In the last few years there has been an explosion in the amount of traffic carried by the Internet which made a huge increase in demand for bandwidth in core optical networks and we have witnessed the extensive deployment of wavelength- division multiplexed (WDM) networks. These systems are configured to allow multiple channels at different wavelengths to share the same optical fiber, increasing the effective transmission rate on that fiber. Optical network architectures can be characterized as opaque or all- optical (transparent) or as a compromise between opaque and transparent networks. Transparent (or all-optical) network architecture was proposed to reduce the associated cost of opaque networks [1]. In transparent WDM networks, data is transmitted from its source to its destination in optical form. Switching/routing operations are being performed in the optical domain without undergoing any optical-to-electrical conversion. According to the state of the technology, various factors degrade the quality of an analog optical signal along its route. As the signal propagates in a transparent way, these impairments accumulate along the path, and limit the system reach and overall network performance. These physical layer constraints may be classified into linear and nonlinear impairments. In a transparent optical network, beside impairments that accumulate along the path there is an impact of failures which also propagates through the network and therefore cannot be easily localized and isolated. Failure management is one of the crucial functions and prerequisite for protection and restoration schemes. In order to realize the vision of transparency while offering strict QoS guarantees, it is necessary to monitor and control optical network.