comsis, which stands for communication systems interactive software, is a computer-aided-design tool based on a time approach. It allows the design, analysis, and performance optimization of optical transmission systems by use of various optical devices. comsis allows scalar or vectorial simulations, depending on whether the polarization is taken into account. An overview concerning the optical component models is given. A new model of an erbium-doped fiber amplifier allows the user to describe the amplifier through either physical or system parameters by using silicate or fluoride glass fibers or any other material, provided the user can give a file that contains the amplifier's characteristics. The new model of a single-mode fiber allows the user to describe chromatic dispersion through a constant, a function, or a file (given by the user) and to take optionally into account the Kerr and the Raman effects and the polarization-mode dispersion. The simulation tools that are used to characterize the quality of an optical transmission system are also presented. To show the system's full range of capabilities in the optical domain, we describe examples of wavelength-division-multiplexing and soliton-transmission systems.