other at every traversed node. The content of one wavelength can be exchanged with the content of another wave- length, and channels, not wavelengths, are added and dropped. Thus, in this new type of ring, the wavelength is only a useful way to divide the avail- able bandwidth SO as to allocate the information, but it has no logical relevance at the net- working level. The role of the wavelength is the same role of a frame time slot in an SDH/ SONET ring. From a logical point of view, if all the wavelengths disappear, and a single channel at very high speed would substitute them exactly, the same ring working would be realized, with the only difference that TDM instead of WDM multiplexing would be used. A possible architecture of an OADM for a CRing is represented in Figure 10.7. On this ground, it is clear that wavelength protection does not exist in CRings; these rings can guarantee survivability either through WDM protection or through channel protection. East side Add/drop West side High speed Channels demux regeneration Channels signal after Channel switch FIGURE 10.7 Possible architecture of an OADM for a CRing. Next Generation Networking: Enabling Technologies, Architectures, and Performances Local add/drop Channel switch fabric Regenerated Channel Channel signal demux FIGURE 10.8 Possible scheme of a line terminal in an ODN. Channel protection is completely analogous to the path protection in SDH/SONET rings, considering the WDM comb as a super-frame. Two- and four-fiber rings can be defined, and in both cases, besides dedicated protection, shared protection can be adopted The case of WDM protection is analogous to the SDH/SONET MS ring, and also in this case, two- and four-fiber rings can be considered with dedicated and shared protection. The block scheme of an ODN regenerator is shown in Figure 10.8: regeneration happens when the bits are regenerated via a detection and information estimate. Before the injec- tion of the regenerated signal into the output fiber, a signal processing section assures the 