What is Power over Ethernet (PoE)?
Learn what PoE is, how detection and classification work, and 802.3af/at/bt power tiers — with networking interview Q&A.
Expected Interview Answer
Power over Ethernet (PoE) is a technology that delivers electrical power alongside data over the same twisted-pair Ethernet cable, letting devices like IP cameras, VoIP phones, and wireless access points run without a separate power adapter.
A PoE-capable switch or midspan injector (the Power Sourcing Equipment, or PSE) first negotiates with the connected device (the Powered Device, or PD) using a low-voltage detection signal to confirm it actually wants power, avoiding damage to non-PoE hardware. Once confirmed, DC power is injected onto the same copper pairs already carrying Ethernet data, either the spare pairs (in 10/100BASE-T) or phantom-powered over the data pairs themselves (in Gigabit and above). Standards like IEEE 802.3af (up to about 15.4W), 802.3at/PoE+ (up to 30W), and 802.3bt/PoE++ (up to 90-100W) define increasing power budgets for hungrier devices such as PTZ cameras or thin clients. Because power and data share one cable, PoE dramatically simplifies deployment of ceiling-mounted access points and cameras where running a separate electrical outlet would be impractical or expensive.
- Eliminates the need for a separate electrical outlet at the device
- Simplifies and speeds up installation of APs and cameras
- Centralizes power management and backup (UPS) at the switch
- Standardized negotiation protects non-PoE devices from damage
AI Mentor Explanation
PoE is like a single boundary rope carrying both the crowd’s cheers and the electricity for the stadium floodlights at once, instead of running a separate cable for each. Before the lights draw power, the ground staff run a quick safety check to confirm a floodlight tower is actually connected, so a spare rope end never gets accidentally energized. Once confirmed, both signal and power travel down the exact same line to the tower. A single cable doing two jobs is exactly the efficiency PoE brings to network wiring.
Step-by-Step Explanation
Step 1
Detection
The PSE (switch/injector) sends a low-voltage probe signal to check whether a real PoE-compatible device is connected.
Step 2
Classification
The device signals its power class so the PSE can allocate an appropriate power budget.
Step 3
Power-up
DC power is applied over the spare or data pairs of the Ethernet cable once negotiation succeeds.
Step 4
Data and power coexist
Ethernet frames and DC power travel simultaneously over the same cable to the powered device.
What Interviewer Expects
- Explains PoE delivers power and data over one Ethernet cable
- Mentions the detection/classification handshake before power-up
- Names at least one standard (802.3af/at/bt) and rough wattage tiers
- Gives a real use case (AP, IP camera, VoIP phone)
Common Mistakes
- Thinking any Ethernet cable automatically carries power with no negotiation
- Confusing PoE wattage classes (802.3af vs 802.3at vs 802.3bt)
- Assuming PoE only works over Gigabit links
- Forgetting that non-PoE devices are protected by the detection handshake
Best Answer (HR Friendly)
“Power over Ethernet lets a single network cable carry both internet data and electrical power at the same time, so devices like a Wi-Fi access point or security camera do not need their own wall outlet. The switch first checks the device actually supports it before sending power, so it is safe for regular equipment too. It just makes installing network gear in places like ceilings or hallways much simpler.”
Code Example
# Show PoE power delivery status per interface (Cisco-style)
show power inline
# Example output:
# Interface Admin Oper Power(W) Device Class
# Gi1/0/1 auto on 6.5 IP Phone 2
# Gi1/0/2 auto on 12.9 AP 4
# Gi1/0/3 auto off 0.0 n/a n/a
# Force-enable PoE on a specific port
interface GigabitEthernet1/0/3
power inline autoFollow-up Questions
- What is the difference between 802.3af, 802.3at, and 802.3bt?
- How does a PoE switch avoid powering a non-PoE device by mistake?
- What happens if a PoE budget is exceeded across all switch ports?
- What is a PoE injector versus a PoE-enabled switch?
MCQ Practice
1. What does Power over Ethernet primarily deliver over a single cable?
PoE delivers both DC electrical power and Ethernet data over the same twisted-pair cable.
2. What is the purpose of the PoE detection phase?
Detection verifies a genuine powered device is present so non-PoE equipment is not accidentally energized.
3. Roughly how much power does the original IEEE 802.3af standard provide?
802.3af provides up to about 15.4W at the source, with about 12.95W guaranteed at the device.
Flash Cards
What is PoE? — A technology delivering both electrical power and Ethernet data over one cable.
What device supplies PoE power? — The PSE (Power Sourcing Equipment) — typically a PoE switch or injector.
Why is detection needed? — To confirm a real PoE device is connected before energizing the line, protecting other equipment.
Name a PoE standard. — 802.3af (~15.4W), 802.3at/PoE+ (~30W), 802.3bt/PoE++ (up to ~100W).