work. Although some of the well-known routing protocols developed for the IP networks can be adopted, it has to be noted that optical technology is essentially an analog rather than digital technology, thus routing is strongly influenced by the transparency degree of the network. To be specific, several types of transparency can be defined at different network layers. Examples are as follows: Service transparency: the layer ability to work ignoring the services managed by the client layer Protocol transparency: the ability to transport any client layer protocol Bit Rate transparency: the ability to work at any bit rate within a given maximum Optical transparency: the absence of network elements where the signal is elec- tronically regenerated On the ground of the aforementioned definitions, any OTN network is service and pro- tocol transparent, while it is not bit rate transparent. The transparency that is relevant discussing about routing is the optical transparency. The optical layer of a network can be divided into optically transparent isles, which are network areas where the optical signal is never electronically regenerated. The optical transparency areas can be SO small as a single DWDM link connecting two electrical core OXCs or SO big as an entire routing zone where optical interconnected rings are deployed. While the optical signal traverse a single optical transparent area, transmis- sion impairments accumulated along the optical paths and this has to be taken into account while calculating the route (see Chapter 10 for details). Under the point of view of the rout- ing strategies, ASON supports hierarchical, source-based, and step-by-step routing. In the first case, CCs are related to one another in a hierarchical manner as we have seen defining administrative domains and routing areas. Source routing is based on a federa- tion of distributed connection and routing controllers. The path is selected by the first con- nection controller in the routing area. This component is supported by a routing contr