g. The situation is shown in Figure 6.39. In this figure a signal is added at node A and extracted at node C, but the extraction filter, being not ideal, allows a small part of the sig- nal to pass through along the ring fiber until it arrives at node A again. At this point, if the suppression of the OADM is not sufficiently high, lasing action can start on the frequency My, ruining completely the ring working. Naturally, here it is not possible to stop the channel bandwidth since it should be pos- sible to have the channel wavelength available over the entire ring. Add signal Drop signal FIGURE 6.39 Recirculation mechanism due to imperfect optical filtering in optical transparent rings. Transmission Systems Architectures and Performances At this point, either a set of switches are deployed in the nodes to stop all the pass- ing through bandwidths that are not occupied by a real channel or the ring is accurately dimensioned, taking into account aging and other potential parameter changes, to make sure that lasing condition never starts whatever is the channel load and routing. Last but not least, in designing optical rings it has to be taken into account that poten- tially the signal could traverse a great number of filters, one for each traversed node. The bandwidth of a cascade of several filters is smaller than the bandwidth of a sin- gle filter, for example, the cascade of n single pole filters has a bandwidth Bn given by Bn = where B is the bandwidth of the single filter. Thus, the design of the OADM filtering characteristic has to be made carefully. This implies also that in optical rings ASE coming from amplifiers nearer to the receiver is filtered by a larger filter with respect to the ASE coming from far amplifier, and this effect has to be taken into account when evaluating the SNo. Just to make an example, let us imagine having a cascade of OADMs with the structure of Figure 6.38 and that the filtering characteristic of an AWG can be approximated with a Gaussian function. Let us imagine that the signal