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The increasing demand in the Internet network for real-time multimedia data traffic with high quality is pushing the limits of existing network structure. Optical packet switching (OPS) is considered as an attractive technology for future optical networks. Due to the high transmission capacity, network management is becoming more and more important. In this context, fault identification and performance monitoring are essential network management functions. In this paper, we propose a monitoring system that not only performs the failure detection, but also assesses the quality of a set of backup paths to infer the network state from the result of a set of end-to-end measurements. This feature allows service providers to establish new or backup paths based on quality requirements. In particular, specific test packets are sent over the supervisory wavelength and processed in the end nodes by means of an optical correlator. Then the optical signal-to-noise ratio (OSNR) of the test packets is estimated from the noise statistics of the autocorrelation pulse peak power. A fiber Bragg grating-based correlator was fabricated to experimentally demonstrate the successful operation. Experiments performed on a 40 Gb/s system confirm the viability of this approach, obtaining direct OSNR monitoring with an error < 0.5 dB.