In all-optical network

the signal degradation occurs during transmission in non-ideal medium along a lightpath. When the signal degradation reaches a certain level

the lightpath is not usable and is blocked due to transmission impairment in the physical layer. To provide high quality connection

the quality of signal should be taken into consideration. Optical signal-to-noise ratio and polarization mode dispersion are considered to be the most important constraints. There are two impairment-aware RWA algorithms: the best-path algorithm and the first-fit algorithm