Network operators deploy optical monitors to ensure uninterrupted network operation and high quality of service. To achieve this they seek efficient design solutions that also maximize the benefit of their investments. In this work we present a monitoring technique that utilizes partial physical layer information generated by only a small set of monitors deployed in a mesh optical network to assess the quality of transmission (QoT) of all the established connections. The proposed method focuses on the placement of the monitors and on the minimization of the required monitoring equipment. We develop a heuristic that takes advantage of the attribute of certain end-to-end impairments that accumulate additively along the established lightpaths in order to find the optimum locations of a reduced number of available monitors. When monitoring a subset of the established lightpaths, it is possible to estimate the monitored QoT-related metric for all lightpaths leveraging the correlation between the connections sharing common links. The proposed algorithm efficiently selects the monitor locations that maximize the estimation accuracy. Extensive simulation studies show that the heuristic provides solutions close to the optimum and demonstrate that only a fraction of all the available monitor locations (1/4 or 1/3) need to be equipped, leading to significant cost savings. The monitor placement solutions are evaluated for core optical networks of different scales in the presence of static and incremental traffic.
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