We introduce SSMS, a multiband optical fiber simulator entirely developed in MATLAB. SSMS solves the generalized nonlinear Schrödinger equation relying on the 4th order Runge-Kutta method in Interaction Picture (RK4IP) with adaptive step size approach and compare it with the widely used split-step Fourier method (SSFM). The simulator is validated considering S+C+L multiband transmission. Results show that the RK4IP method is approximately 10 x faster than the traditional SSFM model for a similar level of accuracy.