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Current wavelength routed optical networks serve a vast variety of clients, carrying connections belonging to different quality of service (QoS) classes. However, optical channel, i.e. lightpath, requests with different QoS requirements compete for the same network resources. Proper policies are, therefore, necessary to assure the client's service level requests, e.g. blocking probability, when setting up lightpaths. We propose and evaluate a strategy for multiple-class optical channel provisioning. The separate wavelength pool provisioning (SWAP) strategy logically separates into different wavelength pools the resources available for the allocation of optical channels that carry connections belonging to different QoS classes. In this way the network itself provides different class connections with classes of lightpaths characterized by different service levels, e.g. blocking probability. Numerical results show that the SWAP strategy, suitably adjusting the pool sizes, inherently differentiates the blocking probability experienced by different class lightpaths, while encouraging backup resource sharing. Moreover it eliminates any impact of low class lightpath requests variation on high class lightpath blocking probability, without significant performance degradation. Finally, with the SWAP strategy the lightpath setup is fast, since a lightpath route is searched in a limited wavelength pool.