 wavelengths represented by dotted and dashed lines. Let us assume that a new wavelength is to be set up, connecting nodes A and E. Two links exit from node A, one of which (A-F) is fully exploited. Thus, the new path has to start on link A-B. Once in B, the wavelength has to reach D and finally E. This is the only available path, since the link C-D is also completely exploited. Unfortunately, this path does not allow wavelength continuity to be realized since wave- length 1 is used in the link D-E, while the wavelength 2 is used in the link B-C. The impossibility of creating the required wavelength path is a network blocking, since if wavelength conversion is made available into the OXCs, the path would be exploitable. Both routing sensitive to transmission impairments and blocking due to the wavelength plane structure could be solved with the help of two optical devices: the transparent regen- erator and the wavelength converter (WC). A great amount of research has been devoted to these key technologies, but to my knowledge, no commercial product has been introduced into the market up to now. In the following sections, we will analyze some key technological results in these two areas, and then we will discuss a few transparent OXC architectures among the great number of proposals that have been made. Finally, we will consider the problem of design and optimization of an optical transpar- ent network. We will close the section with a few considerations on optical time division multiplexing (OTDM). 10.3.1 Enabling Technologies for the Transparent Optical Transport Network Besides the improvement in the dimension of switches and in general in the loss of all the optical elements, the effectiveness of the T-OTN depends critically on the availability of two optical components: the transparent regenerator and the WC. Telecommunication Networks A standard electronic regenerator is based on the so-called three R (3R) operation: repower, reshape, retime. This means that the output signal is ideally amplified to the requ