 DWDM is used in the core of metro and regional network and CWDM in the metro access when single wavelength SDH/SONET systems are not sufficient to lect the traffic. However, exceptions are frequent, where, for example, CWDM is used in the core of small local networks or light DWDM systems are used to collect signals in big business centers where there is a huge amount of generated traffic. Telecommunication Networks 6.3.5.1 Transmission in Dense Wavelength Division Multiplexing Metropolitan Ring The main difference from a transmission point of view between a ring and a point-to-point link is that in a ring different optical channels, routed through the ring, generally have dif- ferent sources and destinations. Moreover, the ring close topology allows spurious signal components and optical noise potentially to circulate along the ring. The first point can represent a serious problem for amplified transmission. As a matter of fact, when designing a point-to-point system we have seen that the gain of EDFA ampli- fiers is flattened by the use of gain flattening filters SO to exactly compensate the unbalance of the considered amplifier. This is possible in a point-to-point system since the power of the channels traveling through a certain amplifier is fixed. The only thing that has to be managed is the pro- gressive addition of other channels. Generally in a point-to-point system there is a rigid channel upgrade plan that prescribes the order in which the channels have to be added. In correspondence to the number of working channels, the element manager tunes the set- ting of the amplifiers SO to maintain the required flatness. A similar strategy is not possible in a ring network, due to the fact that the DWDM comb transiting through a generic node is formed by channels injected into the ring in differ- ent nodes and thus having very different powers. The lack of uniformity of optical power among the channels transiting in a node has an impact on the EDFAs' gain flatness. As a matter of fact, the gain of an amplifier