ecification, VME customers demand that products they buy are compatible with their current and future PCI devices. 6.5.4 System Thermal Design As we have seen in Section 6.5.1, a typical DWDM system is constituted by one or more racks having inside subracks where the units are packed once designed on a card. In a central office racks by different systems are aligned in order to gain access to power supply and external lines. This means that the fan tries that are designed to cool the system have to be assisted by a careful system design if they have to provide optimized performances. The thermal design generally starts from the requirements imposed by the system mechanical design, operational environment, and heath production of the various cards. For reference, the environment requirements standardized by ETSI in Europe and by Telcordia in North America are summarized in Table 6.21. Transmission Systems Architectures and Performances TABLE 6.21 Environment Requirements for Telecommunication Equipment Standardized by ETSI in Europe and by Telcordia in North America Requirements Specifications NEBS Level 3 ETSI Class 3.1 Normal Environ. temp. (C) Relative humidity (%) Short Environ. temp (C) Relative humidity (%) Altitude 60-1800n at 40°C 70-106 1800-4000m a 30°C (environ. temperature) Acoustic noise at 78dB at 27°C 7.2 Bells at 25°C attended room Rate of temp. changes (°C/min) Maximum environ. 40°C up to 96h temperature Maximum surrounding air flow (m/s) In particular inlets and exhaust air flows have to be carefully placed inside the system structure and correctly dimensioned. If the rack is open, that is, no closed doors in front or in the rear, the best direction of inlets and exhaust air flows is front-bottom-in to rear-top-out, since this direction is privi- leged by the natural property that the air flow, hot after cooling the equipment, tends to move vertically up. Since generally more than one subrack is stacked in a rack, the design of air flow has to take into account the cooling of each subrack. The be