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A quality-of-transmission- (QoT-) aware routing and wavelength assignment (RWA) technique suitable for multifiber wavelength-routed all-optical networks supporting class-based lightpaths (LPs) is proposed, where QoT is evaluated not only for any candidate LP being set up but also for already-set-up LPs. In this paper, QoT refers to the quality of transmission at the physical layer and quality of service (QoS) refers to the network-level quality issues. Four performance improvement techniques are proposed: (1) preempting an existing low-quality (LQ) LP by a high-quality (HQ) connection considering bandwidth utilization to provide better QoS for HQ traffic; (2) class-based buffering of connection requests in edge switches to provide the chance of setup for the LPs that are going to be blocked, and therefore, improving the performance of the LP setup; (3) differentiated routing in which LQ LPs are routed through large dispersion paths and HQ LPs through small dispersion paths; and (4) wavelength ordering to reduce crosstalk in switches and interchannel crosstalk on fibers. Our performance evaluations show the effectiveness of the proposed improvements in reducing blocking probability.