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Existing research demonstrated that an effective routing and wavelength assignment (RWA) scheme and a wavelength converter placement algorithm are the two primary vehicles for improving the blocking performance in a wavelength-routed all-optical network. However, these issues have largely been investigated separately, in particular, the RWA has seldom considered the existence of wavelength converters. In this paper, we argue perhaps for the first time, that an effective RWA algorithm needs to take into account the presence of wavelength conversion as the latter is usually done at much earlier stage during the capacity planning. We proceed to show that existing dynamic RWA algorithms largely fail in the presence of wavelength conversion. We then propose a weighted least-congestion routing and first-fit wavelength assignment (WLCR-FF) RWA algorithm in conjunction with a simple heuristic wavelength converter placement algorithm called minimum blocking probability first (MBPF) that considers both the distribution of free wavelengths and the lengths of each route jointly. We further introduce an analytical model that can obtain the blocking performance of the proposed WLCR routing algorithm. Using both analysis and simulation, we carry out extensive numerical studies over the typical topologies including the ring, mesh-torus, and two mesh topologies, the 14-node NSFNET and the 19-node European Optical Network (EON); we compare the performance of proposed algorithm with a wide variety of existing routing algorithms including static routing, fixed-alternate routing and least-loaded routing algorithms. The results conclusively demonstrate that the proposed WLCR-FF algorithm can achieve much better blocking performance in the environment of sparse or/and full wavelength conversion.