Page 305 - Optical Communications Essentials

Optical Networks

Optical Networks 295

Primary 2-fiber
bidirectional loop
4p
1p
Node
8p 5p
4s 1
1s
8s 5s
Secondary 2-fiber
bidirectional protection loop
Node Node
4 2

6p 2p
7s 6s
7p 3s Node 2s
3
3p
Figure 17.10. Architecture of a four-fiber bidirectional line-
switched ring (BLSR).

Primary 2-fiber loop
Node
1
2-Fiber
protection
loop
Node Node
5 2
Protection switching
Node 4 Node 3

Break in
Card failure primary fiber
Figure 17.11. Reconfiguration of a four-fiber BLSR under trans-
ceiver or line failure.

on the primary ring fails in either node 3 or 4. In this situation the affected
nodes detect a loss-of-signal condition and switch both primary fibers connecting
them to the secondary protection pair, as shown in Fig. 17.11. The protection
segment between nodes 3 and 4 now becomes part of the primary bidirectional
loop. The exact same reconfiguration scenario will occur when the primary fiber
connecting nodes 3 and 4 breaks. Note that in either case the other links remain
unaffected.
Now suppose an entire node fails, or both the primary and the protection
fibers in a given span are severed, which could happen if they are in the same
cable duct between two nodes. In this case the nodes on either side of the failed

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