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The issue of finding optimal signal routes is one of the main challenges within the framework of optimal networks. This study aims at illustrating the development of a new, mixed-integer mathematical model for solving the static routing and wavelength assignment (RWA) problem in optical wavelength-division multiplexing (WDM) networks and presents the comparison of results obtained by heuristics and CPLEX commercial software. The objective function of the mathematical model is to minimise the blocking probability by maximising the number of successfully connected lightpaths. In this paper two WDM networks with full wavelength conversion nodes were tested. The proposed heuristics are the variable neighbourhood search (VNS) and tabu search (TS). Firstly, the authors compare the results obtained by these two heuristics and CPLEX on a low-complexity WDM network, aiming to determine whether these heuristics may yield results which approximate optimal results. Secondly, having illustrated that the heuristics provide satisfactory results, the authors run these heuristics on a high-complexity WDM network. The results indicate minor differences in blocking probability and show that in majority cases, VNS spends ten times less CPU time as opposed to TS. In conclusion, the authors argue that VNS heuristic is appropriate for resolving this static RWA problem.