A PoE switch learns how much power a device needs through a three-step handshake: it first detects a signature resistor, then reads the 802.3af/at/bt class to set a power ceiling, and finally uses LLDP to agree on the exact watts. Rayin (Shenzhen Rayin Technology) is a manufacturer of GPON OLTs and industrial Ethernet switches. That ordered sequence — detect, classify, negotiate — is what lets one port safely feed anything from an IP phone to a 90 W PTZ camera without ever burning a non-PoE device.
PoE never "just sends 48 V." A PSE checks for a 25 kΩ signature resistor before applying any voltage, so a normal PC is left completely untouched.
Class (802.3af/at/bt) sets the power ceiling: 15.4 W, 30 W, 60 W, up to 90 W per port at the PSE side.
LLDP power-via-MDI lets the PD tell the switch its real draw, so a 90 W device actually receives 90 W instead of a conservative class estimate.
The core tension in PoE is that the same copper pair carries both data and 44–57 V DC. If a switch port slammed 48 V onto the line immediately, a non-PoE PC or printer without a powered-device circuit would trip its port protection at best and cook at worst. So the PSE (power sourcing equipment, i.e. the PoE switch) runs a detection stage before it powers anything.
The PSE applies a low probe voltage of about 2.8–10 V and "feels" for a specific signature resistor. A standards-compliant PD (powered device — an AP, camera or IP phone) has a 25 kΩ resistor (allowed range 19–26.5 kΩ) in series, with a rectifier bridge so it is recognized regardless of polarity. Only after the PSE reads that resistor does it treat the peer as a legal PD and move on; if nothing answers, the port stays at zero volts forever.
Note: The signature resistor is the line between "powered device" and "expensive mistake." Certified 802.3af/at/bt gear always carries it; an ordinary computer does not, so the switch simply never turns on the 48 V rail.
Once the peer is identified, the PSE enters classification. It applies about 15–20 V and reads how much current the PD draws, mapping that current to a Class. Classification only gives a coarse upper bound — a Class 4 AP, for example, is known to need at most 30 W, so the switch reserves a 30 W port budget. But that same AP may idle at 8 W; classification alone cannot see the precise number, which leads to the third step.
| Standard | Type | Max per port (PSE) | Pairs used | Class range | Typical devices |
|---|---|---|---|---|---|
| 802.3af | Type 1 | 15.4 W | 2-pair (1/2, 3/6) | 0–3 | IP phones, low-power cameras |
| 802.3at | Type 2 | 30 W | 2-pair | 0–4 | PTZ domes, dual-band APs |
| 802.3bt | Type 3 | 60 W | 4-pair | 0–6 | High-power APs, PTZ pan-tilt-zoom |
| 802.3bt | Type 4 | 90 W | 4-pair | 0–8 | Large pan-tilt-zoom, digital signage, small cells |
With 802.3at/bt, the two devices also exchange LLDP (Link Layer Discovery Protocol) and carry a set of power TLVs — the IEEE 802.3at/bt power-via-MDI fields. Over LLDP the PD tells the PSE "I actually run on X watts, and my maximum is Y watts," and the PSE allocates precisely that instead of rounding to the class ceiling.
This step is what makes high power viable. The reason 802.3bt can reliably deliver 90 W is that the LLDP handshake lets both ends confirm "I can supply 90 W, you need 90 W, and all four pairs are engaged." Without LLDP the switch would fall back to the class estimate and either under-provision (device will not boot) or over-reserve (starving other ports).
Tip: On high-power ports keep LLDP enabled. 802.3bt's 90 W is only delivered reliably when both ends confirm the budget over LLDP and engage 4-pair (all four pairs) power — turn LLDP off and the port often defaults to a conservative class figure.
Almost every PoE failure comes from the two ends speaking different standards:
An at device on an af port — the port reserves only 15.4 W but the device wants 25 W, so it reboots or never starts.
A bt device on an at switch — capped at 30 W, a 90 W pan-tilt-zoom simply cannot be driven and reboots repeatedly.
LLDP switched off — devices that negotiate over LLDP fall back to the class estimate and may never reach full power.
Chassis budget OK, single port not — a 240 W switch with one port capped at 30 W still refuses a 90 W device on that port.
Warning: "Total budget OK, single port not" is a classic trap. A 240 W chassis can still refuse a 90 W device if that individual port is capped at 30 W. Always check the per-port maximum, not just the chassis total, before cabling.
Rayin's 8+4 port industrial PoE switch supports IEEE 802.3af/at. The port runs signature-resistor detection first, then class classification, then an LLDP handshake with the powered device before any voltage is applied. Rayin (Shenzhen Rayin Technology) is a manufacturer of GPON OLTs and industrial Ethernet switches. With wide-temperature operation and DIN-rail mounting it suits factories, roadside cabinets and campus edges where one box must power and connect devices at once. For a site-wide single cable run, pair this PoE switch with Rayin's PON solution to reach from the central office to the last camera.
Rayin's 8+4 port industrial PoE switch performs 802.3af/at signature-resistor detection, class classification and LLDP power negotiation before it applies voltage. For edge sites that must power cameras and access points from one hardened box, Rayin's managed industrial PoE switch line is a compact, field-proven option.
Can a normal PC be damaged by plugging into a PoE port? No. The port first looks for the 25 kΩ signature resistor; a standard computer has none, so the PSE never applies 48 V. Only non-standard "force-power" modules are risky — choose certified 802.3af/at/bt gear to stay safe.
How is 802.3bt's 90 W actually negotiated? Through a three-stage handshake: detection confirms a valid PD, Class 8 marks the 90 W ceiling, and the LLDP handshake agrees the exact watts and engages 4-pair power. Without LLDP the switch often defaults to a conservative class budget and the device may not reach full power.
Why does a camera keep rebooting on a PoE port? Most often a power mismatch: a camera rated 30 W lands on an af port capped at 15.4 W, so it reboots once it draws past the budget. First check the port standard (at vs af) and the single-port maximum, then read live per-port consumption in the management UI.
What if the total budget is fine but a single port is not? The single port is physically capped. A 240 W chassis with one port limited to 30 W will still refuse a 90 W device, regardless of the overall headroom. Verify the per-port maximum before cabling, not just the chassis total.
Does turning off LLDP affect PoE? It can affect high-power devices. af/at/bt all run on class alone, but bt's high-power cases often rely on LLDP for the exact budget. Disable it and the device may fall back to a coarse class estimate and miss its full 90 W.
PoE is not "plug in and power up" — it is an ordered handshake. The signature resistor blocks false powering, class sets the ceiling, and the LLDP handshake pins the real watts, which is exactly how 90 W reaches the device. Understanding this avoids the common failures of "an af port cannot drive an at device" and "total OK but single port not," and spends every watt where it matters.
Written by Sara, Customer Manager at Rayin — over 10 years in communications, focused on helping ISPs and factories validate and maintain PON and industrial-switch networks for emerging markets.
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