Off-line Survivable Impairment-aware Routing and Wavelength Assignment

Physical impairments caused by noise and signal distortions affect the quality of an optical signal. The effect of physical impairments becomes more significant with an increase in distance and bit rates. In order to minimize the bit error rate (BER), an optical signal may need to be regenerated after a certain distance. This is mainly achieved through re-amplification, re-shaping, and retiming, which are collectively known as 3R regeneration. In order to guarantee a certain BER, system vendors offer a certain level of optical signal-to-noise ratio (OSNR) at the output of a system. In a system where signal power levels are low enough that nonlinearities can be neglected, OSNR is an important parameter to measure the quality of an optical signal [12]. It is customary to place amplifiers at several points along a fiber link in order to overcome fiber losses. The segment of a link between two consecutive amplifiers is known as a fiber span. However, optical noise is added by each amplifier along a fiber. This noise is referred to as Amplifier Spontaneous Emission (ASE). ASE degrades the OSNR and is reflected in that measure. In practice, vendors generally provide bounds on the length of a transparent (non-regenerated) path and number of spans in order to ensure an acceptable level of OSNR [13].