ded to host the card and put it in the right conditions to work. Let us consider a standard sub-rack with its backplane and cooling system (i.e., with cooling devices mounted). Let us image to equip the sub-rack with all the control cards necessary to the system we are considering. Switching Systems: Architecture and Performances At the end we will have a sort of system shell, whose cost we assume to be C, ready to work from the control and thermal point of view, and with N free slots where interface and signal processing cards (comprising switching cards) have to be plugged to obtain a functional system. Let us imagine that all the cards are equal in shape (even if the definition is easily gen- eralized to different cards shape) and that each card will have a small thickness and is axb cm wide. In these conditions, the use cost of one full slot of the platform is (7.31) where As is the area of a card filling completely the slot. When a card is inserted into the sub-rack occupying an empty slot or part of it (there are platforms allowing to put small cards one over the other in the same slot), it consumes a platform cost equal to (7.32) where S is the number of slots occupied in the sub-rack by the new unit A is the card area The presence of the areas also allows the comparison of the utilization cost of different platforms having different dimensions of the cards and, as a consequence, different back- planes and cooling systems. In order to use Equation 7.32 it is needed to have costs of the different elements of the platform. They critically depend on the technical solutions used to implement the back- plane and the cooling system, and the ratio between these costs are different among dif- ferent solutions. Thus, general results can be achieved only in a parametric way, SO driving the target of the development toward the most critical elements of each solution. Thus, it is frequently useful to rewrite Equation 7.32 in the following form: (7.33) where the addenda composing the cost of the platform are as follows