In this study, we analyze the network design problem arising in Wavelength Division Multiplexing (WDM) networks where tra c is static, wavelength interchanging is allowed and the location and number of the wavelength interchangers are to be determined. Given a topology and tra c data, we try to nd the ber and wavelength interchanger con guration with the minimum cost, that can establish all given connections. We present di erent formulations of the problem and some valid inequalities. Finally, we propose a heuristic method of generating feasible solutions, apply the method on three di erent topologies with varying tra c data, and present the results. The method is based on the idea of partitioning the problem into two; routing problem and wavelength assignment and interchanger location problem. Our results prove to be close to the lower bounds we generate, and indicate that the ber cost performance of the case where all nodes are wavelength interchangers can be attained using a relatively small number of wavelength interchangers.