 In general, given N input/output DWDM systems with W wavelength each, it is possible to divide the NW input channels into S groups of M channels each SO that NW = MS and to assign a separate optical switch to each channel group. This channel aggregation is, in general, useful when M<W, SO that the blocking inside the OXC is less probable than in the case of M = W [59, 60]. The situation M < W is also favored by the fact that, in general, the output DWDM ports of an OXC are much less than the channels of each WDM comb (as seen in the example we have done in this section). The S wavelength subsets can also be called switching planes (SP) of the OXC and the OXC switch fabric can be viewed as a set of S noninteracting switch fabrics. This abstract model of the OXC is shown in Figure 7.36. If S = 1, the switch is strictly nonblocking while increasing S the blocking probability inside the switch increases. The ratio u between the number of possible switch states and the number of all the permutations of the C = WN input channels is given by S(C/S)! (7.38) where the factor n = 1/S (0<n < 1) : and can be called OXC inner connectivity. Let us assume that the wavelength channels arriving at the input of the OXC have the same probability to be directed toward any output direction but have no requirements in terms of wavelength on the output DWDM system. Blocking in an OXC based on the Telecommunication Networks Input port 1 Switch Output port 1 planes Input port 2 Output port 2 FIGURE 7.36 Architectural scheme of an opaque OXC with a switch fabric segmented in switching planes. SP architecture occurs when a channel belonging to an SP cannot be routed toward the wanted direction, due to the fact that all the SP exits toward that direction are occupied, when free exits exist toward that direction on other SPs. In order to evaluate the blocking probability let us assume that the OXC is in a non- blocked state before the tentative of setting up a further channel that provokes blocking. Let us assume, as it is frequent in pract