Assessment of QoT Penalties Caused by Band Failures in Multi-Band Optical Line Systems

Ultra-wideband (UWB) optical transmission systems rely on accurate quality-of-transmission (QoT) estimation to enable automation and reliable lightpath provisioning. While existing physical-layer models typically assume fixed spectral loading conditions, real networks are subject to dynamic changes, including the failure or shutdown of entire spectral bands due to maintenance, reconfiguration, or faults. In this work, we investigate the impact of spectral band failures on the generalized signal-to-noise ratio (GSNR) in multi-band optical networks. Using a numerical propagation framework that accounts for amplified spontaneous emission, nonlinear interference, and stimulated Raman scattering, we analyze single-and multi-band failure scenarios in a five-band UWB system. Our results show that band failures induce significant GSNR redistribution across the remaining spectrum, driven by Raman power transfer and nonlinear effects, leading to both local and global performance variations. These findings highlight the need to explicitly account for spectral failure scenarios in QoT estimation and network design to ensure robust and conservative operation