The design of ICSSM, a nonreal time computer-aided simulation and analysis tool for communications systems, is presented, ICSSM is capable of supporting modeling, simulation, and analysis of any system representable in terms of a network of multiport functional blocks. Its applicability is limited only by the modeler's ingenuity to decompose the system to functional blocks and to represent these functional blocks algorithmically. ICSSM has been constructed modularly, consisting of five subsystems to facilitate the tasks of formulating the model, exercising the model, evaluating and showing the simulation results, and storing and maintaining a library of modeling elements, analysis, and utility subroutines. It is written exclusively in ANSI Standard Fortran IV language, and is now operational in a Honeywell DPS 7/80 M computer under the MULTICS Operating System. Description of a recent simulation using ICSSM and some generic moduels of general interest developed as a result of the modeling work are also presented.