures greatly increases as the number of nodes
within an FDDI network increases. If a single failure occurs, the ring wraps
and becomes a single ring. If another failure should occur, the ring is seg-
mented and separated into two rings.
Shining a Light on FDDI
Failed
Station
Port Port
A B
B A
Port Port
How FDDI Works
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