In a wavelength-routed optical network,optical signal is transmitted over the entire optical lightpath assigned by routing and wavelength assignment(RWA) algorithm.While propagation through the network,the signal may degrade in quality as it encounters crosstalk and amplified spontaneous emission noise due to a number of fiber segments,passive and active optical components.Since these impairments continue to degrade the signal quality as it progresses toward its destination,the received bit error at the destination node might become unacceptably high.A novel attempt to evaluate network performance while taking into consideration the physical layer limitations was presented.The reasonable node and transmission link model were difind.Then a novel lightpath quality evaluation algorithm under linear impairment consideration was provided,which is used to estimate online BER on candidate routes and wavelengths before setting up a lightpath.The validity of the algorithm was validated by simulation and experiments.