We discuss the propagation of optical solitons in a monomode fiber as a model of long-distance-high-bit-rate transmission system. We give several new results which did not appear in our previous papers on this subject, such as (1) a derivation of the perturbed nonlinear Schr6dinger equation from the Maxwell equation, (2) on the integrahility of the perturbed nonlinear Schrodinger equation, (3) a discussion of the soliton as a stable fixed point of certain infinite-dimensional map generated by a transmission system with periodic excitations.