This issue has to do with the type of air-conditioning system in your building. All modern central heating and air-conditioning systems pump conditioned air through enclosed ducts. The conditioned air flows through the building, where users receive the benefits of its coolness (or warmth), and then it returns to the main blower . At the main blower, most of the returning air is recirculated back through the fans. Only a little bit of fresh outside air is mixed into the return flow. This saves a lot of energy compared to continually cooling 100% fresh outside air. The plenum issue has to do with how the used air returns to the fans. Building architects have the option of letting air return to the main blower through the natural open space above the false ceiling in a multistory office building (or the building attic in a one-story structure). If the false ceiling (or attic) space is used for returning air, you have a plenum-return system . The false ceiling (or attic) space is called a plenum . The alternative is to return the air through separate, dedicated return ducts. If you have separate return ducts, the plenum is nothing but a big dead airspace.
When a fire happens in the plenum of a plenum-return building, smoke from flammable materials in the plenum are sucked directly into the main blower, which distributes the deadly smoke instantly throughout the building. As a safety measure, plenum-return systems are therefore required to use nonflammable materials in the plenum. The insulation used in most old category 3 cables is polyvinyl-chloride (PVC), which emits dangerous gases when burned. Such cables are not permitted in plenum-return air systems. Plenum-rated cables must be made of something other than PVC. Unfortunately, the materials used to make plenum-rated cables are heavy, stiff, and somewhat more expensive than PVC.
POINT TO REMEMBER
Transmission Line Parameters
Pcb (printed-circuit board) Traces
Generic Building-Cabling Standards
100-Ohm Balanced Twisted-Pair Cabling
150-Ohm STP-A Cabling
Time-Domain Simulation Tools and Methods
Points to Remember
Appendix A. Building a Signal Integrity Department
Appendix B. Calculation of Loss Slope
Appendix C. Two-Port Analysis
Appendix D. Accuracy of Pi Model
Appendix E. erf( )