CWT Power over Ethernet (PoE) & Cabling Infrastructure 2 — Questions and Answers
Question 1: What is the maximum PSE power output per port defined by IEEE 802.3bt Type 4 (Class 8)?
- 60 watts
- 75 watts
- 90 watts (Correct answer)
- 100 watts
Correct answer: 90 watts
IEEE 802.3bt Type 4 delivers up to 90W at the PSE using all four wire pairs, enabling high-power devices like multi-radio access points and PTZ cameras.
Question 2: In IEEE 802.3af/at Alternative A power delivery mode, which wire pairs carry the power signal?
- The data-carrying pairs (pins 1, 2, 3, 6) (Correct answer)
- The spare pairs (pins 4, 5, 7, 8)
- All four pairs simultaneously
- Pins 3 and 6 only
Correct answer: The data-carrying pairs (pins 1, 2, 3, 6)
Alternative A delivers power on the same pairs used for data transmission by superimposing a DC offset, while Alternative B uses the spare pairs.
Question 3: What is the function of PoE PD power classification?
- To allow a PD to signal its maximum power requirement to the PSE (Correct answer)
- To authenticate a PD's security certificate
- To identify the cable category the PD requires
- To set the physical connector type on the PD
Correct answer: To allow a PD to signal its maximum power requirement to the PSE
PoE classification allows a PD to communicate its power class (0–8) to the PSE so the switch can allocate the correct wattage and manage its total power budget efficiently.
Question 4: A wireless access point is PoE Class 3 under IEEE 802.3af. What is its maximum allowable power draw at the device?
- 6.49 watts
- 12.95 watts (Correct answer)
- 15.4 watts
- 30 watts
Correct answer: 12.95 watts
Under IEEE 802.3af, a Class 3 PD can draw a maximum of 12.95W at the device after cable losses, making it the highest class under the original PoE standard.
Question 5: What is the primary purpose of a PoE splitter?
- To split one PoE port into multiple non-PoE data ports
- To separate power from data so non-PoE devices can use PoE infrastructure (Correct answer)
- To increase PoE output beyond the rated standard wattage
- To convert PoE to wireless charging for mobile devices
Correct answer: To separate power from data so non-PoE devices can use PoE infrastructure
A PoE splitter separates the DC power from the data signal on an Ethernet cable, providing a standard data port and a separate DC power connector for non-PoE-capable devices.
Question 6: Which physical factor most significantly causes power loss across a PoE cable run?
- Cable jacket color
- Conductor resistance of the cable (Correct answer)
- Cable manufacturer brand
- Cable bend radius
Correct answer: Conductor resistance of the cable
Conductor resistance causes power loss proportional to the square of the current (P = I²R), which is why wire gauge (AWG) and cable quality directly impact PoE efficiency.
Question 7: What best practice should be followed when planning power budgets for a PoE switch in a wireless deployment?
- Assume all ports will always draw maximum rated power simultaneously
- Calculate only the average power draw of connected devices
- Sum device power requirements by class and add 20–25% headroom (Correct answer)
- Rely on DHCP to dynamically manage port power allocation
Correct answer: Sum device power requirements by class and add 20–25% headroom
Summing per-device class power requirements and adding 20–25% headroom prevents power budget exhaustion, accommodating startup surges and future device additions.
CWT Power over Ethernet (PoE) & Cabling Infrastructure 2
What is the maximum PSE power output per port defined by IEEE 802.3bt Type 4 (Class 8)?