Lesson 1.1h: Power over Ethernet (PoE)

1. What is PoE?

Power over Ethernet (PoE) is a technology that allows network cables to carry electrical power (DC) alongside data over a single twisted-pair Ethernet cable, such as Cat5e or Cat6.

Thanks to PoE, endpoints do not need a separate power adapter or a nearby electrical outlet.

Example:

1

PoE Switch ---- [Ethernet Cable] ---- Access Point

In this scenario, the Ethernet cable transmits network data and simultaneously supplies enough electrical power for the Access Point to operate.

2. The Role and Benefits of PoE

PoE is highly utilized for devices that need to be installed in locations where pulling electrical wiring is difficult or expensive. Common endpoints include:

• Wireless Access Points (ceiling-mounted)
• IP Phones (desktop)
• IP Cameras, PTZ Cameras (outdoor, wall/pole-mounted)
• Door access control systems
• IoT devices (smart sensors)
• Thin clients or small displays

Key Benefits of PoE:

• Cost and Time Savings: Eliminates the need to hire electricians to install separate power outlets.
• Flexibility: Devices can be easily relocated wherever a network cable can reach.
• Centralized Power Management: Power is sourced from the Switch, simplifying administration.
• High Reliability: Endpoints can remain operational during a power outage if the PoE Switch is backed by a UPS (Uninterruptible Power Supply).
• Remote Management: IT can easily reboot a frozen device remotely by simply toggling (shutdown/no shutdown) the PoE status on the switch port.

3. Key PoE Components

PoE relies on the interaction between two primary components:

3.1. PSE (Power Sourcing Equipment)

The PSE is the device that supplies PoE power.

• The most common example in enterprise networks is a PoE Switch.
• If you have a standard Non-PoE Switch, you can use a PoE Injector.

What is a PoE Injector? A PoE Injector is an intermediary device that takes data from a non-PoE switch, combines it with power from an external wall adapter, and sends both (Data + Power) down the cable to the endpoint. Non-PoE Switch ---- (Data) ---- PoE Injector ---- (Data + Power) ---- Access Point

3.2. PD (Powered Device)

The PD is the device that receives power from the PSE.

• Examples: IP Phones, Wireless APs, IP Cameras. The PD accepts both data and power through a single cable connection.

4. PoE Standards to Remember

Understanding PoE standards is critical for designing networks and selecting compatible equipment. It is important to note that the power at the PSE is always higher than the actual power received at the PD due to power dissipation (loss) over the length of the cable.

[!NOTE] The terms UPOE or UPOE+ are primarily used within the Cisco hardware ecosystem to refer to high-power standards. In the broader market and according to IEEE terminology, these are commonly referred to as PoE++.

5. Understanding the Power Budget

This is a highly practical concept when purchasing and configuring PoE Switches. A PoE Switch has a limit on the total amount of electrical power it can supply to all connected devices, known as the Power Budget.

Real-world Example:

• You have a 24-port switch that supports PoE+ (max 30W/port).
• The power budget of this switch is 370W.
• Although each port supports up to 30W, multiplying it out (24 × 30W = 720W) far exceeds the switch’s capacity.
• Therefore, the Switch cannot supply the maximum 30W to all 24 ports simultaneously.

Network Design Consideration: You must calculate the actual power consumption of all connected PDs. For instance, if you connect 24 IP Phones (consuming 5W each), the total is 120W -> the 370W switch can easily handle it. If you connect APs requiring 30W each, you can only power a maximum of about 12 APs on that switch.

6. How the PoE Process Works

PoE is designed to be intelligent and safe. You do not have to worry about “frying” a non-PoE laptop if you plug it into a PoE port. The PSE always performs checks before delivering power.

1. Detection: The PSE checks if the device on the other end is a valid PD (by detecting a specific 25k ohm resistance).
2. Classification: The PSE determines how much power the PD requires, allowing the switch to allocate its power budget accordingly.
3. Power On: If everything is valid, the PSE begins applying DC power to the cable.
4. Monitoring: The PSE continuously monitors the power draw.
5. Power Off: If the PD is unplugged or draws excessive current, the PSE immediately stops supplying power to ensure safety.

[!IMPORTANT] PoE and Ethernet Cables PoE works perfectly over twisted-pair copper cables (Cat5e, Cat6, Cat6A) up to the standard Ethernet distance of 100 meters. However, with high-power standards like PoE++ (60W/90W), you must pay special attention to the quality of the copper cores and heat dissipation, especially if cables are densely bundled together.

[!NOTE] Exam Essentials

• Function: Transmits both data and power simultaneously over a single network cable.
• Components: PSE (Power Sourcing Equipment - supplies power) and PD (Powered Device - receives power).
• Standards (IEEE): 802.3af (PoE, 15.4W at PSE), 802.3at (PoE+, 30W at PSE), and 802.3bt (PoE++, 60W/90W at PSE).
• Power Loss: The power received at the PD is always lower than the power delivered by the PSE.
• Power Budget: The maximum total power the Switch can provide. Not all ports can output maximum power simultaneously.

References

• CCNA Exam Topics v1.1 (Cisco)
• Cisco Power over Ethernet (PoE) Solutions (Cisco)
• Cisco UPOE+: The Catalyst for Expanded IT-OT Convergence White Paper (Cisco)