ul design tool, but it also provides the means of describing in detail the functional architecture of the network using a formal instrument. This technique was introduced in the field of telecommunications by the International Standard Organization (ISO) with the intent of describing in an abstract way and stan- dardizing the computer networks [13-15]. The resulting standard was called Open Systems Interconnection architecture (ISO-OSI). The abstract methodology followed by ISO was later also adopted by international tele- communication union-telecommunication (ITU-T) (the international standard organiza- tion for telecommunications) that applied it to TDM transport networks extending it and introducing a set of ad hoc definitions and procedures. What came out was a general synthesis technique that has a wide use, much wider than the original intention of its creators. As abstract methodology, it is useful every time a com- plex function has to be realized by designing a system starting from the building blocks performing much simpler functionalities. Other examples of use of this model are software design [16], study of brain function- alities, and of artificial neural networks [17,18]. Naturally, the extension of the layering methodology to all these different applications frequently requires customizations of the method and introduction of ad hoc elements. To introduce the layering methodology, let us imagine to have a complex function F(x) operating over some input a (in general, constituted by a collection of elements) in order to produce an output B (also a set of elements). Just to be specific, it can be the function of a WDM transmission system whose input are the data emitted by the signal sources feeding the system and the output are the data delivered at the destination equipment. In this case, a is the set of bit streams representing the input data, one for each wavelength, and B is the set of bit streams representing the data delivered at the destination. Let us image also that the function has to be