nverging the evolution of router architectures and IP networks. Networks. IEEE, 21 (4), 8-14 (2007). Next Generation Transmission Systems Enabling Technologies, Architectures, and Performances 9.1 Introduction In Chapter 6 we have discussed Dense Wavelength Division Multiplexing (DWDM) tech- nologies for implementing high capacity very long reach optical transmission systems. In that chapter, it has been shown as transmission systems with an overall capacity in excess of 2 Tbit/s and a reach longer than 2000 km can be designed using direct detection and in-line optical amplification. From the point of view of the product capacity-reach it seems that these figures are suf- ficient to satisfy the needs of telecom carriers for the time being. As a matter of fact, the need of a new generation of transmission systems does not emerge from the need of increasing the transmission capacity or the system reach, but comes from other reasons. The fast increase of the traffic in American and European networks is due neither to the increase of the network nodes, whose number remains almost constant, nor to the increase of the network subscribers, that is very slow. What is driving the traffic increase is the increase in the bandwidth required by each subscriber. This means that the traffic pattern remains statistically constant, while the service bun- dle offered by carriers differentiates, requiring to the transport layer to increase more and more the number of wavelengths routed along the same network routes. If the number of wavelengths along a network route is huge, they are demultiplexed and multiplexed uselessly several times, since they should not to be separated. Moreover, every time they traverse a wavelength switch, several management controls have to be done to monitor the correct routing of every wavelength, even if they are directed toward the same direction. Last, but not least, the switching error probability increases with increasing the number of wavelengths. The request to bundle together in the same network 