The analog character of transparent (i. e. nort-3R-regenerated) optical transmission systems leads to limitations of the transmission distance that strongly depend on system design and choice of the system parameters (power level, percentage of dispersion compensation, etc.). To simplify network design and operation, it is proposed to compose a network using a cascade of well designed and optimised basic transmission sections. The optimisation procedure is described in details, and implications for network operation and management are given.