Survivable embedding of logical topology in WDM ring

We consider the design of survivable logical topologies over physical WDM ring networks. The logical topology consists of the same set of nodes as the physical topology, and the links of the logical topology are lightpaths in the physical topology. The logical topology is said survivable if the failure of any single physical link does not disconnect the logical topology. In this paper, we consider tthe following problem. Given a logical topology with lightpath end-nodes, route the lightpaths to make the logical topology survivable if possible. Otherwise, determine and embed the minimum number of additional lightpaths to achieve survivability.