This part of IEC 61280 defines the averaged Q-factor and provides a procedure to measure it by using amplitude histogram parameters such as the standard deviation and average level. The averaged Q-factor and amplitude histogram parameters are useful for monitoring changes in optical signal quality in installed optical networks. The average Q-factor is correlated with the traditional Q parameter of a given optical channel. With the averaged Q-factor test method signal, quality degradations due to optical signal-tonoise ratio (OSNR) degradation and to waveform distortion can be monitored: • OSNR degradation, due to the following causes: – accumulation of amplified spontaneous emission (ASE); – transmission line loss; – channel crosstalk; • waveform distortion, due to the following causes (usually simultaneously present): – chromatic dispersion; – polarization mode dispersion (PMD); – optical nonlinear effects. NOTE 1 In some cases the main cause of signal degradation (OSNR degradation or waveform distortion) can be identified by suitable processing of the measured data NOTE 2 This method is insensitive to the optical signal quality variations created by timing jitter.