We investigate the properties of a semiconductor laser with arbitrary levels of external optical feedback, by means of an iterative travelling-wave (ITW) model. Bifurcation diagrams and the corresponding Lyapunov exponent are used to describe the behavior of the feedback laser as the external feedback reflectivity increases from 0 to 1. Similarities and differences between the ITW model and the Lang-Kobayashi (LK) model are also reviewed. To determine the critical value of the external reflectivity, beyond which the LK model would no longer be valid, we propose a criterion based on the iterates of the phase maps for both the compound-cavity modes (ITW model) and the external-cavity modes (LK model). Preliminary results show that this critical value may correspond to the so-called « -value», which is defined as the onset value of the first chaotic region.