The typical telecommunication boxes for the indoor applications are small with the height less than 4RU (7 in.). The “RU” stands for the rack unit which is 1.75 in. high. However, for simplicity, “U” is commonly used to replace “RU”. One of the unique features of these typical boxes is the printed circuit boards being in horizontal positions. As in the rack configurations, the cooling schemes of the system can be either force air cooling with fans or passive cooling under natural convection.