CWT Power over Ethernet (PoE) & Cabling Infrastructure 1 — Questions and Answers
Question 1: Which IEEE standard originally defined Power over Ethernet (PoE), delivering up to 15.4W per port?
- IEEE 802.3af (Correct answer)
- IEEE 802.3at
- IEEE 802.3bt
- IEEE 802.11ac
Correct answer: IEEE 802.3af
IEEE 802.3af was the first IEEE standard to define PoE, specifying up to 15.4W of power per port at the PSE.
Question 2: What is the maximum power output per port at the PSE specified by IEEE 802.3at (PoE+)?
- 15.4 watts
- 25.5 watts
- 30 watts (Correct answer)
- 60 watts
Correct answer: 30 watts
IEEE 802.3at (PoE+) doubled the original PoE power ceiling, specifying a maximum of 30W per port at the power sourcing equipment.
Question 3: Under IEEE 802.3af, what maximum power is guaranteed at the powered device (PD) after accounting for cable losses?
- 15.4 watts
- 12.95 watts (Correct answer)
- 25.5 watts
- 30 watts
Correct answer: 12.95 watts
After resistive losses in the cable, a PD is guaranteed a maximum of 12.95W, even though the PSE outputs up to 15.4W.
Question 4: Which device is used to add PoE capability to an existing non-PoE switch infrastructure?
- PoE splitter
- PoE midspan injector (Correct answer)
- Power distribution unit (PDU)
- Uninterruptible power supply (UPS)
Correct answer: PoE midspan injector
A PoE midspan injector sits between a non-PoE switch and the PD, injecting DC power onto the Ethernet cable without requiring a PoE-capable switch.
Question 5: What is the minimum recommended cable category for IEEE 802.3at (PoE+) installations?
- CAT3
- CAT5
- CAT5e (Correct answer)
- CAT6A
Correct answer: CAT5e
CAT5e is the minimum recommended cabling for PoE+ deployments, providing adequate conductor gauge and insulation to minimize resistive power losses.
Question 6: What is the maximum standard cable run length permitted for PoE power delivery?
- 50 meters
- 75 meters
- 100 meters (Correct answer)
- 150 meters
Correct answer: 100 meters
PoE power delivery follows the same 100-meter (328 ft) maximum cable run as standard Ethernet, beyond which voltage drop and data signal degradation become problematic.
Question 7: How does a PoE Power Sourcing Equipment (PSE) detect a valid Powered Device (PD) before applying full power?
- It sends a high-voltage pulse to stress-test the cable
- It applies a low-voltage DC signature detection signal (Correct answer)
- It requires manual port configuration by the administrator
- It uses DHCP to query the device type
Correct answer: It applies a low-voltage DC signature detection signal
A PSE applies a low-voltage DC detection signal to identify the characteristic resistive signature (approximately 25kΩ) of a valid PD before committing full power.
CWT Power over Ethernet (PoE) & Cabling Infrastructure 1
Which IEEE standard originally defined Power over Ethernet (PoE), delivering up to 15.4W per port?