BASIC TOKEN RING NETWORKS
Page A-6
Reliability
Since token ring networks depend on ring topology for proper
operation, the entire network is vulnerable to the frailties of each ring
segment. Arranging the ring as a star, using concentrators, and
providing trunk coupling units for the station connections reduce the
risk of a single failing node bringing the entire network down. To
further reduce this vulnerability, a redundant data path is provided
in the main ring trunk cabling.
While our theoretical ring required media capable of only one-way
traffic to achieve the circular flow of data, actual token ring
applications use media that provides two ring paths, a primary ring
and a backup ring. This backup ring is used to restore the continuity
of the ring in the event of a failed trunk segment (broken trunk
cable). Figure A-4 illustrates how the open ends of the ring can be
wrapped into the backup ring, restoring continuity through the
creation of a new ring. (Some devices will wrap automatically when a
problem is detected. Others require human intervention to restore
the ring.) The ability to wrap and bypass trunk segments introduces
other problems. The recovery process produces a much longer
physical trunk cable length. When this length exceeds the maximum
drive distance, the problem must be solved by the network designer
by adjusting cable lengths or installing repeaters in the ring.
Figure A-4. Wrapping a Broken Ring
WRAP
CONCENTRATOR #1
PRIMARY RING
BACKUP RING
RING-IN
RING-OUT
RING-IN
RING-OUT
RING-IN
RING-OUT
CONCENTRATOR #2
CONCENTRATOR #3
