The emergence of self-oscillating modes in data-transmission networks negatively affects characteristics of these networks. As a result the task of identifying zones of self-oscillations’ origin and studying the self-oscillation parameters becomes relevant. The study of the self- oscillating modes is complicated by the significant nonlinearity of the original system. The study of self-oscillating modes could simplify the transition to the linearized model; however, the self-oscillating mode disappears during the linearization. As an alternative, it is proposed to use an harmonic linearization approach which takes into consideration both the linearized part of the equations and the nonlinearity that influences them. This paper describes the preparation for the application of the harmonic linearization method, namely the linearization of the original nonlinear model.