Simplifying Network Installations
Power over Ethernet (PoE) allows electrical power to be transmitted with data over a standard . In suitable deployments, this removes the need for a separate local power supply for devices such as IP cameras, VoIP phones, wireless access points, and IoT sensors.
Power delivered through network cabling can simplify installation and allow devices to be placed away from a nearby power outlet. The resulting infrastructure cost depends on the cabling, PSE, protection, and installation requirements.
How PoE Works: Key Components and Process
A PoE-enabled system consists of two primary types of devices:
- Power Sourcing Equipment (PSE): This is the device that supplies power over the Ethernet cable. PSEs can be integrated into a network device, such as a PoE switch, or can be a standalone device, known as a PoE injector, which adds power to an existing Ethernet line.
- Powered Device (PD): This is the end device that receives both power and data from the Ethernet cable, such as an IP camera, VoIP phone, or wireless access point. PDs are designed to safely accept this power and use it for their operation.
PoE detection and classification
To limit the risk of applying power to a non-PoE device, the PSE performs detection before applying operational power. An optional classification step then communicates the PD power requirement:
- Detection: The PSE sends a low voltage signal to check if the connected device is PoE-compatible. The PD responds with a detection resistance of about 25 kΩ.
- Classification: Once a PD is detected, an optional classification step communicates its power requirement to the PSE. The PD signals its power class by drawing a specific amount of current.
- Power-up: If detection and classification are successful, the PSE supplies full operational power to the PD.
Evolution of PoE Standards
The IEEE 802.3 working group has standardized several PoE amendments to meet different power requirements. The available power depends on the type, class, PSE, PD, and cable path. The values below refer to a compliant copper channel up to 100 m. For Type 1, IEEE 802.3af permits Category 3 cabling for a 10BASE-T interface, while Type 2 to Type 4 deployments normally use balanced cabling of Category 5e or better. Cable construction, bundle size, and ambient temperature affect loss and heating.
| Parameter | PoE (IEEE 802.3af) | PoE+ (IEEE 802.3at) | PoE++ (IEEE 802.3bt) |
|---|---|---|---|
| IEEE type and classes | Type 1, classes 0 to 3 | Type 2, class 4 | Type 3, classes 5 and 6; Type 4, classes 7 and 8 |
| Powered pairs | 2 pairs | 2 pairs | Type 3: 2 or 4 pairs; Type 4: 4 pairs |
| Power at PSE Output | 15.4 W | 30 W | Type 3: 60 W, Type 4: 90 W |
| Guaranteed power at PD input | 12.95 W | 25.5 W | Type 3: 51 W, Type 4: 71.3 W |
| Voltage at PSE | 44 to 57 V | 50 to 57 V | Type 3: 50 to 57 V; Type 4: 52 to 57 V |
| Maximum current per pairset | 350 mA | 600 mA | Type 3: 600 mA; Type 4: 960 mA |
Note: The guaranteed PD input values include the allowed loss in a copper channel up to 100 m. The difference between PSE and PD power is mainly caused by path resistance, and the actual result also depends on the PSE and PD class.
Advantages and Challenges of PoE
Advantages:
- Flexibility and Convenience: Devices can be installed in optimal locations, such as high on walls or ceilings for access points and cameras, without worrying about nearby power outlets.
- Potential cost and time savings: A PoE deployment can avoid a separate power outlet at the device location. Installation time and cost still depend on electrical and cabling requirements.
- Reliability and centralized power management: PoE devices can be connected to a central via the PoE switch, so the network can remain operational during a power outage if the UPS capacity is sufficient.
Challenges:
- Power Budgeting: The total power required by all connected PDs must not exceed the total power budget of the PSE (PoE switch). Careful planning is needed for large installations.
- Distance limitation: A standard Ethernet channel is limited to 100 meters (328 feet), normally comprising up to 90 meters of permanent link and up to 10 meters of cords. Power delivered to the PD decreases with channel length because of cable resistance.
- Heat dissipation: Bundling multiple high-power PoE cables raises conductor temperature and resistance. Bundle size, cable construction, ambient temperature, and installation limits must be considered when selecting and installing the cabling.