Definition
PoE (Power over Ethernet) provides DC power while transmitting data through the network cable, compliant with IEEE 802.3af/at standards. Edge gateways, LoRa gateways, and other devices can choose PoE power where network cables are available, eliminating the need for separate power lines and simplifying deployment. In areas without network cables, a 4G uplink combined with 12V power or solar solutions can be used.
Technical Specifications
IEEE 802.3af (PoE): maximum 15.4W, approximately 12.95W at the powered device; 802.3at (PoE+): maximum 30W, approximately 25.5W at the powered device; 802.3bt (PoE++): maximum 90W. Power Sourcing Equipment (PSE) includes switches or injectors; Powered Devices (PD) include cameras, LoRa gateways, APs, IP phones, etc. DC power is transmitted through unused wire pairs in the network cable (4/5, 7/8) or the data wire pairs without affecting data communication.
Use Cases
Deployment of IP cameras, ONVIF devices; LoRa gateways, edge gateways; wireless APs, IoT gateways. In scenarios with Ethernet wiring, PoE can simplify power supply and reduce adapters and cabling. Industrial scenarios require industrial-grade PoE switches that support wide temperature ranges and IP protection.
Classification, Detection, and Cable Drop
Detect first, classify, then apply power
Power over Ethernet does not mean the cable is live all the time. The power sourcing equipment first puts a small detection voltage on the pairs and, only after it sees the powered-device signature resistance, moves on to classification. During classification the powered device reports the power class it needs, and the sourcing equipment raises the pair to the working voltage for that class. Forcing power without this step can damage a network port that does not support being powered. Data still travels as a differential signal. DC sits on the common mode of the pairs, so the same cable can carry frames and power together, provided both ends use the pairs in the same mode.
Power in the standard is what the sourcing end delivers, not what the powered end actually receives. Cable resistance consumes part of it. A camera at the end of a hundred-meter run receives less than the switch port's stated power. Classes run from a little over ten watts, to several tens of watts, and higher. At startup a powered device may also exceed the average it declared at classification. A port that negotiates successfully is not proof that both steady state and startup are covered.
Add the switch budget up port by port
The condition for use is that the chassis power budget, the class of each port, and the cable length are satisfied together. A switch may say every port supports a high power class while the total budget covers only a few of those ports at full load at once. Cameras that turn on heaters or illuminators at night draw more power. Normal by day and a mass restart at night means the budget was measured in daylight. Outdoor cable and poor connectors have higher resistance and a larger drop. If the industrial site also cascades switches, recompute the budget at every hop. Do not assume an uplink port will pass power onward.
If an industrial router or an edge host is only a data device and is not itself a power source, an injector or a switch that supplies power is required in between. The injector's power class has to cover the device at the far end. Do not insert another injector on a port that is already supplying power. Two DC sources stacked on the same pairs damage the port circuitry.
Cable loss, mode, and a failed powered-device detect
A failed detection is usually not a dead device. It is a wiring order or a mode that does not match. The source uses only the spare pairs, the powered device detects only on the data pairs, negotiation never completes, the link light blinks, and the device stays dark. Non-standard proprietary power at a higher voltage, connected to a standards-compliant powered device, damages it outright. Another error is to treat a port that the switch has shut off after the budget was exhausted as a network fault. Staff unplug and replug the cable, while the switch is actually cycling power because total power is over the limit.
A cable longer than the design, or two lengths joined together, pushes resistance over the limit. Voltage at the powered end falls below the working threshold and the device starts repeatedly. Measure voltage at the far end. Do not stop at a switch display that says power is being supplied. If the shield is used as the DC return, interference enters video and serial links. Finally, check grounding. When the lightning ground of an outdoor camera and the switch ground differ by a large potential, DC and surge arrive together at the port circuitry. The power function itself does not stop a lightning strike.
The classification current of a powered device only says which class it "wants." The sourcing end may refuse a higher class. After negotiation fails, some devices still try to start at the lowest voltage. The link comes up and the device does not run stably. Read the class actually granted on the switch. Do not stop at "the cable is plugged in." At the instant a backup supply switches over, the sourcing end may run detection again, the camera restarts, and the recording misses a few seconds. See that from a recording-gap alarm on the storage side, rather than replacing the camera as if it had failed.