Information Technology Reference
In-Depth Information
Asynchronous bandwidth is allocated using an eight-level priority scheme. Each station is assigned an
asynchronous priority level. FDDI also permits extended dialogues, where stations may temporarily use
all asynchronous bandwidth. The FDDI priority mechanism can essentially lock out stations that cannot
use synchronous bandwidth and have too low an asynchronous priority.
Fault-Tolerant Features
FDDI provides a number of fault-tolerant features, the most important of which is the dual ring . If a
station on the dual ring fails or is powered down or if the cable is damaged, the dual ring is automatically
wrapped (doubled back onto itself) into a single ring, as shown in Figure 5-4. In this figure, when Station
3 fails, the dual ring is automatically wrapped in Stations 2 and 4, forming a single ring. Although
Station 3 is no longer on the ring, network operation continues for the remaining stations.
Figure5-4
Station Failure, Ring Recovery Configuration
Station 1
MAC
B
A
Station 4
Station 2
Ring wrap
Ring wrap
A
B
MAC
MAC
B
A
AB
Failed station
Station 3
Figure 5-5 shows how FDDI compensates for a wiring failure. Stations 3 and 4 wrap the ring within
themselves when wiring between them fails.
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