: the mechanics cost, the control cards cost, the backplane cost, and the cooling system cost. To provide an example of the use of the platform cost of a card, let us image that a designer has to design a medium capacity backbone router, and the decision has to be made as to whether to integrate into the router platform the DWDM line cards or to inte- grate only gray interfaces, SO to use in the network transponders to enter into the DWDM systems. The "gray" interfaces are single wavelength, short-reach interfaces, that are called "gray" to underline that the emission wavelength is generally in the second or in the third window, but there is no strict control on it. Telecommunication Networks To simplify the situation, that is in reality very complex, let us image that the relevant costs are C: the cost of one transponders CGI: the cost of one optical gray interface Cint: the cost of a DWDM interface integrated into the router line card Moreover, let us call Nint: the number of line interfaces of the router Ngr: the number of gray optical interfaces in a line card Nwdm: the number of DWDM interfaces in a line card It could seem that the problem is decided by the following inequality Cint = CTx + CG (7.34) whose meaning is that, if the sum of the costs of a transponder plus a gray interface smaller than the cost of an integrated DWDM interface, the transponder solution is imple- mented, otherwise it is implemented with the integrated DWDM interfaces solution. In reality, this kind of analysis is quite inaccurate, due to the great difference in the platform cost of a transponder mounted in a DWDM optimized platform and a line card interface. The correct comparison has to be carried out considering also platforms costs. Thus, let us call PTX the platform cost of a transponder, Pint the platform cost of a DWDM interface integrated into the router line card, and PGI the platform cost of a gray interface integrated into the router line card. Generally, it is PGI<Pint due both to the better quality electronics needed to