nection request established Datain Adaptation Adaptation Data out function to function to client layer clientlayer Channel layer Variety of channels speeds Wavelength layer A wavelength is the transport unit Transport layer FIGURE 10.5 Scheme of a possible sub-layering of the ODN physical layer. Next Generation Networking: Enabling Technologies, Architectures, and Performances All the operations on the channels, grooming, for example, are carried out at the channel layer. Below the channel layer, we have the wavelength layer where all the operations involv- ing a whole wavelength, comprising channel multiplexing in the wavelength frame and channel demultiplexing before passing them at the upper layer, are carried out. Finally, the transport layer manages the whole WDM comb, and is in charge of transmit- ting it from the near end to the far end of the transmission system. No switching at the transport layer its foreseen in this version of the ODN architecture. 10.2.2.3 ODN Network Elements and Data Plane The main physical layer switching elements of the ODN are the optical cross connect (OXC) and the OADM. A possible OXC architecture that seems suited for application in the ODN exploits the interface sparing allowed by PICs and uses fast CMOS circuits to implement a lower speed digital switch fabric using deserializers to parallelize the data on the single wavelengths. The scheme of this type of ODM OXC is reported in Figure 10.6. From the figure, it is clear that the OXC is equipped with two superimposed switch fabrics, one working on wavelength and the other on channels. Using the upper fabric, the OXC is able to implement grooming, while the presence of the lower one significantly reduces the overall number of needed cross-points with respect to a machine working only on channels. Even if an OXC is a powerful machine allowing efficient grooming and channel-based restoration to be implemented, it is also a complex machine to develop and an expensive equipment to install. Thus, it will be not a surprise if the fi