1.5 Compare and contrast transmission media and transceivers
Wired Media
📘CompTIA Network+ (N10-009)
Introduction
When installing network cables inside a building, the environment in which the cables are placed is extremely important. Two main types of cable ratings are used depending on where the cable is installed — Plenum-rated cables and Non-plenum-rated cables.
Understanding the difference between these two cable types is essential for both safety and compliance with building fire codes. This topic is frequently tested in the CompTIA Network+ exam.
1. What is a Plenum Space?
A plenum space is the area in a building used for air circulation in heating, ventilation, and air conditioning (HVAC) systems.
- Typically, it is the space above ceiling tiles or below raised floors where air flows back to the HVAC system.
- Because air moves through this space, if a fire starts here, toxic fumes can spread quickly throughout the building.
That’s why any cable placed in a plenum space must have special fire-resistant insulation to reduce smoke and harmful gases.
2. What is a Plenum Cable?
A Plenum-rated cable (CMP – Communications Multipurpose Plenum) is designed for use in plenum spaces.
Key Features:
- Fire-resistant: The insulation is made from materials like Teflon (FEP) that resist fire and do not release much toxic smoke.
- Low-smoke and low-toxicity: If the cable burns, it produces minimal smoke and fewer toxic fumes.
- Higher cost: Because of its specialized materials and safety features, plenum cable is more expensive.
- Usage: Installed in areas with airflow (above ceilings or below floors used by HVAC systems).
Fire Rating:
- The National Electrical Code (NEC) identifies plenum cables with the marking CMP.
- CMP cables meet the highest fire safety standards among copper network cables.
Example (IT Context):
In a corporate office, if Ethernet cables are run above the ceiling tiles in a return air plenum, network installers must use CMP-rated Cat 6 cables to comply with building safety codes.
3. What is a Non-Plenum Cable?
A Non-plenum-rated cable (CM or CMR) is designed for spaces that do not carry environmental air.
- These cables are commonly installed inside walls, conduits, or riser shafts (vertical spaces between floors).
Key Features:
- Standard insulation: Usually made of PVC (Polyvinyl Chloride).
- Less fire resistance: The insulation can burn more easily and release thicker, more toxic smoke.
- Lower cost: Non-plenum cables are less expensive because they don’t require special fire-resistant materials.
- Usage: Used in areas where air does not circulate freely, such as inside walls or conduits.
Fire Rating:
- Non-plenum cables are often marked as CM (Communications Multipurpose) or CMR (Communications Multipurpose Riser).
- CMR cables are rated for vertical installations (between floors) but not for plenum spaces.
Example (IT Context):
In a server room where cables run inside walls or through conduits, installers can use CMR-rated Cat 6 cables since these areas don’t circulate air.
4. Key Differences: Plenum vs. Non-Plenum
| Feature | Plenum Cable (CMP) | Non-Plenum Cable (CM/CMR) |
|---|---|---|
| Fire Resistance | High – made from flame-retardant material | Moderate – burns more easily |
| Smoke & Toxicity | Low smoke, low toxicity | High smoke, more toxic fumes |
| Material | Teflon (FEP) or low-smoke PVC | Standard PVC |
| Cost | More expensive | Less expensive |
| Usage Area | Air circulation spaces (above ceilings, under floors) | Inside walls, conduits, or riser shafts |
| Marking | CMP | CM or CMR |
| Building Code Compliance | Meets strict fire code standards | Not for plenum use |
5. Riser-Rated (CMR) vs. Plenum-Rated (CMP)
You might also see riser-rated cables (CMR) on the exam. These are a middle option between plenum and basic PVC cables.
- Riser-rated cables are used for vertical spaces between floors (risers).
- They provide some fire resistance but cannot be used in plenum spaces.
- CMP cables can replace CMR cables, but not the other way around.
Rule of Thumb:
If in doubt, using CMP cables is always safe because they meet the highest safety standard.
6. Exam Tip for CompTIA Network+
- Plenum = Fire-resistant, air space safe, marked CMP.
- Non-plenum = For general use, cheaper, marked CM or CMR.
- CMP cables can replace CMR cables, but CMR cannot replace CMP in a plenum area.
- Always consider safety and fire code compliance when installing cables in buildings.
- Expect a question asking which cable type is required for installation above ceiling tiles — the correct answer will be Plenum (CMP).
7. Summary
| Aspect | Plenum Cable (CMP) | Non-Plenum Cable (CM/CMR) |
|---|---|---|
| Purpose | Safe use in air-handling spaces | Use in walls, risers, conduits |
| Fire Safety | High fire resistance | Moderate to low |
| Smoke Emission | Minimal | More smoke and toxic gases |
| Cost | Higher | Lower |
| Compliance | Meets strict fire codes | Limited fire code compliance |
✅ Key Takeaway for the Exam:
When network cables are installed in any area where air circulates (like above ceilings or under raised floors), you must use Plenum-rated (CMP) cables because of fire safety regulations. Non-plenum cables (CM/CMR) are acceptable only in spaces without air circulation.
