These concepts
extend upon SONET bidirectional line-switched ring (BLSR) operation via the
automatic protection switching (APS) protocol. In particular, SPRING achieves spatial
reuse to improve wavelength efficiency and provides multiple protection levels; wavelength
plans route bidirectional demands along the same set of nodes and allow working
/protection traffic to travel in both directions. In contrast, UPSR cannot provide such
reuse since connections traversing different ring segments are unable to use the same
wavelength. Overall, SPRING architectures include two- and four-fiber variants that
operate at the fiber and wavelength levels [3]. These designs also permit wavelength
sharing (backup multiplexing) between multiple working paths and/or lower-priority
traffic. Hence, operators can differentiate protection levels to meet a broader set of
customer demands such as dedicated (platinum), shared (gold), unprotected (silver),
and preemptable (bronze). A major drawback of SPRING, however, is again the lack
of related standards. Therefore, this design is only supported by a few vendors, and
interoperability is extremely low.
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