Abstract
The physical layer of a wavelength-division-multiplexed optical distribution network for smart-city Internet-of-Things access is almost always reported from a single simulation run, which says nothing about its stability. This paper characterized an eight-channel C-band link over 25 km of standard single-mode fibre, modelled in OptiSystem, across thirty independent replications yielding 240 per-channel Q-factor observations. Every channel cleared the Q ≥ 6 threshold in every replication, with a system mean of 8.66, a per-channel 95% confidence interval never wider than 0.46, and a worst realization of 6.64 on the weakest channel, so the link held a stable margin rather than crossing the boundary between runs. The per-channel Q-factors followed a U-shape across the wavelength grid with its minimum at the centre, and a short-reach reference at 1 km showed that two thirds of the edge-to-interior asymmetry was established within the first kilometre. A launch-power sweep placed the usable ceiling between 10 and 13 dBm. A control experiment with the fibre nonlinearity disabled collapsed the edge-to-interior ratio from 1.354 to 1.038, establishing that 89% of the asymmetry was nonlinear in origin, while a four-wave-mixing coherence length of 0.74 km against the 25 km span identified cross-phase modulation as the mechanism.