tter solution should be independent cooling flux for each subrack. In order to implement this kind of air flow design some space has to be foreseen between adjacent racks and this space should be organized SO to render thermally independent adjacent subracks. If a similar solution is not possible, the design can be carried out by foreseeing a single air flux traversing the whole rack, but in this case, the flux has to be sufficient to assure good cooling even taking into account the heat produced by every subrack. This can be achieved by correctly optimizing the air flow path through the rack and the dimension and speed of each subrack fan set. To comply with NEBS requirement the overall cooling system has to operate correctly in three situations: 1. Worst case environment conditions (taking into account to calculate temperature and humidity, the height on the see) with fans at full speed and a single dirty filter in all installation conditions. 2. One fan failure with the other fans at full speed and the environment at 40°C. 3. Standard conditions (environment at 40°C) with fans at minimum speed, assuring on all the components a temperature compliant with their foreseen live. Telecommunication Networks Thus, the thermal design has to be carried out taking into account effective compliance tests. The approach to the analysis of the cooling capacity of a given air flux is in principle, easy starting from classical fluid motion theory and thermodynamic equations. In particular, assuming laminar motion and calling Z the vertical direction, we can write the following equations for the air flow motion and the heath exchange with the surround- ing system: (Navier-Stokes equation) (fluid mass conservation) (6.104) (principle of energy conservation) where D/Dt = a/dt +V. is the so-called Lagrangian derivative and represents the fact that the equations of motions are all written in the moving fluid particle reference frame ii is the fluid particle velocity in the absolute reference frame (as defined in the Navier- Stokes 