A: The plenum is the space between a suspended ceiling (usually made of foam tiles and fluorescent lights) and the actual solid ceiling. Many buildings use the plenum for air ducts, electrical and data cabling, and plumbing.
Because the plenum usually has a lot of open space and often already contains cables, many IT folks want to put new WiFi access points (APs) there. After all, the AP will be hidden, out of users' reach, and in a great place for wide coverage, right?
Still, you shouldn't put an AP in the plenum without extremely careful planning. Anything in the plenum—including the AP and any related cables—must meet fire code, which varies by location. Proximity to metal conduits, high-voltage electrical runs, and fluorescent lights and ballasts is also a consideration. Plus, there's quite a lot of dust up there. Dust and electronics make bad bedfellows.
Besides the safety and cleanliness issues, the plenum is chock full of metal pipes, copper cables, and steel ventilation ducts and suspension wires. These all have a negative impact on WiFi waveforms. Signals are simply weaker and more prone to corruption when the AP is in this type of environment.
Based on experience, there's always a better place than the plenum for your APs.