Plenum-rated cable
termnetworking
Stands for: Plenum and riser cable ratings (CMP/CMR)
The fire rating that decides where a cable may legally run - plenum (CMP) for air-handling spaces, riser (CMR) for vertical shafts.
Air-handling plenums spread smoke fast, so cable jacketed for those spaces (CMP) must burn slowly and emit little smoke; riser-rated CMR handles floor-to-floor shafts; general-purpose ratings cover the rest. Ratings nest downward - plenum can substitute for riser, never the reverse. Outside North America the parallel vocabulary is LSZH (low smoke, zero halogen). It is the code-compliance dimension of cabling, enforced by inspectors rather than by link testers.
Plenum-rated cable is required in spaces used for air return, typically above suspended ceilings, because those spaces move air throughout a building. The rating is about what the jacket does in a fire: ordinary PVC produces dense toxic smoke that the ventilation system would then distribute everywhere.
This is a building code requirement rather than a networking preference, and it is enforced by inspection. The cable costs several times more, which is why the temptation to run a few metres of ordinary cable through a ceiling is real and why it is a genuinely dangerous economy. If air moves through the space, the cable in it needs the rating, and the question of whether a given ceiling is a plenum is one for the building's mechanical drawings rather than for the network team.
Also known as: plenum, cmp, cmr, riser cable, lszh