UK What PSE and PD mean in PoE
PSE (power sourcing equipment) and PD (powered device) are the two roles in every Power over Ethernet link. The PSE injects DC power onto the Ethernet cable; the PD receives it and runs from it. The terms come from the IEEE 802.3 standards, 802.3af, 802.3at and 802.3bt, which define how the two ends find each other, agree how much power is needed and stay safe while it flows.
Industrial PoE switches are the most common PSE, while IP cameras, wireless access points and VoIP phones are the classic PDs. The roles are not fixed by product type, though. An edge computer with PoE ports is a PSE for the cameras it powers, and the same class of computer can be a PD when it is itself powered over a single cable.
Knowing which end is which matters when sizing a system: the PSE sets the power available, and the PD sets the power needed.
Types of power sourcing equipment
The standards describe two places a PSE can sit in the link.
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Endspan PSE. Power comes from the same device that provides the data port, such as a PoE switch, an NVR with PoE ports or a PoE computer. This is the tidiest option, with one box and one power budget.
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Midspan PSE. A separate injector sits between a non-PoE switch and the PD and adds power to the cable. Multi-port midspans do this for a whole patch panel.
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PoE computer as PSE. Our PoE computers include models with several PoE+ ports, so the computer powers and connects cameras or sensors directly, with no separate switch.
How a PSE and a PD agree on power
A standards-based PSE never powers a cable until a PD has identified itself. First, during detection, the PSE applies a low voltage, about 2.7 to 10 V, and measures the PD’s signature resistance of around 25 kΩ. A device without the signature, such as a laptop, gets data only. Next, in classification, the PSE applies a slightly higher voltage and the PD draws a set current that tells the PSE its class, from Class 0 to Class 8, so the PSE can reserve the right power from its budget. Then the PSE switches on around 48 V DC and keeps monitoring. If the PD is unplugged or stops drawing its maintain-power signature, the port turns off. PoE+ and PoE++ PDs can also refine their request over LLDP once the link is up. Our guide to industrial Power over Ethernet covers the PD and PSE handshake in more depth.
PSE vs PD compared
| Feature | PSE | PD |
|---|---|---|
| Role | Supplies power on the cable | Draws power from the cable |
| Examples | PoE switch, injector, NVR, PoE computer | IP camera, access point, VoIP phone, PoE monitor |
| In the handshake | Detects, classifies, then powers | Presents a 25 kΩ signature and a class |
| Voltage | Outputs about 44 to 57 V DC | Accepts about 37 to 57 V DC |
| Power figure | Per-port maximum and total budget | Class or LLDP request |
| Types | Endspan or midspan; Type 1 to 4 | Type 1 to 4, Class 0 to 8 |
| Key spec to check | Standard and budget | Class and worst-case draw |
Powered devices in industrial systems
PDs are no longer just cameras and phones. 802.3bt raised the power available at the device to about 71 W, which is enough for displays and small computers. Our PoE monitors and PoE panel PCs run from a single network cable, which removes the mains outlet at the point of use and lets a central UPS back up every screen through the switch. A PD of this kind has its own DC-to-DC converter, isolated from the cable as the standard requires, so a fault on the network side does not reach the electronics.
Some devices are both. A PoE pass-through switch or extender draws power as a PD on its uplink and passes some of it on as a PSE to devices further out, which is useful at the end of a long cable run where no power is available.
Sizing a link from both ends
A reliable PoE design checks the PD and the PSE against each other, not each on its own.
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Start with the PD’s worst case. Use the highest draw in the datasheet, such as a camera at night with IR and heaters on, not the typical figure.
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Allow for the cable. The PSE must supply more than the PD receives, because up to 100 m of cable loses some power as heat. The standards’ PSE figures already include this margin.
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Check the PSE’s per-port limit and total budget. A port rated for PoE+ cannot feed a Class 6 PD, and a switch whose budget is already committed may refuse a new device even if the port could handle it.
Quick answers on PoE PD vs PSE
PSE and PD: common questions
What is a PSE in PoE?
The power source. Power sourcing equipment, such as a PoE switch or injector, detects a PoE device on the cable and supplies it with DC power.
What is a PD in PoE?
The device being powered. A powered device, such as a camera or access point, presents a signature and a power class, then runs from the power on the cable.
What is the difference between endspan and midspan?
Where the power is added. An endspan PSE is the switch itself. A midspan PSE is an injector placed between a normal switch and the device.
Can a device be both a PD and a PSE?
Yes. PoE pass-through switches and extenders are powered as a PD on one port and supply power as a PSE on others.
What happens if a PD needs more power than the PSE offers?
It may not start, or may run in a reduced mode. Some devices disable heaters or motors; others refuse to boot. Match the PSE’s standard and budget to the PD’s class.
We match PoE sources to the powered devices on each project and check the power budget end to end as standard. Send us your device list and we will size the system.