Most of the worldwide data and voice traffic is transported using optical fiber. We describe a simulation-based method to estimate the capacity of a fiber channel under current and near-future constraints, especially the use of optical routing via wavelength-division multiplexing (WDM). The simulation incorporates all Kerr instantaneous nonlinear effects, input constellation optimization, and various dispersion maps. The networks turn out to be interference-limited, so a natural next step is to consider what is possible when viewing the system as an interference network while still working with the WDM constraint. Strategies called "interference focusing" are outlined that reduce or remove interference.