1higher, linear and non-linear fibre impairments become prominent factors affecting the signal quality. Thus, new techniques in both physical layer and network layer are necessary for mitigating impairments to accommodate high-speed traffic (Tomkos et al. 2002). On the other hand, the flexibility of modern networks with dynamic and distributed management, where lightpaths can be dynamically and automatically assigned end-to-end, increases the risk of changing path conditions that affect signal quality and consequently quality of service (QoS) over the lifetime of a connection as well as for subsequent connection set-up requests (on-demand routing and wavelength assignment -RWA). For traditional connection provisioning an ideal physical layer, ignoring transmission impairments (Chlamtac et al. 1992) could be assumed because the implicit per hop regeneration (optics-to-electronics-to-optics conversion -O/E/O) compensated signal impairments hop-by-hop in a rather static manner (section commissioning). For end-to-end all-optically switched connections using photonic cross-connect switches (PXC), the conditions change and common practice is not applicable. Anyhow, intelligent connection provisioning is an important traffic engineering problem, and minimising operational cost as well as efficiently utilising network resources is the main driver. In this context, it seems important to have flexible routing protocols that take into consideration the most relevant physical impairments, and are able to exchange messages with their values as part of the route information with others. This condition, if definitely used to calculate routing, will surely assure better success in data delivery over the network (Huang et al. 2005). Irrespective of the control architecture chosen by an operator of an optical network, the included control plane (in charge of setting up and tearing down optical circuits; also known as lightpaths) needs to have a proper set of tools to satisfyingly deal with physical impairments. The most essential tools are: optical signal monitoring, signal processing and impairment compensation techniques (each all-optically and/or electrically). With the help of these tools, it is possible to predict the QoT (quality of transmission) attainable for a lightpath at a given time (i.e. the latest network-wide monitoring instant), and the control plane can route requests across the network