1. A method of assessing chromatic and modal dispersions in a multimode fiber for assessing a fiber power penalty at a given bit error rate of the multimode fiber to predict multimode fiber behavior in an optical communication system, the method comprising: providing the multimode fiber and an artificial light source before a first step; applying artificial light from the artificial light source to the multimode fiber before the first step; the first step of measuring differential mode delay by utilizing a test machine to measure a set of elementary fiber responses corresponding respectively to different offset launches of the artificial light over a core radius into the multimode fiber; a second step of generating a global fiber response by applying, to the set of elementary fiber responses, a set of weighting coefficients and time delays respectively depending on the different offset launches of the elementary fiber responses; and a third step of computing a parameter representative of the fiber power penalty from the global fiber response; wherein, in the second step of generating a global fiber response, the set of weighting coefficients includes several subsets of weighting coefficients that are time delayed relative to one another, wherein at least one relative time delay is not set to zero, and wherein the weighting coefficients of each subset respectively depend on the different offset launches of the elementary fiber responses, wherein fiber selection is made based on the computed parameter being at least an effective bandwidth and the fiber selection includes fibers having the effective bandwidth between 3000 MHz-km and 6000 MHz-km at 850 nm; and wherein the method achieves a correlation between predicted values and actual values of the multimode fiber in the optical communication system.