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In all-optical networks with no wavelength converters, signals are switched optically inside the nodes and therefore propagate over hundreds or thousands of kilometers with no electrical regeneration. Over such distances, physical impairments, such as intersymbol interference (ISI), amplifier noise, and leaks within nodes (cross-talk), accumulate and can lead to serious signal degradation, resulting in poor quality of transmission (QoT) as measured by signal bit-error rates. The role of routing and wavelength assignment (RWA) algorithms is to accommodate incoming calls in optical networks over a route and a wavelength. RWA algorithms block calls if a continuous wavelength from the source to the destination cannot be found (wavelength blocking) or when the QoT of the call is not acceptable (QoT blocking). Evaluating RWA algorithms via simulations is possible but time consuming, and hence analytical methods are needed. Wavelength blocking has been studied analytically in the past, but QoT blocking has never been analytically modeled to our knowledge. In this paper, we present an analytical method to evaluate blocking probability in all-optical networks, accounting for physical layer impairments. Our physical layer model includes ISI and noise, two static effects that only depend on the network topology, and also cross-talk, which depends on the network state. Simulations on three different topologies with various numbers of channels, representing small- to large-scale networks, show that our technique is suitable for quick and accurate dimensioning of all-optical networks: the accuracy of the blocking rates computed with the analytical method, taking only seconds or minutes to run, is the same as that of simulations, which take hours to run.